Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

General Part
Structure of the Human Body
Tissues - Epithelial Tissue

Like Connective Tissue, Epithelial Tissue is among the phylogenetically oldest Tissues. It covers the body surfaces bordering the external environment (Skin, mucous membranes) and also forms a component of Serous Membranes and most glands.

All types of epithelium share several common structural features, namely:

1. Arrangement as continuous sheets or cords in which epithelial Cells are in close contact with one another.

2. Attachment to the underlying connective tissue, from which the epithelium is separated by a specialized laminar Structure known as the basement membrane.

3. Absence of Blood Vessels. Oxygen and nutrients diffuse from the capillaries of the connective tissue across the basement membrane, while Metabolic waste products from the epithelial cells are removed in the opposite direction.

4. Cellular polarity, which is reflected in the structural differences between the lower (basal) and upper (apical) poles. The Nucleus, Endoplasmic reticulum, and most Cell/35.html">Mitochondria are typically localized in the basal region of the epithelial cells, whereas other Organelles are found in the apical region.

5. Structural variation among cells within a tissue layer (anisomorphism). Stratified epithelia exhibit vertical anisomorphism (from the deep layers to the superficial ones), whereas simple epithelia display horizontal anisomorphism (within the plane of the epithelial sheet).

Epithelial Tissues constitute renewing cell populations with varying rates of turnover, as they contain cambial cells (undifferentiated cells capable of proliferation). Consequently, many epithelia exhibit a high capacity for reparative regeneration.