The above description corresponds basically to the appearance of the resting mammary gland. Pregnancy induces a considerable growth of the epithelial parenchyma leading to the formation of new terminal branches of ducts and of alveoli in the first half of pregnancy. Growth is initiated by the elevated levels of oestrogen and progesterone produced in the ovaries and placenta. Concurrently, a reduction in the amount of intra- and interlobular connective tissue takes place. The continued growth of the mammary glands during the second half of pregnancy is due to increases in the height of epithelial cells and an expansion of the lumen of the alveoli.
Secretion is stimulated by prolactin .
Prolactin secretion in turn is stimulated by sensory stimulation of the nipple, which also initiates the so-called milk ejection reflex via the secretion of oxytocin from the neurohypophysis. Milk is ejected from the glandular tissue into the lactiferous sinuses - now it's up to the baby to get things out. See if you can identify resting alveoli and lactiferous sinuses. Draw a nice lactiferous duct and, if possible, a few resting alveoli. Identify the secretory alveoli and interlobular ducts in the slide of the lactating mammary gland.