Types of epithelia

By

Dr. mohamed sabaa ch.

M.BCH.B M.Sc.Pathology

- Epithelia can be divided into 2 main groups according to their structure and function:

1- covering(or lining).

2- glandular .

Covering or lining epithelia: 1- the cells are organized in layers that cover the external surface or line the cavities of the body.

2- they are classified according to the ……………..and ……………..of the cells in surface layer.

3- they classified according to no. of cells layers into :

A- Simple (only one layer).

B- Stratified(2 or more layer).

4- Based on the shape they classified into :

A-Cuboidal.

B- Squamous.

C- Columnar.

D- Transitional.

Simple squamous epithelia

1-cells of single layer are flat and usually very thin, with only the thicker cell nucleus appearing as a bulge to denote the cell.

2- specialized as lining of vessels and cavities .

3- also regulate substances which can enter underlying tissue from the vessels or cavity.

4- often exhibit transcytosis.

5- example : mesothelium , endothelium lining the inner surface of cornea.

1- the cells vary in their height but are roughly as tall as they are wide.

2- their greater thickness often includes cytoplasm rich in mitochondria. What is the benefit.

3- example : renal collecting tubule, pancreatic duct.

Simple cuboidal epithelium

1- the cells are taller than they are wide .

2- such cells are usually highly specialized for absorption, with microvilli .

3- they always have tight and adherent junctional complexes at their apical ends , but are often loosely associated in more basolateral areas.

4- this allow for rapid transfer of absorbed material.

5- the additional cytoplasm in columnar cells allows additional mitochondria and other organelles needed for absorption .

Simple columnar epithelium

Stratified squamous epithelium

1- the very thin surface cells of stratified squamous epithelia can be :

A- Keratinized

(rich in keratin intermediate filaments).

B- Nonkeratinized .

A- The stratified squamous keratinized epithelium:

1- found mainly in the epidermis of skin.

2- its cells form many layers.

3- the cells closer to the underlying connective tissue are cuboidal or low columnar .

4- the cells become irregular in in shape and flatten ? Why?

5- this surface layer of cells helps protect against water loss across this epithelium , also protect against easy invasion by microorganisms.

B- The stratified squamous non keratinized epithelium:

1- lines wet cavities

(………… , …………..)

2- the surface layer are living cells containing much less keratin and retaining their nuclei.

Stratified cuboidal and stratified columnar :

1- rare.

2- stratified columnar can be found in the conjunctiva lining the eyelids , where it is both protective and mucus secreting .

3- stratified cuboidal epithelium is restricted to large excretory ducts of sweat and salivary glands.

Transitional Epithelium or Urothelium

1- lines only urinary bladder , ureter ,and upper part of urethra.

2- k.k by superficial layer of domelike cells that are neither squamous nor columnar, they also called umbrella cells.

They are protective against hypertonic and cytotoxic effects of urine.

Transitional epithelium

Psedostratified columnar epithelium

Glandular Epithelia 1- formed by cells specialized to secrete.

2- the molecules to be secreted are generally stored in the cells in small membrane bound vesicles called secretory granules.

3- they may synthesize , store , and secrete

-proteins(eg , in the pancreas)

-lipids(eg , adrenal , sebaceous gland)

-complexes of carbohydrate and proteins(salivary gland).

- The ……….......… gland secrete all of them

- Some have low synthetic activity like……………….

4- we have :

-unicellular glands which consist of large isolated secretory cells .(goblet cell )

- multicellular glands which have clusters of cell.

Gland formation

A- Exocrine glands:

1- retain their connection with the surface epithelium.

2- the connection taking the form of tubular ducts lined with epithelial cells through which the secretions pass to the surface.

3- they have secretory portion which contain

the cells specialized for secretion , and ducts transport the secretion out of gland.

Exocrine glands classification

Structural classes

1- the ducts can be simple (unbranched) or compound(with 2 or more branches).

2- secretory portion can be tubular(either short or long and coiled) or acinar (round or globular).

3- either type of secretory portion may be branched.

Functional classification

Glands: Merocrine 1- they secrete products , usually containing proteins ,by means of exocytosis at the apical end of the secretory cells. 2- most exocrine glands are merocrine. 3- they are further classified according to the nature of the protein or glycoproteins secreted and the resulting staining properties of the secretory cells into : a- serous (example , acinars cells of pancreas and parotid salivary gland). b- mucous (goblet cells)

:glands Holocrine

1- their secretion produced by the disintegration of the secretory cells themselves as they complete differentiation which involve becoming filled with product. 2- sebaceous glands of hair follicles are the best example of holocrine glands.

Apocrine glands -the secretory product is typically a large lipid droplet and is discharged together with some of the apical cytoplasm .

Endocrine glands

1- they lost their connection to the surface from which they originated during development. 2- they are producers of hormones. 3- hormones diffuse into blood and bind specific receptors in the body. 4- define (paracrine , autocrine)