The breasts, located on the front of the chest, are medically known as the mammary glands. The term “breast” is sometimes used to refer to the area at front of the chest.
What are the anatomical features of the breast?
The mammary gland is made up of lobules, glandular structures that produce milk in females when stimulated to do so. The lobules drain into a system of ducts, connecting channels that transport the milk to the nipple.
Between the glandular tissue and ducts, the breast contains fat tissue and connective tissue.
Both males and females have breasts; the structure of the male breast is nearly identical to that of the female breast, except that the male breast tissue lacks the specialized lobules, as there is no physiologic need for milk production by the male breast.
Abnormal enlargement of the male breasts is medically known as gynecomastia.
The breast does not contain muscles. Breast tissue is located on top of the muscles of the chest wall. Blood vessels and lymphatic vessels (a system of vessels that drains fluid) are located throughout the breast.
The lymphatic vessels in the breast drain to the lymph nodes in the underarm area (axilla) and behind the breast bone (sternum).
In females, milk exits the breast at the nipple, which is surrounded by a darkened area of skin called the areola. The areola contains small, modified sweat glands known as Montgomery’s glands.
These glands secrete fluid that serves to lubricate the nipple during breastfeeding.
The breast is a mass of glandular, fatty and connective tissue. The breast is made up of:
- lobules – glands that produce milk
- ducts – tubes that carry milk from the lobules to the nipple
- fatty and connective tissue – surrounds and protects the ducts and lobules and gives shape to the breast
- areola – the pink or brown, circular area around the nipple that contains small sweat glands, which release (secrete) moisture as a lubricant during breast-feeding
- nipple – the area at the centre of the areola where the milk comes out
Ligaments support the breast. They run from the skin through the breast and attach to muscles on the chest.
There are several major nerves in the breast area, including nerves in the chest and arm. There are also sensory nerves in the skin of the chest and axilla.
The lymphatic system of the breast
The breast has many blood vessels and lymph vessels. Lymph vessels are thin tubes similar to blood vessels. They collect and move lymph fluid away from the breast into small bean-shaped masses of lymphatic tissue, called lymph nodes, in the area around the breast.
The lymph vessels and lymph nodes are part of the lymphatic system, which helps fight infections.
The breast lymph nodes include:
- supraclavicular nodes – above the collarbone
- infraclavicular (or subclavicular) nodes – below the collarbone
- axillary nodes – in the armpit (axilla)
- internal mammary nodes – inside the chest around the breastbone (sternum)
Axillary lymph nodes
There are about 30–50 lymph nodes in the axilla. The number varies from woman to woman.
The axillary lymph nodes are divided into 3 levels according to how close they are to the pectoral muscle on the chest:
- level I (low axilla) – located in the lower or bottom part of the armpit, along the outside border of the pectoral muscle
- level II (mid axilla) – located in the middle part of the armpit, beneath the pectoral muscle
- level III (high axilla) – located below and near the centre of the collarbone, above the breast area and along the inside border of the pectoral muscle
When breast cancer spreads, it usually goes to level I lymph nodes first, to level II next and then to level III.
How does breast tissue develop?
Breast tissue begins to form in the fourth week of fetal life. In the fetus, breast tissue develops along two “milk lines” that start at the armpit and extend to the groin.
Uncommonly, an extra (ancillary) breast can develop along this line. On the skin surface, an extra nipple (supernumerary nipple) may develop along this line.
Breast tissue changes at different times during a woman’s life. It changes during puberty, during the menstrual cycle, during pregnancy and after menopause.
Female breasts do not begin growing until puberty (around 10–12 years of age). At this time, the breasts respond to hormonal changes (mostly increased estrogen and progesterone) in the body and begin to develop.
During puberty, the breast ducts and milk glands grow.
The breast skin stretches as the breasts grow, creating a rounded appearance. Young women tend to have denser breasts (more glandular tissue) than older women.
In older women, much of the glandular and ductal tissue is replaced with fatty tissue and breasts become less dense. Ligaments also lose their elasticity when women age, causing the breasts to sag.
The size and shape of women’s breasts vary considerably. Some women have a large amount of breast tissue and have larger breasts.
Others have a smaller amount of tissue with little breast fat. A woman’s breasts are rarely the same size. Often one breast is slightly larger or smaller, higher or lower or shaped differently than the other.
Hormones and the breast
Estrogen is the main female hormone. It influences female sexual characteristics, such as breast development, and it is necessary for reproduction.
Most of the estrogen in a woman’s body is made by the ovaries, though a small amount is made by the adrenal glands.
Progesterone is the other female sex hormone made in the ovaries. Its role is to prepare the uterus (womb) for pregnancy and the breasts for producing milk for breast-feeding (lactation).
The breast tissues are exposed to monthly cycles of estrogen and progesterone throughout a woman’s childbearing years.
In the first part of the menstrual cycle, estrogen stimulates the growth of the milk ducts.
Progesterone takes over in the second part of a woman’s menstrual cycle, stimulating the lobules.
After menopause, the monthly cycle of estrogen and progesterone end. However, the adrenal glands continue to produce estrogen so that a woman keeps her sexual characteristics.
The breast’s main function is to produce, store and release milk to feed a baby. Milk is produced in lobules throughout the breast when they are stimulated by hormones in a woman’s body after giving birth.
The ducts carry the milk to the nipple. Milk passes from the nipple to the baby during breast-feeding.
How are human breasts different from other species?
In other primates (such as apes), the breasts develop only when they are producing milk. After the young have been weaned, the breasts flatten again.
In humans, the breasts enlarge at puberty and stay enlarged throughout a woman’s life.
What are the most common medical conditions affecting the breasts?
Breast health is a source of concern for most women. Although breast cancer is a fairly common malignancy affecting one out of every eight women in the U.S. at some point in life, benign (non-cancerous) conditions of the breast are much more common.
In fact, most masses and lumps in the breasts are not cancer. Breast cancer occurs in males as well, but it accounts for a small percentage of all breast cancers.
Among the benign breast conditions, cysts and fibrocystic changes are common. One type of benign tumor in particular, known as a fibroadenoma, is common in young women. Infections of the breast tissue can also occur, particularly during breastfeeding.
Mastitis is the medical term for inflammation of the breast.