Breast tissue sits on the front of the chest wall, spanning from the second rib at the top down to the sixth rib at the bottom. Horizontally, it stretches from the edge of the breastbone to the mid-axillary line, which is roughly the middle of your armpit. This footprint is larger than most people expect, covering a significant portion of the upper chest rather than just the visible mound.
The Boundaries of the Breast
The breast occupies a fairly consistent region regardless of breast size. Its upper border sits at the level of the second rib, just below the collarbone. The lower border reaches the sixth rib, roughly where the underwire of a bra would sit. On the inner side, the tissue extends to the lateral edge of the sternum (the flat bone running down the center of your chest). On the outer side, it reaches the mid-axillary line, a vertical line dropping from the center of the armpit.
This means breast tissue doesn’t just exist within the visible breast mound. It extends above, below, and to the sides of what you can see and feel as the obvious breast shape. The upper and outer portions, in particular, spread further across the chest than many people realize, which is why thorough breast exams cover a wide area from the collarbone to below the bra line and from the breastbone to the armpit.
What Sits Beneath the Breast
The breast doesn’t attach directly to muscle. It rests on the pectoralis major, the large chest muscle, but is separated from it by layers of connective tissue called fascia. There’s a front layer of fascia in front of the breast gland and a back layer behind it. Between the back layer and the chest wall muscle lies a small space filled with loose connective tissue, sometimes called the subglandular space. This pocket of loose tissue is what allows your breast to move freely over the chest wall rather than being rigidly fixed in place.
Running throughout the breast are fibrous bands called Cooper’s ligaments. These form a three-dimensional net that holds the fatty tissue and glandular tissue in shape. They anchor to the skin on one end and to the pectoral muscle fascia on the other. Cooper’s ligaments are what give the breast its structure and support. Over time, stretching of these ligaments contributes to natural changes in breast shape.
How Tissue Is Distributed Across the Breast
The breast is typically divided into four quadrants: upper outer, upper inner, lower outer, and lower inner. These quadrants don’t contain equal amounts of glandular tissue. MRI studies measuring the ratio of glandular tissue to total breast volume show that the lower outer quadrant has the highest concentration of glandular tissue, averaging around 27% glandular density. The upper outer quadrant, somewhat surprisingly, has the lowest at roughly 15 to 16%. The two inner quadrants fall in between, at about 20 to 24%.
The rest of each quadrant is composed of fat, which makes up the majority of breast volume in most people. The ratio of fat to glandular tissue varies enormously from person to person and changes with age, hormonal shifts, pregnancy, and body weight. Younger women tend to have denser, more glandular breasts. After menopause, glandular tissue gradually replaces itself with fat, which is one reason mammograms become easier to read in older women.
The Area Near the Armpit
You may have heard the term “axillary tail of Spence,” a supposed tongue of breast tissue that angles up from the outer breast into the armpit. This concept appears in most anatomy textbooks. However, recent anatomical research has challenged whether this tail actually exists as a continuous extension of the breast. A study published in Plastic and Reconstructive Surgery Global Open found that in dissections and clinical examinations, no subjects had a continuous band of fat connecting the breast to the axilla. Instead, any fullness near the armpit was a separate lateral chest wall fat pad, not a direct extension of the breast.
This distinction matters clinically. Lymphatic drainage channels do travel from the breast through the armpit area, which is why the axillary lymph nodes are important in breast cancer staging. But the tissue itself near the armpit may not be breast tissue in the traditional sense. Cancers found in that area may arise from separate embryonic remnants of breast tissue rather than spreading from the main breast.
Breast Tissue in Men
Men have breast tissue in the same anatomical location as women, spanning from the second to the sixth rib between the sternum and the mid-axillary line. The key difference is composition. Male breast tissue is predominantly fat with a few rudimentary ducts and supporting tissue. It contains no lobules, the milk-producing units that develop in the female breast.
That said, palpable breast tissue is far more common in men than most people assume. Estimates suggest 50 to 70% of the male population has some degree of detectable breast tissue, a condition called gynecomastia when it becomes noticeable. This tissue sits in the same subareolar region, just beneath the nipple and areola, and is composed of the same basic cell types found in female breast tissue, just in far smaller quantities.
Breast Tissue Outside the Breast
In some people, breast tissue develops in locations well beyond the normal breast region. This is called ectopic breast tissue, and it traces back to early embryonic development. During the fifth and sixth weeks of gestation, a structure called the mammary ridge (or milk line) forms. This ridge runs from the armpit down the front of the torso to the inner thigh. Normally, all of this tissue regresses except for the two spots that become the breasts. When that regression is incomplete, extra breast tissue or even extra nipples can persist anywhere along this line.
The armpit is the most common site for ectopic breast tissue. It can also appear in the groin, vulva, perineum, or thigh. Rare cases have been documented on the face, neck, ear, scapula, knee, and foot. Ectopic breast tissue is not just a cosmetic curiosity. It can swell and become tender during hormonal changes like menstruation or pregnancy, and it carries the same risk of developing breast disease, including cancer, as tissue in the normal location.