The mammary glands develop from ectodermal ridges that form on the ventral surface of the embryo. They extend laterally from forelimb to hindlimb and constitute the milk line. These buds occur in multiple pairs and begin appearing at around 5 weeks of gestation. Most of the pairs regress during fetal development, except for one pair in the pectoral region that eventually develops into the two mammary glands. If buds along the milk line do not completely disappear, they will develop into ectopic glandular tissue. This ectopic mammary tissue is usually seen at the extreme ends of the mammary ridge, usually in the axilla or the vulva in the adult. This can occur as extra breasts (polymastia) or nipples (polythelia). Accessory mammary tissue is found in 2 - 6% of women.
At midpregnancy, the two fetal mammary buds destined to form breasts begin to grow and divide. Fifteen to 25 secondary buds are formed that are essentially the duct system in the mature breast.
Each secondary bud elongates into a cord, bifurcates and differentiates into two concentric layers of cuboidal cells and a central lumen. Secretory epithelium is derived from the inner layer of cells. These cells are responsible for milk production during lactation.
The outer layer of cells becomes myoepithelium and is responsible for the mechanism of milk ejection.
In the last 2 months of gestation, canalization of these epithelial cords occurs, as well as development of the branching lobuloalveolar glandular structures. The lactiferous ducts converge to form the mammary pit. Near birth, evagination of the mammary pit forms the nipple.
- Embryology / Development
- Adult Breast Morphology
Nipple / Areola
Blood Supply of the Breast
Innervation of the Breast
- Thoracic Wall
Muscles and Associated Structures
Axillary Lymph Nodes
Internal Mammary Lymph Nodes
In the female, adolescent breast development (thelarche) occurs with onset of cyclical estrogen and progesterone. Ductal growth and differentiation of periductal stroma is influenced by estrogen, growth hormones and glucocorticoids. With onset of ovulation, progesterone stimulates development of the alveoli and prepares the breast for possible future lactation.
Mary L. Gemignani
Memorial Sloan-Kettering Cancer Center