Textbook of Veterinary Histology (Part I) Beni-Suef University PDF

Beni-Suef University Faculty of Veterinary Medicine Department of Cytology & Histology TEXT BOOK OF VETERINARY HISTOLOGY (Part I) Prepared By Prof. Dr Dr/ Khalid M. Mazher Prof. Dr/...

Beni-Suef University Faculty of Veterinary Medicine Department of Cytology & Histology TEXT BOOK OF VETERINARY HISTOLOGY (Part I) Prepared By Prof. Dr Dr/ Khalid M. Mazher Prof. Dr/ Taghreed M. Nabil Prof. Dr/ Usama K. Moawad Dr/Abdelrazek H. Abdelrazek Dr/ Randa M. Hassan (2022- 2023)  Contents NO. Contents Page 1 Introduction 1 2 Cytology 2 3 Epithelial tissue 17 4 Connective tissue 28 5 Blood 50 6 Muscular Tissue 64 7 Nervous tissue 73 8 Nervous system 90 9 Lymphatic system & 104 Immunity 0  Introduction Histology is a branch of the main science biology dealing with the microscopic and ultra structures of all body tissues and cells which in accordance form the organs and systems. Also it is defined as microscopic anatomy dissecting the cells and tissues that build up the organs and systems. The studying of the normal structure of tissues (histology) as a basic science is very important as it helps in detecting and identifying the diseased tissues, explains the physiology and biochemistry of different cells and tissues, facilitates the study of pathogenesis of the diseases and illustrates the relation between the pathogenic microorganisms and the cells in addition to explaining the optimum roles for fattening and reproduction. In addition, there is a progressive need to the science of histology to overcome the untreatable diseases by the stem cell technology. During the last few years the science of histology is greatly expanded parallel to the great advancement in microscopic tools and techniques as electron microscopy, immunocytochemistry, image analysis, immunogold electron microscopy, nanotechnology and stem cell technology. Our curriculum includes Cytology, General histology and Special histology. The general histology includes epithelial tissue, connective tissue, muscular tissue and nervous tissue. The special histology represents the different systems of the body. In this note we try to present a very short, simple and basic histological information provided with the intended learning outcomes (ILOs) of each chapter according to NARS rules that helping the students to be aware of the basic objectives. With my best wishes and congratulations Prof. Dr. khaled Mazher 1  Cytology At the end of this chapter, the student should be able to:
Give a brief note about the cell constituents.
Classify the cytoplasmic organelles and compare between them.
Define briefly each cytoplasmic organelle, draw a labeled diagram of this organelle and list its functions.
Compare between the organelles and inclusions.
Give a full account about the nucleus and its constituent.
Explain the modification of the cell constituents to perform specific functions with examples. The term cytology (CYTO=CELL, OLOGY=SCIENCE) deals with the microscopic and ultrastructure of the different cell types and their constituents in addition to their functions and functional correlates. THE CELL Definition: the cell is structural and functional unit of any living organism. In some animals the cell can exist independently. Shape: the cells take different shapes according to its function and location. It may be spherical, star shape, cuboidal, oval, columnar, flat or polyhedral. Cell size: In mammals, cells are rarely seen by naked eye. They are measured by microns. The smallest cell is measured about 4 µm (granular cells in cerebellum) and the largest one is about 300 µm (megakaryocyte in bone marrow). Some cells are greatly elongated so that their length is measured in centimeters (skeletal muscle cells) or even in meters (nerve cells). Function: all functions that carried out by the organs are in fact related to the cells of these organs as secretion, contraction, respiration, excretion, sensation, growth, reproduction and absorption. Structure: the living cells either eukaryotic (contain true nucleus) or prokaryotic (contain only chromatin material). The animal cell is eukaryotic type consists of cytoplasm and nucleus. THE CYTOPLASM The cytoplasm of any cell has three basic components: 1. Cytosol: it is the fluid component consists of water, lipid, proteins, carbohydrates, RNA, minerals, ions, salts, metabolites, O2and CO2. The cytosol plays an important role in cell metabolism and cell movement. 2. Organelles: they are living and permanent structures essential for all nucleated cells. They also have vital functions and are metabolically active. 3. Inclusions: they are non-living and temporary structures not essential for each cell. They result from the cell activity and are metabolically inactive. 2  The cytolasmic organells Definition: they are living and permanent intracellular structures that carry out the vital functions of the cell. They are classified according to the presence or absence of membrane into membranous and non-membranous organelles. Membranous organelles Non membranous organelles - Surrounded with membrane - not surrounded by membrane - Have metabolic enzymes - free from enzymes - It includes: - It includes:- 1-Cell membrane 1-Ribosomes 2-Mitochondria 2-Cytoskeleton: 3-Rough endoplasmic reticulum a-microtubules 4-Smooth endoplasmic reticulum b- microfilaments 5-Golgi apparatus 6-Lysosmes The membranous organelles 1-The cell membrane It also called plasma membrane or plasmalemma. Definition: it is a limiting membrane surrounding the cell cytoplasm externally. It measures about 7.5-10 nm in thickness. LM: cannot be seen by light microscopy while the cell coat (the external part of the cell membrane) can be demonstrated when stained with PAS (pink) or silver stains (black). EM: It appears as trilaminar membrane as it formed from two electron dense dark lines separated by an electron lucent clear one. It also called unit membrane. 3 Molecular structure of the cell membrane: The cell membrane consists of lipid, protein and carbohydrates. 1-lipid components: includes phospholipids and cholesterol. a- phospholipid molecules: appear as two layers (lipid bilayer), each layer has polar and non polar ends - hydrophilic polar ends: they are charged ends like water and directed outward the cell membrane. - hydrophobic non polar ends (tails): they are non charged, do not like water and directed inward the cell membrane, so that the tails of the two layers facing each other. b-Cholesterol molecules: they are insert themselves among the tails of the phospholipids. 2- Protein component: according to its location in the cell membrane, the proteins of the cell membrane have two forms: a- Integral protein: embedded in the lipid bilayer and some of them extend across the cell membrane from side to side (transmembranous protein). b- Peripheral protein: small molecules loosely attached to the surfaces of the cell membrane. 3- Carbohydrate component: present only at the external surface of the cell membrane. It is formed of: a- Glycoproteins: oligosaccharide chains linked to the protein moloecules b- Glycolipids: oligosaccharide chains linked to the phospholipid molecules.
Cell coat (glycocalyx): it represents the external surface of the cell membrane formed of glycoproteins and glycolipids. It includes specific receptors for drugs and hormones. Functions of the cell membrane: 1- Passive transport: water, ions and dissolved gasses enter the cell according to their concentration. 2- Facilitated transport: through the integral proteins e.g. glucose and amino acids. 3- Active transport: against the concentration gradient so it needs energy as sodium / potassium pump. 4- Endocytosis: bulk uptake of substance across the cell membrane, it may be: 4  a) Phagocytosis: The cell engulfs an extra cellular material as bacteria, virus or damaged cell. The engulfed material called phagosome. b) Pinocytosis: The cell engulfs fluid material. The engulfed material called pinosome. 5- Exocytosis: a membranous cytoplasmic vesicle fuses with the cell membrane to release its content outside the cell. Functions of the cell coat: 1-Cell immunity 2- Cell adhesion 3- Cell protection 4- Cell recognition. 2- Mitochondria Definition: one of the membranous organelles responsible for energy production. They consider the power house of the cell. Site: they present in the part of the cytoplasm where the energy is needed as beside rER in secretory cells, between myofibrils in muscle cells and in the basal infolding in the ion transport cells. Number: their number varies according to the activity of the cell. Mitochondria are self- replicating organelle, so they can increase their number as they contain DNA. LM: - appear as dark blue dots when stained with Iron Hematoxylin. - appear as green dots when stained with genus green. EM:- appear as rounded or elongated, double membranous structure of about 1um in diameter and 10 um in length. - The outer mitochondrial membrane: is smooth and permeable - The inner mitochondrial membrane: forms shelf-like folds or cristae with variable number to increase the surface area for oxidative phosphophosphorylation. - Mitochondrial matrix: contains oxidative enzymes, DNA, RNA and electron dense granules rich in calcium. Functions: 1- Production of energy from metabolites present in the cytoplasm by Kreb`s cycle. 2- Storage of energy as ATP molecules by oxidative phosphorylation. 5  3- Rough endoplasmic reticulum (rER) Definition: one of the membranous organelles responsible for protein synthesis. Present mainly in protein secreting cells. Site: present around the nucleus and associated with the nuclear membrane. LM: either localized or diffused basophilic dots localized as in pancreatic cells (basal basophilia) or diffused as in plasma cell (diffused basophilia). EM: appear as parallel flattened membranous sacs or cisternae connected with each other by short canaleculi. These membranous sacs are studded with polyribosomes on a special site which give this organelle its rough appearance. Functions: 1- Synthesis of protein for export outside the cell as polypeptide hormones and digestive enzymes. 2- Synthesis of hydrolytic enzymes of primary lysosome. 3- Packing of formed proteins in membranous vesicles to be delivered to Golgi apparatus as transfere vesicles. 4- Act as intracellular pathway for some substances. 4- Smooth endoplasmic reticulum Definition: one of the membranous organelles responsible for steroid synthesis Site: well developed in steroid hormone secreting cells (adrenal cortex, testis and ovary), fat cells and liver cells. LM: cannot be seen. EM: Branching and anastomosing intercommunicating tubules with variable shaped cisternae. They may be continuous with rER but they lacking attached ribosomes. Functions: 1- synthesis of steroid hormones as cortisone, esterogen and progesterone. 