1. Dep. Of Histology & Embryology
Epithelial tissue
Dep. Of Histology & Embryology

2. Emphases of today’s lecture
1. Classification of epithelial tissue
2. Membrane specializations of
epithelial cells

3. What is epithelium?
Conception: Epithelium is a tissue composed of closely aggregated polyhedral cells and very little extracellular matrix.
Distribution: Cells cover body surfaces, line body cavities, and constitute glands.

4. Functions of Epithelium
Protection- main function of covering E.
Secretion- main function of glandular E.
Absorption- intestinal, distal renal tubule,
alveolar E.
Excretion- urine, sweat and co2
Receptor function

5. Characteristics of Epithelium
1. They are closely apposed adhere to one another by means of specific cell to cell adhesion molecules that form specialized cell junctions.

6. 2. They exhibit functional as well morphologic polarity.
Free surface (Apical domain)
Basal surface (Basal domain)
Lateral surface (Lateral domain)

7. 3. Epithelium is avascular, their basal surface is attached to an underlying basement membrane. nourished by diffusion.

8. Classification of Epithelium
1. Covering epithelium
2. Glandular epithelium
3. Sensory epithelium
4. Germinal epithelium
5. myoepithelium

9. Covering epithelium
Covering epithelia appears as members whose boundaries at free and luminal surfaces face either air or fluid. cover the ext. and int. surface of body serve as barrier membranes.

10. Classification of Covering Epithelium
The traditional classification of epithelium is descriptive and based on two factors:
Simple- single layer
Stratified- multi layer
1. Number of cell layers
Squamous- flat
2. Shape of the component cells
Cuboidal- square
Columnar- rectangular

12. Simple epithelium Simple squamous E Simple cuboidal E
Simple columnar E
Pseudostratified ciliated columnar E
Endothelium
Mesothelium

13. Simple squamous epitheliumB C

14. Simple squamous epithelium
Typical location:
Endothelium: epithelium lining the inner surface of
vascular system, heart, blood vessel, lymphatic
Mesothelium: epithelium lining body cavities, pericardial,
pleural, peritoneal
Major function:
Exchange, lubrication, barrier.

15. Simple cuboidal epithelium
Typical location:
exocrine glands, kidney tubules, pancreas, ducts of many
glands
Major function:
Absorption, secretion

16. Simple columnar epitheliumB A
Typical location:
Small intestine, colon, stomach, gallbladder
Major function:
Absorption, secretion

17. Pseudostratified columnar ciliated epithelium
Containing different types of cells:
columnar, goblet, fusiform and basal cells
Typical location:
Trachea and bronchial tree, ductus deferens
Major function:
Secretion, conduit

19. · Stratified epithelium
Stratified squamous epithelium
Transitional epithelium
Non-keratinised
Keratinised

20. Stratified squamous epithelium
2 types:
a) Non-keratinised (mucous type)
moist cavities or orifices , sufrace cell are
viable and contain nuclei but lack keratin.
b) Keratinised (cutaneous type)
entire exposed surface of body, cells of
surface layer contain keratin but no nucleus.

21. Stratified squamous epitheliumC A B
Typical location:
Epidermis, oral cavity, esophagus, vagina
Major function:
Barrier, protection

22. Keratinised

23. Transitional epithelium
A highly distensible stratified epithelium with large polyploid (nucleus, cell or organism that has more than two haploid sets of chromosomes ) superficial cells that are cuboidal in the relaxed state but broad and squamous in the distended state; occurs in the kidney, ureter, and bladder.

24. · Transitional epithelium
Superficial cells A B
Typical location:
Urinary tract (renal calyces, ureters, bladder, urethra)
Major function:
Barrier, distensible property

26. Summary 1 Epithelium: covering epithelium and glands
Classification of covering epithelium
simple squamous
stratified cuboidal
columnar

27. Specialization of Epithelium
1. Modification of free surface
Microvilli
Stereocilia
Cilia

28. Microvilli:
Microvilli are finger-like cytoplasmic projection on the free surface of most epithelial cells.1.4 μm long
Function: increase the surface areas
Distribution: striated border: intestinal epi. cell
brush border: proximal renal tubule
Goblet cell A B

31. Stereocilia
Long, nonmotile process of cells of epididymis and ductus deferens

32. Cilia
Cilia are mobile projections of cell membrane protruding from free surface μm long
Function: moving fluid and particles along epithelial cells
Distribution: epithelial cells of respiratory tract and female reproductive system. A B

35. 2. Specialization in cell to cell adhesion
(lat. Surface)
Tight junction (zonula occludens)
Intermediate junction (zonula adherens)
Desmosome (macula adherens)
Gap junction (communication junction)

38. Tight junction (zonula occludens)
-Belt shape, expand around the apex,
-Higher magnification EM reveal that membranes are in contact only at this points.
-It also serve as barrier to prevent solutes passing out to in or vice versa.

