The anatomy and physiology of the breast

A woman's  breasts are a complex system - with everything working together so she can produce milk for her baby and breastfeed.

Fascinating and complex, breasts are a wonder of nature that begin developing when we are embryos in our mother's womb. They starting as thickened tissue, growing into buds and then developing glands, milk ducts, the nipple-areola complex and pigmentation.

By the time a girl has gone through puberty, her body has begun producing the estrogen and progesterone needed for her breasts to grow and develop - with the estrogen working to stimulate the development of milk ducts, and the progesterone responsible for the development of milk glands.

An outer view

Naturally, the breast is covered with skin, though the skin that surrounds the nipple - the areola - is quite different from "normal" skin. This darker pigmented skin is home to the Montgomery tubercles, a collection of sebaceous, sweat and accessory glands that come into action during pregnancy, and which can appear as small bumps. They produce an antibacterial oily substance that helps protect the nipple, and a light scent to help mom's baby locate her nipple.

The areola is also where smooth muscle fibers live that give nipples the ability to become erect. This happens when the nipple is stimulated, thanks to sensory and tactile nerve endings that respond to the lightest touch.  Within the nipple are 10-15 pores - the final point of the breast's milk ducts, from which baby can feed.

Inside the breast

Traveling back, inside the breast, these milk ducts lead back to lobules - the milk producing glands. Such glandular tissue makes up about 10-15% of a woman's breast. The breast is also made up of fatty tissue, and fibrous bands of connective tissue, known as Cooper ligaments, that support the breast's weight and give it shape.

During pregnancy, the fibrous bands of connective tissue decrease - and the lobes of glandular tissue increase. As baby grows during its first months in the womb, milk ducts sprout and branch out. As the pregnancy progresses and mom's levels of progesterone increases, the milk lobes continue to grow - and colostrum begins to form within the milk gland, in small alveoli.

As the baby develops, colostrum production continues. Secretory cells also develop, to produce and secrete milk proteins. Mom's levels of oxytocin increases during the last trimester, resulting in a change in the milk ducts, which propels the milk forward to the nipple-areola complex.

Blood and lymph flow

All these glands and tissues depend on a good blood supply. About 60% of the blood flow to the breast come from the internal mammary artery and vein, running along either side of the sternum. Another 30% is fed to the breast through the axillary artery and vein, running from the armpits. Lateral branches of the intercostal arteries - arteries feeding the ribs - are also a source of blood to the breasts.

Breasts and their nipple-areola complex are also connected to a complex lymphatic system. The Sappey's plexus is a superficial network that drains lymph from the breast's skin, nipple and areola.

This lymph flows into a deeper lymph system that is responsible for the breast tissue. About 87% of lymphatic fluid is drained through the armpit lymph system - the other 3% travels through internal mammary lymph nodes.

After birth

Once baby is born, the breasts are ready for action. Suddenly mom's levels of estrogen, progesterone and placental lactogen drop - as her prolactin levels increase. This is a sign to the milk glands to make milk for her newborn baby. At this point, colostrum - and later on, milk - fills the alveoli and milk ducts.

All that's needed now it for baby to stimulate her mom's nipple. The happy hormone, oxytocin, gets to work stimulating the flow of milk - as the cells surrounding the milk ducts contract. And baby can happily drink her mother's milk and thrive.