2- Detoxication of drugs, hormones and some chemicals in liver cell. 3- Act as a pump releasing calcium ions during muscle contraction. 4- Synthesis of cholesterol and phospholipids. 5- Act as intracellular pathway for some substances. 5- Golgi apparatus Definition: one of the membranous organelles responsible for secretion, so it is well developed in secretory cells. Site: - in pancreatic cells present supranuclear (in the apical part). - In nerve cells present perinuclear (around the nucleus). 6 LM: it can be demonstrated by silver stains as network of brownish granules and fibers. In plasma cell, which characterized by diffuse basophilia, Golgi apparatus could be recognized as a pale unstained area near the nucleus called negative Golgi image. EM: Golgi apparatus consists of: 1- Golgi stalks or membranous saccules: several parallel, flat and curved saccules with dilated ends. These saccules are interconnected with each other. Each saccule has two surfaces: a- immature convex surface: receives transfer vesicles which carry crude protein from rER. b- Mature concave surface: from which the secretory vesicles bud off carrying protein ready for secretion. 2- Transfer or Micrivesicles: small membrane bound vesicles carrying crude protein from rER to the immature surface of the Golgi stalks. 3- Secretory or Macrovesicles: large membrane bound vesicles arise from the mature active surface of Golgi stalks carrying protein ready for secretion. Functions: 1- concentration, sorage and packing of proteins recived from rER. 2- conjugation of protein with carbohydrates, sulphates and salts. 3- formation of coated vesicles and primary lysosomes. 4- renewal and maintenance of the cell membrane. 6- Lysosome Definition: one of the membranous organelles contains hydrolytic enzymes and responsible for cellular digestion. Number: a huge number of primary lysosomes present in the phagocytic cells as monocyte, neutrophils, lymphocytes and macrophages. Origin: the hydrolytic enzymes are synthesized in the rER then transferred to Golgi apparatus then released as primary lysosomes. LM: can not be seen but the acid phosphatase enzyme inside the lysosome could be detected by histochemical reactions. EM: 1- Primary lysosomes: membrane bound spherical vesicles contain hydrolytic enzymes. They have homogenous electron density. 2- Secondary lysosomes: when the primary lysosome fuses with any intra cytoplasmic vesicle either exogenous or endogenous it becomes a secondary lysosome. The secondary lysosome appear heterogenous in electron density due to digested and undigested particles. 7 Types of secondary lysosomes: a- Heterolysosome: fusion of the primary lysosome with a phagocytic vesicle as dust, bacteria or virus b- Multivesicular body: fusion of the primary lysosome with a pinocytotic vesicle c- Autophagosome: fusion of the primary lysosome with an old organelle as mitochondria or ER leading to its hydrolysis d- Residual body: the undigested materials inside the lysosome as dust retained as residual body then the cell expelled it by exocytosis (cytostol) or it accumulate inside the cell as lipofuscin pigment in nerve cells. Functions: 1- protection of the cell by digestion of the engulfed bacterea or viruses. 2- Maintain the cell health by continuous removal and elimination of the old organelles. 3- Autolysis after cell death the lysosomal enzymes liberated and hydrolyse the cell constituents. 4- Helps in fertilization of the ova by the acrosomal enzymes. 5- Formation of thyroid hormones by digestion of colloid. Non membranous organelles 1- Ribosomes Definition: they are non membranous electron dense bodies present in all cells but increased greatly in protein producing cells. Structure: formed of rRNA from the nucleolus and cytoplamic protein. Both structures conjugate together inside the nucleus to form large and small subunits, then they pass to the cytoplasm for protein synthesis. LM: accumulation of a large number of ribosomes causes cytoplasmic basophilia which may be diffused (embryonic cells), localized (pancreatic cells) or focal (Nissl`s granules of nerve cells). EM: a ribosome appears as small electron dense granule formed of small and large subunits. The large subunit contains a groove in its center housing the polypeptide chain. The two subunits connected together by mRNA. More than one ribosomes are connected together by mRNA forming polyribosomes, the latter may be free or attached. 8 Free ribosomes: scattered singly throughout the cytoplasm. Attached ribosomes: mainly attached to the outer nuclear membrane and the outer surface of rER by the large subunits at glycoprotein receptors. Functions: -Free ribosomes: synthesize protein to be used inside the cell as glycolytic enzymes. The mRNA that carries information about the polypeptide to be synthesized attached to ribosome between large and small subunits, tRNA picks up the specific amino acids and transports them to the ribosome which translate the code into polypeptide chain that extends through the groove of the large subunit. - Attached ribosomes: they form the proteins that secreted outside the cells as hormones and enzymes. 2- Cytoskeleton The term cytoskeleton refers to a complex network of microtubules and microfilaments together with some linked proteins leading to formation of what is called microtrabecular lattice. A- Microtubules Definition: they are fine tubules of variable lengths and regular diameter of about 25nm. LM: can not be seen but they could be demonstrated by immunohistochemistry. EM: they appear as fine tubules formed of alpha and beta tubulin protein molecules arrange into 13 protofilaments. Forms: 1- Dynamic form with continuous assembly and disassembly helping in cell movement 2- Stable or organized form represented by centerioles, cilia and flagella. Functions: 1- Determination of the cell shape. 2- Formation of mitotic spindle during cell division. 3- Intracellular transport of macromolecules. 4- Form the centerioles, cilia and flagella. 9  Centerioles Definition: one of the non membranous organelles formed of highly organized microtubules. LM: appear as two dark bodies near the nucleus when stained with iron hematoxylin. EM: appear as two perpendicular cylinders, each one formed of nine triplets microtubules connected with each other by protein link. The two cylinders are surrounded by matrix of tubulin called centromere. Functions: - During mitosis the centeriole dublicates and each one move towards cell pole and form mitotic spindles. - Form the basal body of the cilia and flagella. Cilia Definition: they are motile processes formed of a core of microtubules covered by cell membrane. Origin: the centeriole duplicate several times forming basal bodies of the cilia that migrate to the apical part of the cytoplasm, then shafts grow up. LM: appear as hair like striations. EM: Each cilium is formed of a basal body, a shaft and a rootlet. 1-The basal body: nine triplets microtubules resembling the centeriole. 2- The shaft: nine doublets microtubules and two singlets at the center. 3- The rootlet: nine singlet microtibules fix the basal body and the shaft to the cytoplasm. Flagella They are very long, motile projections formed of nine dublets microtubules housing two singlets at the center. In vertebrates the flagella is restricted to the sperm only. B- Microfilaments Definition: they are thread like structures distributed allover the cytoplasm and constitute a part of the cytoskeleton. L/M: Some of them appear in ordinary sections (Tonofibrils & Myofibrils), others require a special stain (Neurofibrils are argyrophilic). They appear as very fine lines. E/M: They appear as very fine thin threads of varying length and diameter. In C.S they appear as minute dots. Regarding their arrangement, they are either: - Regularly:- arranged to form myofibrils in muscle cells and tonofibrils in epithelial cells. - Irregularly:- arranged to act as a cytoskeleton. 10 Functions: - Microfilaments in every cell act as a cytoskeleton. - Myofibrils in muscle cells contract muscle fibers. - Tonofibrils in epithelial cells connect adjacent cells together. Cytoplasmic inclusions They are temporary, nonliving and nonessential metabolically inactive materials in the cytoplasm. They exist in 3 different forms: stored food, pigments and secretory granules. 1- Stored food: It is biochemical compounds produced by the cell itself and stored inside it till the time of need. It includes carbohydrates and fats a- Carbohydrates: - They are cytoplasmic inclusion stored in the cells of liver and muscle in the form of glycogen. - They can be demonstrated by LM. As red granules when stained with Best`s carmine or pink granules when stained by PAS stain. - By EM:- They appear as rosette-like aggregations. b- Fats: - They are cytoplasmic inclusions stored in special cells called fat cells or adipocytes as large globules and occasionally present in liver cells as small droplets. - They can be easily demonstrated by LM. as black spots when stained with Sudan black or osmic acid stains, and orange colour when stained with Sudan III. These stains applied after freezing technique as the fat dissolved during preparation with paraffin technique. 2- Pigments: Definition: they are coloured intracellular inclusions either produced within the cell itself (endogenous) or taken from outside (exogenous). Types:a-Endogenous pigments: they are coloured inclusions produced by the cell itself as hemoglobin in RBCs, melanin in melanocytes and lipofuscin in cardiac & nerve cells. b- Exogenous pigments: they are coloured inclusions engulfed or phagocytosed by the cell from outside as dust particles in lung, tattoo marks in skin and carotene pigments in other cells. 3-Secretory granules: Definition: One of the cytoplasmic inclusions which occur in secretory cells as a result of their activity (endogenous in origin). LM: Some granules appear in ordinary sections (zymogen granules stained pink with PAS). EM: each granule appears as vesicle surrounded by a unit membrane and filled with the secretory substance. Functions: Secretory granules possess so many functions depending on the nature of their contents. 11  Nucleus Definition: it is the largest and most prominent and important component of the cell. It is present in all true cells Shape: the nucleus may be spherical, oval, flattened, lobulated, segmented or kidney- shape. Position: The nucleus usually take a central position but sometimes it may be eccentric, basal or peripheral. Number: the cell may be mononucleated (one nucleus in each cell), binucleated (two nuclei in each cell as liver cell) or multinucleated (more than two nuclei in each cell as skeletal muscle cell). Components of the nucleus: 1- Nuclear membrane 3- Nuclear sap 2- Nuclear chromatin 4- Nucleolus 1- Nuclear membrane: It is a dark basophilic line surrounding the other nuclear components By EM.: It appears as double walled membrane formed of outer and inner nuclear membranes separated by perinuclear space of about 50nm. The nuclear membrane is interrupted at intervals by the nuclear pores. - The outer nuclear membrane: appears rough due to presence of attached ribosomes on its outer surface. It also continuous with rER. - The inner nuclear membrane: attached to the peripheral chromatin and continuous with the outer membrane at the nuclear pores. - The nuclear pores: they are circular openings where the inner and outer membranes fuse. They are formed of special protein called nucleoporins. 