Histology Final Content PDF

# Cytology ## The Nucleus - The nucleus is present in all eukaryotic cells except mature erythrocytes and blood platelets. - It contains the genome (genetic information). ### Number The majority of cells have only one nucleus. Some cells e.g liver cells and cardiac muscle fibers are binucleated, and...

# Cytology ## The Nucleus - The nucleus is present in all eukaryotic cells except mature erythrocytes and blood platelets. - It contains the genome (genetic information). ### Number The majority of cells have only one nucleus. Some cells e.g liver cells and cardiac muscle fibers are binucleated, and few others e.g skeletal muscle fibers and osteoclasts are multinucleated. ### Staining The nucleus is basophilic because of its content of nucleic acids (DNA & RNA). ## Interphase Nucleus ### (Non-dividing nucleus) The nucleus of an interphase cell (non-dividing cell in-between the end of one mitosis and the beginning of another), consists of the following components: - Chromatin - Nuclear envelope - Nucleolus or nucleoli. - Nucleoplasm or karyoplasm. ## Nuclear Envelope - It consists of two parallel membranes, each is a unite type membrane of about 7.5nm thickness separated by a 10-30nm space known as perinuclear space. - The inner nuclear membrane is associated with the peripheral chromatin. - The outer nuclear membrane has ribosomes and is often continuous with rough endoplasmic reticulum (rER). - The nuclear envelope is not continuous, but form nuclear pores. ## Chromatin - It is the hereditary material of the cell - In the interphase nucleus, chromatin has two forms: - Euchromatin (extended) which is the active form. - Heterochromatin (condensed) which is the inactive form. ### Structure of chromatin Chromatin is composed mainly of DNA combined with basic proteins known as histones. There are five types of histones which are H1, H2A, H2B, H3 and H4. | | Euchromatin | Heterochromatin | |--------------------|-----------------------|-------------------------------| | 1-demonstration with L/M | - Invisible. | - Visible as Intensely basophilic clumps.| | Activity | - Active. | - Inactive. | | Localization within the interphase nucleus. | - Dispersed. | - Peripheral chromatin in association with inner nuclear membrane. - Chromatin islands of basophilic granular clusters scattered in nucleoplasm - Nucleolar associated chromatin surrounding the nucleolus | ## Nucleolus - It is a dense non-membranous structure located in the nucleus. - It is observed only during interphase and disappears during cell division. - L/M, it is deeply basophilic due to its content of (RNA) - E/M: the nucleolus consist of three distinct components, which are: - Pars amorpha: pale-staining centers with fine threads of chromatin representing chromosomes 13, 14, 15, 21 and 22 in humans (nucleolar organizers) encoding rRNA. - Pars fibrosa: Containing filaments of newly formed rRNA surrounding the fibrillar center. - Pars granulose (nucleolenema) having granular appearance and it is the site in which ribosomal subunits are assembled. # Epithelial Tissue - It is classified according to the cell shape into: - Simple squamous epithelium: - It is made of one layer of flattened plate-like cells with flattened nuclei. ### Sites - Lungs (lining of alveoli). - Endothelium of blood vessels and heart - Simple cuboidal epithelium: - It is composed of a single layer of cuboidal cells with round central nuclei. ### Sites - Thyroid follicle - Simple columnar epithelium: - It is made of a single layer of tall cells with oval basal nuclei. It is subdivided into non-ciliated and ciliated types: - Simple columnar ciliated epithelium: - The cells have cilia on the free surface. ### Sites - Bronchioles of lungs - Simple columnar non-ciliated epithelium: ### Sites - The gastro intestinal tract (from the stomach to the rectum). - Pseudostratified columnar epithelium: - All the cells are attached to the basal lamina but some of them do not reach the surface. - The nuclei appear to lie in various layers and result in the false appearance of stratified epithelium. - This type may be ciliated or non-ciliated. - Pseudostratified columnar ciliated epithelium - This type is usually associated with goblet cells. ### Sites - Respiratory passages e.g.: - Most of nasal cavity - Larynx - Trachea - Bronchi - Pseudostratified columnar non-ciliated epithelium ### Sites - Large ducts ## B- Stratified Epithelium - The epithelium is made up of layers of cells. - It is classified according to the shape of the most superficial layer of cells into squamous, cuboidal, columnar and transitional epithelium. ### 1. Stratified squamous epithelium: - The most superficial layers are squamous or flattened - Stratified squamous non keratinized epithelium ### Sites - Mouth cavity - Vagina. - Stratified Squamous keratinized epithelium ### Sites - Epidermis of skin. ### 2. Stratified cuboidal epithelium - The most superficial cells are cuboidal in shape. ### Site - Lining ducts of sweat glands. ### 3. Stratified columnar epithelium: - The superficial cells are columnar. - Stratified columnar non ciliated epithelium ### Sites - conjunctiva - Stratified columnar ciliated epithelium ### Sites - Fetal esophagus ### 4. Transitional epithelium - This epithelium lines the urinary passages. In the empty or relaxed condition, the epithelium is made of 3-8 layers. The superficial layer is composed of dome shaped cells with 1-2 nuclei. When the organs are distended, the epithelium becomes stretched the superficial layers become squamous. ### Sites - urinary passages: pelvis of the kidney, ureter, urinary bladder. # Connective Tissue ## Connective Tissue Cells ### 1) FIBROBLASTS AND FIBROCYTES - The most common cells in C.T. - There are two different morphologic types: - The active stage is called fibroblast. - The inactive (resting) stage is called fibrocyte. #### Fibroblast: - Origin: undifferentiated mesenchymal cells (U.M.C). - Shape: irregularly branched cells. ##### Nucleus - Large ovoid - Pale staining with extended chromatin - Prominent nucleolus ##### Cytoplasm - Abundant basophilic. - Rich in RER - Well developed Golgi. ##### Function: - Synthesis of collagen, reticular and elastic fibers. - Synthesis of glycosaminoglycans and glycoproteins of the matrix. ### 2) Macrophage - Origin: From monocytes. - Shape: Irregular surface with protrusions and indentations. ##### Nucleus - Eccentric. - Kidney shaped. ##### Cytoplasm - Many lysosomes. - Staining: By injection of vital dyes (trypan blue or Indian ink). ##### Functions: - Defensive through Phagocytosis and digestion of foreign particles by lysosomes. - Repair by removal of the necrotic tissue. ### 3) Mast cell - Origin: From stem cells (U.M.C). - Cytoplasm: filled with metachromatically stained granules. - Staining: Metachromatically with toluidine blue. ##### Function.The granules contain: - Heparin which is anticoagulant - Histamine in hypersensitivity - Eosinophil chemotactic factor ### 4) Plasma cell - Origin: B-lymphocyte. ##### Nucleus - Eccentric. - With condensed chromatin in the form of cart wheel or clock-face appearance. ##### Function: - Synthesis of antibodies # Vascular System ## Blood Vessels - The cardiovascular system is composed of: - The heart. - Blood vessels. - The arteries that carry the blood from the heart to tissues. - The veins that carry the blood from tissues to the heart. - Arterio-venous connections in the form of capillaries. ### General structure of blood vessels ### 1-Tunica intima: (The innermost). - It consists of: - Endothelium: (simple squamous cells). - Subendothelium: loose connective tissue. ### 2-Tunica media: (The middle layer). - It is formed of: - Circular smooth muscle cells. - Variable amounts of elastic fibers. ### 3-Tunica adventitia: (The outermost layer). - It consists of loose connective tissue. It contains small blood vessels (vasa vasorum) that supply the outer part of the wall while the inner part is supplied from the blood in its lumen. ## II- ARTERIES - They are: - Large (elastic) arteries - Medium sized (muscular) arteries - Arterioles - Metarterioles ### LARGE (ELASTIC) ARTERIES: - They are the aorta and its main branches. ### The specific features: - Tunica media: the main component of the wall of the aorta. - It consists mainly of concentric layers of elastic laminae. - Between the elastic laminae, there arecircular smooth muscle - Tunica adventitia: It is relatively thin. ### Large veins (e.g. inferior vena cava): ### The specific features: - Media: It is thin with few لایه of smooth muscle cells. - Adventitia: - It is the thickest layer and contains bundles of longitudinally arranged smooth muscle. - Valves are present. | Structure | Medium sized artery | Medium sized