1. Definition
An epithelium is a layer of cells that lines the surfaces and cavities of organs and structures throughout the body, providing protection, secretion, absorption, and filtration functions. Epithel is one of the four basic tissue types.
2. General characteristics
Epithelial cells are tightly attached to each other, with minimal extracellular space between them, measuring approximately 20 nm. Each epithelial cell is situated on a basement membrane, which provides structural support and separates it from the underlying connective tissue.
Epithelial tissue is avascular, meaning it lacks blood vessels. Consequently, nutrients reach the cells solely through diffusion from the underlying connective tissue, which limits the thickness of the epithelium.
3. Polarization
Epithelial cells are frequently polarized, meaning they have distinct basal and apical surfaces with specialized structures and functions on each side.
3.1.1. Apical: Towards the lumen or outside environment.
The most important apical surface projections are:
- no, or just a few microvilli
- brush border (microvillus)
- stereocilium
- kinocilium
3.1.2. Basal: Towards the basement membrane and underlying connective tissue.
4. Cell contacts
Cell connections serve a variety of functions, including the formation of belts around cells, facilitation of transport, and enabling communication. The most important cell membrane connections are as follows:
- Tight junction (zonula occludens)
- Zonula adherens
- Desmosome (macula adherens)
- Gap junctions
5. Types
- Surface epithelium
- Glandular epithelium
- Myoepithelium
- Sensory epithelium
- Pigment epithelium
- Reticular epithelium
6. Embryolgy
All epithelia can be derived from the three germ layers. The mesoderm gives rise to both the mesothelium (which lines body cavities) and the endothelium (which lines blood vessels and lymphatic vessels).