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Anatomy of a Working Breast

By Anna Edgar
Sebring FL USA
From: NEW BEGINNINGS, Vol. 22 No. 2, March-April 2005, pp. 44-50

Most of us know that breastfeeding is a wonderful way to nurture and provide the best nutrition for baby. But have you ever wondered how it all works? How does the milk really get into the breasts? What keeps it coming time and again? What have people thought about this in the past? What do we now know? This article will address these questions. When you understand the anatomy of a working breast, you will appreciate this amazing process for supporting a new life even more.


People have been interested in the anatomy and physiology of the breast for thousands of years. The earliest medical writings about breasts hail back to ancient Egypt. These writings described how to tell whether the mother's milk was good or bad and how to increase her supply. The author recommended back rubs with fish-infused oil and "sitting cross-legged...while rubbing the breasts with a poppy plant" to increase milk flow (Fildes 1985). A modern commentator, Marilyn Yalom writes, "Both treatments at least had the merit of relaxing the nursing mother," which might, in turn, help her let-down reflex function better, but they probably have no direct effect on milk production. The ancient physician Hippocrates (460-377 BCE) believed that menstrual blood was somehow transformed into human milk. This belief persisted until the 17th century! For example, in his anatomical sketches, Leonardo DaVinci (1452-1519) drew veins connecting the uterus and the breasts.

Even the philosopher Aristotle (384-822 BCE) had thoughts on breastfeeding. He believed that darker-skinned women had healthier milk than fair-skinned women and that babies who drank warmer mother's milk developed teeth at an earlier age. (He was wrong on both counts.) Aristotle also stated that infants should not drink colostrum, a misconception that persists in some cultures today. Commenting on remedies for a low milk supply, Soranus, a gynecologist from antiquity (who practiced c. 100-140), thought breast massage and self-induced vomiting might be helpful, but he rejected "drinks mixed with the ashes of burnt owls and bats" (Soranus 1991). By the 1500s, anatomists were beginning to be on the right track. They learned from dissecting cadavers that breasts were composed of glandular tissue which, they reasoned, "converts the blood brought to them by the veins into milk" (Vesalius 1969).

Many of the early writings about breastfeeding concerned wet nurses: lactating women who were hired to breastfeed another woman's baby. Wet nursing is mentioned in the Code of Hammurabi (c. 1700 BCE), the Bible, the Koran, and the writings of Homer, among many others. Opinions about what kind of women made the best wet nurses specified everything from hair color to breast appearance to whether the woman had borne male or female children (Yalom 1997). During the 1700s, doctors finally started to understand that it is healthier for a mother to nurse her own child than to use a wet nurse and that drinking the mother's colostrum was good for babies (Riordan 2005).

In the last 50 years, medical science has learned a great deal about human milk, particularly in the area of immunology. We now know that colostrum is loaded with antibodies that protect newborns from disease, that mature milk has a perfect balance of nutrition for infants, and that toddler milk becomes more concentrated with immune factors as the toddler begins to nurse less. Milk that is produced after a premature birth is different from the milk of mothers whose babies are born fullterm, and those unique properties are beneficial for fragile premies. The Womanly Art of Breastfeeding states: "No two mothers produce identical milk....The composition of your milk varies from day to day and during different times of the day.... The colostrum your baby receives on the first day of his life is different from the colostrum on day two or three."

Human milk is a complex, living substance that is the key to good health and optimal development for human infants. Because human milk is so important, it is also important to know how a mother's breasts make milk.

Breast Development

Breast development begins already in the womb, in both male and female fetuses. Between the fourth and seventh week of the embryo's life, the outer layer of the skin begins to thicken along a line extending from the armpit to the groin. This forms the mammary ridge. Later, most of this "milk line" fades away, but a small portion remains in the chest and forms 16 to 24 buds that develop into milk ducts and alveoli, the sacs that secrete and store milk in the breast.