39. Tight junction (zonula occludens)
Function: seal the cells

40. Intermediate junction (zonula adherens)
Just below the tight junction
Opposing membrane are separated by a gap of nm filled with glycoprotein
Electron dense material embedding numerous filaments radiating away
Thought to hold adjacent cells firmly together

41. Intermediate junction (zonula adherens)
Function:
adherens, keep the cell shape, transfer cell contract force

42. Desmosome (macula adherens)
plate shape below intermediate junction.
Opposed cell membrane diverge slightly with a space nm
On the inner surface of cell membrane adjacent to desmosomes, there is punctate electron dense material.

43. · Desmosome (macula adherens)
Function: firmly connection, spot-weld type of cell-to-cell attachment

44. Gap junction (communication junction)
Deep part of lat. Cell surface
Not visible under LM
Outer leaflets comes as 2-3 nm
Bridging structure termed connexons
Provide a low resistance pathways between cells.
Transfer of impulses, ions and small molecules.

45. Gap junction (communication junction)
Function: allow communication between cells

46. 3. Specialization in cell to extracellular matrix adhension
(basal surface)
Basement membrane:
-between E. & CT
-all type of E rest on it
Best shown with silver technique and PAS (+ ve)
nm thick

47. Composed of 2 layers Basal lamina - produced by epithelial cells
Reticular lamina – composed of reticular
fibers

49. · Hemidesmosomes Function:
bind the epithelial cells to the subjacent basal lamina

50. Plasma membrane infolding
This structure is well developed in basal surface of epithelial cells in proximal and distal renal tubules.
Function:
increase the basal surface areas facilitate the passage of water and ions from lumen to surroundings capillaries.

51. Summary 2
Specializations of apical domain
lateral domain
basal domain

52. Glandular epithelium
are cells or aggregations of cells whose function is secretion.
Exocrine glands release the secretory product via a system of ducts that opens upon one of the surfaces of the body which are in contact with the external world (skin, gastrointestinal tract etc.).
Endocrine glands release their secretory product (typically hormones) into the spaces between the secretory cells (extracellular space) from which it enters the bloodstream.

53. Formation of glands from covering epithelia
Formation of glands from covering epithelia. Epithelial cells proliferate and penetrate connective tissue. They may—or may not—maintain contact with the surface. When contact is maintained, exocrine glands are formed; without contact, endocrine glands are formed. The cells of endocrine glands can be arranged in cords or in follicles. The lumens of the follicles accumulate large quantities of secretions; cells of the cords store only small quantities of secretions in their cytoplasm.

54. On the basis of the no. of cells in them, exocrine glands can be divided into
Unicellular glands- single secretory cell interposed in the epithelium between cells which have other functions. E.g. goblet cell
2) multicellular glands- are composed of cell clusters. Most glands are multicellular.

55. Exocrine glands have secretory unit or portion (acinus), which contains cells for the secretion process and a duct system which carries the secretion out of the gland.

56. Classification of exocrine glands
A) Simple or compound
simple- connection with the surface is unbranched. E.g. sweat gland
2) compound- connection with the surface is branched. E.g. parotid gland

57. Principal types of exocrine glands
Principal types of exocrine glands. The part of the gland formed by secretory cells is shown in black; the remainder shows the ducts. The compound glands have branching ducts.

58. The secretory portion may have a variety of shapes
The secretory portion may have a variety of shapes. Secretory cells may form
# acini in acinar glands,
# tubes in tubular glands,
# alveoli in alveolar glands.

60. On the basis of secretion, classification of gland:
Serous acinus is composed of serous secretory cells. The acinus produces a serous (whey-like) secration, which is clear, watery fluid containing a proteinaceous component. glands containing only serous acini are termed serous glands.

61. Mucous acinus is composed of mucous secretory cells
Mucous acinus is composed of mucous secretory cells. A mucous acinus secretes the glycoprotin mucin ( mucin + water = mucus) glands containing only mucous acini are termed mucous glands.

62. Mixed acinus is composed of above two types of secretory cells
Mixed acinus is composed of above two types of secretory cells. Glands containing both serous and mucous acini or mixed acini are termed mixed glands.

63. Secretory Mechanisms
The secretory cells can release their secretory products by one of three mechanisms.
1) Merocrine secretion corresponds to the process of exocytosis. Vesicles open onto the surface of the cell, and the secretory product is discharged from the cell without any further loss of cell substance.

64. 2) Apocrine secretion a mechanism in which part of the apical cytoplasm of the cells is lost together with the secretory product. The continuity of the plasma membrane is restored by the fusion of the broken edges of the membrane, and the cell is able to accumulate the secretory product anew. This mechanism is used by apocrine sweat glands, the mammary glands and the prostate.

65. 3) Holocrine secretion designates the breakdown and discharge of the entire secretory cell. It is only seen in the sebaceous glands of the skin.

67. Today’s lecture
Classification of Epithelium: distribution and function
Membrane specialization of epithelial cell
Glandular epithelium

68. Questions What is epithelium? Functions of Epithelium
Characteristics of Epithelium
Classification of Epithelium
Specialization of Epithelium