Glands

structures in animals and humans that elaborate and excrete specific substances and usually play a part in the physiological functions of the organism. Glands that discharge their products on the surface of the body or of mucous membranes through excretory ducts are called glands of external secretion, or exocrines; their products are called secretions. Other glands—endocrines (incretory glands) or glands of internal secretion—have no excretory ducts; the products they manufacture (incretions, or hormones) are secreted into the blood or lymph and distributed with them throughout the body. Some glands selectively absorb from the blood certain end products of dissimilation, concentrate them, and excrete them, thus preventing intoxication of the body by them. Such concentrative glands are the kidneys, sweat glands, and partially the lacrimal glands; the substances they secrete are commonly called excretions.

The formation and secretion of the substances produced proceeds in basically the same manner in all glands; for this reason the term “secretion” is often used to designate all secreted products—secretions, incretions, and excretions, regardless of their physiological significance. The secretions of most glands (for example, the parotid and the pancreas) belong chemically to the proteins; upon dissolving in water, they are secreted in the form of serous fluids. Such glands are often called protein, or serous, glands. Another group is comprised of the mucous glands (for example, those of the esophagus or uterus), which produce mucins and mucoids (substances of the group of glycoproteins). Some glands, the so-called heterocrines, elaborate simultaneously both protein and mucous secretions. The secretions of other glands (sebaceous, in part the lacteals) are lipoid in nature and do not dissolve in water.

The exocrine glands and the majority of endocrines develop as derivatives of epithelial (bordering) tissues; some endocrine glands may originate from other tissues. Thus, the interstitial cells of the sex glands (which participate in elaborating sex hormones) arise from the mesenchyma. The chromaffin cells (comprising the medullary portion of the adrenals and of the so-called paraganglia), which produce catecholamines, are modified nerve (sympathetic) cells. Closely related to them are the neurosecretory cells, which, being neural in nature, are capable of elaborating and sending secretory products into the blood; in vertebrate animals and man such cells are concentrated in the hypothalamus. The neuroglia participates in the construction of certain endocrine glands (the epiphysis and the posterior lobe of the hypophysis).

The principal function of epithelium is the exchange of substances between the body and the environment, including the excretion of products elaborated by epithelial cells. In some epithelial cells the latter function becomes the dominant one, and they are converted to gland cells or uni-cellular glands (for example, goblet cells). Sometimes all the cells of a certain section of an epithelial layer become differentiated as gland cells and begin to discharge a secretion, and a glandular margin develops—for example, the epithelium of the mucosa of the stomach. As a result of an increase in the number of gland cells in a given section of epithelial layer, a glandular depression is formed, which is embedded in the underlying connective tissue and which, in the course of further deepening, assumes the form of a tubule that is separated from the epithelial layer. Elaboration of the secretion is concentrated in the distal portion of this rudiment, which becomes differentiated into a terminal (secretory) section, or adenomere, of the formed gland. The proximal portion of the glandular tubule becomes the excretory duct, whose cells remain hardly differentiated (as a result of which the excretory ducts retain the capacity to proliferate and in many cases become the source of growth and regeneration of the gland). Glands are classified, according to the shape of their adenomeres (elongated or rounded), as tubular or alveolar (spherical adenomeres, often called acini). Glands that consist of one adenomere (including those with a branched adenomere) and an unbranched excretory duct are called simple glands (tubular or alveolar)—for example, the fundic and pyloric glands of the stomach and the glands of the uterus. Glands that consist of a number of adenomeres, whose secretion pours into a common excretory duct via numerous branches, are called compound glands. According to the shape of their adenomeres, compound glands may be tubular (for example, the subglossal salivary glands) or alveolar (the pancreas and the parotid gland). Sometimes in the same compound gland some adenomeres are tubular and others are alveolar—for example, the submaxillary salivary gland. In rare instances, the tubular adenomeres, becoming branched, unite to form a loose network, and the gland becomes a complex meshwork—for example, the liver and the anterior lobe of the hypophysis.

In simple glands with branched adenomeres and in compound glands with many adenomeres, the interstices between them are filled with connective tissue through which blood vessels and nerves pass. Thus, in large multicellular glands, one can distinguish the parenchyma of epithelial origin (adenomeres and excretory ducts) and the connective-tissue stroma, which supports and nourishes the parenchyma. Prolonged reduction of the functional activity of a gland leads to atrophy of the parenchyma, whereas the stroma in such circumstances becomes hyperplastic and begins to replace the atrophying parenchyma (sclerosis or cirrhosis of the gland).