MAMMARY GLAND

milk-producing gland characteristic of all female mammals and present in a rudimentary and generally nonfunctional form in males. Mammary glands are regulated by the endocrine system and become functional in response to the hormonal changes associated with parturition. In the primitive monotreme mammals (e.g., platypus), milk is expressed directly from the ducts onto the fur, from which the young lap it up. Unique in monotremes, the mammae lack nipples and are functional in both sexes. In marsupial mammals (e.g., kangaroo), the mammae are located on the ventral surface of the body and in some species are protected by a skin fold or by a pouchlike structure. The tiny newborn sucks the nipple, which then expands in its mouth and thus attaches the young to the female's body. It remains attached until it is fairly well developed, after which time it nurses at will, as do the more advanced mammals (see suckling). In cattle, horses, and whales, the mammary glands are located in the inguinal (groin) region; in primates, they are on the chest. Most small mammals have several pairs spread along the ventral surface. Mammary glands are derived from a modification of sweat glands. They first appear in embryonic life as clumps of cells proliferating from a longitudinal ridge of ectoderm (the outermost of the three germ layers of the embryo) along the so-called milk line, from the buds, or beginnings, of the lower limbs to those of the upper limbs. The number of these clumps that ultimately become breasts, or mammae, varies with each mammalian species according to the size of its litter. In the human normally only one develops on each side of the chest. A lesser development of one or more breasts (polymastia) or nipples (polythelia) may, however, occur anywhere along the milk line.