At first the lactiferous ducts open into a small pit under the skin, but shortly after birth this is transformed into the nipple (Sadler 2000). The nipple is surrounded by the areola. The mammary glands then remain inactive until puberty.

The next stage of breast development happens as females enter puberty, around 10 to 12 years of age. The breasts start to grow a year or two before a girl begins to menstruate. During each ovulatory cycle, the breast tissue grows a little more. Most growth occurs in adolescence but continues until about age 35 (Riordan 2005). The breast is not considered fully mature until a woman gives birth and begins to produce milk (Love & Lindsey 1995).

The BREASTFEEDING ANSWER BOOK explains that a mature breast is composed of glandular tissue for making and transporting milk; supportive connective tissue; blood, which provides the nutrients needed to make milk; lymph, the fluid that removes waste through the body's lymphatic system; nerves that send messages to the brain; and fat tissue for protection from injury (Mohrbacher &Stock 2003). The glandular tissue is made up of alveoli, which secrete and store milk until surrounding muscle cells eject it into ductules. The ductules then merge into larger ducts, which lead to between five and 10 nipple pores on the surface of the areola. Until very recently, it was believed that besides being stored in the alveoli, milk was also stored in lactiferous sinuses, widened areas in the ducts just behind the nipples. However, recent ultrasound studies have shown that lactiferous sinuses are not permanent structures in the breast (Kent 2002). The milk ducts just behind the nipple widen in response to the milk ejection reflex, but narrow again when the feeding is over and the milk moves back into the alveoli.

One way to visualize the structures of the breast is to picture a tree. The alveoli are the leaves and the ducts are the branches. Many smaller branches merge into a few bigger branches that finally become the trunk of the tree. Similarly, the breast is made up of units called lobes, each one composed of a single major duct with multiple smaller ducts and alveoli leading into it. Most authorities believe that women have approximately 15 to 20 lobes per breast, although one current study suggests that the number may be closer to seven to 10 per breast (Kent 2002).

The areola, the darker area surrounding the nipple, gets its color from the many capillaries under the skin that carry blood to the nipple. Within the areola there are sebaceous glands (which secrete oil to soften and protect the skin), sweat glands, and Montgomery's glands, which are believed to produce a substance that lubricates the nipple and protects it from germs.

Pregnancy and Lactation

The hormones of pregnancy, including estrogen, progesterone, prolactin, and others, cause complex changes to occur in the breast. The various hormones each play a specific part in preparing the body for breastfeeding. However, the change that the majority of women notice first can be summed up in one word: enlargement. During the first trimester of pregnancy, the ducts and alveoli in the breast multiply rapidly. The breasts may be tender, and their size increases in preparation for breastfeeding.

Lactogenesis is the term denoting the origin, or the beginning, of lactation, and it occurs in three stages. Lactogenesis I starts at about 12 weeks before delivery, as the mammary glands begin to secrete colostrum. Breast size increases further as the alveoli become filled with colostrum, but the presence of high levels of the hormone progesterone in the mother's blood inhibits the full production of milk until after birth.

Lactogenesis II begins after birth when the placenta is delivered. Progesterone levels fall while prolactin levels remain high. Prolactin is the main hormone in charge of lactation, and it, in turn, is controlled by hormones secreted by the pituitary, the thyroid, the adrenal glands, the ovaries, and the pancreas. More blood flows to the breasts, carrying more oxygen. Two to three days postpartum, the "milk comes in." The amount of milk produced increases rapidly, and its composition gradually changes from colostrum to mature milk. Sodium, chloride, and protein levels in the milk decrease, and levels of lactose and other nutrients increase. The color gradually changes from the golden yellow typical of colostrum to a bluish white. Since this process is controlled by hormones, the breasts begin to produce milk whether a mother is breastfeeding or not. At this stage of lactogenesis, it is important to breastfeed often (and/or pump, if the baby cannot feed well), because frequent breastfeeding in the first week after birth seems to increase the number of prolactin receptors in the breast. A receptor's job is to recognize and respond to a specific hormone. Having more prolactin receptors makes the breast more sensitive to prolactin, which researchers believe affects how much milk a mother produces in the next stage of lactogenesis.

