Embryology: Breast parenchyma (ducts/acini) originates from ectoderm. The nipple and breast stroma (connective tissue) originate from mesoderm. Primary mammary buds: Develop near the 4th intercostal space (wk 7-8), grow downward into underlying mesenchyme (wk 12). Secondary buds: Grow into the mesenchyme around each primary bud, later branch and canalize to form lactiferous ducts; lactiferous ducts later coalesce at an epidermal pit (primordial nipple) in 3rd trimester. At term, there are 15-20 lobes of glandular tissue.

Puberty: Increased estrogen (also GH, IGF-1) stimulates breast development (thelarche); first secondary sex characteristic to develop in females. There is an increase in fibrous and fatty tissue, followed by ductal elongation (estrogen) and branching (progesterone). The ductal epithelium matures into an outer myoepithelial layer, an inner luminal layer, and an inner secretory layer.

Males: Under the influence of testosterone, the male mammary gland largely involutes, and is primarily composed of ductal structures within collagenized stroma, with none of the lobular elements found in the female breast.

(Javed 2013)

Breast development