A tissue is composed of specialized cells of one type that perform a common function in the body. There are four major types of tissues: (1) Epithelial tissue, also called epithelium, covers body surfaces and organs and lines body cavities; (2) connective tissue binds and supports body parts; (3) muscular tissue contracts; and (4) nervous tissue responds to stimuli and transmits impulses from one body part to another (Table 4.1). In epithelial tissue, the cells are tightly packed, with little space between them. Externally, this tissue protects the body from drying out, injury, and bacterial invasion. On internal surfaces, epithelial tissue protects, but it also may have an additional function. For example, in the respiratory tract, epithelial tissue sweeps up impurities by means of cilia. Along the digestive tract, it secretes mucus, which protects the lining from digestive enzymes. In kidney tubules, its absorptive function is enhanced by the presence of fine, cellular extensions called microvilli.
Epithelial cells readily divide to produce new cells that replace lost or damaged ones. Skin cells as well as those that line the stomach and intestines are continually being replaced. Surprisingly, then, epithelial tissue lacks blood vessels and must get its nutrients from underlying connective tissues. Because epithelial tissue covers surfaces and lines cavities, it always has a free surface. The other surface is attached to underlying tissue by a layer of carbohydrates and proteins called the basement membrane. Epithelial tissues are classified according to the shape of the cells and the number of cell layers. Simple epithelial tissue is composed of a single layer, and stratified epithelial tissue is composed of two or more layers. Squamous epithelium has flattened cells; cuboidal epithelium has cube-shaped cells; and columnar epithelium has elongated cells.
Simple squamous epithelium is composed of a single layer of flattened cells, and therefore its protective function is not as significant as that of other epithelial tissues (Fig. 4.1). It is found in areas where secretion, absorption, and filtration occur. For example, simple squamous epithelium lines the lungs where oxygen and carbon dioxide are exchanged, and it lines the walls of capillaries, where nutrients and wastes are exchanged. Stratified squamous epithelium has many cell layers and does play a protective role. While the deeper cells may be cuboidal or columnar, the outer layer is composed of squamous-shaped cells. The outer portion of skin is stratified squamous epithelium. New cells produced in a basal layer become reinforced by keratin, a protein that provides strength, as they move toward the skin’s surface. Aside from skin, stratified squamous epithelium is found lining the various orifices of the body.
Figure 4.1 Simple squamous epithelium. The thin and flat cells are tightly joined. The nuclei tend to be broad and thin.
Simple cuboidal epithelium (Fig. 4.2) consists of a single layer of cube-shaped cells attached to a basement membrane. This type of epithelium is frequently found in glands, such as salivary glands, the thyroid gland, and the pancreas, where its function is secretion. Simple cuboidal epithelium also covers the ovaries and lines most of the kidney tubules. In one part of the kidney tubule, it absorbs substances from the tubule, and in another part it secretes substances into the tubule. When the cells function in secretion, microvilli (tiny extensions from the cells) increase the surface area of cells. Also, the cuboidal epithelial cells contain many mitochondria, which supply the ATP needed for active transport. Stratified cuboidal epithelium is mostly found lining the larger ducts of certain glands, such as the mammary glands and the salivary glands. Often this tissue has only two layers.
Figure 4.2 Simple cuboidal epithelium. The cells are cube-shaped. Spherical nuclei tend to be centrally located.
Simple columnar epithelium (Fig. 4.3) has cells that are longer than they are wide. They are modified to perform particular functions. Some of these cells are goblet cells that secrete mucus onto the free surface of the epithelium. This tissue is well known for lining digestive organs, including the small intestine, where microvilli expand the surface area and aid in absorbing the products of digestion. Simple columnar epithelium also lines the uterine tubes. Here, many cilia project from the cells and propel the egg toward the uterus, or womb. Stratified columnar epithelium is not very common but does exist in parts of the pharynx and the male urethra.
Pseudostratified Columnar Epithelium
Pseudostratified columnar epithelium is so named because it appears to be layered; however, true layers do not exist because each cell touches the basement membrane. In particular, the irregular placement of the nuclei in comparison to columnar epithelium makes the tissue seem stratified. Pseudostratified ciliated columnar epithelium (Fig. 4.4) lines parts of the reproductive tract as well as the air passages of the respiratory system, including the nasal cavities and the trachea (windpipe) and its branches. Mucus-secreting goblet cells are scattered among the ciliated epithelial cells. A surface covering of mucus traps foreign particles, and upward ciliary motion carries the mucus to the back of the throat, where it may be either swallowed or expectorated.
The term transitional epithelium implies changeability, and this tissue changes in response to tension. It forms the lining of the urinary bladder, the ureters, and part of the urethra-organs that may need to stretch. When the walls of the bladder are relaxed, the transitional epithelium consists of several layers of cuboidal cells. When the bladder is distended with urine, the epithelium stretches, and the outer cells take on a squamous appearance. It’s interesting to observe that the cells in transitional epithelium of the bladder are physically able to slide in relation to one another while at the same time forming a barrier that prevents any part of urine from diffusing into the internal environment.
Figure 4.3 Simple columnar epithelium. The cells are longer than they are wide. The nuclei are in the lower half of the cells.
Figure 4.4 Pseudostratified ciliated columnar epithelium. The cells have cilia and appear to be stratified, but each actually touches the basement membrane.