Structure
The breast is made of glandular lobes set in fat and supported by fibrous septa, all lying superficial to the deep fascia of the chest wall.
Position and extent
The base of the breast extends vertically from the 2nd to the 6th rib and horizontally from the lateral border of the sternum to about the mid-axillary line. About two-thirds lies on the pectoral fascia over pectoralis major; the rest lies on serratus anterior and, inferomedially, on the external oblique aponeurosis. The axillary tail (of Spence) extends up and laterally along the lower border of pectoralis major and pierces the deep fascia to reach the axilla. A layer of loose tissue, the retromammary space, separates the breast from the pectoral fascia and lets it move freely.
Glandular tissue
The gland is divided into around 15 to 20 lobes, arranged radially. Each lobe drains by a lactiferous duct that widens into a lactiferous sinus beneath the areola and opens on the nipple. The functional unit is the terminal duct lobular unit, where most breast cancers begin.
Supporting framework
Fibrous suspensory ligaments (of Cooper) run from the dermis to the deep fascia between the lobes and support the breast.
Nipple and areola
The nipple contains smooth muscle and the openings of the lactiferous ducts. The pigmented areola carries areolar glands (of Montgomery), which enlarge in pregnancy. In males and in females before puberty the nipple usually lies in the 4th intercostal space, though its position varies widely in adult females.
Blood supply and innervation
The breast is supplied from its medial side by the internal thoracic artery and from its lateral side by branches of the axillary artery, with a lesser contribution from the intercostal arteries.
- Medial: perforating branches of the internal thoracic artery, mainly through the 2nd to 4th intercostal spaces.
- Lateral: the lateral thoracic artery and the pectoral branches of the thoraco-acromial artery, both from the axillary artery.
- Deep: lateral mammary branches of the posterior intercostal arteries in the 2nd to 4th spaces.
Veins follow the arteries into the axillary, internal thoracic and intercostal veins. The intercostal veins connect with the vertebral venous plexus, a valveless route that allows breast cancer to spread to the vertebrae without passing through the lungs.
Sensation comes from the anterior and lateral cutaneous branches of the 4th to 6th intercostal nerves. The nipple lies in the T4 dermatome.
Lymphatic drainage
Most lymph from the breast drains to the axillary nodes, with the medial part also draining to parasternal nodes along the internal thoracic vessels.
Lymph collects in a subareolar plexus and in plexuses within the gland. Most passes to the pectoral (anterior) group of axillary nodes and then on through the central and apical groups. Some lymph, especially from the medial quadrants, passes to the parasternal (internal thoracic) nodes. Minor routes cross to the opposite breast or pass down to the anterior abdominal wall and subdiaphragmatic region.
| Surgical level | Position relative to pectoralis minor | Nodes included |
|---|---|---|
| Level I | Below and lateral to its lower border | Pectoral, subscapular (posterior) and humeral (lateral) groups |
| Level II | Deep to the muscle | Central group, interpectoral (Rotter's) nodes |
| Level III | Above and medial to its upper border | Apical group |
Apical nodes drain to the subclavian lymph trunk and then to the venous system at the root of the neck.
Clinical relevance
Breast anatomy explains the signs of breast cancer, the pattern of spread and the nerves at risk in axillary surgery.
- Upper outer quadrant: contains the most glandular tissue, including the axillary tail, and is the commonest site of carcinoma.
- Skin dimpling: a tumour shortening the suspensory ligaments tethers the overlying skin.
- Peau d'orange: blocked dermal lymphatics cause skin oedema pitted at hair follicles.
- Nipple retraction: tumour pulls on the lactiferous ducts.
- Fixation to pectoralis major: invasion of the retromammary space and pectoral fascia makes the lump immobile when the muscle contracts.
- Sentinel node biopsy: identifies the first axillary node draining the tumour.
- Axillary surgery: can injure the long thoracic nerve (winged scapula), the thoracodorsal nerve (latissimus dorsi) and the intercostobrachial nerve (numbness of the medial upper arm), and may lead to arm lymphoedema.
- Accessory nipples: may occur anywhere along the embryological milk line from axilla to groin.