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 com-
• plexity
• a. Simple or branched tubular (intestinal and gastric 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)—andular 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 dur-
• ing secretion
• A. Merocrine (sweat glands, choroid plexus)—no loss of cytoplasm
• B. Holocrine (sebaceous glands)—and 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)—secretion does not contain mucin