EPITHELIAL TISSUE



Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity


Epithelium functions include:

Protection

Absorption

Filtration

Excretion

Secretion

Sensory Reception


Special Physical Characteristics


1. Cellularity

Cellularity means closely packed together cells.





2. Specialized Contacts



Epithelium has specialized proteins embedded in the plasma membrane that help bind cells together and enhance intercellular communication.



Desmosomes are like individual buttons, rivets, or snaps that hold cells together in small areas.


Tight junctions are more like zippers, or strips of velcro that hold cells together.


Gap junctions, when they align, form channels that allow for the passage of small molecules from one cell to another.





3. Polarity





4. Epithelium is Avascular, but Innervated

Epithelial tissue does not have its own blood supply. It is therefore never found in isolation. Epithelial tissue is always bound to connective tissue.




5. Basement Membrane





The basement membrane is a chemical matrix that firmly binds the epithelial tissue to the underlying connective tissue. It also plays a role in regulating the diffusion of substances between the tissues. The basement membrane includes:


Basal Lamina

The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits.


Reticular Lamina

Reticular lamina is a thin extracellular layer that is below the basal lamina, is composed chiefly of collagenous fibers, and serves to anchor the basal lamina to underlying connective tissue.



6. Highly Mitotic





Given the protective function of epithelial tissue, its cells are highly mitotic.