114. TYPES OF TISSUE
a. Tissues. A tissue is composed of a group of cells that are the same or
similar in nature. The various tissues of the body have different characteristics because
the cells that make up these tissues are different both in structure and function. There
are four primary types of tissues: epithelial, connective, muscular, and nervous.
b. Epithelial (figure 17). Epithelial tissue covers the outer surface of the body
and forms the lining of the intestinal and respiratory systems. A special form called
endothelium lines the heart and blood vessels. As serous membranes, it lines the
cavities of the abdomen, the chest, and the heart, and covers the organs that lie in
these cavities. Epithelial tissue forms the glands and parts of the sense organs.
According to its location, this tissue has different functions. As the skin, it protects
underlying structures; in the small intestine, it absorbs; in the lungs, it is a highly
permeable membrane; in glands, it secretes; and in the kidneys and liver, it both
secretes and excretes. There are three types of epithelial tissue based on the shape of
the cells. These are squamous (flat), cuboidal, and columnar. These cells are further
designated as simple if they are arranged in a single layer, or stratified if arranged in
Figure 17. Epithelial tissue.
c. Connective. Connective tissue (figure. 18) is very widely distributed
throughout the body. It binds other tissues together and supports them, forms the
framework of the body, and repairs other tissues by replacing dead cells. Principal
types of connective tissue are osseous (bony), cartilaginous, fibrous, elastic, and
adipose (fatty). Areolar tissue, which lies under the skin and serves to fill many of the
sharp corners and small spaces of the body, is a mixed type composed of fibrous,
elastic, and fatty connective tissue.