Covering and lining epithelia

Covering epithelia may be classified according to the arrangement and shape of their cells.

Simple epithelium is composed of a single layer of cells attached to a basement membrane; it is very delicate and is found where there is little wear and tear (Fig. 3.1).

Fig. 3.1 Types of simple epithelium.

Ciliated columnar epithelium has microscopic hair-like processes projecting from the free surface of the cell. The cilia move together with a wave motion, and mucus and other particles are moved along. This type of tissue is found in the respiratory tract.

Goblet cells are cells that secrete mucim, which dissolves in water to become mucus.

A brush border is found on cells specialized for absorption. These have minute finger-like projections that increase the area over which absorption can occur. They are found in the small intestine.

Stratified epithelium is made up of many layers of cells (Fig. 3.2). The deepest cells, called the germinal layer, lie on the basement membrane and are columnar. As they divide, and this occurs frequently, the parent cells are pushed nearer the surface and become flattened. The cells on the surface are rubbed off and are continually replaced from below. If the surface of the epithelium is dry, as on the skin, the surface cells die because the blood supply is below the basement membrane, and the scaly surface which develops, keratin, constitutes a waterproof layer. If the surface is moist, as in the mouth, the cells survive until they are rubbed off, and so keratin is not formed.

Fig. 3.2 Stratified epithelium.

Transitional epithelium is like stratified epithelium but the surface cells, instead of being flattened, are rounded and can spread out when the organ expands. Transitional epithelium is found lining organs which must expand and must be waterproof, e.g. the bladder.

Glands

Glands develop from epithelial tissues and have the ability to manufacture substances from materials brought by the blood. These substances are called the secretions of the gland. For example, blood contains sodium chloride; from this the gastric glands can make the hydrochloric acid found in the gastric juice, although to obtain hydrochloric acid from sodium chloride in the laboratory is a difficult process. Glands are well supplied with blood vessels, which supply the cells with the materials necessary for making secretions. Glands are of two types: exocrine and endocrine.

Exocrine glands

Exocrine glands (Fig. 3.3) pour their external secretions out through a duct. The secretions of many of these glands contain enzymes, which are chemical substances produced by the cells of the gland. These enzymes produce chemical changes when they come into contact with specific substances but do not themselves enter into the reaction.

Fig. 3.3 Types of exocrine gland.

Simple glands

Simple glands have one duct leading from a single secretory unit:

• Simple tubular glands are found in the walls of the small intestine and the stomach.

• Simple coiled glands pour sweat on to the surface of the skin.

• Simple saccular glands, known as sebaceous glands, secrete a substance called sebum, which lubricates hair and skin.

Compound glands

Compound glands have several secretory units pouring their secretions into a number of small ducts which unite to form a larger duct:

• Compound tubular glands are found in the duodenum.

• Compound saccular glands are also called racemose glands; examples are the salivary glands in the mouth.

Loose connective tissue

This type of tissue, also called areolar tissue, consists of a loose network of both collagenous and elastic fibres with small scattered groups of fat cells and some fibroblasts (Fig. 3.4). Some blood vessels and nerves are found in the tissue but these are not very numerous. Areolar tissue forms a transparent skin, as thin as tissue paper, but very tough, and it is found between and around the organs of the body.

Fig. 3.4 Loose connective tissue.