I was reading on mammary gland development, and I found this (1):
As a girl approaches her teen years, the first visible signs of breast development begin. When the ovaries start to produce and release (secrete) estrogen, fat in the connective tissue starts to collect. This causes the breasts to enlarge. The duct system also starts to grow.
Wikipedia has an article about the duct system(2):
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.
It goes on to say, developmentally, mammary gland epithelium is constantly produced and maintained by rare epithelial cells.
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.
In the end, I understood that the mammary alveoli (containing milk) are not formed during this. During pregnancy, the ductal systems undergo rapid proliferation and form alveolar structures within the branches to be used for milk production.
And from where is the milk produced? (Just for some more detail)
After delivery, 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 nursing infant.
An interesting case is one of neonatal milk.(3)
It is thought to be caused by a combination of the effects of maternal hormones before birth, prolactin, and growth hormone passed through breastfeeding and the postnatal pituitary and thyroid hormone surge in the infant.
The article doesn't mention anything about developement of mammary alveoli, though. How does that happen, then?
I know milk formation primary requires prolactin, oxytocin and indirectly, estrogen.(4) So does this process wait till pregnancy?(If yes, then the lobes and branches are devoid of any lumen, and they remain empty until pregnancy, which seems skeptical to me.)
Can lactation occur without alveoli formation?