Epithelia are layers of cells that line surfaces throughout the body. They are classified based on cell layer number (simple or stratified) and cell shape (squamous, cuboidal, columnar). Epithelia have several important functions, including transcellular transport, absorption, secretion, selective permeability, and protection from injury. They contain specialized structures on their surfaces like tight junctions, microvilli, and cilia that allow them to perform key roles in the body.

1. Histology of Epithelia
By Dr. Mohammed

2. Objectives
What to Learn?
 Definition
 Classification/Types of Epithelia and their locations in the body
 Functions of Epithelia
 Epithelial Surfaces and their functions

3. Epithelia : Overview
 Epithelia are specialized layers that line the internal and cover the external surfaces
of the body.
 An epithelium consists of a sheet of cells lying close together with little
intercellular space

4. Epithelia : Structure
 The basement membrane separates the epithelium from underlying connective
tissue and blood vessels
 Epithelia are avascular and receive nourishment by diffusion of molecules through
the basal lamina

5. Epithelia : Classification
 Epithelia are classified into various types on the basis of:
1. number of cell layers (one cell layer is simple; more than one is stratified)
2. shape of the superficial cells: Pseudostratified epithelia appear to have multiple
cell layers, but all cells are in contact with the basal lamina

6. Epithelia : Classification (according to
no. of cell layers)

7. Epithelia : Classification
(according to shape of cells)
G. Stratified squamous keratinized
B. Simple cuboidal
A. Simple squamous
D. Pseudostratified
columnar epithelium with
cilia
E. Transitional F. Stratified squamous non-
keratinized
C. Simple columnar

8. https://youtu.be/Bi4K_IIJl3w

9. Epithelia : Functions
 Transcellular transport of molecules from one epithelial surface to another occurs by various
processes, including the following:
• Diffusion of oxygen and carbon dioxide across the epithelial cells of lung alveoli and capillaries
• Carrier protein–mediated transport of amino acids and glucose across intestinal epithelia
• Vesicle-mediated transport of immunoglobulin A (IgA) and other molecules
 Absorption occurs via endocytosis or pinocytosis in various organs (e.g., the proximal
convoluted tubule of the kidney).
 Secretion of various molecules (e.g., hormones, mucus, proteins) occurs by exocytosis.
 Selective permeability results from the presence of tight junctions between epithelial cells
and permits fluids with different compositions and concentrations to exist on separate sides of an
epithelial layer (e.g., intestinal epithelium).
 Protection from abrasion and injury is provided by the epidermis, the epithelial layer of the
skin.

10. Epithelial Surfaces
 Epithelial cells have certain specialized structures on their surfaces which help the
epithelium to function
A. Lateral Epithelial surface
B. Basal Epithelial surface
C. Apical Epithelial surface

11. Epithelial Surfaces

12. Lateral Epithelial Surface
 The junctional complex is an intricate arrangement of membrane-associated
structures that functions in cell-to-cell attachment of columnar epithelial cells
 The gap junction (communicating junction) is not part of the junctional complex
and is common in certain tissues other than epithelia (e.g., central nervous system,
cardiac mus- cle, and smooth muscle)

13. Basal Epithelial Surface
A. The basal lamina is an extracellular supportive structure that is visible only by
electron microscopy.
• The basal lamina plus the underlying reticular lamina constitute the basement
membrane which is observable by light microscopy
B. Hemidesmosomes are specialized junctions that mediate adhesion of epithelial
cells to the underlying extracellular matrix.

14. Apical Epithelial Surface
A. Microvilli are fingerlike projections of epithelia approximately 1 µm long that
extend into a lumen and increase the cell’s surface area (e.g., brush border of
kidney proximal tubule cells and the striated border of intestinal absorptive cells
B. Stereocilia are very long microvilli (not cilia) in the epididymis and vas deferens of
the male reproductive tract
C. Cilia are actively motile processes extending from certain epithelia (e.g.,
tracheobronchial and oviduct epithelium) that propel substances along their
surfaces

16. THE END