Cytolasmic Organelles PDF

## The Cytolasmic Organelles **Definition:** They are living and permanent intracellular structures that carry out the vital functions of the cell. **Classification:** They are classified according to the presence or absence of a membrane into membranous and non-membranous organelles. ### Membr...

## The Cytolasmic Organelles **Definition:** They are living and permanent intracellular structures that carry out the vital functions of the cell. **Classification:** They are classified according to the presence or absence of a membrane into membranous and non-membranous organelles. ### Membranous Organelles - Surrounded with membrane. - Have metabolic enzymes. **Includes:** 1. Cell membrane 2. Mitochondria 3. Rough endoplasmic reticulum 4. Smooth endoplasmic reticulum 5. Golgi apparatus 6. Lysosomes ### Non Membranous Organelles - Not surrounded by membrane. - Free from enzymes. **Includes:** 1. Ribosomes 2. Cytoskeleton: - Microtubules - Microfilaments ## The Membranous Organelles ### 1- The Cell Membrane - Also called plasma membrane or plasmalemma. - Definition: It is a limiting membrane surrounding the cell cytoplasm externally. - Measures about 7.5-10 nm in thickness. - Cannot be seen by light microscopy. - The cell coat (the external part of the cell membrane) can be demonstrated when stained with PAS (pink) or silver stains (black). - Appears as a trilaminar membrane under electron microscopy. - Formed from two electron dense dark lines separated by an electron lucent clear one. - Also called unit membrane. ## Phagocytosis and Pinocytosis **a) Phagocytosis:** - The cell engulfs an extra-cellular material as bacteria, virus or damaged cell. - The engulfed material is called a phagosome. **b) Pinocytosis:** - The cell engulfs fluid material. - The engulfed material is called a pinosomes. ## Exocytosis - A membranous cytoplasmic vesicle fuses with the cell membrane to release its content outside the cell. ## Functions of the cell coat 1. Cell immunity 2. Cell adhesion 3. Cell protection 4. Cell recognition ## 2- Mitochondria **Definition:** One of the membranous organelles responsible for energy production. They are considered the power house of the cell. **Site:** - Present in the part of the cytoplasm where energy is needed. - Beside the rough endoplasmic reticulum in secretory cells. - Between myofibrils in muscle cells. - In the basal infolding in the ion transport cells. **Number** - Varies according to the activity of the cell. - Mitochondria are self-replicating organelles, so they can increase their number because they contain DNA. **Microscopy:** - **Light Microscopy:** - Appear as dark blue dots when stained with Iron Hematoxylin. - Appear as green dots when stained with genus green. - **Electron Microscopy:** - Appear as rounded or elongated, double membranous structures of about 1 μm in diameter and 10 μm in length. **Structure** - **Outer mitochondrial membrane:** Smooth and permeable. - **Inner mitochondrial membrane:** Forms shelf-like folds or cristae with a variable number to increase the surface area for oxidative phosphorylation. - **Mitochondrial matrix:** Contains oxidative enzymes, DNA, RNA and electron dense granules rich in calcium. ## Functions 1. Production of energy from metabolites present in the cytoplasm by the Krebs' cycle. 2. Storage of energy as ATP molecules by oxidative phosphorylation. ## 3- Rough Endoplasmic Reticulum (rER) **Definition:** One of the membranous organelles responsible for protein synthesis. Present mainly in protein secreting cells. **Site:** - Present around the nucleus and associated with the nuclear membrane. - Localized or diffused basophilic dots localized as in pancreatic cells (basal basophilia) or diffused as in plasma cells (diffused basophilia). - Appears as parallel flattened membranous sacs or cisternae connected with each other by short canaliculi. - Membranous sacs are studded with polyribosomes on a special site. **Functions:** 1. Synthesis of protein for export outside the cell as polypeptide hormones and digestive enzymes. 2. Synthesis of hydrolytic enzymes of primary lysosome. 3. Packing of formed proteins in membranous vesicles to be delivered to the Golgi apparatus as transfer vesicles. 