2- Nuclear chromatin: It constitutes the genetic materials of the cell consists of chromosomes that formed mainly of DNA and histones. LM: it appears as basophilic masses distributed allover the nucleus and attached to the inner nuclear membrane. It has two forms: a- Euchromatin: lightly stained appears pale basophilic. It contains the active genes that responsible for protein synthesis. The euchromatic nucleus called vesicular nucleus. It is present in active cells. By EM it appears as fine granules. b- Heterochromatin (condensed or inactive): appears dark basophilic as it represents the coiled chromosomes containing inactive genes.By EM it appears as coarse electron dense granules. Functions: 1- carries the genetic information of the organism. 2- controls the process of protein synthesis. 3- formation of different forms of RNA. 12 3- Nuclear sap: It is a colloidal fluid that filled the spaces between the nuclear chromatin and the nucleolus. It consists of nucleoproteins, enzymes, sugars and some ions. It provides a medium for nucleic acid synthesis and transport of RNA to the cytoplasm. 4- The nucleolus: It is a rounded or oval deeply basophilic mass formed of rRNA and surrounded by chromatin material. The nucleus contains one or more nucleoli. By EM: it appears as non membranous oval spongy structure formed of dark and light areas.  The dark area consists of: a- Pars granulosa: mature rRNA. b- Pars fibrosa: newly formed rRNA. c- Pars amorpha (nuclear organizer): DNA filaments.  The light area formed of nucleolar sap. 13  Modifications in cell constituents to perform specific function 1- Steroid hormone-secreting endocrine cells: They are characterized by: - Abundant smooth endoplasmic reticulum, this is because they are specialized for lipid metabolism. - SER are necessary to accommodate the enzyme needed to make cholesterol and modify it to form steroid hormones. - Mitochondria with tubular cristae which involved in steroid hormones synthesis. - Many lipid droplets, this is where cholesterol is stored. - Large Golgi apparatus. - Large vesicular nucleus. e.g., adrenal cortical cells, Leydig cells of testis and leutin cells of the ovary. 2) Protein-secreting cells: - Abundant rough endoplasmic reticulum. - Many mitochondria. - Large vesicular nucleus. - Well-developed Golgi apparatus. - Numerous zymogen granules. e.g. pancreas. 3) Glycoprotein-secreting cells: - Abundant rough endoplasmic reticulum. - Well-developed Golgi apparatus(addCHO). -Numerous apical mucigen granules (glycoprotein). - Oval basal nucleus. e.g. trachea and intestine (goblet cells). 14 4) Ion-transport cells: - Basal infolding (multiple and deep invaginations) of the cell membrane without infolding of the basal lamina. - Abundant elongated mitochondria filling the basal infolding the cell membrane. - Invaginations between adjacent lateral cell surface. - Tight junctions seal the apical portion of the cells. - Numerous microvilli. e.g., kidney (proximal&distal convoluted tubules) and striated duct of salivary gland. 15  Cell renewal The majority of the mature specialized cells are not in the active cycle (mitosis- interphase-mitosis). They perform their specific function in prolonged GI phase. Types of cells according to renewal ability:- 1- Renewable cell:- Some cells are continuously in mitosis with very short GI-phase as stem cells in bone marrow and stratum basalis of epidermis. 2- Potentially renewable cells:- Some specialized cells that go to GO phase (outside the cycle) can return back to continue the cycle under certain conditions as liver cells when exposed to severe degenerative effect the normal cell can divide to restore the normal size. 3- Non-renewable cells:- Contain cells as nerve cell and cardiac myocyte leave the cycle and persist in GO phase and can not return to the cycle. 4- Continuously turn over cells:- Some highly specialized cells are called end cell, they can not divide but can be replaced by other cells as sperms. Cell death The cell life and activity is terminated by either apoptosis or necrosis 1- Apoptosis:- - Also called programmed cell death or physiological cell death which occurs at the end of life span of each cell under the effect of a specific gene on DNA of the cell. - In this type of cell death, the apoptotic cell do not swell but decrease in size together with condensation of the nuclear chromatin which later on dissolute with the cytoplasm forming apoptotic bodies → phagocytosed. 2- Necrosis:- - It is a pathological condition results from toxins, mechanical injury or anoxia. - The necrotic cell swelled, vacuolated then brust and degenerated → phagocytosed. 16  Epithelial tissue At the end of this chapter, the student should be able to:
Mention the different characters of epithelial tissue.
Classify the lining and covering epithelium with examples.
Draw labeled diagrams about the different types of surface epithelium and mention the sites of each type.
Classify the glandular epithelium with examples.
Compare diagrammatically between the different types of glands according to nature of secretion and also the mode of secretion.
Mention the modifications of the epithelial cell surfaces and draw labeled diagrams about each of them. General characters of epithelial tissue: 1- It consists of crowded cells connected with each other by tight junction. 2- It rests on a basement membrane either continuous or interrupted. 3- Epithelial cells are highly regenerative cells. 4- It is originated from ectoderm, mesoderm and endoderm. 5- It is a vascular as blood and lymph vessels cannot penetrate epithelial cells. It receives nutrition by diffusion from the underlying CT. 6- Epithelial tissue is highly enervated. 7- Most of epithelial cells act as surface epithelium while some of them act as glandular epithelium and few of them are specialized epithelium (neuroepithelium & myoepithelium). Classification of epithelial tissue: 1- Lining and covering epithelium A- Simple epithelium: i- Simple squamous epithelium. ii- Simple cuboidal epithelium. iii- Simple columnar epithelium. iv- Pseudo stratified columnar epithelium. B- Stratified epithelium: i-Stratified squamous epithelium. ii-Stratified cuboidal epithelium. iii- Stratified columnar epithelium. iv- Transitional epithelium. 2- Glandular epithelium. 3- Specialized epithelium. a- Neuroepithelium. b- Myoepithelium. c- Reticuloepithelium. 1- Lining and covering epithelium Definition: it is a type of epithelial tissue that lines and covers all body organs as stomach, intestine, urinary bladder and heart. It is classified according to the number of cell layers into simple (one layer) and stratified (more than one layer). 17  A- Simple epithelium Definition: it is a single layer of epithelial cells that lines and covers body organs. It is classified according to the shape of the cell into simple squamous, simple cuboidal, simple columnar and pseudo stratified. i- Simple squamous epithelium Definition: it is one layer of squamous epithelial cells rest on a basement membrane. It has two views surface view and side view. The surface view: The cell shape: the cells are polyhedral and polygonal. The nuclei : the nuclei are rounded and centrally situated. The side view: The cell shape: the cells appeared thin, flattened and attenuated. The nuclei : the nuclei are elongated and centrally situated. Function: 1- It gives a smooth surface for easy passage of fluids as blood. 2- Its smooth surface facilitates the movement of organs as stomach, lungs and heart. 3- Allows easy passage of gasses in the lung. Sites: 1- Endothelium as in blood vessels, heart and lymph vessels. 2- Mesothelium as in pleura, pericardium and peritoneum. 3- Lining the lung alveoli (pneumocytes). 4- Lines the Bowman`s capsule of the kidney & the inner surface of the cornea. ii- Simple cuboidal epithelium Definition: It is a single layer of cuboidal cells rest on the basement Membrane. The cell shape: The cells are squares in cross sections. The nuclei : The nuclei are spherical and centrally situated. Functions: secretion or absorption. Sites: 1- Small ducts of glands as salivary gland. 2- Secretory alveoli of some glands as salivary glands. 3- Thyroid follicles. 4- Distal convoluted tubules of the kidney. 18  iii- Simple columnar epithelium Definition: a single layer of tall columnar cells. The cell shape: the cells are taller than width. The nuclei : the nuclei are oval and basally situated. Types: 1- Simple columnar secretory epithelium: the columnar cells free from cilia or microvilli. Function: Secretory epithelium. Site: Stomach and large ducts of glands. 2- Simple columnar ciliated epithelium: The apical surface of the cells provided with cilia Function: Moves fluids or particles in one direction over the surface epithelium Sites: Lung bronchioles and fallopian tubes. 3- Simple columnar provided with micrivilli: The columnar cells carry numerous microvilli on its apical surface. Function: absorption. Sites: small and large intestine and gall bladder. iV- Pseudostratified columnar ciliated epithelium Definition: it is a single layer of tall and short crowded epithelial cells rest on the same basement membrane but the short cells fail to reach the surface and the nuclei are present at different levels giving a false stratified appearance. The cell shape: there are two types of cells, tall cells reach the surface carrying cilia and short non ciliated cells fail to reach the surface. The nuclei: the nuclei appear at different levels. Types: 1- Pseudostratified columnar ciliated with motile cilia: Site: in upper respiratory tract as trachea, larynx and bronchi. 19 2- Pseudostratified columnar ciliated with non motile (sterocilia) cilia: They are not true cilia but long microvilli. Site: epididymis and vas deferens. Function: moves fluids or particles in one direction over the surface epithelium. B- Stratified epithelium: Definition: the epithelial membrane is formed of more than one layer and classified according to the superficial cells into, stratified squamous, stratified cuboidal, stratified columnar and transitional epithelium. i- Stratified squamous epithelium Definition: it is one of the stratified lining and covering epithelium in which the upper most cells are flattened squamous cells. It may be keratinized or non keratinized. a- Stratified squamous non keratinized epithelium It consists of basal layer, intermediate layers and superficial layers. a. The basal layer: one layer of columnar cells with basal oval nuclei. b. Intermediate layers: several layers of polyhedral cells with central spherical nuclei. c. Superficial layers: few layers of flat cells with elongated nuclei Sites: 1- Oesophagus. 