Last Theoretical Handout (S&F) 2024-2025 PDF

2024-2025 1|Page  Learning outcomes 1- Describe the sturcture and movements of human Skeleton. 2- Identify cell structure 3- Discriminate the structure and function of different cell membrane speciali...

2024-2025 1|Page  Learning outcomes 1- Describe the sturcture and movements of human Skeleton. 2- Identify cell structure 3- Discriminate the structure and function of different cell membrane specializations. 4- Identify cell cycle, karyotyping, chromosomal anomalies and cell renewal. 5- Describe the general characters and functions of epithelium. 6- Differentiate between different CT cells and different types of CT proper. 7- Identify different types of blood cells and their formation 8- Describe the classifications and importance of the four major classes of biological macromolecules (Proteins, carbohydrates, nucleic acids, and lipids). 9- Identify storage and expression of genetic information 10- Describe functions and deficiencies of vitamins 11- Describe different mechanisms of homeostasis, feedback and differentiate between body fluid compartments and its regulation. 12- Describe divisions, functions, receptors, and higher centres of autonomic nervous system. 13- Discriminate different blood components and their functions. 2|Page  INDEX Content page ANATOMY 1 * The anatomical structure of the human skeleton 4 2 * Classifications of joints 12 3 * Muscles 17 HISTOLOGY 4 * The structure of the cell 20 5 * Cytogenetics 58 6 * Character of epithelial tissue (types , function and polarity) 80 7 * Character of connective tissue and its types. 104 8* Blood (cellular element) and Hemopoiesis. 134 134 BIOCHEMISTRY 9 * Carbohydrates chemistry 160 10 * Lipids chemistry 179 11 * Proteins chemistry 198 12 * Nucleic acid chemistry 227 13 * Molecular Biology 240 14 * Enzymes 283 15* Vitamins 299 PHYSIOLOGY 16 * General physiology 321 17* Cell membrane 325 18* Body fluids 337 19 * General organization of body control systems 343 20 * Blood physiology 368 3| P a g e ANATOMY ANATOMY The anatomical structure of the human skeleton 4| P a g e  ANATOMY Classification of Bones The skeleton is the framework that provides structure to the rest of the body and facilitates movement. -The skeletal system includes 206 bones, cartilage, and ligaments. - The functions of the skeletal system are: 1- forms a rigid framework of the body. 2- Provides protection to the organs of the body from external injury. 3- Provides leverage for the body movements. - The skeleton is divided into: 1- Axial skeleton. 2- Appendicular skeleton. Axial skeleton: - It consists of bones and cartilage that lie close to the central axis of body. - It includes skull (eight cranial bones form the cranial cavity and enclose the brain, 14 facial bones form the face), vertebral column (33 vertebrae; 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral bones and 4 fused coccygeal bones), rib cage (21 pairs of ribs and sternum), hyoid bone and auditory ossicles. Appendicular skeleton: - It includes bones of pectoral girdle, upper limb, pelvic girdle and lower limb. Classifications of bones 1-According to the shape (morphological classification). 2-According to the structure. 5| P a g e  ANATOMY 1- Classification according to the shape: depending on the shape and size. The bones are classified into: 1- long bones (Fig.1): Bone which its length is greater than its breadth. They are found in limbs: Parts of long bone (Fig.2): Epiphysis: bones' upper and lower ends Diaphysis: Shaft of long bone Epiphyseal plate: is a plate of hyaline cartilage Fig. 1: Long bone separating epiphysis from diaphysis during growing period and allows growth of bone in length. The metaphysis is the region where the epiphysis joins the diaphysis. 2-Short bones (Fig.3): They have definite shape without length. These bones Fig.2: Parts of the long found in wrist (carpal bones) and foot (tarsal bone). bones 3-Flat bones (Fig.4): Fig.3: Short bone 6| P a g e  ANATOMY Flat bones are thin, flattened, plate like bones and usually curved. Most of the bones of the cranium are flat bones, the scapulae, and the sternum. 4-Irregular bones (Fig.5): They are the bones that have no definitive shape. E.g.: Hip bone and vertebra. 5-Pneumatic bones (Fig.6 &7): Fig. 5: Irregular bone (Hip -These bones contain cavities filled with air. They are mainly located around bone) the nasal cavity, e.g., maxilla, frontal, ethmoid and sphenoid bones. The air-filled cavities in these bones called paranasal sinuses e.g. maxillary air sinus. Fig.7: Maxillary air sinus. Fig.6: Pneumatic bone (Maxilla) 7| P a g e  ANATOMY 6-Sesamoid bones (Fig.8): They are seed like bony nodules embedded inside the long tendon muscles in close relation. to certain joints. They prevent friction of tendon with joints and facilitate movements, e.g. patella at the knee Fig.8: Sesamoid bone joint. (patella). 2- Classification according to the structure 1- Compact bone. 2- Spongy bones. - Compact bone: It forms the outer layer of the bones (cortex of bones) and is responsible for the strength of bones. It is dense with no visible spaces on naked eye examination. - Spongy bone: It is a meshwork of bony spicules. It encloses large spaces filled with red bone marrow. Fig.9: Gross structure of long bone -Most bones of the body have a basic structure of an outer region of compact bone and inner region of spongy bone. For example: 1- in long bones (Fig.9): the shaft consists of compact bone forming a cylinder that surrounds a central cavity called medullary cavity. The end of long bones consists of spongy bone surrounded by a thin layer of compact bone. 2-in flat bones of skull (Fig.10): the spongy bone is sandwiched between the plates of compact bone. Fig.10: Gross structure of skull bone. 8| P a g e  ANATOMY Terms of movements Flexion and Extension (Fig.11,12): Flexion: approximation of the ventral (anterior) surfaces to each other. Extension: is movement of the ventral surfaces away from each other. Fig.11 -There is an exception for this rule; in the lower limb flexion moves the posterior surface near to each other and the reverse occurs in extension. In the trunk there are two types of flexion: forward flexion and lateral flexion.. Fig.12 1) Medial rotation and Lateral rotation (Fig. 13): rotation involves turning or revolving a part of the body around its longitudinal axis, Medial rotation/ internal rotation: the anterior surface of the limb comes towards the midline. Lateral rotation/ external rotation: the anterior surface of the limb goes away from the midline. 2) Circumduction (Fig. 14): It involves sequential flexion, abduction, extension and adduction, so that the distal end of the part moves in a circle. It occurs at the shoulder and hip joints. 3) Adduction and abduction (Fig. 15): Adduction: is the movement of the limb (upper or lower) towards the midline of the body. Abduction: is the movement of the limb away from the midline of the body. 9| P a g e  ANATOMY Fig.13,14, 15 4) Elevation and depression (Fig. 16): as occur in the shoulder joint or in the movement of the mandible (lower jaw). 5) Protraction and Retraction (Fig. 17,18): Protraction: forward movement; as the shoulder girdle in pushing or the mandible bringing the lower teeth in front of the upper teeth. Retraction: is the backward movement of the Fig.16 shoulder girdle or of the mandible to bring the lower teeth opposite the upper teeth. Fig.17 Retraction Inversion and eversion (Fig. 19): occur in the foot only: 10| P a g e Fig.18  ANATOMY Inversion: the sole of the foot turns to the inner side or medially (inwards). Eversion: the sole of the foot turns to the outer side or laterally (outwards). Fig. 19 Supination and pronation (Fig. 20): occur in the forearm. Pronation: the palm of the hand turns backwards. Supination: the palm of the hand turns forwards Opposition (Fig. 10): restricted to the thumb. the palmar surface of the distal phalanx of the thumb with faces those of the other fingers. Fig. 20 11| P a g e  ANATOMY Joints Joints are the regions of the skeleton where two or more bones meet and articulate. Classification of the joints Structural classification (Based on the material binding them and presence or absence of a joint cavity). They are classified into three types: 1. Fibrous 2. Cartilaginous 3. Synovial A. Fibrous joints: The articular surfaces of the bones are bounded together by fibrous tissue. Very little movement can be allowed. The Fig. 21: Gomphosis fibrous joints are subdivided into three types: 1. Gomphosis (Fig.21): limited to the teeth to fix them in their sockets in alveolar margins of the jaws. 2. Syndesmosis (Fig.22): present between the bones of the forearm (Radius and Ulna) and between the bones of the leg (tibia and fibula). The two bones are bounded together by a dense membrane of fibrous Fig. 22: Syndesmosis tissue called the interosseous membrane. Sutures (Fig.23): present between the bones of the skull. 1. F Fig.23 Sutures 12| P a g e  ANATOMY B. Cartilaginous joints The bony surfaces are connected by cartilage. These joints are of two types: 1. Primary cartilaginous joints (Fig.24): the bones are connected by a plate of hyaline cartilage. They are present between the epiphysis and diaphysis of the long bones. They are temporary, Fig. 24: Primary cartilaginous allow growth of long bones in length and disappear at puberty. joint They have no movement. 2-Secondary cartilaginous joints (Fig.25): the ends of the bones are covered by hyaline cartilage, and they are connected by a plate of fibrocartilage. These joints are permanent and permit a little degree of movement. They are present at the joints of the midline of the body e.g., intervertebral discs and symphysis pubis. Fig. 25: Secondary cartilaginous joint 13| P a g e ANATOMY Table 1: Comparison between the primary and secondary cartilaginous joints: Primary cartilaginous joints Secondary cartilaginous joints The bones are connected by hyaline They are connected by a plate of cartilage. fibrocartilage. They are present between the epiphysis They are permanent. and diaphysis of long bones. They are temporary and have no They have a little degree of movements. movement. They allow growth of long bones and They are present in midline of the disappear at puberty. body. C. Synovial joints These joints have a wide range of movements. Characters: (Fig.26) 1. The articular surfaces are covered by hyaline cartilage. 