At the end of week 4 of human embryonic development, paired thickenings
appear in the ectoderm on the ventral aspect of the torso. Extending from the axilla to
the inguinal region, they form the mammary ridges or “milk lines.” Subsequently they
regress and leave a pair of primary mammary buds at the level of the fourth and fifth
inter- costal spaces. The primary buds thicken into lens-shaped mammary placodes.
Epithelial cells invade the underlying mesenchyme during weeks 7 and 8 to form the
primitive mammary disk. In week 9, a surge of mesenchymal proliferation occurs,
coincident with a thinning of the overlying epithelium. A dense mesenchymal stroma
then coalesces around the bud. Between weeks 10 and 12, epithelial buds form, begin
to branch, and extend into the epithelial–mesenchyme bound- ary. By the first half of
the second trimester (weeks 13–20), there are 15–20 solid epithelial cords that
converge at the nipples. Ramification processes continue to week 32, when the cords
undergo apoptosis to establish tubules and alveoli. At birth, male and female mammary
glands are equally formed. There are 20 lactiferous ducts draining into the dimple. In
later stages of the final trimester, the mesoderm underlying the dimple changes it into a
true nipple with an areola. Placental estrogens during the final weeks of gestation cause
breast buds to enlarge to create a true breast nodule at birth, about 1 cm in size, in both
genders.
Keywords: Breast, Buds, Mammary.
