Different forms of epithelium
The free surface of the body is covered by epithelium or tissue. The same tissue also forms the coating of the different body cavities.
Epithelial cells close together with very little intercellular substance so that molecules can not or hardly diffuse between the epithelial cells. Yet, molecules can pass the epithelium, namely via active transport through the epithelial cells, which can absorb molecules on one side and secrete them on the other side. For example, such resorbent epithelium is found in the intestinal wall and kidney.
The coating epithelial cells can be arranged in one or more layers. The cells may have different shapes, ranging from very thin and flattened to high-cylindrical. We therefore speak of a single flat, cubic or cylindrical epithelium. The cells of multilayer epithelia are almost always flattened in the outer layers. Occasionally, multilayer epithelium consists entirely of living cells, as in the cornea, sometimes the cells of the outer layers are dead and consist entirely of celiac material keratin. This is the case with the epithelium that covers the skin, multilayered squamous epithelium.
The major epithelial cell variants are squamous epithelium, cubic epithelium, cylindrical epithelium, transition epithelium and vibratory epithelium.
A special variant of the epithelium is formed by the glandular tissues. Although glandular tissue usually also forms the coating of a body cavity, that of the gland drainage gland, the coating function has become subordinate to the production and excretion of all kinds of substances. Glandular tissue is found in salivary glands, in the pancreas and in the pancreas. Also, the liver, which consists largely of epithelial tissue, is considered to be a complexly constructed gland. Epithelium does not contain blood vessels. Erection and discharge of substances for the epithelial cells occurs via the capillaries in the underlying connective tissue by diffusion.
Sometimes we also find groups of gland cells that are not bound to a drainage tube, but loose in the connective tissue. This is called endocrine glands. The discharge of their secretion products or hormones passes through the blood. Literatuurverwijzingen Also see
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