2. The joint has a fibrous capsule enclosing a joint cavity. 3. The capsule is lined by synovial membrane which is reflected to cover the non-articular bony parts within the joint cavity. 1. Fig.26: Characters of Synovial joints 4. The joint cavity contains a film of serous fluid called synovial fluid secreted by the synovial membrane to lubricate the articular surfaces. 5. The joint is supported by ligaments which may be extra-capsular or intra-capsular. 6. Some joints contain plates of fibrocartilage called intra-articular discs as the knee joint. 14| P a g e ANATOMY 7. Classification of Synovial joint: 1- According to the axis of movements: A- Uniaxial joints: 1- Hinge joint (Fig.27): these joints resemble a hinge of a door. It moves along transverse axis. It permits flexion and extension only e.g., elbow joint. 2- Pivot joint (Fig.28): These joints move along a longitudinal axis and allow rotation only e.g., superior and inferior radio- ulnar joints. Fig.27: Hinge joint (elbow joint) Fig.28: Pivot joint (Superior radioulnar joint) B- Biaxial joints 1- Ellipsoid (Fig.29): It consists of one convex surface that received into one elliptical concavity. It permits flexion, extension, abduction, adduction and rotation in this type is impossible e.g., wrist joint and metacarpophalangeal joints. Fig.29: Ellipsoid joint 2- Saddle shaped (Fig.30): Each of the articular surfaces is concavo-convex. Thejoint) (wrist convexity of one surface is received into the concavity of the other one and vice versa. 15| P a g e ANATOMY It permits flexion, extension, abduction, adduction with some rotation e.g., carpometacarpal joint of the thumb. Fig.30: Saddle joint (carpometacarpal joint of thumb) 3-Condyloid joints (Fig.31): two convex articular surfaces articulate with two concave articular surfaces. Movements of flexion and extension and a minimal degree of rotation are allowed e.g., Knee joint. C- Polyaxial joints. Ball and socket joint (Fig.32): a ball shaped head of one bone is received into a socket-like concavity of another Fig.31: Condyloid joint (Knee joint) bone. This type permits flexion and extension, abduction and adduction, medial and lateral rotation and circumduction. e.g., the hip and the shoulder joints. 2- According to morphology: 1- Simple: only 2 bones articulated with other e.g. Fig.32: Ball and socket joint (hip joint) shoulder joint. 2-compound: more than two pointes articulated with each other.e.g. elbow joint. N.B: Plane joints: The articular surfaces are flat. These joints allow slight gliding or sliding movements e.g. intercarpal and intertarsal joints 16| P a g e ANATOMY Muscles The muscle is a contractile tissue of the body, derived from the mesodermal layer of embryonic germ cells. It forms the red flesh of the body and accounts for about 40% of the body weight. On the basis of morphological and functional characteristics, the muscles are classified into three types: 1. Skeletal muscles (voluntary) 2. Smooth muscles (involuntary) 3. Cardiac muscles (involuntary) The skeletal muscles are attached to the skeleton, the smooth muscles form the walls of the viscera, and the cardiac muscles form the wall of the heart (myocardium). The skeletal muscle has two parts (Fig.33, 34):a-A thickened fleshy part called belly: The belly is the contractile part and is generally attached to the bone that is proximal to the bone that is to be moved. b-A cord or a rope-like fibrous part called tendon: The tendon spans the joint and is attached to the bone that is to be moved. When tendon is flattened, it is called aponeurosis. Fig. 34: Diagram to show the belly and the aponeurosis of the muscle. 17| P a g e ANATOMY APPENDIX STRUCTURES ASSOCIATED WITH MUSCLES A. Tendons 1- Are fibrous bands of dense connective tissue that connect muscles to bones or cartilage. 2- Are supplied by sensory fibers extending from muscle nerves B. Ligaments Are fibrous bands that connect bones to bones or cartilage or are folds of peritoneum serving to support visceral structures C. Raphe -Is the line of union of symmetrical structures by a fibrous or tendinous band as pterygomandibular raphe. D. Aponeuroses -Are flat fibrous sheets or expanded broad tendons that attach to muscles and serve as the means of origin or insertion of a flat muscle E. Retinaculum -Is a fibrous band that holds a structure in place in the region of joints F. Bursae Are fluid-filled flattened sacs of synovial membrane that facilitate movement by minimizing friction. G. Synovial Tendon Sheaths -Are synovial fluid-filled tubular sacs around muscle tendons that facilitate movement by reducing friction H. Fascia -Is a fibrous sheet that envelops the body under the skin and invests the muscles limit the spread of pus and extravasated fluids such as urine and blood. 18| P a g e ANATOMY 1- Superficial Fascia has a fatty Is a loose connective tissue between the dermis and the deep (investing) fascia and superficial layer (fat, cutaneous vessels, nerves, lymphatics, and glands) and a membranous deep layer 2- Deep Fascia 1- Is a sheet of fibrous tissue that invests the muscles and helps support them by serving as an elastic sheath or stocking. 2- Provides origins or insertions for muscles, forms fibrous sheaths or retinacula for tendons, and forms potential pathways for infection or extravasation of fluids. LANDMARKS AND REFERENCE LINES In surface anatomy, palpable structures or visible markings on the surface of the body are used to identif the location of underlying structures. Table 1: Vertebral Spinous Processes and Posterior Landmarks 19| P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY HISTOLOGY &CELL BIOLOGY The structure of the cell The cell is the structural and functional unit of a living organism (Fig.1). In mammals, the cell contains two major compartments: I- Cytoplasm that consists of: A) Cell organelles B) Cell inclusions C) Cytosol II- Nucleus Individual cytoplasmic components are usually not clearly distinguishable in common hematoxylin and eosin-stained sections. The nucleus appears intensely stained dark blue Fig.1. The cell 20| P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY CYTOPLASM A) Cytoplasmic Organelles They are classified according to the presence or absence of membranes into: I. Membranous II. Non- membranous organelles organelles 1. Cell membrane. 1. Ribosomes. 2. Mitochondria. 2. Cytoskeleton: 3. ER I. Microtubules: a. RER. a. Centrioles. b. SER. b. Cilia. 4. Golgi apparatus. c. Flagella 5. Lysosomes. II. Filaments 6. Peroxisomes. 7. Secretory vesicles. 8. Coated vesicles. I. Membranous organelles 1- Cell Membrane (Plasmalemma, Plasma Membrane) It is the limiting membrane of the cell and forms boundary between it and the outside environment. HISTOLOGICAL STRUCURE LM: It is not visualized by LM (very thin). It could be demonstrated by a special stain (e.g., silver stain). EM Cell membrane ranges from 7.5 to 10 nm in thickness. By EM after fixation in osmium tetroxide, it exhibits a trilaminar (Tri lamellar) appearance; two electron dense layers separated by electron lucent layer (Unit membrane). 21 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Molecular Structure: It consists of a lipid bilayer within which proteins are distributed (Fig. 2): A- Lipids: Lipid consider as the fundamental building blocks of cell membrane. They consist of phospholipids, glycolipids and cholesterol. Each phospholipid molecule consists of: i. A polar hydrophilic head. ii. Two long, non-polar hydrophobic hydrocarbon tails. Phospholipids are organized into a double layer: o Hydrophobic tails direct toward the center of the membrane. o Hydrophilic heads direct to the aqueous solution inside and outside the cell. ▪ The trilaminar appearance seen by EM is due to the deposition of osmium on the hydrophilic heads on both sides of the phospholipid bilayer (Fig. 2) Fig. 2. Molecular structure of cell membrane and phospholipid B- Proteins: 22 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Represent the major molecular constitute of cell membrane approximately half of the total membrane mass. 1. Integral proteins: They are firmly embedded in the lipid bilayer. Most of them pass through the whole thickness of the membrane (transmembrane proteins). Transmembrane protein form ion channels and carrier proteins that facilitate the passage of specific ions and molecules across the cell membrane. 2. Peripheral proteins: They are usually located on the cytoplasmic surface of the cell membrane or occasionally on the extracellular surface. C- Carbohydrates: They are polysaccharide components attached to transmembrane proteins (glycoprotein) or phospholipids molecules (glycolipid). They project from the external surface of the cell membrane and contribute to the formation of carbohydrate-rich surface coat or glycocalyx. Functions of Cell Membrane: 1. Keeps constant intracellular environment, which is different from the extra- cellular fluid. 2. Keeps cell shape (epithelial cells, CT cells, nerve cells, RBCs, muscles etc). 3. Specific recognition, via receptors of antigens and foreign substances. 4. Regulatory functions because it has specific signaling receptors. 5. A selective barrier that regulates the passage of certain molecules into and out of the cell. For performing the barrier function, there are three main mechanisms: 23 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Membrane Transport Signal Reception Vesicular Transport I. Membrane Transport: See page….