Milk secreting adaptations of sweat glands.
The basic components of the mammary gland are the alveoli lined with milk secreting epithelial cells and surrounded by myoepithelial cells and a rich capillary network. These alveoli join up to form lactiferous ducts that drain into openings in the areola.
In humans, mammary glands usually number two although polythelia (accessory nipples) and polymastia (accessory glands) can occur anywhere from the knee to the neck in man.
Development of mammary glands depends on testosterone (inhibits mammary gland formation), estrogen (promotes mammary gland formation) and prolactin (which is stimulated by estrogen anyway).
At birth, there are lactiferous ducts but no alveoli. Little branching occurs before puberty when ovarian oestrogens stimulate branching differentiation of the ducts into spherical masses of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels of oestrogen and progesterone cause further branching and differentiation of the duct cells, together with an increase in adipose tissue and a richer blood flow.
Colostrum is secreted in late pregnancy and for the first few days after parturition (birth). True milk secretion begins a few days later due to a reduction in circulating oestrogens and progesterone.