The breast in the male and in the pre-pubertal female is rudimentary but, in the female, it begins to develop significantly at puberty. In the reproductive female, it occupies the superficial fascia over the pectoralis major muscle, and extends roughly from the lateral margin of the sternum to the mid-axillary line, overlying the second to sixth ribs. Non-lactating breasts contain a system of lactiferous ducts embedded in fat. The breast can be divided into about 15-20 lobes which radiate from the nipple and are separated from each other by fibrous septa, or suspensory ligaments. Under normal conditions, the breast can move freely over the underlying muscle. However, a tumour invading the retromammary space can fix the breast to the muscle so that the breast moves when the muscle contracts.
During pregnancy, the deepest parts of the duct system (alveolar ducts) proliferate and differentiate into numerous secretory alveoli that secrete a protein-rich fluid (colostrum) which accumulates in the lumina of the alveoli and ducts, leading to significant breast enlargement. After birth the nature of the secretion changes to produce breast milk. Following menopause, breast size often diminishes.
The blood supply to the breast is generally from vessels in the vicinity. Hence, the internal thoracic, intercostal, lateral thoracic and thoraco-acromial arteries send branches to it. Corresponding veins drain the breast tissue, much of which eventually reaching the axillary vein.
The lymphatic drainage of the breast is of particular clinical importance with respect to the spread of cancer. Metastatic spread along lymphatics leads to the enlargement of nodes, some of which are palpable. For the purposes of generalizing the main pathways of lymphatic drainage, the breast can be divided into a lateral two-thirds and a medial one-third. The medial one-third of the breast drains medially to the (para)sternal nodes which accompany the internal thoracic artery. The lateral two-thirds drains laterally to axillary nodes, of which there are five main groups:
- anterior: deep to pectoralis major
- posterior: along the subscapular vessels
- lateral: along the axillary vein
- central: in the axillary fat
- apical: immediately behind the clavicle, at the apex of the axilla.