(Physiology) II. Signal Reception: See page….(Physiology) III. Vesicular Transport A process involves changes in the plasma membrane at localized sites followed by vesicles formation. There are two main forms of this transport: 1. Endocytosis 2. Exocytosis 1- Endocytosis: brings substances into the cell. Three mechanisms of endocytosis are recognized: a) Phagocytosis b) Pinocytosis c) Receptor mediated endocytosis (clathrin-mediated):{See Coated vesicle page…..:} a) Phagocytosis: means "cell eating."(Fig. 3) It is prominent in macrophages and neutrophils that are specialized for removing foreign bacteria, damaged cells, and unneeded extracellular constituents. Cytoplasmic processes of macrophages extend with the help of actin filaments and surround this foreign material. The edges of the processes fuse enclosing the foreign substance in an intracellular phagosome. 24 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY It may be receptor-mediated process e.g., phagocytosis of microorganisms, or receptors are not involved e.g., phagocytosis of inhaled carbon and dust particles. b) Pinocytosis: means "Cell drinking." (Fig. 3) Small invaginations of the cell membrane form and entrap extracellular fluid substances. Pinocytotic vesicles pinch off from the cell surface. The vesicles have smooth surface in EM (clathrin- independent). It is prominent in endothelial cells lining the blood vessels. Fig. 3. Phagocytosis and Pinocytosis 2- Exocytosis: Exocytosis is the process by which a cytoplasmic vesicle discharges its contents to the extracellular space without affection of cell membrane integrity. Exocytosis is mediated by a number of specific proteins and Ca++ as an increase in cytosolic Ca++ often triggers exocytosis. There are two general pathways of protein exocytosis: 1. Constitutive pathway in which the proteins leave the cell immediately after their synthesis and are not stored as secretory granules in the cytoplasm e.g. secretion of 25 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY antibodies by plasma cells. 2. Regulated pathway in which secretory proteins are stored in the cytoplasm as secretory vesicles e.g. cells of pancreas. This type is controlled by hormonal or neural stimuli. 2- Mitochondria They are important in energy production of cells "power house of the cell". Life span: about 10 days. They contain their own genetic components which is composed of circular DNA, mRNA, tRNA and rRNA. They can divide by simple binary fission like bacteria. Site and Number: They are randomly distributed in the cytoplasm or tend to accumulate near sites of high energy utilization such as: 1. The apical part of ciliated cells. 2. The middle piece of spermatozoa. 3. The base of active transport cells. The number of mitochondria and their cristae are directly related to the activity of the cells as: 1. Cells with a high-energy metabolism (e.g., cardiac muscle cells) have abundant mitochondria with a large number of cristae. 2. Cells with a low-energy metabolism (e.g., osteocytes) have few mitochondria with few number of short cristae. HISTOLOGICAL STRUCTURE LM: They are not visible with H&E stain. They need a special supravital stain as Janus green B. 26 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY They appear as minute rods or spheroid bodies (0.5-1µm wide and up to 10µm length). EM: Mitochondria have a characteristic structure which is formed of: a) Outer smooth membrane: It surrounds the entire mitochondrion. b) Inner membrane: It has numerous folds (cristae) directed to the interior of the mitochondria. Most cells contain mitochondria with flat, shelf-like cristae. Cells that secrete steroids (e.g., adrenal gland) frequently contain mitochondria with tubular cristae. The cristae markedly increase the internal surface area and contain enzymes and other components of oxidative phosphorylation and electron transport systems. c) Intermembrane space: It separates the outer and the inner membranes. d) Matrix Space: The inner membrane encloses it. It is rich in protein and contains mitochondrial DNA and mitochondrial ribosomes (Fig. 4). It has the enzymes for citric acid (Krebs) cycle and lipid oxidation. In many types of cells, the matrix shows rounded electron-dense granules rich in Ca++. 27 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. 4. Electron-micrograph and diagram of mitochondria Function: 1. The main function of mitochondria is the energy production as they produce ATP, the primary source of energy for the cell. 2. The mitochondria store Ca++ and other cations in the dense matrix granules. Also, they regulate the concentration of it and other certain ions of the cytoplasm. 3- Endoplasmic Reticulum a. Rough Endoplasmic Reticulum (RER) Endoplasmic reticulum is an anastomosing network of intercommunicating channels and sacs formed by a continuous membrane, which encloses a space called a cisterna. The name "rough" is due to the presence of ribosomes on its surface (Figs. 5 & 6). HISTOLOGICAL STRUCTURE LM It appears as basophilic areas in the cytoplasm due to presence of many attached ribosomes on its surface. It is prominent in the cells specialized for protein secretion, such as fibroblasts (secrete collagen). 28 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY EM RER consists of flattened membrane limited interconnected sacs like (or parallel stacks); called cisternae. The exterior surface of the membrane is studded with particles called ribosomes (Fig.6) Its limiting membrane is continuous with the outer membrane of the nuclear envelope. Function: The principal function of the RER is to synthesize (by attached ribosomes) and segregate proteins excreted by the cell. Certain modifications of newly formed polypeptides as glycosylation of glycoproteins. Fig. 5: Three-dimensional diagram of RER and SER Fig. 6: Electron micrograph of RER 29 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY b. Smooth Endoplasmic Reticulum (SER) It consists of short anastomosing tubules that are not associated with ribosomes. LM: It cannot be demonstrated by LM. EM: SER is a smooth tubular membranous network within the cell. It differs from RER in: (Fig. 5) 1. SER lacks ribosomes so, it appears smooth. 2. SER cisternae are tubular, interconnected channels of various sizes and shapes , anastomosing and not in stacks. Function: 1. Steroid hormones synthesis e.g., cells of suprarenal gland. 2. Detoxification of drugs by oxidation, conjugation, and methylation of these drugs as in liver cells. 3. Sequestration and release of Ca++ thatregulates muscular contraction. 4. Synthesis of phospholipids for all cell membranes. 5. Glycogen metabolism in the liver cells contains the enzyme glucose-6-phosphatase. 4- Golgi Complex (Golgi apparatus) LM: It is not seen in H&E-stained section (in highly active basophilic cells as plasma cells, it appears as empty space called "negative Golgi image"). It can be demonstrated using silver impregnation method. 30 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Size and site vary according to the type and the activity of cells: 1. Supranuclear: present between the nucleus and the apical cell membrane e.g., cells of epididymis. 2. Infranuclear: present between the nucleus and the basal cell membrane e.g., endocrine cells. 3. Perinuclear: it is demonstrated around the nucleus e.g., Nerve cells. EM: Several flattened disc-shape saccules arranged in stacks. o The periphery of each saccule is dilated. o The apparatus has two faces (Figs.7 & 8): a. Cis- face (entry) is convex and called cis or immature face. b. Trans- face (exit) is concave and called trans or mature face. Transfer vesicles containing the newly synthesized protein arise from RER and are associated with the cis face. Secretory vesicles containing modified protein are associated with the trans face. Function: 1. Modification of cell products through the addition of fatty acids, sulfation, and glycosylation. 2. Concentration and packaging of synthesized proteins into secretory granules. 3. Formation of lysosomes. 4. Recycling and redistribution of the cell membrane. 5. Synthesis of carbohydrates and lipoproteins 31 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. 7: Three-dimensional diagram of Golgi complex Fig. 8: Electron micrograph of Golgi complex 5- Lysosomes Lysosomes are sites of intracellular digestion and turnover of cellular components. They are abundant in phagocytic cells (e.g., Macrophages). Lysosomal enzymes (e.g., acid phosphatase) are capable of breaking down most biologic macromolecules. They are active at an acidic pH. Formation: The enzymes are synthesized and segregated in the RER and transferred to the Golgi 32 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY complex, to be modified and packaged as lysosomes and not secreted by the cell. LM: They can be demonstrated using histochemical reactions for their hydrolytic enzymes. EM: According to the type of lysosome: 1ry lysosomes: - Appear as rounded homogeneous membrane bound structures. - They have not participated in a digestive event. 