Stage III of the lactogenesis process is also known as galactopoiesis. This is the establishment of a mature milk supply. At this time, milk production switches from endocrine (hormonal) control to autocrine control. This means that continued milk production depends more on the ongoing removal of milk from the breasts than on the hormones circulating in the blood. The "supply and demand" principle takes over. The more a mother nurses, the more milk she will produce. If she nurses less, milk production will slow down.

Physiology and Milk Supply

Understanding how milk production works can help a mother ensure that her baby is getting enough milk at the breast. For example, sometimes mothers feel that their baby has completely emptied their breast and that there is no more milk available, even though the baby wants to nurse. Knowing that new milk is constantly being produced in the alveoli will give a mother the confidence she needs to put her baby to the breast, even when it feels "empty." One study found that babies removed an average of only 76 percent of the available milk from their mother's breasts in a 24-hour period (Hartmann et al. 1993).

Emptying the breasts is what keeps milk production going. A baby's sucking sends messages to the brain, which then releases the hormone oxytocin. Oxytocin causes the muscle cells around the alveoli to contract, pushing milk down through the ducts to the nipple. This movement of milk down the ducts is called the milk-ejection reflex. Mothers may experience it as a tingling feeling or a sense of release in the breast -- which is why it is also called the "let-down." The let-down empties the alveoli and makes the milk available to the baby at the nipple. When the alveoli are empty, they respond by making more milk. Recent research suggests that a special protein in human milk, called feedback inhibitor of lactation (FIL), regulates milk production (Wilde 1995). When there is a lot of milk in the breast, FIL inhibits, or prevents, the alveoli from making more. When milk is removed from the breast—and FIL is not there to stop milk production—the alveoli get busy and manufacture more milk. This is why it is important to nurse often and to encourage the baby to empty the breast as much as possible for optimal milk supply.

Another consideration related to milk supply is the breasts' storage capacity. Sometimes small-breasted women worry that they may not be able to make enough milk for their babies, but the milk production process makes adjustments for breast size. Smaller breasts may not be able to store as much milk between feedings as larger breasts, but if they are emptied often enough, they will make as much milk as the baby needs. Women with larger breasts and greater storage capacity may be able to go longer between feedings without affecting their supply. On the other hand, women with smaller breasts may need to nurse more frequently since their breasts fill faster and milk production slows down as the alveoli become full. Frequent nursing is not only good for supply, but it is also a healthy habit that helps mothers avoid plugged ducts and breast infections.

Does a mother need to know how much milk her own breasts can store in order to know how often she should feed her baby? No. Healthy babies with good breastfeeding skills take as much milk as they need when they need it, without mothers giving much thought to the whole process. But knowing how the whole process works can help a mother solve any problems she may be having with milk supply. It can also help her think through some of the myths and misunderstandings people have about breastfeeding. For example, she will know that she doesn't have to wait for her breasts to "fill up" between feedings -- there is always milk there for the baby. She will also know that if her baby seems hungry or is going through a growth spurt, nursing more often will speed up her milk production almost instantly.

How Do Substances Enter Human Milk?

Learning about lactation also helps a woman understand how things such as food proteins, contaminants, and drugs enter human milk. This can help her make informed decisions about what she exposes her body to when she is breastfeeding.

How do substances enter human milk? When someone takes medication or eats food, the substance is usually broken down by the digestive tract and then molecule-sized components of the substance are absorbed into the blood. When these molecules get to the capillaries near the breast tissue, they move through the cells that line the alveoli and into the milk, a process known as diffusion.

This is how ingredients needed to make milk get into the milk, as well as how drugs and other foreign substances enter milk. But many factors influence whether or in what quantity a substance will actually enter the milk. In the first days after birth, there are gaps between the lactocytes, the cells that line the alveoli and either block or allow substances to enter. These gaps mean that substances pass rather freely into the milk in the first days of life. After a few days, the gaps close. From then on, it is harder for substances to cross the barrier between the blood and the milk.