4. Act as an intracellular pathway for some substances. ## 4-Smooth Endoplasmic Reticulum **Definition**: One of the membranous organelles responsible for steroid synthesis. **Site:** - Well developed in steroid hormone secreting cells (adrenal cortex, testis and ovary), fat cells and liver cells. **Microscopy:** - Not visible under light microscopy. - Appears as branching and anastomosing intercommunicating tubules with variable shaped cisternae. - Continuous with the rough endoplasmic reticulum but lacking attached ribosomes. **Functions:** 1. Synthesis of steroid hormones as cortisone, estrogen and progesterone. 2. Detoxification of drugs, hormones, and some chemicals in liver cells. 3. Act as a pump releasing calcium ions during muscle contraction. 4. Synthesis of cholesterol and phospholipids. 5. Act as an intracellular pathway for some substances. ## 5- Golgi Apparatus **Definition:** One of the membranous organelles responsible for secretion; so it is well developed in secretory cells. **Site:** - In pancreatic cells, present supranuclear (in the apical part). - In nerve cells, present perinuclear (around the nucleus). **Microscopy:** - Can be demonstrated by silver stains as a network of brownish granules and fibers under light microscopy. - In plasma cells, which are characterized by diffuse basophilia, Golgi apparatus could be recognized as a pale unstained area near the nucleus called negative Golgi image. - Consists of: - **Golgi stalks or membranous saccules:** Several parallel, flat and curved saccules with dilated ends. - Interconnected with each other. - Each saccule has two surfaces: - **Immature convex surface:** Receives transfer vesicles, which carry crude protein from the rough endoplasmic reticulum. - **Mature concave surface:** Secretory vesicles bud off, carrying protein ready for secretion. - **Transfer or microvesicles:** Small membrane-bound vesicles carrying crude protein from the rough endoplasmic reticulum to the immature surface of the Golgi stalks. - **Secretory or macrovesicles:** Large membrane-bound vesicles that arise from the mature active surface of Golgi stalks carrying protein ready for secretion. ## Functions 1. Concentration, storage, and packing of proteins received from the rough endoplasmic reticulum. 2. Conjugation of protein with carbohydrates, sulfates, and salts. 3. Formation of coated vesicles and primary lysosomes. 4. Renewal and maintenance of the cell membrane. ## 6- Lysosomes **Definition:** One of the membranous organelles containing hydrolytic enzymes and responsible for cellular digestion. **Number:** A huge number of primary lysosomes are present in phagocytic cells as monocytes, neutrophils, lymphocytes, and macrophages. **Origin:** - The hydrolytic enzymes are synthesized in the rough endoplasmic reticulum. - Then transferred to the Golgi apparatus. - Then released as primary lysosomes. **Microscopy:** - Cannot be seen under light microscopy. - The acid phosphatase enzyme inside the lysosome could be detected by histochemical reactions. - Appear as membrane-bound spherical vesicles containing hydrolytic enzymes under electron microscopy. - They have homogenous electron density (primary lysosomes). - Secondary lysosome: When the primary lysosome fuses with any intra-cytoplasmic vesicle, either exogenous or endogenous. - Appears heterogenous in electron density due to digested and undigested particles (secondary lysosomes). ## Types of Secondary Lysosomes **a) Heterolysosomes:** - Fusion of the primary lysosome with a phagocytic vesicle as dust, bacteria, or virus. **b) Multivesicular Body:** - Fusion of the primary lysosome with a pinocytotic vesicle. **c) Autophagosome:** - Fusion of the primary lysosome with an old organelle, such as mitochondria or the endoplasmic reticulum, leading to its hydrolysis. **d) Residual Body:** - The undigested materials inside the lysosome, such as dust retained as a residual body. - Then the cell expels it by **exocytosis** (cytostol). - OR it accumulates inside the cell as **lipofuscin pigment** in nerve cells. ## Functions 1. Protection of the cell by digestion of the engulfed bacteria or viruses. 2. Maintain cell health by the continuous removal and elimination of the old organelles. 3. **Autolysis:** After cell death, the lysosomal enzymes are liberated and hydrolyse the cell constituents. 4. Helps in fertilization of the ova by the acrosomal enzymes. 