Milk Line and Accessory Breast Tissue

Expansion

In the embryo, a mammary ridge (milk line) of ectodermal thickening runs on each side from the axilla to the inguinal region. In humans all of it regresses except a single point on the thorax, which becomes the breast.

Failure of regression gives:

• Polythelia - accessory nipples, the commonest anomaly, occurring in perhaps 1 to 5 per cent of people
• Polymastia - accessory glandular breast tissue, most often in the axilla

Ectopic tissue remains hormonally responsive, so it can enlarge and become tender in pregnancy and lactation, and it can develop the same diseases as the normal breast, including carcinoma. That is why an axillary swelling in a woman needs consideration of accessory breast tissue as well as lymphadenopathy.