The human breasts, or mammary glands, sit on the anterior chest wall over the pectoralis major muscles. They typically extend vertically from the second to the sixth ribs and horizontally from the sternum to the mid-axillary line.
Key surface landmarks include:
Beneath the skin and fatty layers lies the functional glandular tissue of the breast. This internal framework is divided into 15 to 20 distinct lobes arranged radially around the nipple like slices of a pie.
Each lobe subdivides into smaller clusters called lobules. These lobules contain the actual milk-producing secretory units known as alveoli. During pregnancy and lactation, these glandular structures multiply and enlarge rapidly under hormonal influence to prepare for milk synthesis. In non-lactating states, the glandular tissue remains relatively quiescent and much smaller in volume compared to the surrounding adipose tissue.
Once milk is synthesized in the alveoli, it travels through a branching network of tubes toward the exterior. Each of the 15 to 20 lobes drains via its own lactiferous duct.
As these ducts converge near the areola, they widen slightly to form lactiferous sinuses, which serve as temporary storage reservoirs for milk before a nursing infant triggers a let-down reflex. From these sinuses, the pathway narrows again into tiny collecting tubules that pierce the nipple surface to open to the outside environment.
The shape, firmness, and support of the breast rely heavily on non-glandular supporting elements. The stroma consists of adipose tissue, which determines overall breast size, and fibrous connective tissue.
Running through this stroma are suspensory ligaments of Cooper. These fibrous bands extend from the deep fascia of the chest muscle up through the breast tissue to attach firmly to the dermis of the skin. They act as natural internal brassiere straps resisting gravity. When a breast malignancy causes fibrosis and shortening of Cooper's ligaments, it pulls the overlying skin inward, creating the classic clinical sign known as skin dimpling.
Blood reaches the breast through branches of the internal thoracic artery, the lateral thoracic artery, and intercostal arteries. Venous return largely mirrors this arterial network, feeding into the axillary and internal jugular veins.
Lymphatic drainage is clinically crucial, especially regarding breast cancer metastasis. More than 75% of the lymph from the breast drains into the ipsilateral axillary lymph nodes. The remaining lymph flows toward internal mammary, parasternal, and supraclavicular nodes. Sensory innervation is supplied by the lateral and anterior cutaneous branches of the intercostal nerves, particularly the fourth intercostal nerve which supplies sensation to the nipple-areola complex.
Breast tissue undergoes continuous morphological evolution from embryonic development through old age.
Milk production and delivery depend on a precise hormonal interplay between the pituitary gland and the breast.