2- anal canal. 3- External surface of cornea. 4- Vagina proper. b- Stratified squamous keratinized epithelium It resembles the non keratinized epithelium but the superficial flat cell layer is covered by tough non living keratinized or cornified layer Sites: 1- Epidermis of skin. 2- Hard palate. 3- Oral cavity of ruminants. 4- Dorsal surface of the tongue. 20  ii- Stratified cuboidal epithelium Definition: It is one of the stratified epithelium consists mainly of two layers of cuboidal cells. Sites: present in ducts of some glands as sweat gland. iii- Stratified columnar epithelium Definition: it is one of stratified epithelium in which the superficial layer is formed of columnar cells. Sites: 1-Large ducts of glands 2- Male urethra. iV- Transitional epithelium Definition: it is one of the stratified epithelium has the ability to change its form according to the state of the organ. Sites: present in urinary passages as renal pelvis, urinary bladder, ureters and urethra. Structure: according to the state of the organ. In empty bladder (relaxed state): The transitional epithelium consists of 6-8 layers arranged as: a) Basal layer: single layer of high cuboidal cells with spherical central nuclei b) intermediate layers: several layers of polyhedral or bear-shaped cells with central spherical nuclei and wide intercellular spaces containing mucous like substance. c) Superficial layer: single layer of dome-shaped cells with central spherical nuclei and convex apical surface. Some cells are binucleated. In full bladder (stretched state): -The number of layers decreases to 2-3 layers only. -The superficial layers appear flat but with rounded nuclei. 21 Functions: The arrangement of epithelium allows the urinary bladder to distend without rupturing or separating the lining cells. 2- Glandular epithelium Definition: it is a modified type of epithelial tissue in which the cells are specialized for secretion. They are classified according to the presence or absence of duct into: 1- Exocrine gland: the gland is formed of secretory part that produce the secretion and excretory part or duct system that carry the secretion outside the gland as salivary gland. 2- Endocrine gland: the gland is formed only of the secretory part while the duct system is absent and the secretion pass directly to the blood stream as adrenal gland, thyroid gland and pituitary gland. 3- Mixed gland: the gland consists of two parts exocrine part provided with ducts and endocrine part lacking ducts as pancreas and liver. Classification of exocrine glands A- According to the number of cells: 1- Unicellular gland: the gland is formed of one cell scattered singly throughout the epithelial membrane as goblet cell. Goblet cell: Definition: it is a unicellular gland specialized to produce glycoprotein and scattered singly between the epithelial cells of respiratory and digestive tracts Structure: LM: - It has an upper expanded part (the free end) which is enlarged and a lower narrow part contains the nucleus. - The free end contains secretory granules (mucigen granules). - In H & E sections, the upper part appears foamy and basophilic since the granules are not stained leaving empty spaces. - The granules could be stained with some specific dyes as PAS (pink) and Alcian blue (blue). EM: - Each mucigen granule is surrounded by a membrane. - The basal part is cylndrical and narrow and elongated. It is known as the stem of goblet cells. It contains most of cytoplasm and the nucleus is single, basal, oval and vesicular. 22 2- Multicellular glands: the gland is formed of large number of cells as salivary gland. B- According to the nature of secretion: 1- Serous glands (adenomeres). 2-Mucous glands. 3-Mixed glands. Serous gland Mucous gland Seromucoid gland Secrete serous fluid Secrete mucin or Secrete serous and contains enzymes. mucoploysaccharides. mucous secretions. -They are lined with -They are lined with -They present various pyramidal cells. cuboidal cells. orientations: - The nuclei are rounded -The nuclei are flattened in 1. Cell by cell. and basal. shape and peripheral. 2. Acinus by acinus. -The basal cytoplasm is -The cytoplasm is less 3. The mucous unit is basophilic, rich in rough acidophilic and foamy and capped by crescent-shaped endoplasmic reticulum. vacuolated due to dissolved aggregation of serous The apical cytoplasm is mucingen granules. It also cells, crescents of acidophilic, rich in contains few rough Gianuzzi (or demilunes of zymogen granules. endoplasmic reticulum. Gianuzzi). -They have small -They have large diameter -The passage way of diameter and narrow and wide lumens. serous secretion to the lumens. -Number of cells are mucous unit is the -Number of cells is few numerous (8 – 12). intercellular canaliculi. (3–5). - Examples: - Examples: Examples: 1. Pancreas. 1. Palatine s. glands. 1.Mandibular and 2. Parotid gland 2. Brunners gland. Sublingual s. gland. 23 C- According to the mode of secretion: 1- Merocrine gland: The secretion passes through the free surface of the cells by exocytosis without losing any part of the cytoplasm as in pancreas and salivary gland. 2- Apocrine glands: The secretion accumulates inside the apical part of the cell then delivered with this apical part of the cell as in mammary gland and axillary sweat glands. 3- Holocrine glands: The secretory products are discharged together with the whole cells. The basal cells of the gland constantly divide to replace the destroyed cells as sebaceous glands D- According to the shape of secretory part and branching of ducts: 1- Simple gland (single duct and unbranched end-pieces): - Simple tubuler as the intestinal glands. - Simple alveolar as sebaceous gland. - Simple tubulo-alveolar as small salivary glands. - Simple coiled tubular as sweat gland. 2- Branched glands (single duct and branched end-pieces): -Branched tubular as gastric glands. -Branched alveolar as tarsal glands. -Branched tubulo-alveolar as prostate. 3- Compound glands (branched duct and branched end-pieces): - Compound tubular as kidney and liver. - Compound alveolar as mammary gland. - Compound tubulo-alveolar as pancreas. 24  3- Specialized epithelium Definition: it is highly specialized epithelial cells to perform a specific function as sensation, contraction or reticular formation. Types: 1- Neuro-epithelium: it is a special type of epithelium acts as a receptor for sensation. It consists of 3 types of cells: i- Sensory: (neuro-epithelial) cells: they are spindle- shaped cells with acidophilic cytoplasm and oval vesicular nucleus.The free end of the cell carries microvilli (sensory hairlets). ii- The supporting cells: they are columnar slender-shaped cells with oval nuclei. These cells located peripherally to support the sensory cells. iii- Basal cells: polyhedral or triangular cells with rounded nuclei. Site: 1- Tongue as taste buds 2- ear as organ of Corti 3- Olfactory mucosa of the nasal cavity. 2- Myoepithelium: it is a special type of epithelium which is modified to contract. Shape: they are elongated curved cells with oval nuclei and long cytoplasmic processes. 25 Site: around the secretory adenomeres in salivary gland, mammary gland, pancreas and sweat gland. 3- Reticuloepithelium: it is a special type of epithelium forming a network to support other cells. Structure: they are large cells with long and branched cytoplasmic processes which anastomose forming a reticulum to support other cells. Site: lymph node, spleen and thymus gland. Modifications of the epithelial cells to perform specific function A) Apical modifications: They include: 1- Cilia. 2- Microvilli. 3-Sterocilia. 1- Cilia: Described before. 2- Microvilli: Finger-like and cylindrical projections on the surface of certain cells. Their cores contain many actin filaments. Sites: Cells of small intestine and kidney. Function: It increases the cell surface area for absorption. 3- Sterocilia: - They are non-motile and hair-like processes, its core contains actin filaments - They are long microvilli present on the cells lining the epididymis. B) Lateral modifications (cell junction: 1- Zonula occludens (tight junction): - The 2 adjacent cell membrane fuse completely at certain points where the intercellular space is zero. - It completely encircle the cell as a belt. - It prevent the passage of any substance between the cells. 2-Zonula adherens (adherent junction): - The 2 adjacent cell membranes are widely separated by intercellular space of 20 nm filled with adhesive glycoprotein material. - At the cytoplasmic side, there is condensation of tonofilaments. - It encircles the cell as a belt. - it fixes the 2 adjacent cells to prevent their separation. 26 3- Maculae adherents (desmosomes): - The intercellular space is 25 -30 nm which contain granular material and electron opaque central line. - The cytoplasm presents a dense attachment plate in which the microfilaments (tonfilaments) are converged and inserted. Some of them are arranged as a hair pin loop - It does not encircle the cell but scattered as circular spots N.B., The zonula occludens, zonula adherents and the desmosomes are called junction complex in the small intestine. 4- Gap junction (Nexus): - The 2 adjacent cells are separated by a spaces of only 2 nm. - This is junction channels which represent the region of low electrical resistance or coupling. - It permits the passage of ions and impulses between the adjacent cells. B) Basal modifications: They include basement membrane and hemidesmosome. 1) Basement membrane: Definition: It is an extra cellular sheet-like structure which separates the epithelium from the underlying C.T. Structure: E/M: It is formed of 2 layers: a. Basal lamina: formed of protein collagen embedded in glycoprotein. b. Reticular lamina: formed of reticular fibers embedded in a glycoprotein. 2) Hemidesmosomes: - They are similar to half desmosomes. - They fix the epithelial cells to the underlying basement membrane. - The basal part of the cell membrane becomes thickened. In this thickened part the microfilaments are embedded in it forming hemidesmosomes. 3) Basal infoldings: help in increasing the surface membrane for effective basal flow of materials as in kidney tubules and striated ducts of the salivary glands. 27  Connective tissue At the end of this chapter, the student should be able to:
Mention the different characters of connective tissue.
Classify the connective tissue cells and fibers.
Draw labeled diagrams about the different types of connective tissue.
Classify the types of cartilage.
