Development of the mammary glands begins in the male and female embryo in identical fashion. During the fourth to fifth week of fetal development, primitive milk streaks, also known as galactic bands, form [1-4]. These are single, thickened ridges of ectoderm that extend bilaterally from the axillary to the inguinal region. Each band consolidates to form a mammary ridge on the thorax, and the remaining band regresses. At 6-8 weeks, a primary bud forms, with thickening of the mammary anlage, which penetrates into the chest wall mesenchyme. The primary mammary bud gives rise to secondary buds that extend into the surrounding connective tissue and become the lactiferous ducts and their branches. The mesenchyme surrounding the duct systems becomes the fibrous stroma and fat of the breast. Between 12 and 16 weeks of development, mesenchymal cells differentiate into the smooth muscle of the nipple-areolar complex (NAC), and branches link to future secretory alveoli. The secondary mammary anlage then develops with differentiation of hair follicles and sweat glands.
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