2ry lysosomes: - They are formed after combination of primary lysosome with phagosome, pinocytotic vesicle or autophagosome. - They appear as rounded membranous structures heterogeneous in appearance and electron density. Types of Secondary lysosomes (Fig. 9) a) Heterophagic vacuole It occurs when the primary lysosome fuses with a phagosome. It appears as heterogeneous vacuole under EM b) Multivesicular body It occurs when the primary lysosome fuses with a pinocytotic vesicle. c) Autophagic vacuole It occurs when primary lysosomes fuse with the autophagosome (old organelles enclosed in a membrane). Lysosome plays a role in turnover of cytoplasmic organelles Fate of lysosomes (Fig. 9) Residual bodies: 33 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY - After digestion, nutrients diffuse through the limiting membrane of secondary lysosome to the cytoplasm. - Undigestible compounds are excreted by exocytosis except in long-lived cells (e.g., neurons, heart muscle) where they form residual bodies. They often accumulate as lipofuscin pigment. Fig. 9: Different types and fate of lysosomes 6- Peroxisomes (Microbodies) These are membranous organelles similar to lysosomes. They are spherical in shape, however, they differ from lysosomes in: - They are not formed by Golgi apparatus but raised directly from RER. - It is enclosed in a single membrane. - They contain oxidative enzymes. - They are self-replicating as mitochondria. LM: Demonstrated by specific cytochemical stains. EM: 34 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Rounded membrane-bound structures, containing moderate electron dense- material (some of them contain electron dense crystalline material). Function: Their high content of enzymes enables peroxisomes to perform vital role for cell metabolism and protection, so their functions include: 1. Oxidation of some organic substances by removing hydrogen ions by forming H2O2. 2. Elimination of H2O2 which is harmful to the cell by catalase enzyme. 3. In hepatocytes, they degrade several toxic molecules, drugs and alcohol. 4. Lipid metabolism: they are important for formation of bile acids and cholesterols. 5. Formation of myelin sheath 7- Secretory Vesicles (Secretory Granules) They are found in cells that store their product until its release which is signaled by a metabolic, hormonal, or neural message (regulated secretion) such as pancreatic cells and cells of thyroid gland. LM: They can be demonstrated by certain immunochemical methods. EM: They are rounded bodies surrounded by membrane. They have different electron densities and sizes according to the nature of their contents. 8- 8. Coated Vesicles They are involved in receptor-mediated endocytosis of specific molecules by the cell. LM: 35 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY They are demonstrated by immunocytochemistry. EM: They are rounded bodies with a cytoplasmic surface coated with clathrin, and form a network-like basket structure. Receptor-mediated endocytosis (Formation and function): A transport process that allows selected molecules to enter the cell. The outer surface of the cell membrane has receptors for many substances such as protein hormones (ligand). These receptors are either widely dispersed over the surface or aggregated in special regions called coated pits. The coat of the cytoplasmic surface is composed of several polypeptides as clathrin. Binding of the ligand to its receptor leads to the following steps (Fig. 10): 1. The widely dispersed receptors accumulate in the coated pits. 2. The coated pits invaginate inward and pinch off to form coated vesicles that carry the "ligand-receptor complex" into the cell. 3. The coated vesicles lose their clathrin coat. 4. The separated clathrin moves back to the cell membrane to participate in the formation of new coated pits. 5. They fuse with vesicles located in the cytoplasm either near the cell surface (early endosomes) or deeper (late endosomes). 6. Lysosomes fuse with endosomes to separate ligands from their receptors. 7. Receptors return back to the cell membrane. 36 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. 10. Receptor-mediated endocytosis II. Non membranous organelles 1. Ribosomes Ribosomes are non-membranous small electron-dense particles, about 20- 30 nm in size. They are composed of four types of rRNA and about 80 associated different proteins. Formation 1- Ribosomal RNA is synthesized within the nucleolus. 2- The proteins are synthesized in the cytoplasm, then enter the nucleus, and associate with rRNA. 3- Ribosomes (two subunits) leave the nucleus, via nuclear pores, to the cytoplasm. Forms of ribosomes: There are two forms of ribosomes: a. Free ribosomes scattered in the cytoplasm. b. Attached ribosomes to the endoplasmic reticulum. 37 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY LM: They are very small to be seen individually. When aggregate, they cause intense cytoplasmic basophilia due to presence of rRNA. Basophilia may be: o Diffuse (e.g. fibroblast). The cell chapter (2) 37 o Localized (e.g. pancreatic cells). o Spotty (e.g. nerve cell). EM: Ribosome is composed of two subunits: large and small subunits. (Fig. 11). Individual ribosomes are held together by a strand of mRNA to form polyribosomes (polysomes) (Fig.12). RER has receptors (specific glycoproteins, called ribophorins) in its membranes, to which the large subunit of the ribosome binds. Function: 1- It translates (decodes) the message carried by mRNA to amino acid sequence (polypeptide chain). 2- Free ribosomes synthesize proteins designed for use within the cell (e.g., hemoglobin in erythrocytes). 3- Attached ribosomes synthesize: a. Proteins secreted by the cells as pancreatic and salivary enzymes b. Proteins stored in the cells as enzymes of lysosomes. c. Integral proteins of the plasma membrane. 38 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig.11: EM and diagram of Ribosomes Fig 12: Polyribosomes attached to molecule of mRNA 2. Proteasomes Proteasomes are very small organelles composed of protein complexes not associated by membrane. They are responsible for proteolysis of malformed proteins. 3. Cytoskeleton The cytoplasmic cytoskeleton is a complex network of: (Fig.13). 1- Microtubules. 2- Filaments: 39 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY a. Thin filaments (actin filaments). b. Intermediate filaments. c. Thick filaments (myosin filaments). Microtubules It cannot be demonstrated by LM. EM: They are straight structures, 25 nm in diameter. They have a wall of 5nm thick, which surrounds a lumen-lake region. They may exist in two forms: a. Labile population exists freely in the cytoplasm and polymerizes or depolymerizes, depending on temperature, pressure, drugs, etc. changing the shape of the cell (e.g., mitotic spindle). b. Stable population forms the centrioles and the axonemes of cilia and flagella. Fig 13: A diagram showing cytoskeleton in the cytoplasm and its molecular structure Molecular Structure: (Fig.14). Each microtubule is composed of 13 spirally parallel arranged protofilaments (linear 40 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY polymers of α- and β-tubulin protein dimers). Microtubules have proteins associated with their walls called microtubular- associated proteins (MAPS). During polymerization, microtubule shows polarity, so tubulin subunits are added at one end (plus end) and removed from the other (minus end) Tubulin polymerization is controlled by: 1. Concentration of Ca++ 2. Microtubule associated proteins or MAPs Function: 1- Maintain the shape of the cell. 2- Participate in intracellular transport of organelles and vesicles, e.g., axoplasmic transport in neurons and melanin in pigment cells. 3- During cell division, chromosomes move by mitotic spindle. 4- Provide the basic structure for many cytoplasmic components including centrioles, basal bodies, cilia, and flagella. NB: Many antimitotic drugs, which are used to arrest cell proliferation in the treatment of tumors, affect microtubules in different ways: Colchicine, Taxol, and Vinblastine. Fig.14: Molecular structure of microtubule 41 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Centrioles Cylindrical structures composed of short, highly organized microtubules (0.5 µm in length and 0.2 µm in diameter). LM: Each cell contains 2 centrioles. They duplicate before cell division. The centrioles are present in an amorphous protein matrix (pericenteriolar material). The centrioles with pericenteriolar materials are called (centrosome) or microtubule organizing center (MTOC) that found near the nucleus. EM: (Fig. 15). The long axis of one centriole is perpendicular to the other and surrounded by an amorphous material. Each centriole is formed of 9 triplets of microtubules. Each triplet consists of 3 subunits of one complete microtubule (A) and two incomplete microtubules (B&C) microtubules fused to each other and share three protofilaments. Function: 1. During cell division, they form the poles of mitotic spindle where microtubules originate. 2. They form basal bodies that give rise to cilia and flagella. Fig 15: EM and diagram of centriole 42 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Cilia Definition: They are motile hair-like processes projecting from the cell surface. Origin: They arise from a structure called the basal body. Formation of cilia: 1. Multiple centrioles (one for each cilium) migrate beneath the apical cell membrane where they form the basal body. 2. From each triplet of the basal body 2 microtubules grow upwards to form the shaft and one microtubule grow downward to form the rootlet Function: to brush away things (secretion, foreign bodies,) away from the surface of cell sheets. Structure: Formed of highly organized microtubule core covered by cell membrane. Ciliated cells typically possess a large number of motile cilia. Each cilium is formed of: 1. Shaft 2. Basal body 3. Rootlet 1- Shaft: LM: They appear as acidophilic striations on the free surface of ciliated cells. EM: The cilium consists of 9 doublets of microtubules surrounding 2 central singlets of microtubules (axoneme). The microtubules in the central singlet are enclosed within a central sheath made of protein. 43 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Each one doublet is formed of two subunits of microtubules (A, B) (Fig. 16) a. Subunit A:  Complete microtubule, circular in cross section and is composed of 13 protofilaments. b. Subunit B :  Incomplete microtubule, C-shaped in cross section, composed of 10 protofilaments and shares 3 protofilaments of subunit A. Radial Spokes: ▪ Project from subunit A of each doublet toward the central sheath Nexin: ▪ Extends from subunit A of one doublet to subunit B of adjacent peripheral doublet. Dynein: ▪ Pairs of arms formed by the protein dynein, extend from the surface of each subunit A. ▪ They have ATPase activity that provides the energy for the movement of cilia. 