vein | |--------------------------------|----------------------|--------------------| | Lumen | -Narrow | - Wide | | Intima | Well developed internal elastic lamina | Has no internal elastic lamina | | Media | - Thick | - Thin | | Adventitia | As thick as media | Thicker than media. | | Valves | Absent | Present | ## CAPILLARIES - Capillaries are the smallest and thinnest channels in the circulatory system. - Capillaries arise from metarterioles or directly from arterioles. - They recollect forming venules. ### Structure: - L/M: The blood capillary consists of: - Endothelial cells: The cross section usually contains a single layer of 1-3 endothelial cells. - Basal lamina. - Pericytes: - They partly surround the endothelial cells with their long cytoplasmic processes. - They contain actin and myosin and may have a contractile function. - Following tissue injuries pericytes produce smooth muscle cells of the newly formed blood vessels. ### Types of blood capillaries - Continuous capillary - The endothelial cells have no fenestrae (pores) - It is present in brain. - Fenestrated capillary - The endothelial cells have large fenestrae in their walls. - It is present in the renal glomeruli and endocrine glands. - The basal lamina is continuous. - Sinusoidal capillary - It has a tortuous path and wide. - The endothelial cells have multiple fenestrae. - The basal lamina is discontinuous. - They are present in the liver, bone marrow and spleen. # BLOOD - Blood cells (45% of the total blood volume) - Red blood cells (corpuscles) or erythrocytes - White blood cells or leukocytes - Platelets or thrombocytes. ## ERYTHROCYTES ### (Red blood cells, RBCs) - Shape: They are rounded biconcave discs. - Size: - The normal diameter of RBCs varies between 6 - 9 microns (7.5 µm average) these cells are normocytes. RBCs larger than 9 µm in diameter are called macrocytes as in macrocytic anaemia. Those smaller than 6 µm in diameter are called microcytes as in microcytic anaemia. - Number: - 4.5-5.5 millions/mm³ of blood. - Structure: - They are not true cells, as they have no nuclei, or organoids. The cytoplasm contains mainly Hb (cell inclusion) enclosed by cell membrane. - They have biconcave disc shape - Reticulocytes: - These are immature erythrocytes which constitutes normally about 1% of the circulating erythrocytes. Their diameter is slightly larger than mature erythrocyte. - The reticulocytes have no nuclei, but they contain some free ribosomes and few mitochondria. ## LEUKOCYTES (White Blood Cells, WBCs) - Total leukocytic count: the normal total number of leukocytes is 11,000/mm³ of blood. - Differential leukocytic count is the percentage of each type of leukocytes to the total number of leukocytes. ### I) Granular leukocytes: - Neutrophils - 50-70% - Eosinophils - 1-4 % - Basophils - 0-0.5% ### II) Nongranular leukocytes: - Lymphocyles - 20-40% - Monocytes - 2-8 % ## NEUTROPHILS (Polymorphs, Polymorphonuclear leukocytes) - Nucleus: The mature cell has one segmented (lobulated) nucleus formed of 2-5 lobes (usually 3 lobes) - Cytoplasm: - It contains two types of granules: - Specific granules, which are small and more numerous (80%). - Non-specific (azurophilic) granules, which are larger and less numerous (20%). They are lysosomes. - Functions - Antibacterial. - It is the most important in acute inflammation. ## EOSINOPHILS: - Nucleus is bilobed with horse-shoe shaped appearance. - The cytoplasm contains two types of granules: - Specific eosinophilic granules. By electron microscope these granules show central crystalline dense core and contain antiparasitic and histaminase. - Non-specific granules which are lysosomes containing hydrolytic enzymes. - Functions: - Antiallergic. - Antiparasitic. ## BASOPHILS - The nucleus is S-shaped. - The cytoplasm contains two types of granules: - Specific granules which obscure the nucleus. They are similar to those of mast cells containing heparin and histamine. - Non-specific granules which are lysosomes containing hydrolytic enzymes. - Functions: - Secretion of heparin---------anticoagulant. - Secretion of histamine-------causes allergy. ## LYMPHOCYTES ### 1) Small lymphocytes - The nucleus is relatively large and rounded. Its chromatin is so condensed. - The cytoplasm is scanty and has few organelles. ### 2) Large lymphocytes - The nucleus is relatively large and rounded with indentation