Page 28 - Demo 1

Epithelial Tissue

Epithelial ssue, or epithelium, covers the enre surface of the body, lines
body cavies, and forms the internal linings of many organs. Before any
substance can enter or exit the body, for example, in the digesve tract or the
lungs, it must cross an epithelial ssue. A characterisc of all epithelia is that
sheets of cells are ghtly bound together with very lile space between them to
form a barrier. Layers of epithelia are usually only one or a few cells thick.
Epithelial ssues have the ability to regenerate, such that the cells of the
epithelial layers are constantly being replaced throughout the life of the
organism. The epithelial lining of the digesve tract, for example, are constantly
replaced every few days. The epidermis of the skin is renewed every two
weeks. The liver, a gland formed from epithelial ssue, is able to regenerate
parts of itself that have been surgically removed.

Based on the arrangement of its layers, epithelial ssue may be classified
as simple, pseudostrafied, or strafied. Simple epithelium is only a single cell
layer thick. This includes the membranes that line the lungs and the major
cavies of the body. Pseudostratified epithelium appears like mulple layers due
to the varying posions of the nucleus in the cells but actually has only one.
Pseudostrafied epithelium lines part of the respiratory tract. Stratified
epithelium is made up of more complex epithelial cells which are several layers
thick. This type of epithelium is found on the skin, or epidermis. Several layers
are necessary to provide sufficient cushioning and protecon, and to promote
skin cell regeneraon.

Simple Pseudostrafied Strafied

Squamous Cuboidal Columnar

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