Breast tissue originates from ectodermal layers. During the sixth week of embryogenesis, epidermal cells migrate into the underlying mesenchymal tissue and form primitive mammary ridges. These ridges initially extend from the axilla to the base of the lower limb bud. The validity of this ‘theory’was derived from animal study and its validity in humans is now being challenged.
However, congenital lesions in the human are overwhelmingly confined to the axillo-pectoral region.
By the tenth week, these structures atrophy at their upper and lower borders, leaving ridges in the pectoral region only. The nipple and areola overlie a bud of breast tissue composed of both the primary mammary ducts and fibrous stroma. Between 10 and 20 weeks, approximately 15 - 20 major ducts form and then bifurcate into tubules which form the basis of the duct system.
The major ducts at this stage have only small vesicles at the distal ends and no lobular development is present. Each duct system opens separately into the nipple. The areola develops at approximately 5 months of gestation.
Interactions between stroma, connective tissue, the vascular network and specialized fat cells result in the development of the breast mammary gland.
Rudimentary sebaceous glands and Montgomery’s tubercles are present. The breast bud is usually not palpable until 34 weeks of gestation and measures approximately 3mm at 36 weeks and 4 - 10mm at 40 weeks. The final change is completed by the time of birth.
Revision date: July 9, 2011
Last revised: by Janet A. Staessen, MD, PhD