at one side. Its chromatin is less condensed than that of the small lymphocytes. - The cytoplasm is basophilic because it contains an increased amount of free ribosomes. - Functions: - Humoral immunity by B-lymphocytes. - Cellular immunity by T-lymphocytes. ## MONOCYTES - The nucleus is oval or kidney shaped and is eccentrically placed. - The cytoplasm contains lysosomes. - Functions: - Phagocytosis. - Production of mono-nuclear phagocytic system. # LYMPHATIC TISSUE AND SKIN ## Differentiation of lymphocytes - T & B-lymphocytes originate in the bone marrow. - T cells migrate to thymus, while B-cells remain in the bone marrow. - T and B-lymphocytes are programmed. ### Activated lymphocyte: - When a programmed B or T-lymphocyte is activated, it changes to a much larger cell with basophilic cytoplasm and lightly stained nucleus (more extended chromatin). ### Activation and differentiation of B – lymphocyte produces - Plasma cells that produce antibodies. - B-Memory cells for secondary immune response. ### Activation and differentiation of T-lymphocyte produces - Cytotoxic or killer cells. They attack foreign cells, for example: - Cancer cells - Virus infected cells - Transplanted cells - T-helper cells: play an important role in activation, proliferation and differentiation of B-cells and T-cells. They are killed by the virus that causes the acquired immuno-deficiency syndrome (AIDS). - T-suppressor cells: They inhibit T-helper and B-cells and so suppress both humoral and cellular immune response. - T-Memory cells for secondary immune responses ## Lymphoid organs ### THYMUS - At puberty it atrophy. - Formed of lobules continuous with each other. #### The cortex - The cortex is the peripheral dark zone because it contains small lymphocytes. #### The medulla: - It is lightly stained because it contains large activated lymphocytes - It contains Hassle's corpuscles which consist of degenerated cells. ### Functions of thymus - Maturation of T-lymphocytes responsible for cellular (cell-mediated) immunity ### LYMPH NODES - Lymph nodes are encapsulated, kidney shaped lymphatic organs. - #### The cortex consists of - Lymphatic follicles - Lymphatic Cortical sinuses #### The medulla consists of: - Lymphatic cords - Lymphatic Medullary sinuses #### Functions of lymph nodes - Immunologic reaction, both cellular and humeral. - Filtration of lymph by macrophages. ### TONSILS - They are aggregates of lymphoid tissue beneath the epithelium. #### Palatine tonsils: - They are in the oropharynx. - They are covered by stratified squamous epithelium #### Lingual tonsils: - They are in the base of the tongue. - They are covered by stratified squamous epithelium. #### Pharyngeal tonsil (adenoids): - They are in the nasopharynx. - They are covered by pseudostratified columnar ciliated epithelium. ### SPLEEN - The spleen is the largest lymphatic organ in the body. - A fresh section of the spleen shows dark red tissue (the red pulp), within which white spots (white pulp) are observed. #### A- THE WHITE PULP - It is formed of lymphatic follicles. #### B - THE RED PULP - It is composed of: - Blood sinusoids - The splenic cords (Billroth cords) #### Functions of the spleen - Cellular and humoral immunity. - Immunologic filtration of blood by removal of old cells. - Haematopoietic function in fetus. ## Skin - The skin is the heaviest single organ of the body - It consists of: - Epidermis: an epithelial layer of ectodermal origin. - Dermis: a connective tissue layer of mesodermal origin. ### I- EPIDERMIS - It is the outer layer of the skin and is a keratinized stratified squamous epithelium which consists of - keratinocytes - Non-keratinocytes which are melanocytes, and Langerhens' cells. - Keratinocytes are arranged in 5 layers: - Stratum basale (germinal layer) - It consists of a single layer of columnar cells - Stratum spinosum (prickle cell layer) - It is formed of several layers of polygonal cells. - Stratum basale and deeper part of stratum spinosum are called together Malpighian layer - Stratum granulosum (granular layer) - 3 to 5 layers of flattened polygonal cells releasing lipid-containing substance in the intercellular space. This substance forms a barrier against invasion with foreign materials. - Stratum lucidium (clear layer) - It is translucent, thin layer of acidophilic flattened cells containing eleidin, against invasion