The process of diffusion allows good things, such as antibodies, to easily enter colostrum and mature milk. Antibodies are a kind of protein found in blood that help the body fight infection. They are found in greater concentrations in human milk at the beginning and at the end of lactation. One important antibody, secretory immunoglobulin A (SIgA), is both synthesized and stored in the breast. SIgA is in good company with about 50 other antibacterial factors, many of which enter human milk from the mother's blood. And that is not including the factors that haven't been identified yet! This is one of the great advantages of breastfeeding. All women transfer antibodies to their babies during pregnancy and birth, but breastfeeding prolongs the time in which the mother's body helps protect the baby from illness.

However, other substances also enter the breast by diffusion. One fairly common belief about breastfeeding is that if mothers eat foods that make her gassy, such as broccoli or cabbage, the baby will be gassy. Is this true? No. Gas from a mother's intestinal tract cannot pass into her blood and deposit itself into the breast for her baby to drink. However, when food is digested, some of the proteins do enter the blood and may then pass into a mother's milk. Some babies may be sensitive to a particular protein and react with gas and fussiness. If a baby has an obvious reaction every time a mother eats a certain food, she can eliminate that food from her diet, but it is important to remem-ber that, for most fussy and gassy babies, the problem stems from some source other than mother's milk. Allergic reactions to substances in mother's milk may also appear as skin, respiratory, and intestinal problems. When a family has a known history of an allergy to a specific food, mothers are advised to avoid that food during breastfeeding.

With all that said, the average nursing mother should feel free to eat whatever she likes and be assured that the vast majority of babies do not have any problems with food proteins. (For more information, see "Allergies and the Breastfeeding Family" in the July-August 1998 issue of New Beginnings.)

Medications the mother takes may also get into her milk by crossing from blood through the lactocytes into the alveoli. According to Thomas Hale, PhD, author of Medications and Mothers' Milk, there are several factors that influence drug transfer into milk. One important determinant is the level of the drug in the mother's blood. When the concentration of drug is high in the blood, more of it will diffuse into the milk where the concentration of the drug is low. Diffusion tries to keep the concentration of a substance equal on both sides of whatever barrier is between them. So, as the concentration of a substance in the mother's blood begins to drop, the particles of the substance in the milk will diffuse back into the blood, and the concentration in the milk will drop as well.

This is important to understand. Mothers sometimes think that after they drink a glass of wine the alcohol stays in their milk until that milk is removed from the breast. As a result, they may hesitate to breastfeed their baby and decide to pump the milk out instead. In reality, the level of alcohol in the milk drops as a mother's blood levels drop. It takes two to three hours for a 120-pound woman to eliminate from her body the amount of alcohol found in a single glass of wine or beer. When the alcohol is eliminated from her blood, it is also gone from her milk.

Some other considerations affecting how much of a drug enters a mother's milk include the molecular weight of the drug (i.e., how big the molecules are), protein binding, and lipid (fat) solubility. Drugs with low molecular weights transfer into milk more easily. Drugs that are highly protein-bound are mostly "stuck" to proteins in the plasma and are not free to enter breast milk. Human milk contains more lipids than plasma, so drugs that are lipid soluble may concentrate in the milk fats. In Medications and Mothers' Milk, Hale writes that most medications are compatible with breastfeeding. When they are not, an alternative medication is almost always available. As with all medications, women will want to consult with their doctors. (For more information on medications and breastfeeding, see "Medications and the Breastfeeding Mother" in the November-December 2003 issue of New Beginnings.)

Medical science now comprehends more about the physical process of lactation than ever before. We understand many of the structures in the breast, and we know a great deal of helpful information about how those structures work with regard to milk supply. Compared to previous generations, we have a good understanding of how substances enter mother's milk. This allows us to have more successful experiences with breastfeeding and to solve problems when they arise. It also allows us to better appreciate the process when things are going smoothly!


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