Describe the haversian system
Compare diagrammatically between the different types of bone. General characteristics of the connective tissue: The connective tissue is mesodermal in origin. It is called connective, because it is specialized to connect, bind, support and protect the other tissues of the body. It may be vascular (loose connective tissue), highly vascular (bone) or avascular (cartilage). It consists of cells, fibers and intercellular substances (matrix) The C.T is classified according to the relative amount of matrix, the type and arrangement of fibers and the abundance and kinds of cells. (I) Connective tissue cells: They are all mesodermal in origin except melanocytes are ectodermal in origin. (I) They are classified according to function into: A) Cells responsible for generation and maintenance of C.T: 1. Undifferentiated mesenchymal cells (UMCs). 2. Fibroblasts. 3. Osteoblasts. 4. Chondroblasts. 5. Endotheial cells. 6. Pericytes. B) Cells responsible for defense and immunity: 1. Macrophages. 2. Plasma cells. 3. Mast cells. 4. Leucocytes. C) Cells responsible for storage: 1. Fat cells ( adipocytes). 2. Pigment cells ( melanocytes). 28 
They are classified according to their state into: Fixed cells Free cells 1. undifferentiated mesenchymal cells 1- Mast cells. 2. fibrobasts 2- plasma cells. 3. fat cells 3- leucocytes. 4. fixed macrophages 4-freemacrophage. 5. pericytes 6. endothelial cells 7. reticular cells 8. pigment cells(melanocytes) 9. chondroblasts 10. osteoblasts 29  1. Mesenchymal cells - They are undifferentiated embryonic connective tissue cells.they are numerous in embryos while in adults these cells are localized at: - Bone marrow (blood cells). - Around blood vessels ( pericytes). Structure: - LM: Shape: they are small irregular branching, star shaped cells. Nucleus: is large, oval or spherical and lightly stained as well as centrally located with prominent nucleolus. Cytoplasm are pale basophilic. EM: They contain few cytoplasmic organelles,abundant ribosomes and fine chromatin in the nucleus Functions: They have the ability to divide and differentiate into any other type of C.T cells so it is considered as mother or stem cells. 2. Fibroblasts: Origin: UMCs Sites: the most common type in the C.T proper Structure: LM: Shape: They are large irregular or fusiform branching cells with long and thin cytoplasmic processes. cytoplasm: is basophilic. Nucleus: is large, oval, and vesicular with prominent nucleolus. EM:- Their cytoplasm rich in organoids responsible for protein synthesis i.e. well developed Golgi apparatus, RER, mitochondria ,ribosomes and secretory granules. - Their nuclei are euchromatic - Their ultrastructural features give indication of active protein synthesis. N.B., aged fibroblasts are called fibrocytes which are inactive cells, small in size, flattened in shape with fewer cytoplasmic processes. The cytoplasm is lightly acidophilic with little and ill developed cytoplasmic organelles. Their nuclei are small flat and deeply stained. 30 Functions: 1. These are the principle cells of the connective tissue which produce all types of C.T fibers (collagen, elastic and reticular) also responsible for production of amorphous ground substances. 2. Fibroblasts are essential during wound healing and repair of C.T after injury. 3. Adipose cells (fat cells) Origin: they are developed from mesenchymal cells by gradual accumulation of lipid droplets in the cytoplasm. Then the lipid droplets either fuse with each other forming one large fat globule called white fat cells or persist as multiple fat droplets scattered in the cytoplasm called brown fat cells. a. White fat cells: - They are called unilocular adipocytes. Sites: most abundant in white adipose C.T Structure: LM: Shape:They are spherical in shape when present singly or polyhedral in clusters. Cytoplasm:They contain single large fat globule which fill the cell surrounded by very thin rim of acidophilic cytoplasm. Nucleus is compressed, flattened and bulged peripherally. In H&E stained sections the fat globule is dissolved during dehydration leaving large empty space so the fat cells have a signet ring appearance. In frozen sections, they can be stained orange by Sudan III or black by Sudan black or osmic acid stains. The fully formed fat cell is incapable of mitotic division. EM: - A fat droplet appears as electron dense non memberanous inclusion occupies most of cytoplasm. - Cytoplasmic organelles as well developed mitochondria , SER and Golgi apparatus are present Functions: 1. Storage of fat. 2. Support and protect of some organs as kidneys. 3. Source of energy and heat. 31 b. Brown fat cells: They are called multilocular adipocytes. Sites: present mainly in brown adipose tissue. Structure : LM: Shape: are rounded or polygonal in shape small in size. cytoplasm : contains multiple small lipid droplets also, have abundant cytochromes – rich mitochondria which give the brownish coloration of the cells. Nucleus : is spherical and eccentrically situated Functions: They provide more heat & energy for rodents & hibernating animals. 4- Fixed macrophages (Histocytes or Clasmatocytes) Origin: they are originated from monocytes (a type of white blood cells) which migrate to the loose C.T and become fixed when attached to the C.T fibers. Structure: LM: Shape: are large branched cells with irregular cell boundaries due to presence of pseudopodia Nucleus: They have small, eccentric darkly stained, kidney shaped nuclei. Cytoplasm is acidophilic, granular and vacuolated. It can be demonstrated by using special stains as Indian ink or trypan blue, phagocytic granules were seen in the cytoplasm of histocytes. EM: The cytoplasm contains numerous lysosomes, primary and different types of secondary lysosomes, phagosomes, residual lysosomes, multivesicular bodies and autophagic vacuoles. Also, well developed RER, prominent Golgi apparatus.were present in the cytoplasm. 32 Functions: 1. They are phagocytic cells which can phagocytose and digest foreign particles. 2. clean wounds from debris. 3. Interaction with lymphocytes and neutrophils by producing interleukins which stimulate attraction of neutrophils and mitosis of lymphocytes. 4. Many histocytes fuse with each other to form multinucleated cells called foreign body giant cell which engulf large foreign bodies. 5- pericytes - They are believed to be pluripotential cells as UMCs of adult tissue. Site: they wrap around the blood capillaries and sharing the basal laminae of endothelial cells. Structure : LM: They are flattned, branched cells and lightly stained cytoplasm with oval nuclei. EM: The cytoplasm contains few organelles as Mitochondria, RER & euchromatic nucleus. Functions: 1- They are considered as undifferentiated mesenchymal cells in adult tissue which can be differentiated into various types of other cells as fibroblasts or smooth muscle cells. 2-They are contractile cells which leading to vasoconstriction. 6. Endothelial cells Sites: lining all blood vessels and heart. Structure: they are simple squamous epithelial cells having little cytoplasm with few organelles and flat nucleus. Functions: - They can be differentiated into new endothelial cells. - They produce special type of collagen (type IV) - They produce their own basal lamina. 7. Reticular cells sites: present in the stroma of most organs. structure: stellate shaped cells with long and branched cytoplasmic processes which forming a reticulum with reticular fibers. 33 Functions: 1- Have a supportive function. 2- Have a phagocytic function. 8. plasma cells origin: developed from B- lymphocytes. Sites: numerous in lymphoreticular tissue. Structure: LM: Shape: ovoid cells. Cytoplasm: deeply basophilic with pale (unstained area) called negative Golgi image. The nucleus is rounded ,eccentric and having a characteristic feature which looks like a spokes of cart-wheel or clock-face due to regular radial arrangement of chromatin along the inner surface of the nuclear memberane. This feature is used to identify the plasma cell. EM: - They have EM features of protein producing cells as extensive granular RER, GA, mitochondria and ribosomes. Functions: - Synthesis and secretion of antibodies which play important role in humral immunity (body defense against bacterial infections). 9. Mast cells origin: differentiated from haemopioteic stem cells. sites: loose C.T and in vicinity of the small blood vessels. Structure: LM: Shape: they are large spherical or ovoid cells with few cytoplasmic processes. Cytoplasm is engorged with a huge number of large, coarse metachromatic granules i.e., the color of granules is not the true color of the dye used to stain them. Nucleus is small, eccentric and pale which obscured by these granules. EM: Have EM features of secretory cells as well developed RER; mitochondria, Golgi apparatus and electron dense memberane- bounded secretory granules. 34 Functions: Mast cells play an important role in detecting foreign substances and intiating a local inflammatory response in the tissue in the form of: Secretion of histamine which causing vasodilatation, increasing permeability of blood capillaries during allergic reaction. Secretion of heparin which prevent the blood clotting (anticoagulant substance). Secretion serotonin which cause contraction of smooth muscle. Secretion of eosinophil chemotactic factor which attract eosinophils to site of allergy. 10. pigment cells (Melanocytes) origin: are ectodermal in origin which derived from the neural crest. melanocytes are melanin forming cells. Site: located in-between the basal cells of the stratified epithelium of the epidermis, hairs, iris and choroids of eye. Structure: LM: small cells with rounded cell bodies and small, central nucleus possess long branched cytoplasmic processes. Cytoplasm contain melanin granules (melanosomes) which difficult to be seen with H&E. It can be identified by only by histochemical methods, DOPA reaction that blackens the cells and also detecting of tyrosinase enzyme. EM: Melanocytes have the typical EM features of protein synthesizing cells as prominent GA, abundant RER many mitochondria, ribosomes and secretory vesicles. Functions: - Melanocytes are responsible for sythesis of melanin which is essential to absorb and disperse the harmfull effect of ultraviolet rays and thus prevent the sunburn. N.B: melanophores or melanophages differ from the melanocytes which are mesodermal in origin. They are called C.T macrophages in dermis and considered as melanin eating cells they phagocytose melanin paricles from disintegrating or aging melanocytes.These cells lacking tyrosinase enzyme and showed –ve DOPA reaction. 