Epithelial Tissue Lecture 2025 PDF

Epithelium Dr. Sally Selim Dr. Nadia El- Akabawy Characteristic features of epithelial tissue: - Very cellular, little intercellular space. - Cells rest on BM. - Numerous nerve endings. - No lymph vessels. - Avascular. - Cells show polarity. - Display surface modifications. - Many functions;...

Epithelium Dr. Sally Selim Dr. Nadia El- Akabawy Characteristic features of epithelial tissue: - Very cellular, little intercellular space. - Cells rest on BM. - Numerous nerve endings. - No lymph vessels. - Avascular. - Cells show polarity. - Display surface modifications. - Many functions; Absorption; intestine Protection; skin Secretion; thyroid Exchange; lung Epithelia can be divided into 4 main groups: Covering or Lining Epithelia: Secretory Epithelia and Glands: Myo- Epithelia: Neuro- Epithelia: 1- Covering or Lining Epithelia: 1.Simple squamous epithelium: Cells have the appearance of thin scales. Cell nuclei: flat Site: a-Endothelium lining of capillaries. b-Lung alveoli: gases diffuse plus segments of kidney tubules. c-Mesothelium; the surface layer of the serous membrane that lines body cavities and internal organs. Function: provide a smooth and protective surface. 2.Simple cuboidal epithelium: Cell nuclei: round and central. Site: a- Kidney tubules. b- Ducts of the glands. c- Thyroid follicles. Function: active in the secretion and absorptions of molecules. 3.Simple columnar epithelium: = (Non- ciliated) Cell nuclei: elongated and basal. Site: a- some sections of the digestive system. [[[ b- some sections of the female reproductive tract. Function: active in the secretion and absorptions of molecules. 4. Ciliated columnar epithelium : Cell nuclei: elongated and basal. Cells have cilia on their apical surfaces. Site: a- Fallopian tubes. b- Some parts of the respiratory system. Function: beating of the cilia helps to remove particulate matter. 5.Pseudostratified columnar epithelium: Appears to be stratified. But instead, consists of a single layer of irregularly shaped and differently sized columnar cells. The arrangement gives the appearance of stratification. But in fact, all the cells are in contact with the basal lamina. Site: a- In the respiratory tract, where some of these cells have cilia. 1. Stratified squamous epithelium: The deeper layers are columnar. In proceeding towards the surface, the cells become increasingly flattened (squamous). This type may be keratinized or non- keratinized. Site: a- Mammalian skin: keratinized. b- Mouth cavity: non- keratinized. 2. Stratified cuboidal epithelium: Can be found in certain glands and ducts. Uncommon in the human body. 3. Stratified columnar epithelium: The same as stratified cuboidal epithelium. 4. Transitional epithelium: So- called because of the gradual changes in the shapes of the apical cells. As the urinary bladder is empty, it appears thicker and more multi-layered. When the bladder fills up with urine and more stretched out, the epithelium is less stratified. Site: Only in the urinary system, specifically the ureters and urinary bladder.  These cells are called transitional as they undergo a change in their shape and structure. Facet cells or Umbrella cells: the cells of the superficial cell layer, lines the lumen. This layer is fully differentiated, dome shaped with many bi- nucleated cells. The urothelium is the most impermeable membrane in the mammalian body. It acts as an osmotic barrier against the contents of the urinary tract, transitional epithelium is relatively impermeable to water and salts. 2- Secretory Epithelia and Glands: Secretory Epithelia and Glands: -Some epithelial cells specialized to perform secretory function. -Such cells, present singly or in groups, constitute glands. # Based on number of secretory cells, glands are classified as: 1. 1- Unicellular: -Interspersed amongst other (non-secretory) epithelial cells. -Can be found in the epithelium lining the intestines (Goblet cells). 1. 2- Multicellular: -Most glands are multicellular. -Glands develop as diverticula from epithelial surfaces. -The ‘distal’ parts of the diverticula develop into secretory elements, while the ‘proximal’ parts form ducts through which secretions reach the epithelial surface, e.g., lacrimal gland, parotid gland. # Based on mode of secretion, glands are classified as: 1. 1- Endocrine glands: 1. 