Lecture 4a

1. Lecture 4a Epithelial Tissue

2. Epithelial Tissue • Overview: • Characteristics and functions of epithelia • Cell junctions • Classification of epithelia • Exocrine glands

3. Four types of tissues in the body • Groups of cells similar in structure and function • The four types: • Epithelial • Connective • Muscle • Nerve

4. Remember the levels of organization

5. What is an Epithelium? • Epi = “on” or “around” • Thele = “nipple” • Covers the external body surface (epidermis), lines cavities and tubules, and generally marks off our insides from our outsides • Other examples?

6. Epithelial Tissues – two types • Epithelia: • layers of cells covering internal or external surfaces • Glands: • structures that produce secretions

7. Characteristics of Epithelia • Cellularity: composed of cells bound by cell junctions • Polarity: apical and basal surfaces • Attachment: via basal lamina to underlying connective tissue • Avascularity: no blood vessels (but richly innervated) • Regeneration: germinative cell division

8. Free Surface and Attached Surface • Polarity: • apical and basolateral surfaces Figure 4–1

9. Repairing and Replacing Epithelia • Epithelia are replaced by division of germinative cells (stem cells) • Near basal lamina

10. Functions of Epithelial Tissue • Provide physical protection • Control permeability • Move fluids over the surface • Provide sensation (e.g. neuroepithelia) • Produce specialized secretions (glandular epithelium)

11. Specializations • Microvilli increase absorption or secretion • Cilia (ciliated epithelium) move fluids

12. Effective Barriers • Physical integrity is maintained by: • intercellular connections • attachment to basal lamina • maintenance and repair

13. Cell Junctions

14. Cell junctions • Tight Junctions – surround cells, waterproof • Isolates wastes in the lumen • Gap junctions – allow rapid communication • Desmosomes – tie cells together with great strength (like rivets) • Hemidesmosomes attach Figure 4–2b

15. Desmosomes • CAMs, dense areas, and intercellular cement Figure 4–2d

16. Attachment to Basal Lamina • Hemidesmosomes Figure 4–2e

17. Classification of epithelia • Cell shape • Squamous: flat • Cuboidal: square • Columnar: tall • Layers of cells • Simple: one layer of cells (what is a function?) • Stratified: many layers of cells (what is a function?)

18. Classes of Epithelia • Based on shape and layers Table 4–1

19. Classification of Epithelia • Simple or stratified Figure 4.1a

20. Classification of Epithelia • Squamous, cuboidal, or columnar Figure 4.1b

21. Simple Squamous Epithelia • Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm Look like a fried egg from the top • most delicate • Diffusion, friction reduction • Special names Mesothelium: • lines body cavities (e.g. peritoneum, pleura) Endothelium: • lines heart and blood vessels

22. Epithelia: Simple Squamous Figure 4.2a

23. Simple Squamous Epithelium Figure 4–3a

24. Epithelia: Simple Cuboidal • Single layer of cube-like cells with large, spherical central nuclei • Function in secretion and absorption • Present in kidney tubules, ducts and secretory portions of small glands, and ovary surface

25. Epithelia: Simple Cuboidal Figure 4.2b

26. Simple Cuboidal Epithelium • Kidney tubules Figure 4–4a

27. Epithelia: Simple Columnar • Single layer of tall cells with oval nuclei • May contain microvilli • Goblet cells are often found in this layer • Function in absorption and secretion • Line digestive tract and gallbladder, small bronchi, uterine tubes, and some regions of the uterus

28. Epithelia: Simple Columnar Figure 4.2c

29. Epithelia: Simple Columnar • Intestinal lining

30. Epithelia: Pseudostratified Columnar • Single layer of cells with different heights; all touch the basal lamina but some do not reach the free surface • Nuclei are seen at different layers • Function in secretion and propulsion of mucus • Present in the male sperm-carrying ducts (nonciliated) and trachea (ciliated)

31. Epithelia: Pseudostratified Columnar Figure 4.2d

32. Pseudostratified Columnar Epithelium • Trachea Figure 4–5b

33. Epithelia: Transitional • Several cell layers, basal cells are cuboidal, surface cells are dome shaped (or flat) • Stretches to permit the distension of the urinary bladder • Lines the urinary bladder, ureters, and part of the urethra

34. Epithelia: Transitional Figure 4.2f

35. Epithelia: Transitional • Urinary bladder Figure 4.2f

36. Stratified epithelia

37. Epithelia: Stratified Squamous • Thick membrane composed of several layers of cells (the only one with more than 2 or 3 true layers) • Functions in protection of underlying areas subjected to abrasion • Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells)

38. Epithelia: Stratified Squamous Figure 4.2e

39. Epithelia: Stratified Cuboidal • Quite rare in the body • Found in some sweat and mammary glands • Typically two cell layers thick • Only top layer is cuboidal

40. Stratified Cuboidal Epithelium • Sweat gland ducts Figure 4–4b

41. Epithelia: Stratified Columnar • Limited distribution in the body • Found in the pharynx, male urethra, and lining some glandular ducts • Also occurs at transition areas between two other types of epithelia

42. Stratified Columnar Epithelium • Rare • Salivary gland duct Figure 4–5c

43. Epithelia: Glandular • A gland is one or more cells that makes and secretes an aqueous fluid • Classified by: • Site of product release – endocrine or exocrine • Relative number of cells forming the gland – unicellular or multicellular

44. Glandular Epithelia • Endocrine and exocrine glands Figure 4–6

45. Glands • Endocrine • Ductless glands that produce hormones • Secretions include amino acids, proteins, glycoproteins, and steroids • Exocrine • More numerous than endocrine glands • Secrete their products onto body surfaces (skin) or into body cavities via ducts • Examples include mucous, sweat, oil, digestive, and salivary glands • The only important unicellular gland is the goblet cel • EXAMPLES?

46. Goblet Cell Figure 4.3b

47. Glands are classified based on mode of secretion - 3 types

48. Modes of Secretion • Merocrine – products are secreted by exocytosis (e.g., pancreas, sweat, and salivary glands) • Holocrine – products are secreted by the rupture of gland cells (e.g., sebaceous glands) • Apocrine – products acumulate in the top of the cell and then it breaks down

49. Summary • Epithelial tissue structures and functions • Cell junctions • Classification by cell shape and layers • Glands