2- Basal body: ▪ It has a similar structure to the centriole. 3- Rootlet: ▪ Group of un- organized microtubules. Flagella Flagella possesses the same structure as cilia. Flagellated cells have only one flagellum (100 µm). In human, the sperms are the only cell type with a flagellum. Clinical Note: Immotile cilia syndrome: it is characterized by male infertility due to immotile spermatozoa and chronic respiratory infections caused by the lack of the cleansing action of cilia in the respiratory tract 44 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig.16: Diagram of cilia LS, TS and its molecular structure Filaments LM: They are seen by using special stains as silver or immunocytochemical stains. EM: They have different distributions according to the cell type and variable thickness according to the type of filaments. Thin Filaments (microfilament, actin) Actin filaments are present as thin filaments (5-7 nm in diameter) and can be described in two forms: 1. Stable form as in muscle cells. (See muscular tissue). 2. Unstable form, they dissociate and reassemble according to the cell activity: a. A thin sheath just beneath the plasmalemma. These filaments play a role in membrane activities such as endocytosis and exocytosis. 45 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY b. Associated with several cytoplasmic organelles, vesicles, and granules to play a role in moving and shifting cytoplasmic components. c. With myosin, they form ring of filaments (under the cell membrane) whose constriction results in the cleavage of mitotic cells. Intermediate Filaments LM: They could be demonstrated by immunocytochemical stains. EM: They appear with an average diameter of 10-12nm. Types: Several proteins that form intermediate filaments have been isolated and localized: - Keratins in skin. - Vimentin filaments are characteristic of cells of mesenchymal origin. - Desmin is found in smooth muscle. - Glial filaments in nervous tissue. - Neurofilaments are present in nerve cell body and processes. Thick Filaments (myosin) Formed of myosin II molecules consisting of two identical heavy chains (each has a small globular head and a long rod- shaped segment that twisted together as myosin tails) and two pairs of light chains (associated with myosin heads). B) Cytoplasmic Inclusions Inclusions are temporary non-living components of the cytoplasm. Mostly, not surrounded by membrane (some of them aremembrane-bound e.g., pigment granules) 46 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Not sharing in metabolic activity. Types: 1. Stored metabolic products: a. Carbohydrate (glycogen granules) b. Lipids (lipid droplets) c. Proteins (secretion granules) 2. Pigments: a. Endogenous pigments (synthesized by the cell) ashemoglobin, melanin, and lipofuscin. b. Exogenous pigments (from outside the body) ascarotene and carbon particles 3. Crystals. 1. Stored Metabolic Products a. Glycogen It is the storage form of carbohydrates, mainly in liver and muscle cells (Fig.17). LM: It cannot be demonstrated by H&E stain because it dissolves inwater. It appears as minute red particles using special stain as Best's carmine and magenta color with PAS stain. EM: It appears as electron-dense aggregates known as the β-rosette arrangement or as large clusters of α-particles. b. Lipid Droplets They are stored in adipose tissue, adrenal cortex and liver cells (Fig.18). LM: It cannot be demonstrated by H&E stain because it dissolves in alcohol. 47 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Sudan III (stained orange), Sudan black and osmium (stained black) can demonstrate it. EM: It appears as homogenous electron lucent vacuoles. c. Secretion Granules They include mucous droplets and certain hormones. Under stimulation, these granules are periodically released into the extra- cellular environment. 2. Pigments a. Endogenous pigments Hemoglobin: The most abundant and important pigment in the body. It is present in RBCs as red pigment. Melanin: Melanin pigment is demonstrated in: 1. Skin. 2. Pigment epithelium of the retina 3. Substantia nigra of the brain Function: 1. Protects the skin against sun rays. 2. Prevent blurring of vision in the eye. Lipofuscin Pigment: It is derived from secondary lysosomes and represents deposits of indigestible substances. It is formed by fusion of several residual bodies. It appears as yellowish-brown substance mainly in long-lived cells(e.g., neurons, cardiac muscle). It is increased in quantity with the increase of the age. 48 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 1 2 Fig.17: Glycogen (1. EM, 2. Best’s Carmine) 1 2 3 Fig.18: Lipid Droplets (1.Sudan Black, 2. Sudan III. 3. H&E) b. Exogenous Pigments Carotene Pigment: Taken in vitamin A-rich food as carrots (and all orange- c o l o r e d fruits and vegetables). Excess carrot intake changes the color of skin to yellow. Carbon Particles: With smoking, carbon particles blacken the lungs. Pigments can be introduced into the skin (tattoo marks). 3. Crystals They are present in some cells as Sertoli cells and interstitial cells of the testis. 49 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Cytoplasmic Matrix (Cytosol) The cytosol is a colloid substance in which cytoplasmic organelles, cytoskeleton and inclusions are suspended. It constitutes about 50% of the total volume of the cell. Functions: 1. It coordinates the intracellular movements of organelles. 2. It provides the consistency of the cytoplasm. 3. It provides a framework for organization of certain enzymes such as those of the glycolytic pathway. 4. Transduction of signals from cell membrane to specific sites inside the cell THE NUCLEUS The nucleus is the most prominent structure in the cell. It controls all cell structures and activities. LM: Stain: the nucleus is basophilic in H&E-stained sections. o Deeply basophilic (condensed): in the inactive cells o Lightly basophilic (vesicular): appears pale with prominent nucleolus, in the active cells. Shape: It may be rounded, oval, flat, kidney shaped, lobulated, bilobed, spindle or irregular. Size: It may be small, medium sized or large. Location: It may be central, eccentric (peripheral), basal or apical. Number: Most cells contain one (mono-nucleated). 50 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Some cells contain two nuclei (bi-nucleated). Other cells have more than two nuclei (multinucleated). RBCs have no nuclei (not considered true cells). EM: It is composed of: (Fig. 19) 1. Nuclear envelope. 2. Chromatin. 3. Nucleolus. 4. Nuclear matrix. Nuclear Envelope It is a double membrane; inner and outer, separated by a narrow inter-membranous or peri-nuclear space (40-70 nm). The two membranes fuse to form the nuclear pores that provide controlled pathway between the nucleus and the cytoplasm (bidirectional). a. The Inner Membrane: It is closely associated with a protein structure called the fibrous lamina which functions to: 1. Help stabilizing the nuclear envelope. 2. Provide a definite localization of chromatin within the nuclei in non-dividing cells. b. The Outer Membrane: It contains polyribosomes and is continuous with the RER. c. The Nuclear Pores The two membranes fuse to form the nuclear pores (octagonal pore complex) that provide controlled pathway between the nucleus and the cytoplasm (bi-directional). 51 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY The pores show an octagonal pore complex consisting of: a. Annulus is formed of eight large protein granules arranged in pattern that supports and defines the inner and outer rings of each pore. b. Eight radially arranged spokes extend from the eight granules into the center of the pore. c. A centrally located electron-dense granule (plug). Function of nuclear pores: 1. Allow passage of mRNA 2. Allow free passage of ions and molecules up to 9 nm without consuming energy. 3. Allow receptor-mediated active transport of molecules larger than 9 nm (energy is utilized to open nuclear pores). Chromatin The genetic material of the cell (DNA) is present in the form of chromosomes that are visible at cell division. In the non-dividing stage (interphase), chromosomes are present in the form of chromatin. Chromatin is composed mainly of coiled strands of DNA double helix bound to basic proteins (histones and non-histones). The chromatin pattern of a nucleus has been considered a guide to the cell activity. Two types of chromatin can be identified according to the degree of chromosome condensation: - Heterochromatin. - Euchromatin. 52 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig.19: The nucleus EM Heterochromatin It represents the inactive form of DNA It is the highly coiled portion of the chromosomes. LM: deep basophilic clumps. EM: electron dense coarse granules. According to the location (site) of heterochromatin in the nucleus there are three types: 1. Peripheral chromatin: it is adjacent to the nuclear envelope. 2. Nucleolus associated chromatin: which is adjacent to the surface of the nucleolus. 3. Islets of chromatin: they are scattered in the nuclear sap. Euchromatin It represents the active form of DNA. It is the extended (uncoiled) portion of the chromosomes. LM: lightly basophilic (vesicular, open face nucleus). EM: fine electron lucent dispersed granular material 53 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Nucleolus The nucleolus is a spherical structure in the nucleus. It is rich in rRNA and proteins. LM: - It appears as intense basophilic structure inside the nucleus. EM: - it consists of 3 distinct components: a. Nucleolar Organizing Regions (NOR's) b. Granular components c. Fibrillar components a. Nucleolar Organizing Regions (NOR's): Each nucleolus contains from one to several nucleolar organizers. They appear as pale-staining regions composed of DNA which code for rRNA. They contain rRNA genes. b. Granular Components Consists of 15-20 nm granules (maturing ribosomes). The granular material consists of ribosomal subunits that have already been formed but matured and are waiting to be exported to the cytoplasm. c. Fibrillar Components: It is densely- packed 5-10nm fibrils (thread-like) It is composed of rRNA molecules and associated proteins (non maturing ribosomes). Function: It is involved in rRNA synthesis. 