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AN9.1-3 | Pectoral region — SDL Guide (Part 3)
Development of the Breast, Age Changes, and Congenital Anomalies
Embryological development:
The breast develops from ectodermal thickening along the mammary ridge (milk line) — a pair of ridges running from the axilla to the groin, appearing in week 6 of embryonic development.
- In humans, only ONE pair of breasts develops at the 4th intercostal space level. The rest of the ridge normally regresses.
- The breast starts as an epithelial bud that sinks into the underlying mesenchyme and branches to form the 15–20 lactiferous ducts by birth.
Hormonal milestones and age changes:
• Puberty: oestrogen drives duct and fat growth; progesterone promotes lobular development; breast grows through 5 Tanner stages
• Pregnancy: massive alveolar and ductal proliferation under oestrogen, progesterone, prolactin, and human placental lactogen
• Lactation: prolactin drives milk synthesis; oxytocin triggers myoepithelial cell contraction (milk ejection reflex)
• Menopause: oestrogen withdrawal causes glandular atrophy; post-menopausal breast appears more radiolucent on mammogram
Congenital anomalies:
| Anomaly | Definition | Clinical significance |
|---|---|---|
| Polythelia | Extra nipples along the milk line | Common (1–5%), usually benign |
| Polymastia | Extra breast tissue along the milk line | Can develop cancer; may lactate in pregnancy |
| Amastia | Absent breast and nipple | May be part of Poland syndrome |
| Gynaecomastia | Breast enlargement in males | Oestrogen excess (liver disease, drugs, puberty) |
| Inverted nipple | Nipple fails to evert | Normal at birth; NEW inversion in adult = malignancy until proven otherwise |
Figure: Development of the Breast, Age Changes, and Congenital Anomalies
Figure: Congenital anomalies: