Epithelial Tissue

Epithelial tissue consists of cells that form membranes, which cover and line the body surfaces, and of glands, which are derived from these membranes. There are two categories of glands. Exocrine glands (exo = outside) secrete chemicals through a duct that leads to the outside of a membrane, and thus to the outside of a body surface. Endocrine glands (from the Greek endon = within) secrete chemicals called hormones into the blood.
Epithelial Membranes
Epithelial membranes are classified according to the number of their layers and the shape of the cells in the upper layer. Epithelial cells that are flattened in shape are squamous; thosethat are taller than they are wide are columnar; and those that are as wide as they are tall are cuboidal. Those epithelial membranes that are only one cell layer thick are known as simple membranes; those that are composed of a number of layers are stratified membranes.

Different types of simple epithelial membranes. (a) Simple squamous, (b) simple cuboidal, and (c) simple columnar epithelial membranes. The tissue beneath each membrane is connective tissue.

Epithelial membranes cover all body surfaces and line the cavity (lumen) of every hollow organ. Thus, epithelial membranes provide a barrier between the external environment and the internal environment of the body. Stratified epithelial membranes are specialized to provide protection. Simple epithelial membranes, in contrast, provide little protection; instead, they are specialized for transport of substances between the internal and external environments. In order for a substance to get into the body, it must pass through an epithelial membrane, and simple epithelia are specialized for this function For example, a simple squamous epithelium in the lungs allows the rapid passage of oxygen and carbon dioxide between the air (external environment) and blood (internal environment). A simple columnar epithelium in the small intestine, as another example, allows digestion products to pass from the intestinal lumen (external environment) to the blood (internal environment).
Dispersed among the columnar epithelial cells are specialized unicellular glands called goblet cells that secrete mucus. The columnar epithelial cells in the uterine (fallopian) tubes of females and in the respiratory passages contain numerous ciliathat can move in a coordinated fashion and aid the functions of these organs.

The epithelial lining of the esophagus and vagina that provides protection for these organs is a stratified squamous epithelium. This is a nonkeratinized membrane, and all layers consist of living cells. The epidermis of the skin, by contrast, is keratinized, or cornified. Since the epidermis is dry and exposed to the potentially desiccating effects of the air, the surface is covered with dead cells that are filled with a water-resistant protein known as keratin. This protective layer is constantly flaked off from the surface of the skin and therefore must be constantly replaced by the division of cells in the deeper layers of the epidermis.

A stratified squamous nonkeratinized epithelial membrane. This is a photomicrograph (a) and illustration (b) of the epithelial lining of the vagina.

The epidermis is a stratified, squamous keratinized epithelium. Notice the loose connective tissue dermis beneath the cornified epidermis. Loose connective tissue contains scattered collagen fibers in a matrix of protein-rich fluid. The intercellular spaces also contain cells and blood vessels.

The constant loss and renewal of cells is characteristic of epithelial membranes. The entire epidermis is completely replaced every 2 weeks; the stomach lining is renewed every 2 to 3 days. Examination of the cells that are lost, or “exfoliated,” from the outer layer of epithelium lining the female reproductive tract is a common procedure in gynecology (as in the Pap smear).
In order to form a strong membrane that is effective as a barrier at the body surfaces, epithelial cells are very closely packed and are joined together by structures collectively called junctional complexes. There is no room for blood vessels between adjacent epithelial cells. The epithelium must therefore receive nourishment from the tissue beneath, which has large intercellular spaces that can accommodate blood vessels and nerves. This underlying tissue is called connective tissue. Epithelial membranes are attached to the underlying connective tissue by a layer of proteins and polysaccharides known as the basement membrane. This layer can be observed only under the microscope using specialized staining techniques.