Mammals are literally named after the gland that feeds their young
The word mammal comes from the Latin mamma, meaning breast. Every mammal has mammary glands, but they vary wildly: platypus-like monotremes ooze milk from modified skin glands with no nipples, cows gather several glands into one udder, and in a few bat species even the males have working glands.
Mammals fall into three groups: monotremes, metatherians and eutherians. In most of the latter two, only females have functional mammary glands. Primates such as humans and chimpanzees carry them in breasts, ruminants like cows, goats and deer in udders, and dogs, cats, rabbits and pigs in rows of dugs. The glands are a specialised apocrine type, and many accounts describe them as modified sweat glands.
Inside, milk starts in alveoli, hollow pockets a few millimetres across lined with secreting cells and wrapped in contractile myoepithelial cells. Alveoli cluster into lobules, each drained by a lactiferous duct opening at a pore on the nipple. When the hormone oxytocin arrives, the outer cells squeeze and push milk toward the nipple. A human breast normally holds one complex gland made of 10 to 20 simple ones.
Both sexes start with the same rudimentary duct tree at birth. Growth then depends on hormones: oestrogen and growth hormone must work together, since neither alone can drive duct development, and much of growth hormone's effect runs through insulin-like growth factor 1. These signals peak at puberty, when most branching happens and ducts spread through the surrounding fat pad like the limbs of a tree. Androgens such as testosterone block oestrogen's effect, which together with low oestrogen explains why male glands stay undeveloped.
Pregnancy triggers rapid proliferation and the formation of milk-producing alveoli within the branches, and after birth luminal cells secrete milk until weaning ends lactation. The glands first make colostrum, the initial milk released after giving birth. After menopause, falling oestrogen, growth hormone and IGF-1 cause the tissue to shrink. Laboratory studies in embryos trace the earliest steps to milk lines forming between fore and hind limbs, followed by placodes that eventually become nipples.
Source: Mammary gland
