The first stage in the development of mammary glands is the appearance of mammary ridges (also called mammary lines) during the 4th week of development. The mammary ridges are thickened strips of surface ectoderm that extend from the base of the forelimb bud (the future axillary region) to the region medial to the hindlimb bud (the future inguinal region). In lower animals several mammary glands are formed along each mammary ridge. In human the ridge disappears except for small part in the pectoral region, where breasts develop.
The remnant of the mammary ridge produces primary bud of the
mammary gland. This bud grows down into the underlying mesenchyme. By the 12th week the primary bud gives rise to 15 to 25 secondary buds each of which elongates into a solid epithelial cord. Each cord represents a future lactiferous duct and a lobe of the mammary gland. The secondary buds give rise to tertiary buds which keep on branching throughout the fetal life. The duct system slowly acquires lumen by canalization of the solid cords which is induced by placental hormones entering the fetal circulation. The mesenchyme surrounding the ducts gives rise to the connective tissue and fat of the mammary glands.
During late fetal period the epidermis at the site of origin of the mammary glands becomes cornified. Hollowing produces a depression, called mammary pit, into which 15 to 25 lactiferous ducts open. Soon after birth proliferation of the connective tissue underlying the pit causes its elevation (above the level of the adjacent skin) to form the nipple. The areola becomes recognizable as a raised circular area of skin surrounding the mammary pit. This area is free of hair primordia but develops large branched acinar glands (called glands of Montgomery).
At birth the rudimentary mammary glands are similar in both sexes. In the male the glends normally remain rudimentary throughout life. In the female the mammary glands enlarge rapidly during puberty and become hemispherical in shape. This growth takes
place due to further branching of the duct system and deposition of fat under the influence of increased level of estrogens produced by the ovary. This state of the female breast is called inactive or resting state.
The mammary glands attain full development during pregnancy when the duct system grows further and acini first become recognizable at the ends of the intralobular ducts. These changes are brought about by further elevation of circulating estrogen level and sustained increase in the level of progesterone. Following childbirth, lactation (ie, production of milk) starts in response to prolactin secretion from the anterior pituitary. Following the cessation of suckling, the mammary glands undergo regressive changes and revert to the state of inactive breast