54 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Nuclear Matrix (Nucleoplasm) The matrix that fills the space between the chromatin and the nucleolus. It is composed mainly of proteins (have enzymatic activity), metabolites, and ions. Functions of the nucleus: 1. Carries all genetic information and hereditary characters of the organism. 2. Directs formation of RNA, so, controls all proteins synthetic processes in the cell 3. Plays a major role in cell division and cell cycle 55 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY PROTEIN SYNTHESIS Organelles sharing in protein synthesis a. Ribosomes b. RER. c. Golgi apparatus d. Transfer and secretory vesicles Steps of protein synthesis: 1. The information coded in the DNA molecule (e.g., GGC), is transcribed (copied) into the mRNA (e.g. CCG) (Fig. 20). 2. The mRNA leaves the nucleus to the cytoplasm via nuclear pores. 3. In cytoplasm there are rRNA (ribosomes); free and attached to endoplasmic reticulum, and a pool of amino acids (20 types) 4. Each type of amino acid is attached to one arm of its own tRNA, the other arm bears the anticodon. 5. The anticodon of tRNA is able to pair in a complementary fashion with mRNA three letter codon specifying particular amino acid (e.g. GGC for proline with CCG of mRNA). 6. As the mRNA moves relative to ribosome, the various amino acids specified by the code words along its course are brought together in the proper order and linked to one another to form what will become a polypeptide chain. i.e., Amino acids are arranged in a certain sequence on the ribosomes, to form a specific polypeptide chain (protein). 7. The formed protein will concentrate at one end of RER, where budding occurs forming transfer vesicles. 8. The transfer vesicles carry small quantities of protein and fuse with the forming (immature) face of Golgi apparatus. 9. Proteins accumulate within the saccules of the Golgi membranes and are concentrated 56 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY as they pass through the Golgi complex. 10. At the mature face, the Golgi saccules expand as bud off to form secretory vesicles. 11. A secretory vesicle (granule) moves to the surface of the cell, where its membrane fuses with the plasma membrane and opens to the exterior (exocytosis). 12. Secretory vesicles which contain hydrolytic enzymes stay in the cytoplasm forming lysosomes. Fig. 20: Protein synthesis 57 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY CYTOGENETICS CHROMOSOMES The word "chromosome" comes from chroma=color and soma=body. They vary in numbers and shapes between the different organisms. They are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells. They either may be unduplicated (single linear threads) during interphase or duplicated, which contain two copies joined by a centromere (copied during S phase). Each chromosome is formed of a single long molecule of DNA (2 nm in diameter) packed (coiled several times) according to the type of chromosome. The chromosomes appear as chromatin granules in the nuclei of cells during the interphase stage of the cell cycle. Each nuclear DNA is coiled in a complex manner and constitutes a thread-shaped structure called a chromosome. During cell division each chromosome appears as two long parallel strands called chromatids, that are held together at one spot called centromere. Interphase Chromosome (Un-duplicated chromosome) During interphase (non-dividing phase of the cell cycle), the chromosomes are extended, and the chromatin exists as long thin tangled threads in the nucleus so that individual chromosomes cannot be distinguished. Different regions of the interphase chromosomes condense (heterochromatin) and not condense (euchromatin) according to function of the cell. The levels of DNA packing in interphase chromosomes are: 1. Nucleosomes: The first and most basic level of chromosome organization. 58 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY If the chromatin is unfolded partially, it can be seen under the electron microscope as a series of “beads on a string”. The string is DNA (linker DNA), and each bead is a “nucleosome” that consists of 2 turns of DNA double strand (146 nucleotide pairs) wound around a protein core formed of eight histone proteins (2xH2 A&B, H3 and H4) (Fig. 21). Another histone (H1) binds to the linker DNA and to the nucleosome surface. The formation of nucleosomes converts a DNA molecules into a chromatin threads of 11-nm in diameter and about one-third of its initial length Fig. 21: Nucleosome 2. 30-nm chromatin fibers: Helical coiling of nucleosomes (six nucleosomes/ turn) by the help of H1. This generates more highly condensed fibers of 30- nm diameter. Mitotic (double) Chromosome The two DNA molecules produced during S phase of the cell cycle are separately folded more and more to produce sister chromatids, held together at their centromeres. The levels of DNA packing in metaphase chromosomes are: (Fig. 22). 1.Nucleosomes 2. 30-nm chromatin fibers 59 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 3.300 nm fibers are formed by looping of the 30 nm fibers. 4.700 nm fibers Structure Stucture of metaphase chromosome: It consists of two sister chromatids attached to each other at the centromere. The centromere divides the chromatid into: 1. Two short arms above the centromere and called p arms (p from the French word petit = small) 2. Two long arms called q arms (q follows p in the Latin alphabet). Fig. 22: Organization of chromosome (DNA packing) Chromosome Number: Gamete cells (ova and sperm) have haploid number (1n); 23 chromosomes Somatic cells have diploid number (2n); 23 pairs of chromosomes: 1. 22 pairs are called autosomes (44 chromosomes) 2. One pair is called sex chromosomes: ▪ Female has two identical X chromosomes (XX) ▪ Male has one X and Y chromosomes (XY) 60 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Polyploidy cells (4n, 8n 16n ….): somatic cells which have more than diploid number of chromosomes (megakaryocytes and some liver cells are normally polyploidy). Classification: According to the size, they are classified into: large, medium sized and small or short chromosomes According to the position of the centromere: 1. Metacentric: centromere at the middle of the chromatids, so the two arms are equal. 2. Submetacentric: centromere nearer to one end, so the chromatids are formed of short and long arms. 3. Acrocentric: centromere close to one end, so the chromatids are formed of very short and long arms. Sex chromatin It is a heterochromatin mass observed in female cells but not in male cells. This chromatin clump represents one of the two X chromosomes present in female cells. This X chromosome remains tightly coiled and visible under the microscope, whereas the other X chromosome is uncoiled and not visible. Sex chromatin can be studied by: 1. Examination of specimens of the cells lining the internal surface of the cheek. Sex chromatin appears as a small granule attached to the nuclear envelope (Barr body) (Fig.23). 2. Examination of blood smears shows the sex chromatin as a drumstick-like attachment to the nuclei of the neutrophils (Fig.24). Male has one X chromosome and therefore has no sex chromatin. Female has two X chromosomes therefore has one Barr body. 61 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Triple X syndrome has three X chromosomes therefore has two Barr bodies. So, number of X chromosome equal sex chromatin number plus one. Fig.23: Barr body Fig.24: Drumstick 62 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY CELL DIVISION Cell Cycle Definition: A series of phases that take place in a cell leading to its replication. It is the vital process by which the zygote develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed. The cell cycle can be divided in two brief periods (Fig. 25): A) Interphase. B) Mitosis (M) phase. A) Interphase After cell division, each daughter cell begins the interphase. During this stage the cell performs its function (absorption, secretion ….etc) or prepares itself for initiating a new cell division through the following phases: G1 phase (G for gap). S phase (S for synthesis). G2 phase. 1. G1 Phase (G for Gap): The first phase within interphase. It is the longest phase of the cycle for each cell Its duration is variable among different cell types. It starts from the end of M phase until the beginning of DNA synthesis. The cell synthesizes various enzymes required in the S phase, mainly those needed for DNA replication. G0 Phase: Non-proliferative cells (fully differentiated) enter the quiescent G0 state from G1. 63 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY The metabolic activities of the cell (which decreased during M phase) resume at a high rate. It may remain quiescent for long time, possibly indefinitely as neurons and cardiac muscle cells. 2. S Phase (S for Synthesis): It starts when DNA synthesis begins. Its duration is relatively constant, among the cells of the same species. When it is complete, the amount of DNA is doubled (4c), and number of chromosomes (2n) remains the same. The centrioles also are self- duplicated during this phase Protein synthesis is very low except for histone production (most of which is produced during the S phase). 