11. Free macrophages Origin: derived from blood monocytes then migrate to the C.T tissue and transformed into macrophages. Sites: - They described as free cells as they are wandering in C.T. - They take different names according to the tissue located in: 35 In liver …………… Von Kupffer cell. In lung …………… dust cell. In spleen ……….... Littoval cell. Functions : have the same structure and function of fixed cells. Leucocytes They will disscused in details in the next chapter (blood). (II) Connective tissue fibers: There are three types: a) White collagen fibers. b) Yellow elastic fibers. c) Reticular fibers. a) White collagen fibers - They are the most numerous of C.T fibers. - They are found in all types of C.T except blood. - They are called white because in fresh state their color is white.when condensed as in tendons and ligaments. - They produce gelatin (glue) on boiling. - They are destroyed by acids and alkalies - They are tough and resist stretch. - They are classified into 5 main types according to the site and the cell of origin: type I, type II, type III, type IV, type V. Structure: LM: - They are formed of fine parallel fibrils which run in long wavy bundles. The individual fibrils never branch but the bundles branch. - They are stained pink with H&E. while they are stained in different colors with special stains as red by Van Gieson,s stain; green to blue by Masson’s trichrome stain. EM: Each collagen fiber formed of parallel fibrils.Each fibril consists of microfibrils which consist of protein molecules (tropocollagen). The latter arranged in a staggered fashion (stepwise overlapping) so in longitudinal section they show cross- banding or particular periodicity (alternating light and dark bands that are repeated at regular intervals of 64 nm each). Functions: Give strength and rigidity to the tissues and resist a pulling force. 36  b)Yellow elastic fibers - They are less tensile strength than collagen fibers - They are yellow in dense elastic tissue but colourless in individual fibers. - They are thin, long and branched fibers - They can unit to form elastic membrane - They possess considerable elasticity so; they can be stretched to over double length and return to original dimensions without deformity. - They are formed of elastin; a protein which highly resistant to boiling and chemicals. Structure : LM: They cannot be easily identified by H&E but can be selectively stained brown with orcein and weighert elastic stain. EM: They have homogenous appearance and exhibit no periodicity. Functions: Because of their property of easily stretching and recoiling they are found in structures where stretching without breaking is essential as large arteries, external ears and epiglottis. c) Reticular fibers They are very thin, short, highly branched and anastomosing with each other forming a network or reticulum in which the parenchyma is suspended. Structure : LM: They are not stained with H&E, but can be specifically stained; black by silver impregnation methods (Gomori,s reticulin stain) so, reticular fibers are called argyrophilic or argentaffin fibers. also, they appear purple by PAS technique due to their high content of glycoproteins. EM: Reticular fibers are thin collagen fibers (type III).So; they show the same periodicity of collagen fibers. They are considered as immature form of collagen fibers. Functions: Reticular fibers form fine network in the stroma and parenchyma of important organs specially lympho reticular tissue, glandular structures, and in sheaths around blood vessels, muscles and nerves. Also they share in the formation of the basement membrane. III) Amorphous ground substance (matrix) - It is the interstitial substance of the connective tissue in which cells and fibers are embedded. - It varies widely in amount and structure. It increased in both areolar and mucous C.T 37  but is minimal in other types. - It is described as an amorphous (shapeless),gellike substance. It is homogenous and transparent and is seldom seen in routine histological sections. Structure: The ground substance is a complex mixture of water, mineral salts, glycoproteins, and mucopolysaccarides which recently is described as glycosaminoglycans (GAG). The latter appear in to forms either: a. Sulfated as (chondrotin sulfate , keratin sulfate , heparan sulfate) which persent in cartilage, bone) b. Non- sulfated as (hyalourinic acid): which predominant in soft tissues as embryonic tissues, loose C.T, synovial fluid, vitreous body. Functions: 1. It controls passage of pathogens from C.T into blood stream. 2. It allows the diffusion of oxygen, nutrients waste products between the blood & cells. Types of connective tissue According the relative amount of basic components of the C.T (cells, fibers and matrix) C.T is classified into: I) Emryonic C.T. 1- mesenchymal C.T 2- Mucous C.T II) Adult proper C.T 1. Loose C.T 2. White dense fibrous C.T 3. Dense elastic C.T 4. Reticular C.T 5. Adipose C.T III) Adult supportive C.T 1- Cartilage. 2. Bone. I) Embryonic C.T 1 Mesenchymal C.T: It is unspecilized connective tissue of early weeks of embryonic life. Cells: It is formed from undifferentiated mesenchymal cells (mitotically active cells) few fibroblasts Matrix: abundant non sulfated amorphous ground substance filing wide spaces between the cells. 38 Fibers:It contains only few, thin mostly collagenus fibers. 2- Mucous (mucoid , gelatinous , myxomatous ) C.T: It is mostly common in embryonic tissues and rare in adults. structure: - Cells: polymorphic cells with numerous interconnected processes as: mesenchymal Cells, young fibroblasts, macrophages and some lymphocytes. - Fibers: very fine collagen and reticular fibers but it cannot be identified under LM because the refractive index of the fine collagen fibers is equal to that of the amorphous ground substance. - Matrix: abundant, homogenous, jelly-like amorphous ground substance, rich in GAG and glycogen which give a mucuin staining reaction. Sites: In embryos: umbilical cord (Wharton,s jelly). In Adults: comb & wattles of cock, macroscopic papillae of omasal laminae. II) Adult proper connective tissue 1- loose (areolar) C.T: - It is the typical and most common type of C.T proper - It is called loose or areolar because it contains spaces or cavities which accommodate large amount of fluids and gasses. - It is well vascularized. Structure: - cells: contains all types of C.T cells mostly fibrobasts. - Fibers: All types of C.T fibers mostly collagen fibers which are loosely interwoven with each other leaving spaces or cavities between them - Matrix: non sulfated amorphous ground substance. Sites: present everywhere in the body except the brain. Forms the submucosal layer of most hollow organs, the interstitial tissues of some organs, around blood vessels & nerves and between muscle bundles and layers. 39 2- Dense white fiberous C.T: - It is dense because the collagen bundles are very compact and closely packed to each other. - According to the arrangement of collagen bundles, there are two types: A) Dense regular white fibrous C.T: Structure:- Fibers: It is formed of closely packed collagenus bundles arranged regurally parallel to each other in the same plane. - Cells: it contains numerous fibroblasts which arranged or squeezed between the packed collagen fibers called (tendon cells). In LS: They appear long flat cells with blunt ends In CS: the cells appear stellate in shape with wing – like cytoplasmic processes extending between the adjacent parallel collagen fibers they are called (bird cells). - Matrix: minimal amount of non sulfated mucopolysaccarides Sites: It found in organs can withstand stretch in one direction e.g. tendons of muscles and ligaments of joints also in fibrous coat of eye (cornea &sclera). B) Dense irregular white fibrous C.T: It is formed of densely packed collagen bundles arranged irregularly and interwoven in different directions. - Cells: many types of C.T cells are found in narrow spaces between the fibers but the fibroblasts predominate. - Matrix: minimal amount of non sulfated mucopolysaccarides. Sites: it is found where tensions are exerted in different directions as: fibrous capsules of various organs (liver, lymph nodes etc), dermis of skin, fibrous sheath of cartilage (periconderium) and bone (periosteum). 3. Dense yellow elastic C.T: Fibers: elastic fibers are the predominant type, so it appears yellow in fresh state. According to the arrangement of the elastic fibers, there are three types: 40 a) Dense regular elastic C.T: elastic fibers form cords parallel to each other and connected by loose C.T sites: in funicular part of ligamentum nuchae. b) Dense irregular elastic C.T: elastic fibers are closely packed and irregularly anastomosed leaving pores in some parts of formed lamellae. Sites: in lamellar part of ligamentum nuchae. c) Fenestrated elastic memberane: elastic fibers are condensed forming zigzagged flattened membranes provided pores in some parts. Collagen fibers attach them. Sites: in large arteries like aorta. 4. Adipose (fatty) C.T : Adipose tissue is almost completely formed of closely packed adipocytes. They are subdivided into lobes and lobules by thin sheaths of vascularized loose C.T septa. Adipose tissue is classified into two types according to the type of fat cells which present: i) White adipose tissue: It is formed of white fat cells with their characteristic features which discussed before. Sites: It is widely distributed in the body e.g. around kidneys and blood vessels, in the mesentery and in the stroma of inactive mammary gland. ii) Brown adipose tissue: It is formed of brown fat cells with their characteristic features which discussed before. Sites: Common in rodents and hibernating animals. Functions: 1. Adipose tissue is the primary site of fat metabolism and storage in the body. 2. Act as heat insulator 3. Support organs as kidneys and intestine. 5- Reticular C.T: It forms a network of thin reticular fibers associated with reticular cells (their structures are discussed before). Which intermingled by a wide variety of cells e.g. histocytes, erythrocytes and leukocytes. It can be recognized by silver impregnation method 41 Sites: It forms the basic tissue of solid organs as all lymphoreticular organs, endocrine glands, kidneys, liver, lung …etc to support the functioning cells. III) Adult supportive connective tissue A) Cartilage Definition: it is the specialized type of C.T responsible for supportive role. General characters: 1- It is rigid, but with some degree of flexibility because the matrix is formed of chondrotin sulfate (sulfated GAG). 2- It is avascular however the cells receive their oxygen and nutrients by the diffusion from the perichondrium (a vascular C.T layer that surrounds the cartilage). 