5. Formation of thyroid hormones by digestion of colloid. ## Non-Membranous Organelles ### 1- Ribosomes **Definition:** They are non-membranous electron dense bodies, present in all cells but increased greatly in protein producing cells. **Structure:** - Formed of ribosomal RNA from the nucleolus and cytoplasmic protein. - Both structures conjugate together inside the nucleus to form large and small subunits. - Then they pass to the cytoplasm for protein synthesis. **Microscopy:** - Accumulation of a large number of ribosomes causes cytoplasmic basophilia, which may be diffused (embryonic cells), localized (pancreatic cells) or focal (Nissl's granules of nerve cells) under light microscopy. - Appear as small electron dense granules formed of small and large subunits under electron microscopy. - The large subunit contains a groove in its center, housing the polypeptide chain. - The two subunits are connected together by messenger RNA. - More than one ribosome can be connected together by messenger RNA, forming **polyribosomes**, which may be free or attached. ## Functions - **Free Ribosomes:** Synthesize protein to be used inside the cell, such as glycolytic enzymes. - **Attached Ribosomes:** Form the proteins that are secreted outside the cells as hormones and enzymes. ### 2- Cytoskeleton **Definition:** Refers to a complex network of microtubules and microfilaments, together with some linked proteins, leading to the formation of what is called the microtrabecular lattice. #### **A-Microtubules** **Definition:** Fine tubules of variable lengths and regular diameter, about 25 nm. **Microscopy:** - Cannot be seen under light microscopy. - Demonstrated by immunohistochemistry. - Appear as fine tubules formed of alpha and beta tubulin protein molecules arranged into 13 protofilaments under electron microscopy. **Forms:** 1. **Dynamic form:** Continuous assembly and disassembly, helping with cell movement. 2. **Stable or organized form:** Represented by centrioles, cilia, and flagella. **Functions:** 1. Determination of the cell shape. 2. Formation of the mitotic spindle during cell division. 3. Intracellular transport of macromolecules. 4. Form the centrioles, cilia, and flagella. #### **B-Microfilaments** **Definition:** They are solid, thread-like structures about 7 nm in diameter, composed of the protein actin, found in all cells. **Microscopy:** - Cannot be seen under light microscopy. - Demonstrated through the use of immunofluorescence or special staining techniques, such as fluorescent phalloidin. - Appear as fine filaments under electron microscopy. **Forms:** 1. **Bundles:** Can be arranged in parallel bundles or networks. 2. **3-dimensional networks:** Can form a three-dimensional network in the cytoplasm. 3. **Contractile bundles:** Associated with other proteins, such as myosin, to form contractile bundles in muscle cells. **Functions:** 1. **Cell shape:** Maintain and alter cell shape. 2. **Cell movement:** Assist in cell movement, such as amoeboid movement or the movement of cytoplasmic components. 3. **Muscle contraction:** Form the contractile filaments responsible for muscle contraction. 4. **Cytoplasmic streaming:** Play a role in cytoplasmic streaming, the directed flow of cytoplasm within cells. 5. **Cell division:** Involved in cell division by forming the contractile ring that pinches the cell in two during cytokinesis ## Classification of Cells According to Their State **Fixed Cells:** 1. Undifferentiated mesenchymal cells 2. Fibroblasts 3. Fat cells 4. Fixed macrophages 5. Pericytes 6. Endothelial cells 7. Reticular cells 8. Pigment cells (melanocytes) 9. Chondroblasts 10. Osteoblasts **Free Cells:** 1. Mast cells 2. Plasma cells 3. Leucocytes 4. Free macrophages ## Connective Tissue **General Characteristics:** - Mesodermal in origin. - Specialized to connect, bind, support, and protect the other tissues of the body. - May be vascular (loose connective tissue), highly vascular (bone) or avascular (cartilage). - Consists of cells, fibers, and intercellular substances (matrix). - The relative amount of matrix, the type and arrangement of fibers and the abundance and kinds of cells determine the type of connective tissue. ## (1) Connective Tissue Cells ### I) Classified According to Function #### A. Cells Responsible for Generation and Maintenance of C.T. 1. Undifferentiated mesenchymal cells (UMCs) 2. Fibroblasts 3. Osteoblasts 