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During the 6th week of gestation, a pair of longitudinal thickening of the epidermis develop on the ventral surface of the embryo, extending from the axilla to the medial thigh, called "mammary ridges" (or "mammary line", “milk lines"). In large part these milk lines later atrophy, leaving only the part in the pectoral region, where the breasts will develop.
The nipple-areolar complex development begins about in the 14th week of gestation, with the differentiation of mesenchymal cells into smooth muscle components.
Sometimes may happens that other segments of the ridge, which do not correspond to the breast, remain giving rise to the development of supernumerary nipples most commonly in the axilla or inframammary fold, but they may occur anywhere along the milk line, from the axilla to the groin. This not uncommon condition is called polythelia (similarly may develops accessory mammary glands: this more rare condition is called polymastia).
The development of the nipple-areolar complex is followed by the development of special apocrine glands into the Montgomery glands, some large intermediate-stage sebaceous glands that are transitional between sweat glands and mammary glands, capable of secreting milk, that opens at the Morgagni tubercles, which are small papules on the areola. Pigmentation of the nipple-areolar complex develop between the 32nd and the 40th week. In adult women the nipple contains the outlets for 15-20 lactiferous ducts arranged cylindrically around the tip.
The nipple-areolar complex consists of numerous vessels, intermixed with plain muscular fibers, which are principally arranged in a circular manner around the base, or even radiating from base to apex. The nipple-areolar complex also contains many sensory nerve endings, smooth muscle, and an abundant lymphatic system called subareolar plexus or Sappey's plexus. Nipple-areolar anatomy is remarkably variable in dimension, texture, and color across ethnic groups and among individuals.
The physiological purpose of nipple-areolar complex is to deliver milk produced by the mammary glands during lactation.
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