The Milk-Secreting Cells of the Mammary Gland Develop Through the Proliferation of Epithelium into Hollow Structures Called Alveoli

Embryonic ectoderm is the source of the mammary glands. The mammary ectoderm is first represented by parallel linear thickenings on the ventral belly wall. The continuity of the ridge that is formed is broken into the appropriate number of mammary buds, from which the functional part of the mammary gland will be derived.

The parenchyma, or milk-secreting cells, of the mammary gland develops through the proliferation of epithelial cells that arise from the primary mammary cord. The epithelial cells eventually form hollow, circular structures called alveoli, which are the fundamental milk-secreting units of the mam­mary gland (Figure 39-1). In concert with this develop­ment, an enlarged area of epithelium, the nipple, which is the external connection to the internal milk-secreting system, develops on the surface. In males, although nipples often develop, the underlying primary mam­mary cord does not develop into substantial glandular tissue.