Animal Tissues

It is traditional to divide all animal tissues into four categories: epithe¬lium, connective tissue, muscle, and nerve, when they are being studied under compound binocular microscopes. Each of these, particularly the second, is a diverse assemblage containing numerous subtypes. It should be emphasized that the subtype classification is based primar¬ily on vertebrate animals, especially human beings, and that its appli¬cation to other animals, particularly the lower invertebrates.

EPITHELIUM
Epithelial tissue forms the covering or lining of all free body surfaces, both external and internal. The outer portion of the skin, for example; is epithelium, as are the linings of the digestive tract, the lungs, the blood vessels, the various ducts, and the body cavity. Epithelial cells are packed tightly together, with only a small amount of cementing material between them and almost no intercellular spaces, as seen under compound binocular microscopes. Thus they provide a continuous barrier protecting the underlying cells from the external medium. Because anything entering or leaving the body must cross at least one layer of epithelium, the permeability characteristics of the cells of the various epithelia play an exceedingly important role in regulating the exchange of materials between different parts of the body and between the body and the external environment.

Since one surface of an epithelium is generally exposed to air or fluid and the opposite surface rests upon other cell layers, and since the epithelium plays, a crucial part in the directional passage of mate¬rials, it is no surprise that epithelial cells should show significant differences between their free ends and their attached ends when they are being examined under compound binocular microscopes. Often highly specialized, the free ends commonly bear cilia, hairs, or short fingerlike processes.

It is customary to group epithelial cells into three categories: squa¬mous, cuboidal, and columnar. Squamous cells are much broader than they are thick and have thee appearance of thin, flat plates. When viewed under compound binocular microscopes, cuboidal cells are roughly as thick as they are wide and, as their name implies, have a rather square or cuboidal shape when viewed in a section perpendicular to the tissue surface; in surface view, however, they look like polygons, often with six sides. Columnar cells are much thicker than they are wide and in vertical section look like rectangles set on end when viewed under compound binocular microscopes.

Epithelial tissue may be only one cell thick, as studied under compound binocular microscopes, in which case it is called simple epithelium, or it may be two or more cells thick and is then known as stratified epithelium. There is, in addition, a third category, called pseudostratified epithelium, in which the tissue looks stratified, but actually is not; whereas in true stratified epithelium only the cells in the lowest layer are in contact with the underlying membrane, in pseudostratified epithelium all the cells are in contact with it. The various types of epithelia are named on the basis of cell type and number of cell layers as they are seen under a microscopes; it is the cells in the outermost layer of stratified epithelium that determine whether it will be called squa¬mous, cuboidal, or columnar. Regardless of type, epithelium is usually separated from the underlying tissue by an extracellular fibrous base¬ment membrane.

Epithelial cells often become specialized as gland cells, secreting substances at the epithelial surface. Sometimes a portion of the epithelial tissue folds inward, and a multicellular gland is formed.