The breast is a modified sweat gland which originates from the ectodermal layer of the embryo between the fifth and sixth week of gestation. It arises from the 'milk lines', which are two ridges of ectodermal thickening, running from the axilla to the groin. Although most of the 'milk line' eventually disappears, a prominent ridge remains in the pectoral area to form the breast. The ectoderm in this area subsequently grows into the underlying mesoderm as a series (15-20) of buds. These ectodermal buds, which are initially solid, eventually form the lactiferous ducts and their associated alveoli. The adjacent mesenchyme develops into the surrounding adipose and connective tissues. During the final 2 months of gestation, the ducts become canalised and a 'mammary pit' is formed in the ectoderm. The lactiferous ducts subsequently communicate with the mammary pit.

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