Page 123 - Atlas of Histology with Functional Correlations

SECTION 1 Classification of Epithelial Tissue

LOCATION OF EPITHELIUM

The four basic tissue types in the body are the epithelial, connective, muscular,
and nervous tissue. These tissues exist and function in close association with one

another.

The epithelial tissue, or epithelium, consists of sheets of cells that cover the
external surfaces of the body, line the internal cavities and the organs, form
various organs and glands, and line their ducts. Epithelial cells are in contact

with each other, either in a single cells layer or in multiple cell layers. The
morphology of any epithelium, however, differs from organ to organ, depending
on its location and its function. For example, epithelium that covers the outer
surfaces of the body and serves as a protective layer differs from the epithelium
that lines the internal organs or their ducts. In certain organs, epithelial lining has

a specific name. As an example, the epithelium that lines the interior of all the
blood and lymph vessels is called endothelium. Epithelium that lines the
abdominal, pericardial, and pleural (lung) cavities is called mesothelium. Both

endothelium and mesothelium in most cases exhibit thin or simple squamous
epithelium.

Epithelium is avascular in most areas of the body; it does not have a direct
blood supply except in the inner ear. Here, an area called stria vascularis
exhibits a rich capillary network and is, therefore, vascular epithelium in

contrast to other nonvascular epithelial lining. In areas where epithelium does
not receive direct blood supply, oxygen, nutrients, and metabolites diffuse into
the epithelial linings from the blood capillaries located in the underlying
connective tissue. In contrast to the other basic tissues, epithelial cells exhibit a

high mitotic rate with continuous cell renewal and replacement of the worn-out
cells.

Figure 4.1 shows different types of epithelia in selected organs.

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