4. Chondroblasts 5. Endothelial cells 6. Pericytes #### B. Cells Responsible for Defense and Immunity 1. Macrophages 2. Plasma cells 3. Mast cells 4. Leucocytes #### C. Cells Responsible for Storage 1. Fat cells (adipocytes) 2. Pigment cells (melanocytes) ## **b. Brown Fat Cells** - Also known as multilocular adipocytes. - Present mainly in brown adipose tissue. - Rounded or polygonal in shape, small in size. - Cytoplasm contains multiple small lipid droplets. - Contains abundant cytochromes, which give the brownish coloration of the cells. - Contains abundant mitochondria. - Nucleus is spherical and eccentrically situated. ### **Functions:** - Provide more heat and energy for rodents and hibernating animals. ## **4- Fixed Macrophages (Histocytes or Clasmatocytes)** **Origin:** Originate from monocytes (a type of white blood cell), which migrate to the loose connective tissue and become fixed when attached to the connective tissue fibers. **Structure:** - Large branched cells with irregular cell boundaries, which were created by the presence of pseudopodia. - Nucleus is small, has an eccentric darkly stained appearance and is kidney shaped. - Cytoplasm is acidophilic, granular, and vacuolated. - Can be demonstrated by using special stains as Indian ink or trypan blue. - Phagocytic granules are present. - They contain numerous lysosomes, primary and different types of secondary lysosomes, phagosomes, residual lysosomes, multivesicular bodies, and autophagic vacuoles. - Contains well developed rough endoplasmic reticulum, prominent Golgi apparatus, and mitochondria. ### Functions: 1. Phagocytize and digest foreign particles and clean wounds from debris. 2. Interact with lymphocytes and neutrophils by producing interleukins which stimulate their attraction and mitosis. 3. Fuse with each other to form multinucleated cells called foreign body giant cells, which engulf large foreign bodies. ## **5- Pericytes** - Belived to be pluripotential cells, acting like undifferentiated mesenchymal cells of adult tissue. - Wrap around the blood capillaries and share the basal laminae of the endothelial cells. - Flattened, branched cells with lightly stained cytoplasm and oval nuclei. - Contain few organelles, including mitochondria, rough endoplasmic reticulum, and euchromatic nucleus. ### Functions 1. Differentiate into various types of other cells, such as fibroblasts or smooth muscle cells in adult tissue. 2. Contractile cells, leading to vasoconstriction. ## **6. Endothelial cells** - Simple squamous epithelial cells lining all blood vessels and the heart. - Small cytoplasm content, few organelles, and flat nucleus. ### Functions: - Can be differentiated into new endothelial cells. - Produce special type of collagen (type IV). - Produce their own basal laminae. ## **7. Reticular cells** - Present in the stroma of most organs. - Stellate-shaped cells with long, branched cytoplasmic processes forming a reticulum with reticular fibers. ## **Classification of Bone** ### **A) Compact Bone** - Solid and hard. - Present in the shafts of long bones, outer and inner tables of flat bones of the skull, and the outer covering of vertebrae, ribs, etc. - Covered externally with periosteum and internally with endosteum. - Characterized histologically by the regular arrangement of bony lamellae into a definite pattern called the **Haversian system** or osteon. #### **Haversian System (Osteon):** - **Structural unit of compact bone.** - Consists of a central Haversian canal, surrounded by concentrically arranged bony lamellae (calcified collagen fibers that encase osteocytes inside lacunae and canaliculi). - Haversian canal runs parallel to the long axis of bone and contains blood vessels, lymph vessels, and nerves. - Haversian bony lamellae are formed of calcified collagen fibers, parallel to each other in one lamellae and perpendicular to those in the next one. - Appear alternately striated and punctuated in cross-sections. - Each osteocyte lies in a small space called a lacuna. - Processes extend inside fine tubules called canaliculi in the hard matrix through which the osteocyte processes are connected with each other by gap junctions. - Unlike cartilage, osteocytes cannot divide. - There is a single cell in each lacuna (no cell nests are present). - Resemble to osteoblasts under electron microscopy, but with less