Final answer:
The hormone primarily responsible for milk letdown or ejection is oxytocin, which is released from the neurohypophysis in response to an infant suckling and causes myoepithelial cells to contract, facilitating milk ejection.
Step-by-step explanation:
The principal hormone directly concerned with milk letdown, or ejection, is oxytocin. This hormone is released by the neurohypophysis (posterior pituitary) during breastfeeding. When the infant suckles, sensory nerve fibers in the areola trigger a neuroendocrine reflex that causes milk secretion from lactocytes into the alveoli. Oxytocin then stimulates myoepithelial cells around the alveoli, causing them to contract and eject milk into the lactiferous ducts. This allows milk to collect in the sinuses and be discharged through the nipple pores. It is important to note that while prolactin, a hormone produced by the anterior pituitary, is crucial for milk production, it is oxytocin that plays a key role in the actual process of milk letdown.