The breast is made up of three main types of tissue: fat, glandular tissue, and connective tissue. Fat makes up the largest portion, typically around 70% of breast volume in most people, while glandular tissue accounts for roughly 30%. A layer of skin encloses everything. The exact ratio varies widely from person to person and changes with age, hormonal shifts, and body composition.
Fat, Glandular Tissue, and Connective Tissue
Despite a long-held assumption that breasts are roughly 50% fat and 50% glandular tissue, imaging studies show the actual breakdown skews heavily toward fat. In most people, breast composition is closer to 70% fat and 30% glandular tissue. Even in denser breasts, the glandular fraction seldom exceeds 50%.
Fat (adipose tissue) gives the breast most of its size and soft texture. It fills the spaces between and around the glandular structures, cushioning them. This is why breast size can change noticeably with weight gain or loss.
Glandular tissue is the functional part of the breast, responsible for producing milk. Connective tissue, made largely of collagen, acts as the structural scaffold that holds everything together and gives the breast its shape.
How the Milk-Making System Is Organized
Each breast contains 15 to 20 lobes arranged in a pattern somewhat like the sections of an orange. Each lobe is divided into smaller units called lobules, which contain tiny bulb-shaped glands at their ends. These are the glands that actually produce milk.
From the lobules, small tubes called milk ducts carry fluid toward the nipple. The ducts from each lobe converge and eventually open at the nipple’s surface through 10 to 20 tiny pores. This entire system sits dormant for most of a person’s life and only becomes fully active during pregnancy and breastfeeding.
The Support Structure
Connective tissue runs throughout the breast in several forms. Two layers of fascia, sheets of tough collagen, sandwich the breast tissue: a deep layer sits against the chest muscles, and a superficial layer lies just beneath the skin. These two layers meet at the collarbone and at the fold under the breast.
Running between them are Cooper’s ligaments, a three-dimensional mesh of collagen fibers that span the entire breast volume. These ligaments create small pockets that hold fat in place and connect the skin to the deeper structures. They’re the reason breasts hold their shape against gravity, and their gradual stretching over time is a major cause of sagging.
Behind all of this, between the breast and the pectoral muscle, sits a thin layer of loose connective tissue called the retromammary space. This layer allows the breast to move freely over the chest wall rather than being fixed in place.
The Nipple and Areola
The nipple is a small cylindrical projection, averaging 10 to 12 millimeters wide and 9 to 10 millimeters tall. It contains no subcutaneous fat. Instead, the skin sits directly on a thin layer of smooth muscle fibers arranged in two directions: circular fibers (called the muscle of Sappey) and radial fibers (the muscle of Meyerholz). These muscles are what cause the nipple to become erect in response to cold, touch, or stimulation.
The areola, the pigmented ring surrounding the nipple, averages 3 to 6 centimeters across. Its surface has small raised bumps created by sebaceous glands called the glands of Montgomery. During pregnancy, these glands enlarge noticeably. They secrete an oily substance that keeps the areola lubricated, which becomes especially important during breastfeeding. The nipple itself lacks these sebaceous glands, which is why nipple skin tends to be drier.
Blood Supply and Sensation
The breast has a rich blood supply drawn from multiple arteries. The largest contributor is the internal mammary artery, running behind the breastbone, which provides at least 60% of the breast’s blood through branches that perforate the chest wall. Additional blood comes from arteries along the side and back of the chest.
Veins mirror this supply in two networks. Superficial veins run just beneath the skin, forming a web around the areola. Deeper veins drain into the chest wall. These superficial veins sometimes become visible through the skin, particularly during pregnancy when blood flow increases.
Sensation across the breast comes from branches of the intercostal nerves, the same nerves that run between the ribs. The nipple specifically gets its sensation from a nerve branch at the fourth rib level. This is why certain surgeries near that nerve can affect nipple sensation.
The Lymphatic Network
Woven throughout the breast is a network of lymphatic vessels, thin channels that carry fluid and immune cells away from the tissue. These vessels run through the superficial layer of tissue between the skin and the breast, with some passing through the breast tissue itself.
Nearly all of these lymphatic collectors drain toward lymph nodes in the armpit (axilla), typically converging on one or a few “sentinel” nodes near the edge of the pectoralis minor muscle. In some people, almost the entire breast drains to a single sentinel node. A smaller set of lymphatic vessels follows the internal mammary blood vessels and drains toward nodes behind the breastbone. This drainage pattern is the reason breast cancer often spreads first to the armpit lymph nodes.
How Breast Composition Changes Over Time
Breast tissue is not static. It responds to hormones throughout the menstrual cycle. During the luteal phase (the two weeks before a period), glandular tissue swells with fluid, the tissue layers become more distinct, and the connective tissue around them becomes edematous, or waterlogged. This is what causes the breast tenderness and fullness many people notice before their period.
Over a lifetime, the most significant change is a process called lobular involution. As people age, particularly after menopause, the glandular lobules gradually shrink and are replaced by fat. This is why breasts typically become softer and less dense with age. However, this process doesn’t happen on a predictable schedule. Research from the Mayo Clinic found that 40% of postmenopausal women haven’t completed involution, and these women face a higher risk of breast cancer.
Breast Density Categories
Because the ratio of glandular tissue to fat varies so much between individuals, radiologists classify breast density into four categories when reading mammograms:
- Almost entirely fatty: very little glandular or connective tissue
- Scattered fibroglandular elements: mostly fat with some areas of denser tissue
- Heterogeneously dense: large areas of glandular and connective tissue with some fat
- Extremely dense: very little fat, predominantly glandular and connective tissue
Density matters for two reasons. Dense tissue appears white on a mammogram, the same shade as tumors, making cancers harder to spot. And higher density is independently associated with increased breast cancer risk. Many states now require that mammography reports include a density notification so patients can discuss supplemental screening options with their provider.