prominent rough endoplasmic reticulum and Golgi apparatus. ### **Functions:** - Considered as bone maintaining or preserving cells. - Maintain the bone matrix by the formation of collagen fibers and glycoproteins. - Maintain the hardness of the matrix by the continuous deposition of calcium salts. ### **B) Spongy Bone (Cancellous Bone)** - Appears as thin irregular branching and anastomosing bony specules or trabeculae, leaving spaces which house blood forming cells of bone marrow. - Doesn't have the regular arrangement of bony lamellae. - There are no Haversian systems. - Present in the epiphysis of long bones, short bones, and flat or irregular bones. ## **4- Osteoclasts** - Giant, multinucleated cells. - Originate from fusion of many blood monocytes. - Nestled in shallow depressions on bone surfaces called Howship's lacunae near bone marrow cavities. ### **Structure:** - Large cells (up to 150 μm in diameter), with foamy acidophilic cytoplasm and multiple nuclei (up to 50). - Have a striated or brush border facing the bone surface called a ruffled surface. - Contain numerous mitochondria, well-developed rough endoplasmic reticulum, an active Golgi apparatus, and various sized vacuoles and many lysosomes. ### Functions: - Also known as bone eating cells. - Responsible for bone remodeling during growth or after fractures by releasing lytic enzymes to break down the bone matrix and release minerals (bone resorption). ## **Classification of Cartilage** - Specialized type of connective tissue responsible for supportive role. - Rigid, but with some degree of flexibility because the matrix is formed of chondroitin sulfate (sulfated glycosaminoglycan). - Avascular: The cells receive their oxygen and nutrients by the diffusion from the perichondrium (a vascular connective tissue layer that surrounds the cartilage). - Consists of chondroblasts and chondrocytes, whose function is to produce the cartilage matrix (a combination of collagen, elastic fibers, and ground substance). - Three types: - Hyaline cartilage - Elastic cartilage - Fibrocartilage ### **a) Hyaline Cartilage** - Most common type of cartilage. - Glassy, translucent, bluish white appearance in the fresh state. - Forms most of the temporary skeleton in embryos and is replaced by bone after birth. - Persists as articular cartilage of joints, costal cartilages, and cartilages of the respiratory passages (nose, trachea, bronchi, and most of the cartilages of the larynx) in adults. ### **b) Elastic Cartilage** - Same structure as hyaline cartilage, except its matrix contains an extensive network of elastic fibers running in different directions around the lacunae. - Contains 1-2 chondrocytes in each lacuna. - Surrounded by perichondrium. - The elastic fibers are responsible for the elasticity of this type of cartilage and its yellow color. ### **c) Fibrocartilage** - Has no perichondrium. - Contains a combination of hyaline cartilage and dense connective tissue. - Contains both chondrocytes and fibroblasts. - Matrix contains abundant collagen fibers and fewer chondrocytes, which are arranged in parallel bundles. ## **Classification of Epithelial Tissue** **Definition:** A single layer of epithelial cells that lines and covers body organs. ### **I) Lining and Covering Epithelium** - Classified according to the number of cell layers: - **Simple epithelium:** One layer of cells. - **Stratified epithelium:** More than one layer of cells. #### **A. Simple Epithelium** **i. Simple Squamous Epithelium** - Single layer of thin, flattened epithelial cells resting on a basement membrane. - Polyhedral and polygonal in shape. - Rounded nuclei centrally situated. - Thin, flattened, and attenuated in side view with elongated nuclei centrally situated. ### Functions 1. Smooth surface for the easy passage of fluids, such as blood. 2. Facilitates the movement of organs, such as the stomach, lungs, and heart. 3. Allows easy passage of gases in the lungs. ### Sites: 1. **Endothelium:** Lines blood vessels, the heart, and lymph vessels. 2. **Mesothelium:** Lines the pleura, pericardium, and peritoneum. 