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Understanding Epithelial Tissue: Features, Classification, and Specializations

Epithelial tissue is characterized by condensed cell layers with minimal extracellular matrix, polarized structures, and an avascular nature yet is rich in innervation. It plays vital roles in protection, secretion, absorption, and sensory functions. The classification includes covering epithelium, glandular epithelium, and sensory epithelium, further divided based on cell layers and shapes such as simple squamous, cuboidal, and columnar types. Epithelial tissues exhibit various specializations like microvilli, cilia, and intercellular junctions essential for their functions in different organs.

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Understanding Epithelial Tissue: Features, Classification, and Specializations

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  1. Chapter 2Epithelial tissue

  2. 1.General feature: 1) contain more cells and less extracellular ground substance 2) Polarisaton: ---free outer surface: face the surface of the body or the lumen of an organ or gland ---basal surface: face underlying connective tissue 3)  Avascularity, but innervation: ---no blood vessels ---rich in nerve terminals 4) Cellular Layer + Basement Membrane 5) Functions: protection secretion absorption sensory reception

  3. 2.Classification of Epithelium 1)Covering epithelium: the epithelium which cover body surface or line the inner surface of body cavities, tubes and sac. 2)Glandular epithelium: the epithelium which main function is secretion. 3)Sensory epithelium: the epithelium which has special sensory function. * Myoepithelial cell

  4. 3. Classification of covering epithelium: According to the number of layer and shape of cells Simple epi.: ---simple squamous epi. ---simple cuboidal epi. ---simple columnar epi. ---pseudostratified ciliated columnar epi. Stratified epi.:---stratified squamous epi. ---stratified columnar epi. ---transitional epi.

  5. Cell shape

  6. Cell Layers Simple Stratified

  7. 1)simple squamous epi: ---structural feature: /one layer flattened cells, cell border are interdigitate closely /with flattened ellipsoid nucleus

  8. ---Distribution: • mesothelium: the simple squamous epi. which line the inner surface of body cavities such as thoracic, abdominal cavities and pericardiac . • endothelium: the simple squamous epi. which line the inner surface of cardiovascular and lymph vessels. • other place: alveoli, parietal layers of renal capsule. ---function: a) transport of materials b) facilitates movement of viscera

  9. Vascular endothelium

  10. Mesothelium on abdominal cavity

  11. 2)simple cuboidal epi.: ---structural feature: • one layer of cells, with same height and width , hexagonal in shape. • spherical centrally-located nucleus

  12. ---distribution: /the renal tubule /thyroid /the some ducts of glands ---function: covering and secretion renal tubule thyroid

  13. 3)simple columnar epi.: ---structural features: • one layer of columnar cells, with basally located ovoid nucleus

  14. ---distribution: gastrointestinal tract gall bladder uterus ---function: secretion and absorption goblet cell: scattered, secreting granules-mucus goblet cell simple columnar epi

  15. four types of cells 4)pseudostratified ciliated columnar epi.: ---Structural feature: 1, Four types of cells columnar cell (ciliated); goblet cell fusiform cell; basal cell: pyramid-shaped 2, Every cell locate on basement membrane: Simple epi.

  16. ---distribution: inner surface of large duct of respiratory passages trachea bronchi nasal The epithelium of trachea

  17. 5)stratified squamous epi.: ---structural features: • deepest (basal) cells: one layer of cuboidal cells • the cells in intermediate regions: several layers of polygonal –shaped cells • to the surface: more and more flattened cells

  18. ---distributon: • non-keratinised: mouth, pharynx, oesophagus, urethra and vagina • keratinised: the surface of body, make up the skin karatinised non-karatinised

  19. 6)transitional epithelium: • Flexible qualities ---ncluding the number of layers and shape of cells

  20. ---distribution: bladder

  21. in the contractedbladder in the distendedbladder

  22. 4. Epithelial specializations • Sides of cells: • --- Apical (free) • --- Basal • --- Lateral

