Introduction

The secretion of milk depends on the cellular coordination of a host of synthetic and secretory processes that combine to produce a fluid rich in lipid, carbohydrate, proteins, minerals, vitamins, growth factors, and protective substances. In humans, this fluid is capable of providing the full-term human infant all the nutrients required for the first 4–5 months of life as well as offering significant protection against infectious disease. Milk delivery to the infant depends on two separate processes, milk secretion and milk ejection. Milk is secreted more or less continuously by specialized epithelial cells that line the lumina of the breast alveoli (or acini). Prolactin, secreted by the anterior pituitary, is the major hormone that regulates the synthesis and secretion of milk products by mammary alveolar cells. The alveoli are surrounded by myoepithelial cells that contract in response to oxytocin to force the milk out of the alveoli into the milk ducts and thence to the nipple. This process, called the “let-down reflex,” is brought about by episodic secretion of oxytocin secreted from the posterior pituitary. To make clear how these processes work, in this article the anatomy of the secretory apparatus will be described, followed by a brief description of human milk composition and a discussion of the mechanisms and regulation of both secretion and let-down. We will then summarize the initiation of lactation, a process that requires a series of carefully programmed functional changes in the breast that take place during the first week postpartum and transform a prepared, but nonsecretory gland, into a fully functioning organ.