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gland,organ that manufactures chemical substances. A gland may vary from a single cell to a complex system of tubes that unite and open onto a surface through a duct. The endocrine glands, e.g., the thyroid, adrenals, and pituitary, produce hormones that are secreted directly into the bloodstream (see endocrine systemendocrine system
, body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones.
..... Click the link for more information. ). Exocrine glands secrete their substances onto an external or internal body surface. Most exocrine glands, e.g., the salivary and lacrimal glands, release their secretions through ducts. However, some open directly onto a body surface, as in the sebaceous glands of the skin and the digestive glands of the intestinal mucosa. A simple exocrine gland may consist only of a tube lined with secretory cells. In more complex types, clumps of cells produce the secretion and a duct or system of ducts discharges the secreted material. Some glands have dual functions, e.g., the liver, pancreas, ovary, and testis produce both a secretion that is emitted through a duct and a hormone that is taken up by the blood. Such structures are called mixed glands. Among the substances produced by exocrine glands in humans are sweat, lubricants like mucus and tears, and digestive juices. There are specialized exocrine glands in the animal world that produce such substances as the shells of bird eggs, spiderwebs, and the cocoons of the silkworm larvae. Simple glands are also common in the plant kingdom. The sweet nectar of flowers and the resinous pitch of pine trees are substances produced by plant glands.
A structure which produces a substance or substances essential and vital to the existence of the organism and species. Glands are classified according to (1) the nature of the product; (2) the structure; (3) the manner by which the secretion is delivered to the area of use; and (4) the manner of cell activity in forming secretion. A commonly used scheme for the classification of glands follows.
- I. Morphological criteria
- A. Unicellular (mucous goblet cells)
- B. Multicellular
- 1. Sheets of gland cells (choroid plexus)
- 2. Restricted nests of gland cells (urethral glands)
- 3. Invaginations of varying degrees of complexity
- a. Simple or branched tubular (intestinal and gas-tric glands)—no duct interposed between surface and glandular portion
- b. Simple coiled (sweat gland)—duct interposed between glandular portion and surface
- c. Simple, branched, acinous (sebaceous gland)—glandular portion spherical or ovoid, connected to surface by duct
- d. Compound, tubular glands (gastric cardia, renal tubules)—branched ducts between surface and glandular portion
- e. Compound tubular-acinous glands (pancreas, parotid gland)—branched ducts, terminating in secretory portion which may be tubular or acinar
- II. Mode of secretion
- A. Exocrine—the secretion is passed directly or by ducts to the exterior surface (sweat glands) or to another surface which is continuous with the external surface (intestinal glands, liver, pancreas, submaxillary gland)
- B. Endocrine—the secretion is passed into adjacent tissue or area and then into the bloodstream directly or by way of the lymphatics; these organs are usually circumscribed, highly vascularized, and usually have no connection to an external surface (adrenal, thyroid, parathyroid, islets of Langerhans, parts of the ovary and testis, anterior lobe of the hypophysis, intermediate lobe of the hypophysis, groups of nerve cells of the hypothalamus, and the neural portion of the hypophysis)
- C. Mixed exocrine and endocrine glands (liver, testis, pancreas)
- D. Cytocrine—passage of a secretion from one cell directly to another (melanin granules from melanocytes in the connective tissue of the skin to epithelial cells of the skin)
- III. Nature of secretion
- A. Cytogenous (testis, perhaps spleen, lymph node, and bone marrow)—gland “secretes” cells
- B. Acellular (intestinal glands, pancreas, parotid gland)—gland secretes noncellular product
- IV. Cytological changes of glandular portion during secretion
- A. Merocrine (sweat glands, choroid plexus)—no loss of cytoplasm
- B. Holocrine (sebaceous glands)—gland cells undergo dissolution and are entirely extruded, together with the secretory product
- C. Apocrine (mammary gland, axillary sweat gland)—only part of the cytoplasm is extruded with the secretory product
- V. Chemical nature of the product
- A. Mucous goblet cells (submaxillary glands, urethral glands)—the secretion contains mucin
- B. Serous (parotid gland, pancreas)—the secretion does not contain mucin