with foreign materials. - Stratum corneum (horny layer) - It consists of many layers of keratin. - Renewal of the epidermis occurs every 15-30 days by mitosis of the cells of Malpighian layer (stratum basale and stratum spinosum). - Melanocytes - They are derived from the neural crest. They migrate to the skin early in development. - They are located between the cells of stratum basale and in hair follicles. - Melanocytes synthesize melanin causing pigmentation of the skin. - Langerhans' cells - They are located between cells of stratum spinosum. - They are the skin macrophages. ### II- DERMIS - It is subdivided into two indistinct layers. - Papillary layer: - It is the outer layer beneath the epidermis. - It consists of loose connective tissue. - Reticular layer - It is the deeper thick layer. - It consists of dense connective tissue #### SWEAT GLANDS - They are simple coiled tubular glands. - There are two types - Merocrine sweat glands - Develop from epidermis. - They are widely distributed in the skin. - Apocrine sweat glands - Develop from hair follicles. - They have limited distribution, in axilla and pubic regions. - They usually open into hair follicle, above the opening of sebaceous glands. #### Sebaceous glands - They are absent in thick skin. - They develop from the hair follicles. - They are holocrine branched acinar glands. - Their ducts open into the upper part of the hair follicles. - The sebum oils the hair and lubricates the skin and have antibacterial and antifungal effect. | | Thick Skin | Thin Skin | |---------------------------|-------------------------|--------------------------------------------------------------------------| | Site | covers palms and soles | covers the whole body except palms and soles | | Epidermis | Thicker than dermis | Thinner than dermis | | stratum granulosum | Well developed | Not well developed | | stratum corneum | Thicker | Thinner | | Stratum lucideum | Apparent | Not apparent | | Dermis | Contains many sweat glands | Contains fewer sweat glands, no Contains hairs, hair follicles and sebaceous glands | | | No hair follicles no sebaceous glands . | | | | | | # Cartilage - Cartilage is a special type of connective tissue. - Origin: Mesenchymal cells (UMCs). ### Cartilage is formed of: - Cells (chondrocytes). - Extracellular matrix(ECM). - The extracellular matrix is - avascular (no blood vessels) - firm, - rubbery. - It consists of fibers and ground substance. ## Cartilage types - There are three types of cartilage according to the type of fibers in the matrix - Hyaline cartilage (collagen type II). - Elastic cartilage (collagen type II+ elastic fibers). - White fibrocartilage (collagen type 1). - The perichondrium is a dense connective sheath covered all cartilage **except** articular cartilage and white fibrocartilage. ### Hyaline cartilage - Hyaline cartilage is the most common type. ### Sites: - Costal cartilage - Most of the fetal skeleton - Articular cartilage - Epiphyseal plate of long bone - Wall of large respiratory Passages: (nose, larynx, trachea and bronchi). #### Hyaline cartilage structure (perichondrium) - Perichondrium: surrounds cartilage - Outer fibrous layer: - Formed of dense irregular connective tissue and blood vessels. - Function: - Nutrition of avascular cartilage. - Inner chondrogenic layer: - formed of chondroblasts #### Hyaline cartilage structure (Chondroblasts) - Chondroblasts: Immature cartilage cell - Origin: Mesenchymal cells (UMCS) & pericytes. - Site: In the inner layer of the perichondrium. ##### LM appearance: - Shape: oval. They are not present inside lacunae. - Nucleus: central, oval and vesicular. - Cytoplasm: basophilic ##### EM appearance: - Show the characters of protein secretory cell (RER, Golgi complex, and mitochondria). ##### Function: - Synthesis and secretion of extracellular matrix (ECM) of cartilage - Divide and differentiate to chondrocytes. - It is responsible for appositional growth (addition new layers on the surface) of cartilage and repair of damage. #### Hyaline cartilage structure (Chondrocyte) - Chondrocytes: Mature cartilage cell - Origin: chondroblasts. - Site: Inside the matrix. ##### LM appearance: - Cell shape: small and oval cell Nucleus: central dark nucleus. - Cells are present inside lacunae. - Cells are present in groups (up to 8 cells in one lacuna called cell nests). - Cytoplasm: pale