Mammary Gland Development is characterized by the unique process by which the epithelium invades the stroma. The development of the mammary gland occurs mainly after birth. During puberty, tubule formation is coupled with branching morphogenesis which establishes the basic arboreal network of ducts emanating from the nipple.[1]
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Embryonic mammary gland development can be divided into a series of specific stages. Initially, the formation of the milk lines that run between the fore and hind limbs bi-laterally on each side of the midline occurs around embryonic day 10.5 (E10.5). The second stage occurs at E11.5 when placode formation begins along the mammary milk line. This will eventually give rise to the nipple. Lastly, the third stage occurs at E12.5 and involves the invagination of cells within the placode into the mesenchyme, leading to a mammary anlage (biology).[2]
Postnatally, the mammary ducts elongate into the mammary fat pad. Then, starting around four weeks of age, mammary ductal growth increases significantly with the ducts invading towards the lymph node. Terminal end buds, the highly proliferative structures found at the tips of the invading ducts, expand and increase greatly during this stage. This developmental period is characterized by the emergence of the terminal end buds and lasts until an age of about 7–8 weeks.
By the pubertal stage, the mammary ducts have invaded to the end of the mammary fat pad. At this point, the terminal end buds become less proliferative and decrease in size. Side branches form from the primary ducts and begin to fill the mammary fat pad. Ductal development decreases with the arrival of sexual maturity and undergoes estrous cycles (pro-estrous, estrous, met-estrous, and di-estrous). As a result of estrous cycling, the mammary gland undergoes dynamic changes where cells proliferate and then regress in an ordered fashion.[3]
Figure Legend: Mammary gland whole mounts from BALB/c mice (Mus Musculus). A) 3 week old mammary gland. B) 6 week old mammary gland. C) 10 week old mammary gland. D) Mammary gland @ Pro-Estrous. E) Mammary gland @ Estrous. F) Mammary gland @ Met-Estrous. G) Mammary gland @ Di-Estrous
During pregnancy the ductal systems undergo rapid proliferation and form alveolar structures within the branches to be used for milk production.
After birth, lactation occurs within the mammary gland; lactation involves the secretion of milk by the luminal cells in the alveoli. Contraction of the myoepithelial cells surrounding the alveoli will cause the milk to be ejected through the ducts and into the nipple for the suckling pups.
Upon weaning of the pups, lactation stops and involution of the mammary gland occurs. This process involves the controlled collapse of mammary epithelial cells where cells begin to apoptosis in a controlled manner, reverting the mammary gland back to a pubertal state.