3- like all connective tissue cartilage is formed of : Cells are called chondroblasts& chondrocytes Fibers: are either solely collagen or combined with elastic fibers depending on the cartilage type. Matrix: is formed of chondrotin sulfate.which causes the basophilia of the matrix. 4- There are three types of cartilage depending on the type of fibers embedded in the matrix : a) Hyaline cartilage b) Elastic cartilage c) Fibrocartilage. 5- All types of cartilage are surrounded by perichonderium except at the articular suface of joints and fibrocartilage. a) Hyaline cartilage It is the most common type. It is called hyaline because in fresh state it has glassy, translucent, bluish white appearance. Sites: In embryos: it forms most of temporary skeleton of embryos that replaced by bone after birth. In adults: it is persists as articular cartilage of joints, costal cartilages and cartilages of respiratory passages (nose, trachea, bronchi and most of the cartilages of larynx). 42 Structure: 1-perichonderium (peri= around) It is a vascular connective tissue membrane, which surrounds the cartilage except the articular surfaces. It is formed of two layers: - Outer fibrous layer: it is formed of dense irregular C.T, fibroblasts, rich in blood vessels and nerves. - Inner cellular (chondrogenic) layer: it is less fibrous and highly cellular (rich in chondrogenic cells) which differentiate into chondroblasts. The perichonderium is absent in articular hyaline cartilage. Functions of perichonderium: 1- Nutrition of cartilages 2- Growth and regeneration of cartilages 3- Attachment for muscles. 2- Cartilage cells: a) Chondrobasts (young chondrocytes): - it is found singly parallel to the inner surface of the perichonderium they are developed from UMCs. - It is characterized by: flattened or oval cells with deeply basophilic cytoplasm - The nucleus is oval, lightly stained with prominent nucleoulus - It has the ulrastructural features of the protein synthesizing cells (euchromatic nucleus, well developed Golgi apparatus, abundant RER, ribosomes and mitochonderia). Functions: 1- They secrete cartilage matrix 2- Essential in growth of cartilage from outside (appositional growth). b) Chondrocytes: - They are mature cartilage cells. - It is differentiated from chondroblasts. When the chondroblasts secrete enough matrix surrounding them, they become mature cells (chondrocytes). 43 - They present singly or in groups (up to 8 cells) surrounded by a condensed matrix called lacuna that forming isogenous groups or cell nest. LM: they are rounded cells with lightly stained, vacuolated cytoplasm.They have rounded, central and darkly stained nucleus. EM: they have the ulrastructural features of protein forming cells (abundant ribosomes, well developed RER and Golgi apparatus, mitochondria as well as glycogen granules and fat droplets). Functions: 1- secretion and maintainace of cartilage matrix 2- Responsible for growth of cartilage (interstitial growth). Matrix: it is formed by cartilage cells.it consists so sulfated GAGs (chondrotin sulfate) and fine collagen fibers which have the same refractive index of the surrounding matrix so the fibers cannot be seen at LM level. b) Elastic cartilage Structure: - It has the same structure of the hyaline cartilage, except its matrix contains an extensive network of elastic fibers run in different directions around the lacunae. - The lacuna contains only 1-2 chondrocytes. - All elastic cartilages are surrounded by perichonderium. - These elastic fibers are responsible for the elasticity of this type of cartilage and for its yellow color. Sites: 1- Ear pinna (external ear). 2- Eustachian tube 3- Epiglottis and some of laryngeal cartilage. C) White fibrocartilage. Structure: - It has No perichondrium. 44 - It has an intermediate character between the hyaline cartilage and dense regular C.T. - It is formed of thick, compact collagen fibers arranged in parallel bundles. They are separated from each other by narrow clefts that contain the encapsulated chondrocytes arranged singly or in rows. Sites: It attaches bone to bone to provide restricted mobility under great mechanical stress. 1- invertebral discs. 2- Pubic symphysis. B) Bone - It is hard, strong specialized supportive C.T. - Unlike cartilage, bone is highly vascular tissue. - It is formed of bone cells and bone matrix. - Bone is covered externally with periosteum and internally with endosteum. Structure: 1- Periosteum: - It is a vascular fibrous C.Tmembrane which covers the bone externally except the articular surfaces. - It is formed of two layers: a) Outer fibrous layer which is formed of dense irregular C.T rich in blood vessels. b) Inner cellular (osteogenic) layer:which is mainly formed of osteogenic cells and osteoblasts. 2- Bone matrix: - It is formed of closely packed layers of calcified of C.T (called bony lamellae) embedded in amorphous calcified intercellular substance. - It consists of organic components (collagen fibers type 1 which give acidophilic reaction in decalcified bone sections with H&E, and sulfated GAGS) and inorganic components ( bone minerals as calcium phosphates, carbonates etc which is the cause of bone hardness) 45  - Endosteum: it lines the internal surface of bone (bone marrow cavities). It is similar in the structure to the inner osteogenic layer of periosteum. 3- Bone cells: There are four types: 1- Osteogenic cells: - They are mesenchymal stem cells which are differentiated to other types of bone cells. - They are present in the inner layer of the periosteum and the endosteum. - They are characterized by flattened cells with lightly basophilic cytoplasm and flattened deeply stained nucleus. - They have ultrastructural features of mitotically active cells. - They are active either during growth of bone or in the repair of born fractures. 2- Osteoblasts: - They are originated from the osteogenic cells. - They are young bone cells found on the surfaces of growing bones. Structure: LM: - They are large rounded branched cells which intercommunicate by their cytoplasmic processes. - The nucleus is oval, eccentric and lightly stained. - The cytoplasm is deeply basophilic (due to RNA) and contains alkaline phosphatase which attracts the deposition of bone salts. - A negative Golgi image is seen close to the nucleus. EM: they have all features of actively secreting cells: Well developed RER, large GA, many ribosomes, mitochondria and many secretory granules. Function: They are termed bone forming (building ) cells as they are responsible for producing of organic matrix of bone (osteoid).; i.e. collagen fibers type1 and GAGs. They are transformed into osteocytes when the bone matrix surrounds them and become inside the lacunae. 3- Osteocytes: They are mature bone cells which are differentiated from the osteoblasts. Structure: LM: - They are smaller than the osteoblasts. - They are branched ovoid cells, with oval, dark nucleus and less basophilic cytoplasm. 46 - Each osteocyte lies in a small space called lacuna and its processes extend inside fine tubules called canaliculi in the hard matrix.through which the osteocyte processes are connected with each others by gap junctions. - Unlike cartilage, osteocytes cannot divide so there is a single cell in each lacuna (no cell nests are present). EM: They resemble to the osteoblasts but with less prominent RER, Golgi apparatus. Functions: They are considered as bone maintaining (preserver) cells as: 1-They maintain the bone matrix by formation of collagen fibers and glycoproteins 2-They maintain the hardness of the matrix by continous deposition of Ca salts. 4- Osteoclasts - They are giant, multinucleated cells. - They are originated from fusion of many blood monocytes. - They are nestled in shallow depressions on bone surfaces called Howship,s lacunae near bone marrow cavities. Structure: LM: - They are large cells (up to 150um in diameter), with foamy acidophilic cytoplasm and multiple nuclei (up to 50). - They have a striated or brush border facing the bone surface called ruffled surface. EM:They have numerous mitochondria, well developed RER, active GA, various sized vacuoles and many lysosomes. Functions: 1- They are called bone eating cells. 2- They are responsible for bone remodeling during growth or after fractures by releasing lytic enzymes to break down the bone matrix and release minerals (bone resorption). 47 Types of bone A) Compact bone. B) Spongy bone. A) compact bone: - It is solid and hard. - It presents in the shafts of long bones, outer and inner tables of flat bones of skull and outer covering of vertebrae, ribs, etc. - It is covered externally with periosteum and internally with endosteum (described before). - It is characterized histologically by the regular arrangement of bony lamellae into a definite pattern.called Haversian system or osteon. Haversian system (Osteon): - It is considered the structural unit of compact bone. - It consists of a central Haversian canal surrounded by concentrically arranged bony lamellae (calcified collagen fibers which house osteocytes inside lacunae and canaliculi). - Haversian canal run parallel to the long axis of bone and contains blood vessels, lymph vessels and nerves. - Haversian bony lamellae are formed of calcified collagen fibers are parallel to each other in one lamella and perpendicular to those in the next one. So in C.S the lamellae appear alternately striated and punctuated. 48 Volkmann’s canals: - They are transverse, oblique canals which connect the Haversian canals together and with the periosteum as well as endosteum so compact bone is a highly vascular tissue. - The bone lamellae are organized in 4 lamellar structure from outside to inward: 1- outer circumferential lamellae 2- concentric lamellae (Haversian system) 3- interstitial lamellae. 4- inner circumferential lamellae B) Spongy bone(Cancellous bone) - It appears as thin irregular branching and anastomosing bony specules or trabeculae, leaving spaces which house blood forming cells of bone marrow. This structure gives the spongy appearance. - It doesn’t have the regular arrangement of bony lamellae. - There are no Haversian systems. - It is present in: 1- epiphysis of long bone 2- short bones 3- flat or irregular bones 49  Blood At the end of this chapter, the student should be able to:
Mention the different characters of the blood
Classify the blood cells
Compare diagrammatically between the different types of RBCs
Compare diagrammatically between the different types of WBCs
Mention the process of hemopoeises and granuolopoeises
Blood is a special type of connective tissue. It is so called due to its constituents of cells (RBCs and WBCs), inter cellular matrix (blood plasma) and soluble fibers (fibrinogen).