basophilic. ##### EM appearance: - Show the characters of protein secretory cell (RER, Golgi complex, and mitochondria). - It also contains fat droplets and glycogen. ##### Function: - Synthesis & secretion of ECM of cartilage. - Can divide. - Responsible for interstitial growth of cartilage (adding new matrix) - chondrocytes: - specialized cells that produce and maintain the extracellular matrix - Extracellular matrix: consist of: - Ground substance - Fibers #### Hyaline cartilage structure (Extracellular matrix) - Ground substance: - Macromolecules: - Glycosaminoglycans (GAGs): chondroitin sulphate, keratin sulphate & hyaluronic acid. - Proteoglycans: GAGs bind to protein. - Glycoprotein: chondronectin for adhesion of cells to matrix components (anchor effect). - Tissue fluid: - 75% of matrix weight. - To Allow easy diffusion of O2 and nutrients از blood vessels of perichondrium to avascular cartilage. - Fibers: Fine network of collagen type II - The matrix surrounding each chondrocyte: Glycosaminoglycans rich and collagen poor so chondrocyte in lacunae also called territorial or capsular matrix. ##### Matrix stains: - H&E: it appears basophilic and deeply basophilic around chondrocytes. - Why? due to high content of negative charged chondroitin Sulfate. - PAS: it appears PAS Positive. - Why? due to carbohydrate content. - Toluidine blue: it is metachromatically stained. - Why? due to its proteoglycans content. - Fine network of collagen type II can't be seen, why ? - Submicroscopic dimension. - Same refractive index of ground substance. #### LM: Hyaline cartilage - Fibrous layer - Chondrogenic layer - Perichondrium - Chondroblasts - Cell nest - Matrix - Chondrocyte in lacunae ### Elastic cartilage - LM appearance: - Yellow in fresh State. - More pliable. ### Sites: - Ear pinna. - External auditory canal. - Eustachian tube. - Epiglottis. - Some cartilage of larynx. ### Structure: - Similar to hyaline cartilage **except** its matrix contains: - network of elastic fibers - collagen type II #### LM: Elastic cartilage - Chondrocytes: Large, scattered - Matrix: Elastic fibers and collagen type II - Perichondrium present ### White fibrocartilage - White in fresh state. - In regions subjected to pulling force. ### Sites: - Intervertebral disc. - Symphysis pubis. - Semilunar cartilage of knee. - Near tendon insertion. ### Structure: - Rows of chondrocytes. - Bundles of collagen type 1 (so fibrocartilage matrix is acidophilic). - No perichondrium (blood vessels are present in between collagen fibers). #### 3. White fibro Cartilage - No Perichondrium - Chondrocytes: arranged in linear group separated by collagen fibers - Matrix: Mainly Collagen I type + collagen II type collagen # Bone - Bone is mesenchymal in origin. - It is highly vascular. - It is a specialized connective tissue (CT), formed of: - Bone cells: osteogenic, osteoblast, osteocytes and osteoclasts. - Bone matrix (calcified matrix). - It covers the external surfaces by periosteum and lining the internal surfaces by endosteum. ## I. BONE MATRIX ### 1) Organic matrix (Osteoid tissue): - Fibers (90%): - Collagen type I - It is responsible for acidophilic matrix and high resistance of the bone. - Ground substances (10%): - Macromolecules: GAGs, Proteoglycans & Glycoprotein. - Tissue fluid: More in young and less in old. ### 2) Inorganic matrix (minerals): - Mainly Calcium and Phosphorus insoluble salt, present as hydroxy apatite crystals, lie alongside the collagen fiber to form bone lamellae. - Others salt as citrate, Co4, K, Mg & Na are also present in small amounts. ## Composition of Bone - Organic compounds (mostly collagen) 33% - Total inorganic components 67% | | | |--------------------------------------|----------------------| | Bone Contains ... | | | Calcium 39% | 99% of the body's Calcium | | Potassium 0.2% | 4% of the body's Potassium | | Sodium 0.7% | 35% of the body's Sodium | | Magnesium 0.5% | 50% of the body's Magnesium | | Carbonate 9.8% | 80% of the body's Carbonate | | Phosphate 17% | 99% of the body's Phosphate | ## II. BONE CELLS ### 1) Osteogenic cells: - Origin: UMC. & pericytes. ##### LM: - Shape: Small & oval. - Site: In periosteum (inner osteogenic layer) and endosteum. - Nucleus: Central, oval, vesicular. - Cytoplasm: Basophilic. - EM: abundant free ribosomes. ##### Function: - Divides & differentiates into osteoblast. - In poor blood