2- Exocrine glands: Glands lose all contact with epithelial surface Glands pour their secretions on to an from which they develop, and they pour their epithelial surface, directly or through secretions into blood. “ Duct- less glands “. ducts. Exocrine glands can be further classified based on: A- Branching of ducts: B- Shape of the secretory unit: C- Nature of the secretions: D- Manner of secretions pouring out of the cells: A- Branching of ducts: Simple: All the secretory cells of an exocrine gland discharge into one duct, the gland is said to be a simple gland, e.g., gastric glands, sweat glands. Compound: Several groups of secretory cells, each group discharging into its own duct. These ducts unite to form larger ducts that drain on to an epithelial surface. Such a gland is said to be a compound gland, e.g., parotid B. Shape of the secretory unit: Tubular glands: : Glands with secretory unit tubular in shape. The tube may be straight, coiled or branched, e.g., gastric glands. Alveolar glands: : Glands with secretory unit flask shaped. C. Nature of the secretions: Mucous glands: - Tall cells with flat nuclei at their bases, Large lumen. - Secretion contain mucopolysaccharides. - Secretion collects in the apical parts of the cells >>> nuclei are pushed to the base of the cell, and flattened. - With H& E, secretion remains unstained so that they have ‘empty’ look. Serous glands: - Cells are triangular with rounded, central nucleus, lumen is small. - Secretions are protein in nature. - With H& E, cytoplasm stains bluish. Mixed glands: - Glands contain both serous and mucous elements. D. Manner in which their secretions pouring out of the cells: Merocrine: Secretions are thrown out of the cells by exocytosis, the cell remains intact, e.g., goblet cell. Apocrine: Apical parts of the cells are shed off to discharge the secretion. e.g., mammary glands. Holocrine: Entire cell disintegrates while discharging its secretion, and is seen typically in sebaceous glands. 3- Myo- Epithelia: III- Myo- Epithelia: - Epithelia of many exocrine glands contain contractile cells at the basal ends of the secretory cells. - Long processes of myoepithelial cells are rich in actin filaments and myosin. - Strong contractions in these cells Myoepithelial cells wrapped serve to help propel secretory around milk-producing alveoli in products from acini into ducts. a lactating mammary glands. 4- Neuro- Epithelia: IV- Neuro- Epithelia: - Specialized sensory cells, such as those in ear, nose, and tongue. - Neuroepithelial cells found in the nose are part of the olfactory epithelium that is specialized to perform a receptor function and transmit impulses through the olfactory nerve. Epithelial Cell Polarity & Membrane Specialization:  Epithelial Cell Polarity& Membrane Specialization: 1- Basement membrane (Basal Specialization): - Epithelial cells rest on a thin basement membrane. In multi- layered epithelia, the deepest cells lie on this membrane. - LM: A distinct basement membrane cannot be seen in H&E, but can be well demonstrated using the periodic acid Schiff (PAS). This stains the glycoproteins present in the membrane. [ - EM: A basement membrane has a basal lamina (near to the epithelial cells) and a reticular lamina (near to the underlying connective tissue); formed of type I and III collagen fibers. - The basal lamina is divisible into the: 1- lamina densa: Formed of type IV collagen fibers. 2- lamina lucida: Formed of glycoproteins. Functions of Basement membrane: 1- Good adhesion; on one side to epithelial cells and other to C.T. 2- Barrier to diffusion of molecules. 3- Membranes influence the regeneration of peripheral nerves after injury and re-establishment of neuromuscular junctions. 4- Scaffold that allows rapid epithelial repair and regeneration.  N.B. Deep infoldings of the basal and lateral cell membranes: - Only seen with electron microscopy. - Found in certain cells, whose function is to transport ions across cell membrane. - E.g. Kidney tubules cells >> selectively absorb useful or nutritious components from the glomerular filtrate and retain them in the body. - Also, eliminate toxic or non-useful metabolic waste products such as urea and drug metabolites.  Lateral surfaces specializations Cell Junctions Cell membrane specializations found on the lateral surfaces of epithelial cells (between adjacent epithelial cells). Function: 1- Provide strong attachment to the adjacent cells. 2- Prevent passages of materials through the intercellular space.  Classification: According to the distance between the cell membranes: a) Tight junction (occluding junction) ( zero) b) Adhering junction (anchoring junction) (20nm) c) Gap junction (communicating junction) (2nm) 2- According to the Shape of the Junction: a) Zonula junction (girdle or ring junction) b) Fascia junction (band junction) c) Macula junction (spot junction) - lateral surfaces of epithelial cells have complexes of several specialized intercellular junctions with different functions: A- Tight or occluding junctions form a seal between adjacent cells. B- Adherent or anchoring junctions are sites of strong cell adhesion. C- Desmosomes. D- Gap junctions; channels for communication between adjacent cells. A. Tight junctions; Zonulae Occludens: Present in a definite order at the most apical end of the cells. “Zonula” indicates that the junction forms a band completely encircling each cell. Function: Continuous junctions within cell membranes prevent movement of molecules at the apical surface into the lateral and basal surfaces, and vice versa. B. Adherens