The breasts are paired structures on the anterior chest wall, present in both sexes but more prominent in females following puberty. The breast contains the mammary glands, the key structure in lactation. Breast cancer is the commonest cancer and it is therefore important to have a good understanding of the anatomy of the breast and axilla.
The breast develops from the mammary ridge, a fold in the epidermis that becomes evident at 7 weeks1 as a line running from the root of the forelimb along the side of the embryo toward the hind limb. Most of this ridge disappears promptly but a small area persists and penetrates the underlying the mesenchyme to form sprouts, which then form buds and they in turn form the lactiferous ducts. The ducts initially empty in to a small pit that is transformed in to the nipple soon after birth1,2.
In puberty the breast undergoes change in response to hormone changes, particularly oestrogen, with significant difference between the sexes. The development of the breasts is measured according to the Tanner scale3
Surface anatomy/macro anatomy
Located on the anterior chest wall, the breast has a circular base extending from the 2nd to 6th costal cartilages and from the lateral sternal edges to the midaxillary line, and the tail extending in to the axilla. The breast lies superficial to deep fascia, whilst the tail pierces it.
The mammary glands are modified sweat glands and consist of a series of ducts and secretory lobules (15-20).
Each lobule consists of many alveoli drained by a single lactiferous duct. Each duct contains a dilated section, (the lactiferous sinus) which is located just behind the areola. The lactiferous ducts converge at the nipple like spokes of a wheel4. These subunits combine to form the mass of breast tissue, which is held on the chest wall in its natural resting position by fibrocollagenous septa, also known as the suspensory ligaments of Cooper (do not confuse these with the pectineal ligament, also named for Astley Cooper)
The breast has a rich blood supply. The medial part is supplied by the internal thoracic (or mammar) artery. The lateral part is supplied by four vessels:
- Lateral thoracic and thoracoacromial arteries (from the axillary artery)
- Lateral mammary branches (from posterior intercostals, from the aorta)
- Mammary branch of anterior intercostal (from musculophrenic, from internal thoracic)
Veins correspond to the arteries and drain in to the axillary and internal thoracic veins.
The breast drains in to three groups of nodes:
- Axillary (75%)
- Parasternal (20%)
- Posterior intercostal (5%)
The axillary nodes are divided in to three levels according to their relationship to pectoralis minor (See axilla)
The breast is supplied by the anterior and lateral branches of the 4-6th intercostal nerves. The nipple is in the T4 dermatoma and serves as an accurate sensory level.
Breast development is under hormonal influence as mentioned above. The key difference between males and females is the oestrogen level: a significant proportion of pubescent boys will develop transient gynaecomastia, likely due to dominant oestrogen levels at that point in time.
In pregnancy, a large number of hormones contribute to lactation. The key things to remember are:
- Progesetrone stimulates the growth of the alveoli and lobes AND prevents lactation during gestation. Progesterone levels drop after birth, triggering the production of milk
- Oestrogen stimulates the milk ducts to grow and also prevents lactation. Like progresterone, levels drop after birth and remain low during breastfeeding
- Prolactin increases growth of theductal stuctures, increases insulin resistance and modifies lipid metabolism in preparation for breastfeeds
- Oxycotin causes smooth muscle contraction during birth and is responsible for the milk-ejection reflex during suckling.
After menopause, changes in hormone levels lead to breast atrophy.
Breast cancer is the commonest cancer in the UK, with over 53,000 new cases diagnosed in 2013 and a lifetime risk of 1 in 8 for for women and 1 in 830 for men. The presenting signs and symptoms of breast cancer are commonly due to obstruction of lymphatics: excess lymph build up may cause nipple deviation or retraction, while peau d’orange is due to swelling of the skin between pores. Nipple inversion of tethering of the breast tissues is caused by cancerous invasion and traction of the suspensory ligaments. Metastasis is most commonly to the axillary nodes first then to distant sites.
It is important to understand the presenting signs and symptoms and understand triple assessment as the gold standard for investigating patients: history and examination, radiological imaging and tissue analysis.
Origin: mammary ridge, epidermis
Surface landmarks: 2-6th ribs, edge of sternum, midaxillary line. Circle with a tail leading in to the axilla.
Vessels: internal mammary artery, lateral mammary branches, thoracoacromial and lateral thoracic
Nodes: axillary, parasternal, posterior intercostal
Nerves: 4-6th anterior and lateral intercostal branches. Nipple is T4.
Breast anatomy is clinically important, and understanding the axilla with it is vital. Many books describe a examination technique but for exams it is important to be methodical and thorough whilst maintain comfort and dignity for the patient at a potentially troubling time.