Glands are special cellular structures in the body which are characterized by their secretions in the body of compounds that have very specific role in the body. Glands can secrete their content on one of two possible manners. These are: endocrine and exocrine secretions. Endocrine secretions refer to secretions by the glands which occur directly into the blood stream. Endocrine glands do not have ducts, but they secrete their content into the connective tissue and from there they enter the blood stream where they travel certain distance before binding with their receptors.
These receptors can be on the cellular membrane or inside the cell. The manner of binding will depend on the type of hormone that is secreted by the endocrine gland. Steroid hormones such as estrogens and testosterone are lipid soluble. Therefore, they bind with cellular receptors inside the cell. Polypeptide hormones on the other hand are not lipid soluble and cannot penetrate the phospholipid bilayer of the cellular membrane. As a result, they bind with receptors on the cellular membrane.
Hormones have other ways of making their effect on the cell other than travelling in the blood certain distance before binding to their receptors. One way of interaction of hormones with cellular receptors is when the hormone binds to cellular receptors next to its site of release. In this respect, when they act on cells that are different than the cells that produced them it is called paracrine action. This type of hormone action is manifested by the action of the hormones of the ovaries and angiotensin II in the kidney.
Another type of hormonal action is called autocrine action. This manner of cellular signaling occurs when the hormone acts on the cell that released it. The hormone can be released by the cell and then acts on it or it can act on the cell without ever being released. An example of such a signaling occurs with the hormone insulin that is released by the islet cells of langerhans. After its release by the cell it inhibits insulin further release by the same cell. Also the hormone somatostatin functions in the same way to inhibit its secretion by the cells that produced it. This type of autocrine signaling is important in the case of cancer in which cancer cells synthesize oncogenic products which can stimulate cancer cells division.
The other type of glands which is prevalent in the body is the exocrine type of glands. Exocrine glands secrete their products usually through epithelial ducts that are connected to a surface. The duct can pass the secretory product of the gland with alteration in its content or with modifying its content by absorbing or secreting contents to it. The mechanism of secreting a compound by an exocrine gland involves a merocrine secretion such as occurs in the cells of the exocrine pancreas. In this case the compound to be secreted by the gland is packed into vesicles which can fuse with the cellular membrane, thus releasing its content to the outside of the gland.
Also another type of secretion by the exocrine glands involves holocrine secretion in which the cells that contain the compound to be secreted undergo spontaneous death which leads to the release of all the contents of the cell including its exocrine secretion. Exocrine glands can be multicellular or monocellular. An example of a monocellular gland is Goblet cells of the gastrointestinal tract. This type of glands specialize in secreting mucus to the inner surface of the intestine. This monocellular gland is spread among the columnar cells of the epithelium that lines several surfaces such as the esophagus and the lungs. Goblet cells are found not only in the gastrointestinal tract, but also in the epithelium of the bronchi.
The other type of exocrine glands is the multicellular glands. An example of such an exocrine gland occurs in the lining of the stomach. The secretion by the exocrine gland in the body such as the gastrointestinal tract and the urogenital system and the respiratory tract can be divided into two types. These are mucus and serous secretions. Mucus secretion is usually viscous while serous secretion is watery. Example of mucus secreting gland includes the goblet cells and cells that line the stomach. This is in addition to to cells of the sublingual salivary glands. The viscous nature of the secretion is a result of the glycosylation of the proteins in the mucus. Serous secretions are usually not glycosylated proteins. These are found in the parotid gland and the pancreas.