junction (Zonula Adherens): Encircles the epithelial cell, usually immediately below the tight junction. Function: Firmly anchoring a cell to its neighbors. C. Desmosome: Resembles a single spot and does not form a belt around the cell. At the site of a desmosome, the plasma membrane of each cell is thickened (plaque). The thickened areas of the two sides are separated by a gap of 25 nm. The region of the gap is rich in glycoproteins. Intermediate filaments are inserted into these plaques and loop back towards the cytoplasm. Function: Firm cellular adhesion and strength throughout the epithelium. D. Gap junctions: Mediate intercellular communication rather than adhesion or occlusion between cells. Abundant in many epithelia. Gap junctions consist of aggregated transmembrane protein complexes that form circular patches in the plasma membrane. Communicating Junctions  Transmembrane gap junction proteins, connexins, form hexameric complexes called connexons. Function: Permit intercellular exchange of molecules with small diameters such as ions, move rapidly through gaps. Allowing cells to act in a coordinated manner rather than as independent units. E:Hemidesmosomes Structure: half (½) of desmosomes. Site: between bases of epithelial cells and basal lamina. Function: help to attach bases of epithelial cells to basal lamina.  3- The Apical Cell Surface (Apical Specialization): - Apical ends of many columnar and cuboidal epithelial cells have specialized structures projecting from the cells. - Function: either to increase surface area for absorption or to move substances along epithelial surface. A. Microvilli: Usually of uniform length. Found in cells such as those lining the small intestine. LM: densely packed microvilli visible as brush or striated border projecting into the lumen. With hundreds or thousands present on the end of each absorptive cell.  EM: Microvillus contains bundled actin filaments bound to surrounding plasma membrane by actin binding proteins. Actin filaments insert into the terminal web at the base of the microvilli. Function: 1- Increase the surface area by 20: 30-fold. 2-The thick glycocalyx covering microvilli of the intestinal brush border includes membrane- bound proteins and enzymes for digestion of certain macromolecules. B. Stereocilia: Much less common type of apical process, best seen on the epithelial cells lining the male reproductive system. Resemble microvilli in containing microfilaments and actin-binding proteins, with similar diameters, and with similar connections to the cell’s terminal web. Function: Like microvilli, Stereocilia are much longer and stereocilia increase the cells surface less motile than microvilli. area, facilitating absorption. C. Cilia: Long, highly motile apical structures, larger than microvilli, containing internal arrays of microtubules not microfilaments. LM: hair-like projections from the free surfaces of some epithelial cells. EM: Cilia are made up of microtubules coated by the plasma membrane.  Each cilium: 9 pairs of microtubules that form the outside ring and two central microtubules. This structure is called an axoneme. The 9 outer pairs are made up of motor proteins called dynein. These are large and flexible that allows the cilia to move. Cilia are attached to cell at basal body that is made up of microtubules arranged in 9 triplets.  Basal bodies have a structure similar to that of centrioles, with nine triplets of microtubules. Cilia have dynamic tubulin protofilaments forming rootlets anchoring the entire structure to the cytoskeleton. Function: 1-Cilia move in coordination with one another like a wave. As a result, fluid, mucous, or small solids lying on epithelium can be moved in a specific direction. 4-In some situations, cilia-like 2-Movements of cilia lining the respiratory structures perform a sensory function. epithelium move secretions in the trachea They may be non-motile. and bronchi towards the pharynx. Such ‘cilia’ present on the cells in the 3-Ciliary action helps in the movement of olfactory mucosa of the nose are ova through the uterine tube, and of called olfactory cilia, they are spermatozoa through the male genital tract. receptors for smell. D. Flagella: Larger processes having the same basic structure as cilia. In the human body the best example of a flagellum is the tail of the spermatozoon.  " Immotile Cilia Syndrome" (Kartagener syndrome): Several mutations in the proteins of the cilia and flagella. Responsible for certain symptoms; chronic respiratory infections, male infertility and female infertility.  RENEWAL OF EPITHELIAL CELLS: ✓ Epithelial tissues are renewed continuously by mitotic activity and stem cell populations. ✓ The rate of renewal varies widely; it can be fast in tissues such as the intestinal epithelium, which is replaced Intestinal stem cells reside at the every week, or slow, as in the bottom and replicate daily, generating large glands. new cells to maintain the tissue.