Breast Anatomy  

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 Anatomy of the Breast

General

Anatomy of the Breast

The breast is composed of glandular, ductal, connective, and adipose tissue.  Embedded in the fibrous tissue are fat and lobules which make up the mammary glands, accessories to reproduction in women, but rudimentary and functionless in men.  In men, little fat is present in the breast, and the glandular system normally does not develop.  In women, the breasts are the most prominent superficial structure on the anterior thoracic wall, and the amount of fat in the glandular tissue determines the size of the breasts.  A small part of the mammary gland often extends into the axilla, forming the axillary tail of Spence.

The breasts lie on the deep pectoral fascia (investing the pectoralis major) and the fascia of the serratus anterior.  They are bounded by the clavicle superiorly, the lateral border of the latissimus muscle laterally, the sternum medially, and the inframammary fold inferiorly.  The breast is attached to the dermis of the overlying skin by connective tissue structures known as Cooper's ligaments (aka suspensory ligaments or retinacula cutis), which suspend the breast on the chest wall.  It is these ligaments which pull on the skin, creating the dimpling (or peau d'orange) associated with malignancy.

The mammary glands are modified sweat glands and are composed of 15-20 lobules, each drained by a lactiferous duct.  Each lactiferous duct independently drains on the nipple and is preceded by a small dilated portion known as the lactiferous sinus.  It is in the sinus that milk collects during nursing and is "let down" by the suckling action of the infant.

Blood Supply and Nerves

The blood supply to the breast is derived from 3 sources.  The predominant supply of blood comes from the perforating branches of the internal mammary arteries, derived from the internal thoracic artery.  The breast is further supplied by the lateral thoracic and thoracoacromial arteries (branches of the axillary artery) as well as posterior intercostal arteries (branches of the thoracic aorta).

Venous drainage of the breast is mainly accomplished by the axillary vein.  The subclavian, intercostal, and internal thoracic veins also aid in returning blood to the heart.

The lymphatic drainage of the breast deserves special attention, due to its role in the metastasis of cancer cells.  The majority of lymph (>75%), particularly from the lateral quadrants, drains to the axillary lymph nodes.  The remainder of lymph drains to either the parasternal nodes or the opposite breast (medial quadrants) or the inferior phrenic nodes (lower quadrants).  With the exception of the nipple and areola, lymph from the skin of the breast drains into the axially, inferior deep cervical, infraclavicular, and parasternal nodes (depending on the location of the vessel).

The innervation of the breast is supplied mainly by branches of the 4th through 6th intercostal nerves, which convey sensation to the skin of the breast and sympathetics to the blood vessels and smooth muscle cells in the overlying skin and nipple.  Although not intimately involved with the innervation of the breast, the long thoracic, thoracodorsal, and intercostobrachial nerves are important to visualize as they cross through the anatomic spaces of the breast and axilla, and are thus important to consider during dissection.

Arteries Internal mammary, lateral thoracic, thoracoacromial, posterior intercostal
Veins Axillary, subclavian, intercostal, internal thoracic
Lymphatics Axillary, parasternal, inferior phrenic nodes
Nerves 4th-6th intercostal nerves

Dissection Considerations

The lateral pectoral nerve passes medially around the medial pectoralis minor, and the medial pectoral nerve passes laterally around the pectoralis minor.  Injuries to these nerves are rare.  The thoracodorsal nerve is identifiable medial to the thoracodorsal vein, running along to enter the latissimus dorsi.  Injury to this nerve results in slight weakening of the latissimus muscle.  The long thoracic nerve is located more medially to the axilla.  It runs just beneath the investing fascia or the serratus anterior, medial to the thoracodorsal complex.  Injury to the long thoracic nerve results in winging of the scapula (on extension).  Brachial plexus injuries can be avoided by keeping the superior extent of the axillary dissection inferior to the lower border of the axillary vein.  The skin of the axilla and upper arm is supplied by the intercostobrachial nerve.  This nerve is often sacrificed during axillary node dissection, resulting in numbness of these areas.