Final answer:
Estrogen stimulates the growth of ductal cells, while progesterone promotes the growth of glandular cells in the breasts, preparing the mammary glands for lactation and breastfeeding postpartum.
Step-by-step explanation:
In the structure of the breasts, the growth of the ductal cells is stimulated by estrogen, while the growth of glandular cells is stimulated by progesterone. During pregnancy, the mammary glands undergo significant changes. Estrogen stimulates the expansion and branching of the milk-transporting lactiferous ducts, as part of the body's preparation for lactation. Concurrently, progesterone induces the budding of breast alveoli from the ducts, which become the sites of milk production.
The mammary gland is composed of these ducts and glandular structures known as lobules, formed by clusters of alveoli. Each lobule is made up of milk-secreting cells called lactocytes, which are surrounded by myoepithelial cells. Together, these cells facilitate the secretion and transport of milk towards the nipple.
Moreover, the hormone prolactin plays an important role in milk production, working in concert with estrogen and progesterone to develop the mammary glands and initiate lactation after childbirth, with oxytocin promoting milk ejection during breastfeeding.