supply it differentiates into chondroblasts. ### 2) Osteoblast (bone forming cell): - Origin: Osteogenic cell ##### LM appearance: - Shape: Large, cuboidal & branched. Its processes connected by gap junction. - Site: In active periosteum & endosteum. - Nucleus: Eccentric, round & vesicular. - Cytoplasm: Basophilic. - EM apearance: Protein secreting cell (ribosomes, RER, golgi apparatus, and mitochondria) ##### Function: - Formation & secretion of organic bone matrix (osteoid tissue) - Release of matrix vesicles: Contain alkaline phosphatase which help deposition of Ca crystals in osteoid tissue. - Differentiate into osteocytes (it is non-dividing cell) ### 3) Osteocytes: - Origin: Osteoblast. ##### LM appearance: - Shape: Small, oval & branched. Its processes connected to each other & to osteoblast by gap junction. - Site: In between bone lamellae, Inside lacunae & its processes inside canaliculi. - Nucleus: Central, oval & dense. - Cytoplasm: Less basophilic. ##### EM appearance: few organelles (ribosomes, RER, golgi apparatus, and mitochondria). ##### Function: - Keep the integrity of bone and it is non-dividing cell. ### 4) Osteoclasts: - Origin: Monocyte. ##### LM appearance: - Shape: Large, irregular, motile and the border facing bone is brush or striated border - Site: In Howship's lacuna, on bone surfaces where resorption occur (Endosteum & bone cavities). - Nucleus: Multi-nucleated - Cytoplasm: Acidophilic. - Osteoclast: Multinucleate cell that secretes acids and enzymes to dissolve bone matrix ##### EM appearance: 4 zones are identified: - Ruffled border: Folds into irregular projection. - Clear zone: present around ruffled border. - lack organelles and contains actin filament. - Site of osteoclast adhesion to bone matrix. - Vesicular zone: cut section of folds or lysosomes. - Basal zone: contains nuclei, lysosomes, Golgi, RER & Mitochondria. ##### Function: - Bone resorption. - Phagocytosis. - How bone resorption occurs? - Osteoclasts create acidic media H+ to dissolve Ca. - They secrete collagenase to digest collagen. ##### Function: - Bone resorption. - Phagocytosis. ## PERIOSTEUM & ENDOSTEUM ### PERIOSTEUM - Outer fibrous layer: formed of dense irregular CT and blood vessels. - Inner osteogenic layer: formed of osteogenic cells & osteoblasts. ##### Function: - Provide blood supply. - Appositional growth. - Repair of fracture. - Site of tendon attachments. ### ENDOSTEUM - Lines inner surfaces and cavities of bone. - Vascular CT. - Contains osteogenic cells, osteoblasts and osteoclasts. - Function: Nutrition & appositional growth. ## I. TYPES OF BONE ACCORDING TO MATURITY - **Immature bone (Primary, woven):** - 1st bone formed in embryo & in fracture repair. - Temporary, replaced by mature bone Except in: - Tooth socket. - Near suture line in skull. - Site of tendon insertion. - **Mature bone (Secondary, lamellar):** - Present in adults. | | Mature bone | Immature bone | |------------|-----------------|-----------------| | Collagen | Regular | Irregular | | Cells | Less | More | | Minerals | High | Low | ## COMPACT BONE - **Compact bone Structure:** - **Haversian system (osteon):** - Structural unit of compact bone, it is formed of: - Haversian canal (contain blood vessels & nerves). - 5-20 concentric bone lamellae. - Osteocytes within their lacuna in between lamellae - **Bone lamellae (calcified collagen fiber)** - outer circumferential lamellae; under periosteum. - Inner circumferential lamellae; under endosteum. - Concentric lamellae; around Haversian canal. - Interstitial lamellae; in between Haversian system. - Osteocytes inside lacuna ### Structure: - Haversian canal (longitudinal canal) and Volkmann's canal are lined with endosteum. - They are contain blood vessels, lymphatic & nerves - They connect with each other & with periosteum & endosteum. - The perforating fibers of sharpey: - Bundles of periosteal collagen fibers penetrate the bone matrix binding the periosteum to the bone. ### Sites: - Shaft of long bone (Diaphysis) - outer covering of any cancellous (Spongy) bone (Epiphysis) ## 2. CANCELLOUS BONE (SPONGY BONE) ### Sites: - Flat bones. - Short bones. - Epiphysis of long bone. ### Structure: - Periosteum - Outer thin shell of compact bone. - Bone trabeculae: - Branching and anastomosing. - Formed of irregular bone lamellae and osteocytes in lacuna in between. - Bone marrow spaces: