In women, the breasts are paired subcutaneous structures that lie on the anterior thorax, superficial to the pectoralis muscle. The size and shape of the breast are highly variable between women and also change with age. The breast extends over the lateral margin of pectoralis major and may also spread into the axilla. The breast is notable for the nipple and the surrounding areola, which are both pigmented relative to the surrounding skin. The nipple typically protrudes from the surface of the breast but this is also highly variable.
Each breast contains 15 - 20 lobes of glandular tissue, which also vary in size depending on various factors. Each gland empties via a lactiferous duct which passes to the skin of the nipple. The lobes are surrounded by a specialised stroma which gives off numerous bands of tissue, known as suspensory ligaments, to the underlying fascia and overlying skin.
Boundaries / Relations
The breast is typically bounded by:
- The 2nd rib above
- The 6th rib below
- The lateral margin of the sternum medially
- The mid-axillary line laterally
The relations of the breast are:
- The epidermis of the skin superficially
- The chest wall (consisting of pectoral fascia, pectoral muscles, ribs/costal cartilage, and pleura) deeply
The lactiferous ducts give off numerous branches within each lobe of the gland. Each branch ends in a group of terminal ducts. In men and before menstruation, the ducts are blind ending tubes with no secretory function. Following the onset of hormonal stimulation, the epithelium of the ducts proliferate and form acini. The resulting structure is the terminal duct lobular unit.
The junction of the TDLU with the larger branches of the lactiferous ducts is theorised to be a common site of malignant cell origin
The epithelium is typically bilayered, with an overlying cuboidal/columnar epithelium and an underlying myoepithelial layer. The cuboidal layer is typically simple in the TDLU and bilayered in the larger ducts. The myoepithelial layer is highly visible and gives the impression of a bilayered epithelium, even in the TDLU.
This underlying layer is lost in malignancy, allowing the pathologist to determine whether a gland is malignant or not.
The ends of the lactiferous ducts are lined by stratified squamous epithelium that is continuous with the epidermis.
In males and prepubertal females, the breasts are undeveloped, with blind ending tubules arising from the lactiferous ducts.
Oestrogen stimulates development of the TDLU in adolescent females, which form clusters of cells at the end of each tubule. There is also differentiation of cells in the stroma to adipocytes, causing an enlargement of breast.
During the follicular phase of the menstrual cycle, the stroma reduces in size and the ducts regress, with epithelial cells undergoing apoptosis. When the luteal phase begins, there is proliferation of stroma and ducts. Secretion into the tubules begins. During pregnancy, the cell clusters at the ends of tubules proliferate into alveoli and secretion increases. After lactation ceases, there is reduction in the size and number of epithelial cells with resulting shrinkage of the breast; the alveoli remain and do not return to the pre-lactation state. After menopause, there is loss of glandular tissue (through apoptosis) and replacement with adipocytes, although some ductal tissue remains throughout life.
Mastectomy involves the removal of the breast, possibly with other tissues. Typically, the skin of the breast and the soft tissues superficial to the deep fascia are removed; the pectoral muscles are left in place. Skin sparing mastectomy usually involves reconstruction at the time of surgery. 'Radical mastectomy' is rarely performed and includes removal of the pectoral muscles.
The breast is supplied by several arteries:
- Perforating branches of the internal thoracic artery pass through the fascia between the ribs to supply the medial parts of the breast
- Intercostal arteries give off lateral cutaneous branches which supply the lateral and inferior parts of the breast
- The pectoral branch of the thoracoacromial artery, which passes along the medial border of the pectoralis minor, supplies the superior parts of the breast
- The lateral thoracic artery, which descends along the lateral margin of pectoralis minor, supplies the breast through large lateral mammary branches.
Venous drainage typically accompanies the arteries, to either the axillary vein, intercostal veins or the internal thoracic vein.
Lymphatics of the breast are predominately dermal and valveless. A lymphatic plexus exists around the areola, and collects lymph from vessels that accompany the lobules of the mammary gland. From the plexus, the majority (> 75%) of drainage occurs to the pectoral group of axillary nodes. Lymphatics from the medial part of the breast may pass to internal thoracic nodes, and from there either superiorly to the neck or inferiorly to the abdomen. Other routes of lymphatic travel include:
- Piercing the pectoralis major muscle, arriving at interpectoral nodes (Rotter's node). From here lymph passes to axillary nodes or through the pectoral fascia to infraclavicular nodes.
- Passing directly to infraclavicular nodes, which lie superficial to the fascia between deltoid and pectoralis major and are closely related to the cephalic vein. Infraclavicular nodes drain to the apical group of axillary nodes.
- Skipping the pectoral group of axillary nodes and passing to the central group
- Very rarely, lymph may travel to the opposite breast or axilla via lymphatics that cross the sternum
From the axilla, lymph passes from pectoral nodes to the central and then apical group of nodes. The apical group drains to inferior deep cervical nodes, which lie between the subclavian vein and the internal jugular vein.
The breast is innervated by the ventral rami of T2 to T6. These nerves pass around the chest wall as intercostal nerves, just beneath the associated rib. These nerves give off lateral cutaneous (at the angle of the rib) and anterior cutaeneous branches (near the sternum) to supply the skin of the breast.
Routes of Malignant Spread
Breast cancers may invade locally throughout the breast. Deep invasion may lead to involvement of the deep fascia or pectoralis major muscle. Invasion of the skin can lead to ulceration and may be the presenting symptom in cases of neglected cancers.
Most lymph fluid (75%) from the breast drains to axillary nodes, in general the pectoral group of nodes. If sentinel lymph node testing is performed, the pectoral (or occasionally central) group of nodes is the site of drainage. It is unusual for lymph to spread to interpectoral, infraclavicular or internal thoracic nodes. It is rare for lymphatic spread to occur to the contralateral breast or axilla.
In the axillary nodes, it is usual for the pectoral and central groups to be involved before apical nodes are affected. From the apical group, lymph passes laterally to the supraclavicular group of the inferior deep cervical nodes, located adjacent to the subclavian vessels in the supraclavicular fossa.
Breast cancers may spread to numerous other sites through the blood. It favours the lungs, bones, liver and brain. Lobular carcinoma is particularly good at spreading to the leptomeninges (between the arachnoid and pia mater).