A breast is a mound of tissue on the front of the chest that, in females, contains glands capable of producing milk. Both men and women have breast tissue, but the structure and function differ significantly between the two. The breast sits over the chest muscles, extending roughly from the second to the sixth rib, with the breastbone as its inner border.
What the Breast Is Made Of
Despite its outward simplicity, the breast has a surprisingly complex internal structure. Each female breast contains 15 to 20 sections called lobes, arranged in a circular pattern like the segments of an orange. Each lobe is further divided into smaller sections called lobules, which end in tiny bulbs capable of producing milk. Thin tubes called ducts connect the lobules and lobes, channeling milk toward the nipple.
Fat is the dominant tissue in the breast. On average, about 73% of breast volume is fat, roughly 17% is glandular tissue (the lobes and lobules), and the remaining 10% is skin. Even in women whose breasts appear dense on a mammogram, glandular tissue rarely exceeds 50% of total volume. This fat layer is what gives the breast its size and shape, which is why breast size varies so widely from person to person while having no bearing on the ability to produce milk.
A network of ligaments runs through the breast, anchoring it to the chest wall and the overlying skin. These fibrous bands help maintain the breast’s shape and position against gravity.
How Breasts Produce Milk
The primary biological function of the female breast is lactation. Two hormones drive the process. Prolactin, released from the brain when a baby suckles, signals the glandular tissue to produce milk. The more frequently the breast is emptied, the more prolactin is released, which is why regular feeding or pumping is the single most important factor in maintaining milk supply.
The second hormone, oxytocin, handles delivery. When the nipple is stimulated, nerve signals travel to the brain and trigger oxytocin release. This causes tiny muscle cells wrapped around the milk-producing glands to contract, squeezing milk out through the ducts and toward the nipple. This is commonly called the “let-down” reflex, and many breastfeeding women can feel it as a tingling or tightening sensation.
The Lymphatic System in the Breast
The breast has an extensive network of lymph vessels, tiny channels that carry fluid and immune cells away from the tissue. Most of this fluid drains into lymph nodes in the armpit (axillary nodes), which are organized into three levels depending on their depth and position relative to the chest muscles. A smaller portion drains into nodes along the breastbone (internal mammary nodes) or just above the collarbone.
This drainage network matters because it’s the most common route breast cancer uses to spread beyond the breast. It’s also why doctors check the armpit lymph nodes during breast cancer evaluation.
Male Breast Tissue
Men have breast tissue too, covering the same area of the chest wall as in women. The key difference is composition: male breast tissue is mostly fat with a few ducts and supporting tissue, but it contains no lobules, the milk-producing structures. Without lobules, male breast tissue can’t produce milk under normal circumstances.
In some cases, hormonal imbalances can cause male breast tissue to grow, a condition called gynecomastia. This is relatively common during puberty and in older men, and it involves an increase in both glandular and fatty tissue. Male breast cancer is also possible, though it accounts for less than 1% of all breast cancer cases.
How Breasts Develop
Breast development is one of the first visible signs of puberty in girls, typically beginning between ages 8 and 13. The process unfolds in stages driven by rising estrogen levels. It starts when the brain begins releasing hormones that signal the ovaries to produce estrogen, which in turn stimulates breast tissue to grow.
Development progresses through five recognized stages. It begins with a small, firm “breast bud” forming beneath the areola. Over the following years, breast tissue expands beyond the areola, the areola itself becomes elevated and more defined, and eventually the breast reaches its adult shape with a fully developed nipple. The entire process typically takes three to five years, though the timeline varies considerably.
Changes During Pregnancy
Pregnancy triggers major changes in breast tissue, often noticeable within the first few weeks. During the first trimester, rising estrogen levels cause the ductal system to expand and branch deeper into the surrounding fat, building the infrastructure needed for milk delivery. The breasts increase in size, the areola darkens, and small bumps on the areola called Montgomery glands become more prominent. These glands secrete an oily substance that lubricates and protects the nipple during breastfeeding.
By the end of pregnancy, the lobules have multiplied and matured to the point where they can begin producing colostrum, a thick, nutrient-rich precursor to mature breast milk.
How Breasts Change With Age
Breast tissue undergoes a gradual transformation over a woman’s lifetime. After menopause, when estrogen and progesterone levels drop, the glandular tissue slowly shrinks in a process called involution. The milk-producing lobules decrease in both number and size, the supportive tissue between them is replaced by dense collagen, and eventually fat fills in where glandular tissue used to be.
This is a normal part of aging and explains why breasts often become softer and less firm over time. It also means that mammograms become easier to read in older women, because fatty tissue appears darker on imaging while dense glandular tissue appears white, which can obscure abnormalities.
Breast Cancer Screening
The U.S. Preventive Services Task Force recommends that all women begin mammography screening at age 40 and continue every two years through age 74. This is a shift from earlier guidelines, which left the decision about when to start screening between ages 40 and 50 up to the individual. The updated recommendation reflects evidence that starting at 40 reduces the risk of dying from breast cancer across all populations.