epithelial tissue.pptx

general histology

1. Epithelial tissue By dr Abdiasis Omar Mohamed MBBS

2. Introduction the organs of the human body are composed of only four basic tissue types: Epithelial Connective Muscular N ervous tissues

3. Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface. Epithelia (Gr. epi, upon + thele, nipple) line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue.

4. The principal functions of epithelial tissues include the following: - Covering , lining, and protecting surfaces (eg, epidermis) - Absorption (eg, the intestinal lining) - Secretion (eg, parenchymal cells of glands)

5. Characteristic features of epithelial cells Most epithelia are adjacent to connective tissue containing blood vessels from which the epithelial cells receive nutrients and O2. The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems is called the lamina propria. The area of contact between the two tissues may be increased by small evaginations called papillae projecting from the connective tissue into the epithelium.

6. Epithelial cells generally show polarity. The region of the cell contacting the ECM and connective tissue is called the basal pole and the opposite end, usually facing a space, is the apical pole , with the two poles differing significantly in both structure and function .

7. Basement Membrane The basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules referred to as the basement membrane , a semipermeable filter for substances reaching epithelial cells from below. Nearest the epithelial cells is the basal lamina , a thin, electron-dense, sheet-like layer of fine fibrils, and beneath this layer is a more diffuse and fibrous reticular lamina.

8. basal lamina characteristically include the following: Type IV collagen, Laminin , integrin , Nidogen and perlecan. The more diffuse meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue

9. Intercellular Adhesion & Other Junctions Tight or occluding junctions form a seal between adjacent cells . Adherent or anchoring junctions are sites of strong cell adhesion . Gap junctions are channels for communication between adjacent cells.

10. Zonula occludens (Tight junctions ): The most apical junction form bands completely encircling each cell close off the space between the cells The seal between the membranes is due primarily to direct interactions between the transmembrane protein claudin on each cell The number of these fusions sites is inversely correlated with the leakiness of the epithelium

11. Prevent the flow of material between the base and the apex of the cell in either directions. It contribute to the formation of two separate functional units , apical and basal. Composed of several proteins:- - Junctional associated molecule (JAM)- - Occludins - Claudins

12. Zonula adherens Also encircles the cell Immediately below the zonula occludens Adhesion is mediated by transmembrane glycoproteins of each cell, the cadherins. which lose their adhesive properties in the absence of Ca2 +. Inside the cell, cadherins bind the protein catenin which is linked to actin filaments

13. Desmosome: does not form a belt around the cell Disk like structure that faces mirror image on adjacent cell (20-30 nm gap ). Circular plaque called attachment plaque made up of 12 proteins of anchoring proteins . Intermediate cytokeratin filaments are inserted to the plaque.

14. Hemidesmosome : connection between cell and BL Take the shape of half desmosome . The attachment plaque contain mainly integrin not cadherin.

15. gap junctions : Passageway between two adjacent cells Let small molecules move directly between neighboring cells Cells are connected by hollow cylinders of protein Each junction is composed of 6 connexin form hexameric complexes called connexons proteins that span the membrane and form one half of the junction Permit Cell to cell exchange of low MW molecules such as: ions, cAMP, small hormones . Not present in skeletal muscle.

16. Specializations of cell surface Microvilli: Non-branching fingerlike projections of cell 1 μm high, 0.08 μm wide,at the free surface of the cell . Increase of cell surface Covered by plasma membrane The glycocalyx is thicker contain glycoproteins and enzymes that allow final stage digestion Glycocalyx and microvilli seen under light microscope called striated or brush border .

17. It’s a cluster of actin filaments that crossed linked to each other proteins such as fimbrin and villin and to plasma membran by proteins such as myosin I. Hundred of microvilli can be seen in absorptive cells.

18. Stereocilia Branched non – motile microvilli of cells of epididymis and ductus deference Its function to increase the surface area Cilia Non-branching motile projections Transport of extracellular mass May be numerous 270 per cell in trachea

19. 5-10 μ m tall ,0.2 μ m wide Attached to basal bodies ( analogus to the centrioles) at the apical area Flagella very long cillia 100 μ m , only in spermatozoa

20. Type of Epithelia Covering epithelia Number of cell layer Simple Stratified Shape of the cell at the surface layer Squamous Cuboidal Columnar Glandular epithelia

21. Simple Squamous Epithelium single layer of flat cells with disc-shaped nuclei Special types - Endothelium (inner covering ) slick lining of hollow organs - Mesothelium (middle covering)Lines peritoneal, pleural, and pericardial cavities Covers visceral organs of those cavities

22. Simple Cuboidal Epithelium single layer of cube-like cells with large, spherical central nuclei Function - secretion and absorption Location - kidney tubules - secretory portions of small glands, ovary & thyroid follicles

23. Simple Columnar Epithelium Single layer of column-shaped (rectangular) cells with oval nuclei Some bear cilia at their apical surface May contain goblet cells Function: Absorption; secretion of mucus, enzymes, and other substances Ciliated type propels mucus or reproductive cells by ciliary action

24. Non-ciliated form - Lines digestive tract, gallbladder, ducts of some glands Ciliated form - Lines small bronchi, uterine tubes, uterus

25. Pseudostratified Columnar Epithelium All cells originate at basement membrane Only tall cells reach the apical surface May contain goblet cells and bear cilia Nuclei lie at varying heights within cells Gives false impression of stratification Function: secretion of mucus; propulsion of mucus by cilia

26. Non-ciliated type - Ducts of male reproductive tubes - Ducts of large glands Ciliated variety - Lines trachea and most of upper respiratory tract

27. Stratified Squamous Epithelium Many layers of cells – squamous in shape Deeper layers of cells appear cuboidal or columnar Thickest epithelial tissue – adapted for protection Specific types Keratinized – contain the protective protein keratin Surface cells are dead and full of keratin

28. Non-keratinized – forms moist lining of body openings Function: Protects underlying tissues in areas subject to abrasion Location - Keratinized – forms epidermis - Non-keratinized – forms lining of esophagus, mouth, and vagina

29.   Stratified Cuboidal Epithelium generally two layers of cube-shaped cells Location: - Forms largest ducts of sweat glands - Forms ducts of mammary glands and salivary glands

30. Stratified Columnar Epithelium several layers; basal cells usually cuboidal; superficial cells elongated Function: protection and secretion Location: - Rare tissue type - Found in male urethra and vas deferens, largest ducts of salivary glands, nasopharynx

31. Transitional Epithelium Basal cells usually cuboidal or columnar Superficial cells dome-shaped or squamous Function: stretches and permits distension of urinary bladder Location: Lines ureters, urinary bladder and part of urethra

32. Secretory Epithelia & Glands Epithelial cells that function mainly to produce and secrete various macromolecules may occur in epithelia with other major functions or comprise specialized organs called glands . Secretory cells may synthesize, store, and release proteins (eg, in the pancreas), lipids (eg, adrenal, sebaceous glands), or complexes of carbohydrates and proteins (eg, salivary glands). Epithelia of mammary glands secrete all three substances.

33. Exocrine glands remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver the secreted material where it is used. Endocrine glands lose the connection to their original epithelium and therefore lack ducts.

34. glands can be simple (ducts not branched) or compound (ducts with two or more branches ). Secretory portions can be tubular (either short or long and coiled) or acinar (rounded and saclike); either type of secretory unit may be branched, even if the duct is not branched . Compound glands can have branching ducts and can have multiple tubular, acinar, or tubuloacinar secretory portions.

35. THE END