The mammary gland is a highly specialized exocrine gland that defines the class of animals known as Mammalia. Its primary biological purpose is to produce and secrete milk, a complex nutrient-rich fluid, in a process called lactation. This function provides nourishment for newborn offspring. The gland’s structure is a sophisticated network of tissues that develops in response to hormonal signals, becoming fully mature and functional after pregnancy.
Anatomical Location in Humans
In the human body, the paired mammary glands are situated on the anterior wall of the chest. They are positioned superficially within the fascia of the pectoral region. The gland’s base overlies the large pectoralis major muscle, separated from it by the retromammary space, a layer of loose connective tissue that allows for some mobility of the breast tissue.
The vertical boundary of the gland typically extends from the level of the second rib down to the sixth rib. Horizontally, the tissue spreads from the lateral edge of the sternum to the mid-axillary line, which runs down the side of the torso from the armpit. A small, distinct portion of the gland, known as the axillary tail of Spence, often extends upward into the armpit itself.
The external markers of the gland’s location are the nipple and the areola, the pigmented skin surrounding the nipple. The nipple generally sits near the level of the fourth intercostal space in an adult female. The areola contains modified sebaceous glands, called Montgomery’s tubercles, which secrete a lubricating substance to protect the skin.
Internal Structure and Milk Transport System
The mammary gland itself is composed of glandular, fibrous, and fatty tissues organized to facilitate milk production and transport. The glandular tissue is structurally arranged into approximately 15 to 20 distinct sections called lobes, which radiate outward from the nipple. Each lobe is further subdivided into smaller units known as lobules.
Milk Production and Transport
Within the lobules are the alveoli, which are tiny, hollow sacs lined with milk-secreting cuboidal cells. These alveoli are the specific sites where the components of milk are synthesized and released into the central lumen of the sac. Surrounding the alveoli are myoepithelial cells, which contract in response to the hormone oxytocin to squeeze the milk out of the alveoli and into the collecting system.
The milk transport pathway begins with small ducts draining the lobules, which merge to form larger tubes called lactiferous ducts. These ducts carry the milk toward the nipple, where they slightly widen just beneath the surface to form the lactiferous sinuses, which act as temporary reservoirs.
Suspensory ligaments, also known as Cooper’s ligaments, provide structural support. These ligaments anchor the gland to the underlying pectoral fascia and the overlying skin.
Comparative Locations Across Mammals
The location and number of mammary glands vary significantly across the class Mammalia, reflecting diverse reproductive strategies. The placement of the glands generally occurs along two parallel lines, called the milk line, that run ventrally from the armpit region to the groin. The number of glands typically correlates with the average litter size of the species.
Primates, including humans, usually possess a single pair of glands located in the thoracic region. Conversely, large grazing animals like cattle and horses have their glands restricted to the inguinal region, where they form a single, large structure called the udder. The udder of a cow, for example, is comprised of two pairs of mammary glands that each feed into a separate teat.
Species that produce large litters, such as dogs and pigs, have multiple pairs of glands distributed along the entire length of the ventral milk line. Marsupials, like kangaroos, have glands located inside a pouch, where the underdeveloped young attach to a nipple until they are fully grown. Even monotremes, the most primitive mammals, possess mammary glands, though they lack nipples and secrete milk through specialized pores onto the fur.