Overview
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.
Gross Anatomy
Development
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.
Micro anatomy
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)
Blood Supply
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.
Lymphatics
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)
Nerve supply
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.
Physiology
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.
Clinical Anatomy
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.
Quick Anatomy
Key Facts
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.
Aide-Memoire
Summary
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.