- Development starts at 4th week gestation
- Downgrowth from a thickened mammary ridge of ectoderm along a line from the axilla to the inguinal region
- Supranumerary nipples or glands proper can form at lower levels on this line
- During puberty:
- Lobules form
- Each lactiferous duct is connected to a tree-like system of ducts and lobules intermingled and enclosed by connective tissue to form a lobe of the gland
- During pregnancy:
- Alveoli bud off from smaller ducts
- After menopause:
- Atrophy of lobes and ducts