It is formed of plasma and cellular elements: Plasma (about 55 %) is formed of water, organic substances, inorganic substances, gases, hormones and enzymes. The cellular elements (average 45 %) consist of: 1- Red blood corpuscles or erythrocytes (RBCs). 2- White blood cells or leukocytes (WBCs). 3- Blood platelets or thrombocytes. Functions of the blood: 1- Transport of oxygen, nutritive substances, hormones, enzymes and chemicals to all tissues. 2- Removal of carbon dioxide and waste products through the lungs, kidneys and sweat glands. 3- Control of body temperature. 4- Maintenance of acid-base balance. 5- It protects the body against infections. Red blood corpuscles - They are responsible for the red coloration of the blood, due to the presence of hemoglobin in the cytoplasm. - In mammals they are not true cells, because they lost their nuclei, ribosomes, mitochondria, Golgi apparatus, centrioles, endoplasmic reticulum and lysosomes during development). 50 Shape: - In mammals, they are discoid and biconcave. The central parts are thinner than the peripheral parts. They are circular when seen from above, and biconcave in side view. In slow blood stream and blood films, RBCs adhere together due to their surface tension showing a Rouleaux appearance. They are non nucleated. - In camels, they are elliptical, biconcave and non- nucleated - In birds and fishes, RBCs are oval in shape biconvex and nucleated - In amphibian, RBCs are ovoid in shape, nucleated and biconvex In birds in camel Size: - The normal diameter of RBCs varies from 6 – 9 µm. The average diameter (7.5 µm), and the normal thickness is 1.9 µm at the periphery and 1.1 µm at the center of RBCs. - The largest erythrocytes are of dogs (about 7 µm), while the smallest erythrocytes are of goats (about 4 µm). Number: - Human In men, the number varies from 5 to 5.5 million per cubic milliliter. - In women, the number varies from 4.5 to 5 million per cubic milliliter. - The number is fewer in females due to: 1- The secreted female sexual hormones depress the process of blood development in the bone marrow. 2- Estrous cycle in females. Colour: - The normal color of RBCs is greenish-yellow or orange due to the presence of haemoglobin (Hb), when RBCs are condensed or overlapped it appears red in color. - When The Hb % is normal, the RBCs are called Normochromic. 51 Structure: - RBCs are not true cells, as they not contain nuclei or any other cells organoids. They resemble bags containing mass of haemoglobin surrounded by cell membranes formed of lipoproteins. The cell membrane is affected by changes in the osmotic pressure. In hypertonic solutions, RBCs undergo cremation (shrinkage), while in hypotonic solution (distilled water), they undergo haemolysis. Composition of haemoglobin: - Haemoglobin is the coloring pigment inside RBCs. It is formed of Heme and Globin (protein). The heme consists of iron and tetra pyrol protoporphyrin which has the ability to combine and dissociate O2 and CO2 physically. Life span and fate of RBCs: - They can live about 100 – 120 days. - RBCs damage may occur mechanically by being destroyed within the blood stream. The whole corpuscle or its fragments are engulfed by macrophages, where the haemoglobin is broken into haem (prtoporphyrin + iron) and globin. The protoporphyrin is converted into biliverdin and then bilirubin which will be released into the circulation and forms complexes with the plasma protein, then it is carried to the liver where it is exerted in bile. Function: - The red blood corpuscles carry oxygen from lungs to tissue and CO2 from tissues to the lungs. Adaptation of RBCs structure to perform their functions: 1- The biconcave disc appearance increases the surface area by about 25 % as compared with a spherical cell through these surfaces, gaseous exchange take place. 2- RBCs are flexible in nature which enables RBCs to be squeezed inside narrow capillaries and regain their original shape in wide vessels. 3- The cell membrane of RBCs is formed of lipoproteins, it is highly selective, it allows easy exchange of carbon dioxide and oxygen through it. 4- There is neither nuclei nor cell organoids in RBCs, this prevents RBCs from reproduction and increase the respiratory efficiency by allowing free space for haemoglobin. 52  White blood cells - They are called also leucocytes. - They are colorless, but when they are packed together, they appear white in color. - They contain all cell organoids and the cell inclusions. - They have an amoeboid movement, so they can penetrate through the capillaries. - They are widely distributed outside the blood and lymph vessels. e.g. loose C.T, epithelium, secretions (colostrums). Number of WBCs: - The total leucocytic number ranges from 7.4 thousands (in sheep) to 17.1 thousands (in pig) per cubic milliliter. Types of leucocytes: 1. Granular leucocytes (neutrophils, basophils and eosinophils). 2. Agranular leucocytes (monocytes and lymphocytes). 1. Granular leucocytes a) Neutrophils (Microphage) - They are also called polymorphonuclear leucocytes. - They range 25 – 30 % in cattle, sheep, goats and pigs and 50 – 60 % in horses, dogs and cats and 60-70% in man With L/M: - Neutrophils contain one segmented (lobulated) nucleus, formed of about 3 – 5 segments. - The cytoplasm contains also specific granules that stain with both dyes acidic and basic dyes. They appear light red by Leishman’s stain. 53 E/M: Cytoplasmic granules are of two types: 1- Primary (azurophilic) granules: - They are non-specific granules and appeared larger, fewer in number and darker than other types. - They are considered as primary lysosomes because they are very rich in hydrolytic enzymes. 2- Secondary (specific neutrophilic) granules: - They are more numerous and lighter than primary granules. - They contain collagenase enzymes, alkaline phosphatase and bactericidal substances. * Functions of neutrophils (carried outside blood stream): 1- Phagocytosis and destruction of microorganisms by the specific and azurophilic granules, they provide the first line of defense against invading microorganisms 2- Secretion of proteolytic enzymes, which cause lysis of bacteria and surrounding C.T. 3- Attraction of monocytes to the site of inflammation. 4- Secretion of growth promoting factor (trophones) which help in healing of wounds by stimulation of fibroblasts to form new connective tissue. 5- Stimulation of bone marrow to produce more leucocytes during acute infections and fever conditions. 6- Production of pyrogens which elevate body temperature (fever) to inhibit the growth of bacteria. Life span: 3 – 5 days. The dead neutrophils change into pus cells. b) Eosinophils - Their diameter varies from 10 – 15 µm. - They range from 9 % in cattle and 3.5 % in dogs and man. 54 With L/M: - The nucleus is generally bilobed and called horse-shoe shaped nucleus. - Cytoplasm contains large coarse, specific and refractile acidophilic granules which stained with acidic dyes and appear shiny red. With E/M: - The nucleus consists of 2 lobes connected by a thick chromatin thread. - The cytoplasm contains few mitochondria, few RER, small Golgi bodies, free ribosomes and cytoplasmic granules azurophilic and larg e specific eosinophilic - The cytoplasmic granules are membrane- bound granules. They are spherical, oval or ellipsoidal in shape. They are considered as lysosomes containing histaminase enzyme. - These granules contain a matrix of medium density and often equatorial (central) band of extremely electron-dense crystalloid material. Functions of eosinophils: 1- They are attracted to the site of inflammation (allergic reactions) by the eosinophil chemotactic factor (released by mast cells and basophils) 2- They are concerned with removal of histamine and sulfated substances at the sites of allergy and inflammation (by secretion of histaminase and aryl sulphatase). 3- They ingest antigen-antibody complex formed at the site of allergy and inflammation. 4- They are concerned with inactivation and killing of parasitic larvae. Life span: 8 – 12 days. N.B: Eosinophils are less motile than neutrophils but they may be present normally under the skin, in the intestine, respiratory and female genital tracts. C) Basophils - Their diameter is about 10 – 12 microns. - They constitute about ½ - 1 % of total leucocytic count except in dogs and cats. 55 L/M: The nucleus is irregular in shape (mostly S-shaped) and over shadowed by large basophilic granules present in the cytoplasm. These granules are metachromatic in reaction with certain dyes (toluidine blue) and they can be stained by Giemsa stain. E/M: The cytoplasm contains few cell organelles and few azurophilic granules, in addition to electron-dense granules, similar to those of mast cells. They contain heparin and histamine. Functions of basophils: 1- Secretion of heparin (natural anticoagulant). 2- Secretion of histamine which play a role in allergic control. 3- Secretion of eosinophil chemotactic factor. 4- They are slightly phagocytic cells. Life span: It is about 10 – 15 days. 2. Agranulocytes A) Lymphocytes - Their percentage varies from 20 – 30 % of the total leucocytic count. - They are called lymphocytes because they are present in lymph and blood, and also in connective tissue, lymph nodes, spleen, tonsils, thymus and tissue fluids. - They contain no specific granules in their cytoplasm, but contain small azurophilic granules in about 10 % of both types. - According to the diameter, they are classified into: a. Large lymphocytes (active): - They are active lymphocytes. - About 5 – 10 % in the circulating blood. - With L/M:- The nucleus is large indented and lightly stained. - The cytoplasm is abundant, basophilic and non-granular. - With E/M: - The cytoplasm contains many ribosomes, well-developed Golgi apparatus and mitochondria. - The nucleus is slightly indented and it has a clear nucleolus and slightly condensed chromatin. 56 b. Small lymphocytes: - Each one is about 6 – 8 microns in diameter. - Their percentage is about 15 – 20 % in the circulating blood. - With L/M: - The nucleus is large, rounded, deeply stained and filling the whole cytoplasm. - The cytoplasm is scanty, pale blue and appear as a thin rim around the nucleus. With E/M:- They are of 2 types: - T-lymphocytes. - B-lymphocytes. - Both types contain few mitochondria, few endoplasmic reticulum and large amount of free ribosomes. B-lymphocytes: - Origin: They are derived from the primitive stem cells of bone marrow in mammals. - They become mature in Bursa equivalent organs in birds which in man are the lymph follicles in the small intestine and probably in the appendix, so they are known as the B-lymphocytes. - They constitute the minority of the circulating blood leucocytes. - They have a short life span (about 3 months). Function: They are responsible for the development of humoral immunity. They are stimulated by antigens, and differentiated into B-memory and plasma cells which produce antibodies. T-lymphocytes: - Origin: - They also originate from bone marrow. - They become mature in the thymus, so they are known as T-lymphocytes. - They constitute the majority of the circulating blood leucocytes - They have a long life span and they can live for years. 57 Function: Activated T-lymphocytes differentiated into 5 types of cells involved in cellular immunity: 1. T-killer or rejection cells: which kill bacteria and viruses, also they reject foreign skin grafts and transplanted organs. 2. T-memory cells: They are small and programmed to carry antibody memory against the different antigen. 3. T-helper cells: They activate B-lymphocytes to develop into blast cells, then plasma cells to make specific antibodies. 4. T-suppressor cells: They inhibit the activity of both T and B lymphocytes. There are additional 2 types: 5. T-lymphokines-secretory cells: which secrete lymphokines and interferon. B) Monocytes - They are the largest leucocytes, their diameter is 12 – 18 microns. - Their percentage varies from 3 – 8 % of total leucocytes. - They range from 5.9 % in dogs to 2.5 % in ruminants. L/M: - The nucleus is large kidney- shaped and polar than lymphocytes. - The cytoplasm is non-granular and not clear; it stained grayish blue due to presence of lysosomal bodies present in their cytoplasm rich in acid phosphatase and other enzymes. - It shows positive peroxidase reaction. E/M:- The nucleus is kidney-shaped or horse-shoe shaped, it is slightly indented. The chromatin is distributed in a more delicate network than in large lymphocytes. It is eccentric in position. - The cytoplasm is abundant containing well-developed Golgi bodies, mitochondria, few RER and many electron-dense membranes bound granules (lysosomes). 58 Mobility: They can penetrate through capillaries and venules to enter the C.T They can extend and withdraw their pseudopodia. Function: They are phagocytic in function, as it is considered the precursor of macrophages. Life span: Three days in blood stream. Blood platelets - Diameter: They vary from 2 – 5 µm. - Number: Their number is 200.000 – 400.000 per cubic millliter(average 250,000). - They are called thrombocytes, they are not cells, they are fragments of cytoplasm of large cells called megakaryocytes. L/M: 1. Each blood platelet is oval or rounded in shape. 2. They are formed of peripheral pale clear part called Hyalomere and a central granular basophilic part called Granulomere which give the false appearance of nucleus. E/M: 1. The cell membrane of platelet is irregular and covered with a cell coat 2. The hyalomere or peripheral area of blood platelets contains numerous microtubules and microfilaments as actin and some myosin as well as fragments of Golgi apparatus. - They have a contractile function; they can change the shape of platelets during retraction of blood clot. 3- Th