  23. 1)Specializations of free surface

  24. ①microvilli: ---definition: delicate finger-liked projections of cell-membrane and cytoplasm protruding from the free surface

  25. ---structure: 0.1um in diameter, with different longth. surface: cell membrane with cell coat core: longitudinal microfilament-actin filament fixed on terminal web terminal web: made up of transverse-arranged filament at the apical side of cells

  26. ---function: increase the surface areas, thus aid in the processes of secretion and absorption. ---distribution: /striated border: intestinal epi. cell /brush border: proximal renal tubule Striated border

  27. ②cell coat: ---definition: a thick layer of extracellular glycoprotein ---function: adherence, supporting, protection, exchange of material and recognize

  28. ③     cilia: ---definition: elongated, mobile projections of cell membrane and cytoplasm protruding from free surface

  29. ---structure: • 5-10um long, 300-500nm in diameter • surface: cell membrane • core: microtubules, 9X2+2 • basal body: centrioles-connected with microtubules of cilia

  30. ---function: beat in a rhythmical manner and produce a forward-moving wave ---distribution: epithelial cells of respiratory tract respiratory tract

  31. 2)specializations of the lateral surface

  32. ---intercellular connection of adjacent cells: • non-special manner: the minute space and adherent molecules (glycoproteins, proteoglycans, cadherin) • Special manner: junctional structures

  33. ①Tight junction (zonula occludens): ---structure: • apical part • point-liked fused between adjacent cells • arranged in 2-4 thread-liked structures • form anastomosing network ---function: seal the space between cells

  34. ② intermediate junction (zonula adherens): ---structure: • below the tight junction • a gap of 15-20nm in width with medium electron-density filament material • plaque of electron-dense materials, with attached microfilament-make up of terminal web ---function: /adherens /keep the cell shape /transfer cell contract force

  35. ③desmosome (macula adherens): ---structure: • plate or spot-shaped • a gap of 20-30 nm, with low electron-density filaments interdigitate • attachment plaque • Many tonofilaments are inserted into attachment plaque, each filament make a hairpin loop and then passes back into the cytoplasm • ---function: firmly connection

  36. ④gap junction (communicating junction): ---structure: • the smallest gap of 2-3 nm • connexons: -consist of protein -7~9nm in diameter -composed of 6-subunits of proteins- connexin -2nm channel: electron-lucid central channel ---function: provide a ions and small molecules pathway between cells

  37. connexons

  38. junctional complex: at least two types of junctional structures get together.

  39. 3)specialization of basal surface

  40. ①basement membrane: ---definition: a sheet of membrane-liked amorphous material interposed between epithelium cells and underlying CT. ---structure: • HE: pink colour, hard to see • PAS +

  41. EM: two layers --basal lamina: 20-300 nm, electron-dense, thread-liked and amorphous ground substance, produced by epithelium Cell --reticular lamina: RT+ground substance, produced by CT

  42. ---function: • support, connection, fixation • semi-permeable membrane • induce the movement, proliferation and differentiation of epithelium cell

  43. ② plasma membrane infolding (basal longitudinal striation): ---definition: the infolding of cell-membrane with many mitochondria at the basal surface of epithelium cell

  44. ---function: • increase the basal surface areas • facilitate the passage of water and ions ---distribution: mainly in proximal and distal renal tubule.

  45. ③hemidesmosomes ---is half of desmosome.

  46. SUMMARY 1：

  47. SUMMARY 2：

  48. 5. Glandular epithelium and gland (study yourself ) • glandular epithelium: are specialized for secretion • gland: organs composed mainly of glandular epiithelium

  49. 1) Classification: exocrine gland: discharge the secretion through a duct system endocrine gland: release the secretion directly into blood steam

  50. Simple gland compound alveolar gland compound tubulo-alveolar gland 2) structure of exocrine gland:

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