3. G2 Phase: It starts with the end of S-phase and lasts until the cell enters mitosis. DNA replication is analyzed (checked) for possible errors and any of these errors is corrected. Significant protein synthesis of the microtubules (mainly tubulin) occurs during this phase which are required during mitosis. Fig.25: Schematic representation of the cell cycle. B) M Phase (Mitosis) 64 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY At the end of G2 phase. Mitosis consists of:  Nuclear division (karyokinesis) 1. Prophase 4. Anaphase 2. Prometaphase 5. Telophase 3. Metaphase  Cytoplasmic division (cytokinesis) 1. Prophase: At the onset of prophase, chromatin condenses together into a highly coiled structure called chromosomes (visible with light microscope). Each chromosome consists of two parallel sister chromatids joined at one point along its length called centromere (a constriction composed of highly repetitive DNA sequence). Each chromosome forms two kinetochores at the centromere (the kinetochore is a protein complex that assembles on the chromosome’s centromere). (Fig.26) The two centrosomes (each one contains one pair of centrioles and one microtubule- organizing center) form a spindle of microtubules by polymerization of tubulin protein. The centrosomes move along these microtubules to the opposite sides of the cell. (Fig.27) The nuclear envelope disintegrates and microtubules invade the nuclear space. 65 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig.26: Kinetochore spindles. Fig.27: Prophase 2. Prometaphase (Fig.28 A): The nuclear envelope disappears. Microtubules that become attached to the kinetochore are known as mitotic spindle microtubules, whereas a number of non kinetochore microtubules interact with corresponding microtubules from the opposite centrosome to help in elongating the cell. 3. Metaphase (Fig.28 B): The chromosomes arrange in the metaphase plate or equatorial plane; an imaginary line that is equidistant from the two- centrosome poles. 66 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig 28: (A) Prometaphase Fig 28: (B) Metaphase 4. Anaphase (Fig.29 A): The kinetochore proteins that bind sister chromatids together are cleaved, allowing their separation. These sister chromatids (now become sister chromosomes) are pulled apart. At the end of anaphase, the cell succeeds in separating two identical copies of the genetic material. 5. Telophase (Fig.29 B): Non-kinetochore microtubules continue to lengthen, elongating the cell. Sister chromosomes move to the opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separate sister chromosomes. Both sets of chromosomes, uncoiled back into chromatin. Nucleolus reappears (develops from nucleolus organizing center on 5 pairs of chromosomes 13, 14, 15, 21 and 22). a. Mitosis is complete, but cell division is not yet complete. 67 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. 29 (A). Anaphase Fig. 29 (B). Telophase Cytokinesis: Cytokinesis is not a phase of mitosis, but it is a separate process necessary for completing cell division. A cleavage furrow (pinch) containing a contractile ring formed of actin and myosin filaments attached to cell membrane develops at the metaphase plate. i.e. Cell division consists of: Nuclear division (Mitosis or karyokinesis) Cytoplasmic division (cytokinesis). Regulation of cell cycle Regulation of the cell cycle includes the detection and repairs of genetic damage as well as the prevention of uncontrolled cell division through three checkpoints occur in G1, G2 of interphase and in the middle of mitosis. Meiosis Meiosis occurs in germ cells. During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into 46 (2n) chromosomes, undergoes DNA replication (4c). It is divided into two successive cell divisions without DNA replication in-between, 68 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY resulting in four haploid cells (gametes), each contains one set of chromosomes (1n) and (1c). The chromosomes of each parent undergo genetic recombination during meiosis, and thus each zygote will have a unique genetic blueprint encoded in its DNA. Because meiosis is a "one-way" process, the cell does not enter the cell cycle. However, the preparatory steps that lead to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle (G1, S and G2). A. Meiosis I It occurs after replication of DNA The first meiotic division reduces the number of chromosomes of the original cell (2n) into the half (1n) i.e. reduction division. Meiosis I divided into prophase I, metaphase I, anaphase I, telophase I (Fig. 30). 1. Prophase I: It begins after the DNA has been doubled to 4c in S phase. Homologous chromosomes are paired. The paired chromosomes (one from each parent) are called bivalents or tetrads which have two chromosomes and four chromatids. At this stage, non-sister chromatids cross-over at points called chiasmata (a step unique to meiosis). Prophase I is further subdivided into: a. Leptotene (meaning "thin threads"): Individual chromosomes begin to condense to become visible under microscope as long threads. The two sister chromatids are still indistinguishable from one another. b. Zygotene: The chromosomes line up with each other into homologous chromosomes (synapsis). 69 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY c. Pachytene: Chromosomes become shorter and thicker. Non-sister chromatids of homologous chromosomes randomly exchange segments of genetic information at recombination sites (chiasmata). d. Diplotene: The homologous chromosomes separate from one another The homologous chromosomes of each bivalent remain tightly bound at chiasmata until they are severed in anaphase I. e. Diakinesis: During the diakinesis, chromosomes condense more and more. The rest of the stage closely resembles prophase of mitosis. 2. Metaphase I: The homologous chromosomes align along an equatorial plane that bisects the spindle. 3. Anaphase I: Kinetochore microtubules are shortened, pulling homologous chromosomes apart towards opposing poles forming two haploid sets i.e. each chromosome consists of two sister chromatids. The cell elongates in preparation for division down the center. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. 4. Telophase I: Each daughter cell now has half the number of chromosomes. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis: the furrowing of the cell membrane occurs for creation of two daughter cells. 70 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Fig. 30: Meiosis B. Meiosis II This process is similar to mitosis but differs in: (i) No duplication of its DNA i.e. No S phase before the division. (ii) Number of chromosome is 23. (iii) At anaphase, each chromosome divides at its centromere into two chromatids, and each chromatid migrates to opposite pole as a chromosome (Fig. 30). The cell passes in four stages; prophase II, metaphase II, anaphase II and telophase II. The result is the production of four haploid cells; 23 chromosomes (1n) and haploid amount of DNA (1c), each chromosome consists of one chromatid. Significance of Meiosis: Meiosis facilitates stable sexual reproduction. It generates genetic variation in gametes that promotes genetic and phenotypic variation in a population of offspring. Meiosis in Females: 71 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY It starts in female during intrauterine fetal life in cells known as oogonia. These cells stop at the diplotene stage of meiosis I and lay dormant within the ovary. Just before ovulation, oocytes continue meiosis I and enter meiosis II, then arrested until fertilization that enhances the completion of meiosis II. Meiosis in females differs from the typical meiosis in 1. It takes a long period (due to meiotic arrest that may extend up to 45 years) known as the Dictyate stage. 2. Each oogonium divides twice to form a single oocyte and three polar bodies. Meiosis in Males: It occurs after puberty in precursor cells known as spermatogonia in the seminiferous tubules of the testicles. Each spermatogonium divides twice, continuously without arrest, to form four sperms. It takes approximately 74 days. CELL RENEWAL All specialized cells live in prolonged G0 phase or have made a final exit from the cell cycle and lose their capacity for proliferation. When depletion of these cells occurs, some mechanisms for cell replacement become in action and stimulate the specialized cells themselves or their progenitors to generate new cells. The body cells are classified according to whether they are renewed or not into three different kinds : 1- Non-renewing cells: Highly differentiated cells that leave the cell cycle and never divide again (present in G0). They have no stem cells to replace the damaged cells. 72 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Heart muscles and nerve cells are examples, as they have no mechanism for cell renewal. 2- Potentially renewing cells: Highly differentiated cells present in G0. There is no stem cells to replace the damaged cells They can go back into the cell cycle and divide to replace their loss. Liver cells are an example, as the liver restores its cells after partial removal. 3- Continuously renewing cells: Highly differentiated cells that leave the cell cycle (G0). They require continuous renewal to compensate their continuous loss. They have stem cells to replace the lost cells. Skin, epithelial lining of the intestine and blood cells are examples. Stem cells The cells that stay in the G0 state and can return back to the cycle at the time of need. 1. Unipotential: Stem cells that produce one type of specialized cells e.g. male germ cells produce sperms only. 2. Pluripotential (multipotential): The stem cells give more than one type of cells e.g. hemopoietic stem cell in the bone marrow produces all the types of blood cells (red blood cells, neutrophils, eosinophils, basophils, lymphocytes, monocytes and platelets). Karyotype - The karyotype is a photograph of the chromosomes from a single cell. Indications 1. Identification of sex when the newborn's sex is not clear. 