The mammary crest usually appears as a narrow, microscopic ectodermal thickening during the first 7 weeks of pregnancy and grows caudally as a narrow, linear crest. Mammary lines begin to shorten and ectodermal cells begin to divide and grow into the mesenchymal cell layer. A basement membrane separating the expanding ectodermal crest structure and the underlying mesoderm usually remains. The mammary crest then becomes recognizable in the thoracic region in the human embryo.
A mammary crest usually stops growing at 8 weeks and its length is regressed starting at the caudal end and extending cranially, so that what remains is a round, ectodermic placode where the axilla develops. When shortening of the mammary crest is complete, the structure remains prominent in the areas where the mammary glands eventually form.