3. **Lining of the lung alveoli:** (pneumocytes) 4. **Bowman's capsule:** (kidney) and the inner surface of the cornea **ii. Simple Cuboidal Epithelium** - Single layer of cuboidal cells resting on a basement membrane. - Square in cross sections. - Spherical nuclei centrally situated. ### Functions: - Secretion or Absorption ### Sites: 1. Small ducts of glands as salivary glands 2. Secretory alveoli of some glands as salivary glands 3. Thyroid follicles 4. Distal convoluted tubules of the kidney. **iii. Simple Columnar Epithelium** - Single layer of tall, columnar cells resting on a basement membrane. - Taller than wide. - Oval nuclei basally situated. #### **Types:** 1. **Simple Columnar Secretory Epithelium:** Columnar cells free from cilia or microvilli. - Function: Secretory epithelium - Site: Stomach and large ducts of glands. 2. **Simple Columnar Ciliated Epithelium:** The apical surface of the cells is provided with cilia. - Function: Moves fluids or particles in one direction over the surface epithelium. - Site: Lung bronchioles and fallopian tubes. 3. **Simple Columnar Epithelium provided with microvilli:** The columnar cells carry numerous microvilli on its apical surface. - Function: Absorption. - Site: Small and Large intestine and gall bladder. **iv. Pseudostratified Columnar Ciliated Epithelium** - Single layer of tall and short, crowded epithelial cells resting on the same basement membrane. - Short cells that fail to reach the surface. - Nuclei present at different levels, giving a false stratified appearance. - Tall cells reach the surface, carrying cilia. #### **Types:** 1. **Pseudostratified Columnar Ciliated with motile cilia:** - Site: Upper respiratory tract, such as the trachea, larynx, and bronchi. #### **B. Stratified Epithelium** - Formed of more than one layer and classified according to the superficial cells into stratified squamous, stratified cuboidal, stratified columnar, and transitional epithelium. **i. Stratified Squamous Epithelium** - One of the stratified lining and covering epithelia. - The uppermost cells are flattened squamous cells. - May be keratinized or non-keratinized. #### **a. Stratified Squamous Non-Keratinized Epithelium** - Consists of basal, intermediate, and superficial layers. - **Basal layer:** One layer of columnar cells with basal oval nuclei. - **Intermediate layers:** Several layers of polyhedral cells with central spherical nuclei. - **Superficial layers:** Few layers of flat cells with elongated nuclei ### Sites: 1. Oesophagus 2. Anal canal 3. External surface of the cornea 4. Vagina proper #### **b. Stratified Squamous Keratinized Epithelium** - Resembles the non-keratinized epithelium, but the superficial flat cell layer is covered by a tough, non-living keratinized or cornified layer. ### Sites: 1. Epidermis of skin 2. Hard palate 3. Oral cavity of ruminants 4. Dorsal surface of the tongue **ii. Stratified Cuboidal Epithelium** - Consists mainly of two layers of cuboidal cells. ### Sites: - Ducts of some glands, such as sweat glands. **iii. Stratified Columnar Epithelium** - The superficial layer is formed of columnar cells. ### Sites: 1. Large ducts of glands 2. Male urethra **iv. Transitional Epithelium** - Also known as urothelium. - Ability to change its form according to the state of the organ. ### Sites: - Urinary passages, such as the renal pelvis, urinary bladder, ureters, and urethra. #### **Structure:** - **In an empty bladder (relaxed state):** Consists of 6-8 layers - **Basal layer:** Single layer of high cuboidal cells with spherical central nuclei. - **Intermediate layers:** Several layers of polyhedral or bear-shaped cells with central spherical nuclei. - Contain mucous-like substance. - **Superficial layer:** Single layer of dome-shaped cells with central spherical nuclei and a convex apical surface, some may be binucleated. - **In a full bladder (stretched state):** The number of layers decreases to 2-3 layers only. - Superficial layers appear flat but with rounded nuclei. ### **2- Pseudostratified Columnar Ciliated with Non-Motile (Stereocilia) Cilia:** - Not true cilia, but long microvilli. - Function: Moves fluids or particles in one direction over the surface epithelium. ### **Sites:** - Epididymis and Vas deferens #### **C. Glandular Epithelium** - Specialized epithelial tissue that secretes substances. - Classified according to the presence or absence of a duct into exocrine glands, endocrine glands, and mixed glands. #### **A. Classified According to the Number of Cells:** **i. Unicellular Gland:** One cell that is scattered singly throughout the epithelial membrane. **Goblet