2. Determination of the ability of chromosomes abnormality to pass from the parents to their children. 3. Determination of chromosome abnormalities; one of the causes that prevent pregnancy. 73 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 4. Determination of the causes of congenital anomalies. 5. Determination of the appropriate treatment for some cancers. Steps of karyotype 1. A blood sample is drawn in a heparinized tube to prevent coagulation but sometimes skin fibroblasts, amniocytes or bone marrow cells can be used. 2. The sample is cultured for 3-4 days in the presence of a mitogen-like phytohemagglutinin which stimulates the lymphocytes to proliferate. 3. At the end of the culture period (as there is a huge number of dividing cells), the culture is treated with colchicine (disrupts mitotic spindles and arrests dividing cells in mitosis). 4. The sample is treated with a hypotonic solution (swell cells and nuclei osmotically). So, chromosomes do not lie on top of one another. 5. The swollen cells are fixed with a fixative solution, dropped onto a microscope slide and dried. 6. Slides are stained and photographed using light microscope with its camera. 7. The chromosomes are cut individually and pasted to a sheet in an orderly manner in eight groups from A to G. 8. Alternatively, a computerized digital image analyzer system of the chromosomes can cut and paste and make the karyotype. 9. Karyotypes are presented in a standard form (the total number of chromosomes, followed by sex chromosomes and any autosomal abnormalities). Table 2.1: Distribution of chromosomes among the eight groups. Groups Numbers Centromere Size Group A 1&3 Metacentric Large Group B 2& 4 – 5 Submetacentric Large Group C 6 – 12 Submetacentric Medium Group D 13 – 15 Acrocentric Medium 74 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Group E 16 – 18 Submetacentric Small Group F 19 – 20 Metacentric Small Group G 21 – 22 Acrocentric Small Chromosome Abnormalities Chromosome abnormalities include the gain, loss or rearrangement of amounts of genetic material. They usually result from an error that occurred in a developing ovumor sperm as may divide incorrectly, resulting in an ovum or a sperm with too many or too few chromosomes. When an abnormal cell joins a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. 1. Numerical Abnormalities It is the loss of one chromosome (monosomy) or the gain of an extra chromosome (trisomy). It results in spontaneous abortion, because it affects the copy numberof hundreds or even thousands of genes. The most common numerical abnormalities are listed in table 3.3. Mechanism 1. Nondisjunction Failure of paired chromosomes to separate (disjoin) in anaphase of meiosis I, or failure of sister chromatids to disjoin at either meiosis IIor at mitosis. Nondisjunction in meiosis produces gametes with 22 or 24 chromosomes and fertilization by a normal gamete makes a monosomic (45) or trisomic zygote (47) respectively. Nondisjunction in mitosis in the early stages of embryogenesis produces a mosaic (some cells contain 45 and others contain 46 chromosomes). 75 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 2. Anaphase lag Failure of a chromosome or chromatid to be incorporated into one ofthe daughter nuclei following cell division, as a result of delayed movement (lagging) during anaphase. Chromosomes that do not enter a daughter cell nucleus are lost. Table 2.2: Common syndromes associated with numerical aberrations 2. Structural Abnormalities The structure of chromosome is altered and can take several forms: 1. Deletions: A portion of the chromosome is missed or deleted. Types of deletion include the following: 1. Terminal deletion: occurs at the end of a chromosome (Fig. 31). 2. Intercalary deletion: occurs from the interior of a chromosome (Fig.32). 3. Deletion of part of the short arm of chromosome 5 results in "Cri du chat syndrome"(Fig. 33). Fig. (31)Terminal deletion 76 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. (32) Intercalary deletion Fig. (33) Cri-du- chat syndrome 2. Duplications: A portion of the chromosome is duplicated, resulting in extra geneticmaterial (opposite of a deletion). It arises from unequal crossing-over that occurs during meiosis between misaligned homologous chromosomes (Fig. 34). Fig. (34) Duplication 3. Translocations: When a portion of one chromosome is transferred to non-homologouschromosome. There are two main types of translocations: 1. Reciprocal translocation, segments from two different chromosomes has been exchanged (Fig. 35 A). 77 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 2. Robertsonian translocation, an entire chromosome has attached to another at the centromere (Fig. 35 B). A B Fig. (35) Translocation 4. Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted. Inversions are of two types: 1. Paracentric inversions: do not include the centromere and both breaksoccur in one arm of the chromosome 2. Pericentric inversions: include the centromere and there is a breakpoint in each arm (Fig. 36). 78 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Fig. (36) Inversion 2. Ring chromosome: Two portions of a chromosome have been broken off and the chromosome forms a circle or ring. This can happen with or without loss of genetic material (Fig. 37). Fig. (37) Ring chromosome Fig. (37) Ring chromosome 5. Isochromosome: Created by the incorrect division of the centromere. Normally centromeres divide vertically. In this case it divides horizontally. The affected chromosome lines up at a right 79 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY angle to its normal position, and the centromere is divided in the opposite plane from all the other chromosomes. This leaves the two long arms together and the two short arms together. Thetwo new mirror-image chromosomes are pulled into opposite daughter cells (Fig. 38). This produces two cells, each lacking one arm (e.g. the short arm) and containing an extra arm (e.g. the long arm) of the affected chromatid (or vice versa). Fig. (38) Isochromosome 80 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY EPITHELIUM Epithelium is one of the four primary or basic body tissues. It is formed of a group of adherent cells with minimal or no intercellular substance. General Characters of Epithelium: 1. Epithelial cells are close together and separated by a little (or no) intercellular substance. 2. Cell junctions are present between its cells to help their fixation. 3. Epithelium is separated from the underlying connective tissue by the basement membrane. 4. It has no blood vessels (not vascularized). 5. The cells receive their nutrition by diffusion from underlying connective tissue. 6. It has no lymph vessels. 7. It has numerous nerve endings. 8. Its cells have a high power to regenerate after degeneration. 9. Its cells exhibit polarity. CLASSIFICATION OF EPITHELIUM According to its functions, the epithelium is classified into: I. Surface epithelium (membrane epithelium) II. Myoepithelium III. Neuroepithelium IV. Glandular epithelium I. Surface Epithelium It is the lining epithelium of the tubes and cavities and the covering epithelium of the body surface. 81 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Epithelial cells are arranged to form one continuous layer /or layers resting on a basement membrane. They are classified according to the number of layers into: A. Simple epithelium. B. Stratified epithelium. A. Simple Epithelium It is a surface epithelium in which the cells are arranged in one layer. According to the shape of its cells, it is classified into (Table 1): 1. Simple squamous epithelium. 2. Simple cuboidal epithelium. 3. Simple columnar epithelium. 4. Pseudostratified columnar epithelium. 1. Simple Squamous Epithelium: Shape: flat cells with flat nuclei Sites: 1. Lung alveoli. 2. Endothelium of blood vessels. 3. Serous cavities; pleura, pericardium, and peritoneum (mesothelium). Functions: provides a smooth and thin surface to facilitate filtration and diffusion of water, electrolytes, and transport of gases. 2. Simple Cuboidal Epithelium: Shape: cubical in shape with central and rounded nuclei Sites: 1. Thyroid follicles. 2. Some kidney tubules. 82 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY 3. Ovarian surface. Functions: mainly absorption and secretion. 3. Simple Columnar Epithelium: Shape: tall cylindrical with basal and oval nuclei. Two types of simple columnar epithelium can be identified: 1. Without brush border e.g. epithelium of stomach. 2. With brush border that may be: a. Cilia e.g. epithelium of uterus and fallopian tubes. b. Microvilli e.g. epithelium of small intestine. Functions: the function differs according to its site: a. Secretory e.g. epithelium of the stomach. b. Absorptive e.g. epithelium of small intestine. c. Movement of ova and sperms in the uterus and fallopian tubes (ciliated epithelium). 4. Pseudostratified Columnar Epithelium: It is formed of one sheet of cells which rest on a basement membrane. Some cells are tall with apical nuclei and others are short with basal nuclei (appearance of stratified). Types and sites: a. Non ciliated pseudostratified columnar epithelium: epithelium of large ducts of glands b. Pseudostratified columnar ciliated epithelium: epithelium of trachea. c. Pseudostratified columnar epithelium with stereocilia: epithelium of epididymis. Functions: protective and others according to its site. 83 | P a g e MEDICAL HISTOLOGY AND CELL BIOLOGY Table 1: Simple epithelium (types, sites and shape of cells) Type Sites Shape Simple Squamous 1- Alveoli of the lung 2- Endothelium Flat cells with flat nuclei 3- Mesothelium Simple Cuboidal 1- Thyroid follicles Cubical in shape with central and 2- Some kidney tubules rounded nuclei 3- Ovarian surface Simple Columnar 1- Without brush border in stomach 2- Ciliated in uterus and Tall cylindrical with basal and oval fallopian tubes nuclei 3- With microvilli in small