The mammary glands (also called mammae) are modified sudoriferous (sweat) glands that produce milk for the nourishment of offspring.
They develop from bilateral thickenings of ventrolateral ectoderm of the embryo, the so-called milk lines, which are more correctly referred to as mammary ridges. In carnivores and the sow, the mammary glands develop throughout the axillary to inguinal extent of the ridges, as is appropriate for species that typically deliver multiple fetuses.
However, in most other domestic animals, only the inguinal mammary glands develop, usually two (e.g., mares, ewes, and does) or four (e.g., cows). In the anthropoid apes and the elephant, only two pectoral mammary glands develop.Each gland is composed of a system of ducts connecting masses of secretory epithelium surrounded by connective tissue and fat and supported in a fibroelastic capsule. The proportion of secretory parenchyma to connective tissue is hormonally dictated; during lactation, the mammary gland’s secretory tissues increase in volume. After the end of lactation (when the dam is dry), the secretory tissues regress, and connective tissue constitutes a greater percentage of the gland.
In ruminants and horses, individual glands are associated so closely to one another that they are commonly referred to as a single udder. Even so, the individual nature of glands of the udder is readily appreciated by the presence of
a single teat (papilla) for each gland. A single (as in ruminants) or multiple (as in the mare and sow) duct system may discharge milk at the tip of each teat.
Embryonic ectoderm invaginates along the mammary ridge to become the duct system of individual mammary glands. These invaginations (mammary buds) will ultimately be associated with an individual teat. In all species more buds initially develop than will persist, and these extra buds regress promptly after appearing. Not uncommonly, though, some extra buds persist and produce Supernumerary teats. These extra teats are usually small and not associated with a well- developed gland; because they can interfere with milking, they are usually removed from the udders of cows and does.
The mammary glands of domestic species have a great deal in common, but inasmuch as the udder of the dairy cow has been so dramatically developed to produce milk far beyond that necessary to nourish the cow’s offspring, it is discussed in detail.