An epithelium is
a sheet
of cells that covers a body surface or lines a body cavity
Epithelial tissue occurs in two different forms:
t Covering and lining epithelium covers the outer and inner
surfaces of most body organs. Examples include the outer
layer of the skin; the inner lining of all hollow viscera,
such as the stomach and respiratory tubes; the lining of the
peritoneal cavity; and the lining of all blood vessels.
t Glandular epithelium forms most of the body glands
Epithelial functions:
Protection of the underlying tissues
2. Secretion (release of molecules from cells)
3. Absorption (bringing small molecules into cells)
4. Diffusion (movement of molecules down their concentration
gradient)
5. Filtration (passage of small molecules through a sievelike
membrane)
6. Sensory reception
Special characteristics of epithelia
- cellularity
- specialized contacts
- polarity
- support by connective tissue
- Avascular but innervated
- regeneration
Cellularity.
Epithelia are composed almost entirely of cells.
These cells are separated by a minimal amount of extracellular
material, mainly projections of their integral membrane
proteins into the narrow spaces between the cells.