NOTES ON HISTOLOGY OF EPITHELIAL TISSUE.pptx

Histology of Epithelial tissues

1. TISSUE AND HISTOLOGY CLASS NOTES

2. Levels of Structural Organization Slide 1.3 Figure 1.1

3. Overview Tissues and Histology Tissues - collections of similar cells and the substances surrounding them Major types of adult tissues Epithelial Connective Muscle Nervous Histology: Microscopic Study of Tissues Biopsy: removal of tissues for diagnostic purposes Autopsy: examination of organs of a dead body to determine cause of death

4. Embryonic Tissue 3 major germ layers that form the embryonic disc (source of stem cells ) Endoderm Inner layer Forms lining of digestive tract and derivatives Mesoderm Middle layer Forms tissues as such muscle, bone, blood vessels Ectoderm Outer layer Forms skin and neuroectoderm

5. 1.Epithelial Tissue Cellularity - Consists almost entirely of cells Covers body surfaces, lines hollow organs, and forms glands Outside surface of the body Lining of digestive, respiratory and urogenital systems Heart and blood vessels Linings of many body cavities Polarity - Has apical , basal , and lateral surfaces Rests on a basement membrane Specialized cell contacts bind adjacent cells together Avascular - no blood vessels Regenerative -Replaces lost cells by cell division

6. Functions of Epithelia Protecting underlying structures; e.g., epithelium lining the mouth Acting as barriers; e.g., skin Permitting the passage of substances; e.g., cells lining air sacs in lungs and nephrons in kidney Secreting substances; e.g., pancreatic cells Absorbing substances; e.g., lining of stomach and small intestine

7. Functions of Epithelia Protecting underlying structures; e.g., epithelium lining the mouth Acting as barriers; e.g., skin Permitting the passage of substances; e.g., cells lining air sacs in lungs and nephrons in kidney Secreting substances; e.g., pancreatic cells Absorbing substances; e.g., lining of stomach and small intestine

8. Special Characteristics of Epithelia Figure 4.1

9. Classification of Epithelium Number of layers of cells Simple - one layer of cells. Each extends from basement membrane to the free surface Stratified - more than one layer. Pseudostratified - tissue appears to be stratified, but all cells contact basement membrane so it is in fact simple, some of the cells do not reach the free surface. Shape of cells Squamous - flat, scale-like Cuboidal - about equal in height and width Columnar - taller than wide

10. Classifications of Epithelia

11. Simple Squamous Epithelium Figure 4.3a

12. Simple Cuboidal Epithelium Figure 4.3b

13. Simple Columnar Epithelium Figure 4.3c

14. Pseudostratified Ciliated Columnar Epithelium Figure 4.3d

15. Stratified Epithelia

16. Stratified Epithelia Contain two or more layers of cells Are named according to the shape of cells at apical layer Basal cells are cuboidal Regenerate from below Major role is protection

17. Stratified Squamous Epithelium Description Many layers of cells – squamous in shape Deeper layers of cells appear cuboidal or columnar Thickest epithelial tissue – adapted for protection

18. Stratified Squamous Epithelium Specific types Keratinized – contain the protective protein keratin Surface cells are dead and full of keratin forms epidermis Non-keratinized – forms moist lining of body openings like lining of esophagus, mouth, and vagina, cervix & anal canal Function – Protects underlying tissues in areas subject to abrasion

19. Stratified Squamous Epithelium Figure 4.3e

20. Clinical importance Basal Cell Carcinoma-skin Vitiligo : condition where the skin loses its pigment cells (melanocytes), resulting in white patches. Epithelial Impact: Loss of melanocytes in the epidermis. Eczema Barrett's Esophagus : A condition where the normal squamous epithelium is replaced by columnar epithelium due to chronic acid reflux. Squamous Cell Carcinoma Esophagitis

21. Stratified cuboidal epithelium A thin , stratified epithelium that usually consists of only two or three layers of cuboidal cells. Found lining the larger excretory ducts of exocrine glands such as the salivary glands Gland Tumors : involve changes in the epi.