Cell:** - Specialized to produce glycoprotein and scattered singly between the epithelial cells of respiratory and digestive tracts. #### **Structure:** - **Light Microscopy:** - Expanded part (free end): Enlarged. - Narrow part: Contains the nucleus. - Free end: Contains secretory granules (mucigen granules). - Foamy and basophilic due to the unstained granules. - Basal part: Cylindrical, narrow, and elongated. - Known as the stem of the goblet cell. - Contains most of the cytoplasm. - Single, basal, oval, and vesicular nucleus. - **Electron Microscopy:** - Mucigen granule: Surrounded by a membrane. - Stained pink with periodic acid-Schiff (PAS) and blue with Alcian blue. #### **B. Classified According to the Mode of Secretion:** **i. Merocrine Gland:** - Secretion passes through the free surface of the cells by exocytosis without losing any part of the cytoplasm. - Pancreas and salivary glands. **ii. Apocrine Gland:** - Secretion accumulates inside the apical part of the cell and is then delivered with this apical part of the cell. - Mammary and axillary sweat glands. **iii. Holocrine Gland:** - The secretory products are discharged with the entire cell. - Basal cells of the gland divide to replace the destroyed cells. - Sebaceous glands. #### **C. Classified According to the Shape of the Secretory Part and Branching of Ducts:** **i. Simple Gland:** Single duct and unbranched end-pieces **Types:** - **Simple Tubular:** Intestinal glands. - **Simple Alveolar:** Sebaceous gland. - **Simple Tubulo-Alveolar:** Small salivary glands. - **Simple Coiled Tubular:** Sweat gland. **ii. Branched Gland:** Single duct and branched end-pieces. **Types:** - **Branched Tubular:** Gastric glands. - **Branched Alveolar:** Tarsal glands. - **Branched Tubulo-Alveolar:** Prostate. **iii. Compound Gland:** Branched duct and branched end-pieces. **Types:** - **Compound Tubular:** Kidney and liver. - **Compound Alveolar:** Mammary gland. - **Compound Tubulo-Alveolar:** Pancreas. #### **D. Classified According to the Nature of Secretion:** - Multiple cells form the gland, such as the salivary gland. **i. Serous Gland:** - Serous fluid, containing enzymes. - Lined with pyramidal cells, which have rounded nuclei and basal cytoplasm that is basophilic, rich in rough endoplasmic reticulum. - Apical cytoplasm is acidophilic, rich in zymogen granules. - Small diameter with narrow lumens. - Few cells. ### **Sites:** 1. Pancreas 2. Parotid gland **ii. Mucous Gland:** - Secrete mucin, a complex mucopolysaccharide. - Lined with cuboidal cells, which have flattened peripheral nuclei. - Cytoplasm is less acidophilic and foamy, with vacuoles due to dissolved mucin. - Contains few rough endoplasmic reticulum. - Large diameter with wide lumens. - Numerous cells. ### **Sites** 1. Palatine glands 2. Brunner's gland **iii. Seromucous Gland:** - Secrete serous and mucous secretions. - Cell by cell, acinus by acinus, or the mucous unit is capped by crescent-shaped aggregations of serous cells (crescents of Gianuzzi or demilunes of Gianuzzi). - Serous secretion passes to the mucous unit through the intercellular canaliculi. ### **Sites:** 1. Mandibular and sublingual glands ## **Modifications of the Epithelial Cells to Perform Specific Functions** **A. Apical Modifications:** 1. **Cilia:** Described before. 2. **Microvili:** Finger-like and cylindrical projections on the surface of certain cells. - Cores contain many actin filaments. - Site: Cells of small intestine and kidney. - Function: Increase the cell surface area for absorption. 3. **Sterocilia:** - Non-motile, hair-like processes. - Its core contains actin filaments. - Present on the cells lining the epididymis. **B. Lateral Modifications (Cell Junctions)** **i. Zonula Occludens (Tight Junction)** - The two adjacent cell membranes fuse completely at certain points. - The intercellular space is zero. - Encircles the cell as a belt. - Prevents the passage of any substance between the cells. **ii. Zonula Adherens (Adherent Junction)** - The two adjacent cell membranes are widely separated by an intercellular space of 20 nm, which is filled with adhesive glycoprotein material. - Tonofilaments condense at the cytoplasmic side, encircling the cell as a belt. - Fixes the two adjacent cells to prevent their separation. **iii. Macula Adherens (Desmosomes)** - The intercellular