22. Transitional Epithelium Figure 4.3h

23. Cont …. a.k.a (or urothelium ) form of stratified epithelium found only in the urinary tract highly specialized to accommodate a great degree of stretch and to withstand the toxicity of urine. has some features intermediate (transitional) between stratified cuboidal and stratified squamous epithelia . Cystitis , Urothelial Carcinoma

24. Stratified columnar epi. Features: Consists of two or more layers of cells The outermost layer of cells is columnar Basal Cells ( deeper layers) often consist of cuboidal or polyhedral cells. Function: Protection against mechanical stresses and pathogens due to its multiple layers. Can also participate in secretion and absorption processes in certain locations. Locations: Large Ducts of Some Glands such as the salivary glands, where it aids in the passage and modification of secretions. Conjunctiva of the Eye : Found in the conjunctiva, providing protection and facilitating movement of the eyelids. NB?/ Stratified columnar epithelium is relatively rare compared to other types of epithelial tissues, but it plays important roles in the areas where it's found.

25. Types of epithelium in the eye 1 . Corneal: Stratified squamous non-keratinized epithelium. Function : Protects the cornea from mechanical damage, pathogens, and helps maintain transparency for vision. 2. Conjunctival Epithelium : Stratified columnar epithelium with goblet cells. Function : Produces mucus to lubricate the eye, protecting it from drying out and trapping debris. 3. Lens ( Anterior surface) : Simple cuboidal epithelium. Function : Provides metabolic support to the lens fibers and helps maintain lens transparency.

26. 4. Retinal Pigment Epithelium (RPE): Simple cuboidal to columnar epithelium with pigment granules. Function : Absorbs excess light, supports photoreceptor cells, and participates in the visual cycle. 5. Ciliary Body: Stratified cuboidal epithelium. Function : Involved in the production of aqueous humor and accommodation (focusing the lens). 6. Iris ( Posterior surface) : Pigmented cuboidal epithelium (two layers). Function : Controls the amount of light entering the eye by adjusting the pupil size.

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28. Glandular Epithelium A gland is one or more cells that makes and secretes an aqueous fluid Two types of glands formed by infolding of epithelium: Endocrine : no contact with exterior of body; ductless ; produce hormones (pituitary, thyroid, adrenals, pancreas) Exocrine : open to exterior of body via ducts (sweat, oil) Exocrine glands classified either by structure or by the method of secretion Classified by structure Unicellular: goblet cells Multicellular: sweat, oil, pituitary, adrenal

29. Multicellular Exocrine Glands Classified on the basis of types of ducts or mode of secretion Types of ducts Simple: ducts with few branches Compound: ducts with many branches If ducts end in tubules or sac-like structures: acini . Pancreas If ducts end in simple sacs: alveoli . Lungs

30. Lateral Surface Features Tight junctions Desmosomes Gap junctions

31. Membrane Junctions: Tight Junction Integral proteins of adjacent cells fuse together Completely encircle the cell and form an adhesion belt. Form an impermeable junction. Common near apical region

32. Lateral Surface Features – Cell Junctions Desmosomes – two disc-like plaques connected across intercellular space Plaques of adjoining cells are joined by proteins called cadherins Proteins interdigitate into extracellular space Intermediate filaments insert into plaques from cytoplasmic side

33. Membrane Junctions: Desmosome Figure 3.5b Linker proteins extend from plaque like teeth of a zipper. Intermediate filaments extend across width of cell. Common in superficial layers of skin; skin peels after a sunburn Reduces chance of tearing, twisting, stretching

34. Membrane Junctions: Gap Junction Connexon proteins are trans- membrane proteins. Present in electrically excitable tissues (heart, smooth muscle)

35. Basal Feature: The Basal Lamina  Noncellular supporting sheet between the epithelium and the connective tissue deep to it  Consists of proteins secreted by the epithelial cells  Functions:  Acts as a selective filter, determining which molecules from capillaries enter the epithelium  Acts as scaffolding along which regenerating epithelial cells can migrate  Basal lamina and reticular layers of the underlying connective tissue deep to it form the basement membrane

36. Epithelial Surface Features Apical surface features Microvilli – finger-like extensions of plasma membrane Abundant in epithelia of small intestine and kidney Maximize surface area across which small molecules enter or leave Cilia – whip-like, highly motile extensions of apical surface membranes Movement of cilia – in coordinated waves Stereocilia - in epidydimis

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