space is 25-30 nm and contains granular material and an electron-opaque central line. - The cytoplasm presents a dense attachment plate in which the microfilaments (tonofilaments) are converged and inserted. - Some tonofilaments are arranged as a hairpin loop. - Scattered as circular spots. - Not encircles the cell. - The zonula occludens, zonula adherens, and the desmosomes are called the **junction complex** in the small intestine. **iv. Gap Junction (Nexus)** - The two adjacent cells are separated by spaces of only 2nm. - Junction channels represent the region of low electrical resistance or coupling. - Permits the passage of ions and impulses between the adjacent cells. #### **B. Basal Modifications:** - Include the basement membrane and hemidesmosomes. **i. Basement Membrane** - Extracellular sheet-like structure that separates the epithelium from the underlying connective tissue. - Consists of two layers: - **Basal lamina:** Formed of protein collagen embedded in glycoprotein. - **Reticular lamina:** Formed of reticular fibers embedded in a glycoprotein. **ii. Hemidesmosomes** - Similar to half desmosomes. - Fix the epithelial cells to the underlying basement membrane. - The basal part of the cell membrane becomes thickened, where microfilaments are embedded. **iii. Basal Infoldings** - Increase the surface membrane for effective basal flow of materials in kidney tubules and striated ducts of the salivary glands. ## **8. Plasma Cells** - Developed from B-lymphocytes. - Numerous in lymphoreticular tissue. - Ovoid cells. - Deeply basophilic cytoplasm with a pale (unstained area), called a negative Golgi image. - Nucleus is rounded, eccentric, and has a characteristic feature that looks like the spokes of a cartwheel or clock-face due to regular radial arrangement of chromatin along the inner surface of the nuclear membrane. ### **Functions:** - Have a supportive function. - Have a phagocytic function. ### **Electron Microscopy:** - Contain the EM features of protein producing cells: - Extensive granular rough endoplasmic reticulum. - Golgi apparatus. - Mitochondria. - Ribosomes. ### **Functions:** - Synthesize and secrete antibodies, which play an important role in humoral immunity (body defense against bacterial infections). ## **9. Mast Cells** - Differentiated from hemopoietic stem cells. - Present in loose connective tissue and in the vicinity of small blood vessels. - Large spherical or ovoid with few cytoplasmic processes. - Cytoplasm is engorged with a huge number of large, coarse metachromatic granules (the color of the granules is not the true color of the dye used to stain them.) - Nucleus is small, eccentric, and pale, obscured by the granules. ### **Electron Microscopy:** - Contain features of secretory cells: - Well developed rough endoplasmic reticulum. - Mitochondria - Golgi apparatus. - Electron-dense membrane-bounded secretory granules. ### **Functions:** - Detect foreign substances and initiate a local inflammatory response. - Secrete histamine, which causes vasodilation and increases the permeability of blood capillaries during allergic reactions. - Secrete heparin, which prevents blood clotting (anticoagulant substance). - Secrete serotonin, which causes contraction of smooth muscle. - Secrete eosinophil chemotactic factor, which attracts eosinophils to the site of allergy. ## **10. Pigment Cells (Melanocytes)** - Ectodermal in origin. - Derived from the neural crest. - Form melanin. - Located between the basal cells of the stratified epithelium of the epidermis, hairs, iris, and the choroids of the eye. - Small, rounded cell bodies, with a small, central nucleus and long branched cytoplasmic processes. ### **Microscopy:** - Cytoplasm contains melanin granules (melanosomes) that are difficult to see with hematoxylin and eosin (H&E). - Identified by the dihydroxyphenylalanine (DOPA) reaction, which blackens the cells, or by detecting tyrosinase enzyme. - Contain the EM features of protein synthesizing cells: - Prominent Golgi apparatus. - Abundant rough endoplasmic reticulum. - Mitochondria. - Ribosomes. - Secretory vesicles. ### **Functions:** - Synthesize melanin, which is essential to absorb and disperse the harmful effects of ultraviolet rays and thus prevent sunburn. ### **Melanophores or Melanophages:** - Macrophages in the dermis. - Mesodermal in origin. - Phagocyto