Chapter 49: Breast Anatomy And Embryology

The breast begins to develop in the 4th week of gestation and has an ectodermal origin. Most tissue usually involutes with the exception of a small thoracic portion which penetrates into the underlying mesoderm. The lactiferous ducts are essentially modified sweat glands of ectodermal origin. They develop early in fetal development and toward the end of the gestational period, and acini develop around the tips of the ducts creating the mammary pit. The milk ridge extends from the axilla to the groin (not the fifth rib) and develops in the 6th week (not 8th week). The areolae are usually formed by the 5th month. Polymastia is the term used to describe supernumerary breasts, and can occur anywhere along the milk ridge but is most common on the left below the inframammary crease. Polythelia refers to a condition with supernumerary nipples, and is the most common congenital breast abnormality occurring in 2% of the population.1–3

Tanner described five stages of breast development. The case described represents stage 2, which normally occurs at approximately 10–12 years of age. Tanner’s stages of breast development are as follows: Stage 1: Preadolescent elevation of the nipple is present with no palpable glandular tissue or areolar pigmentation. Stage 2: Glandular tissue is present in the subareolar region, and the nipple and breast project as a single mound. Stage 3: Glandular tissue is increased and the breast and nipple are enlarged, but the nipple and breast contour remains in a single plane. Stage 4: The areolae are enlarged and areolar pigmentation increased, and the nipple and areola form a secondary mound above the level of the breast. Stage 5: Adolescent development is complete, with smooth contour and no projection of areola and nipple.1,2

3. Sadler TW, ed. Langman’s Medical Embryology. 13th ed. Philadelphia: Wolters Kluwer Health; 2014

The nipple contains orifices which drain each main lactiferous duct and these can act as conduits for infection. This has implications for surgery of the breast, in particular when using breast implants. It is common practice to cover the nipple areola with a dressing when performing augmentation via an inframammary fold approach in order to minimize contamination of bacteria from the nipple to the implant pocket. The functional unit of the breast is the lobule, and each one is comprised of hundreds of acini, not just 16–24. Each acinus itself has secretory potential and is connected to lactiferous ducts with interlobular ducts. There are 16–24 main lactiferous ducts and these dilate as they approach the nipple, forming a sinus that functions as a reservoir for milk storage. Montgomery’s glands are large sebaceous glands capable of milk secretion and also serve to lubricate the areola during lactation. They represent an intermediate stage between sweat and mammary glands. Morgagni’s tubercles represent the openings of the Montgomery’s glands and are located at the periphery of the areola. Fat is the major component of breast tissue and provides most of the bulk, shape, consistency, and contour of the breast.1,2

The breast parenchyma is supplied by the internal mammary and intercostal perforators medially and by the lateral thoracic, thoracoacromial, thoracodorsal, and intercostal perforators laterally (Fig. 49.1). Its rich blood supply and arterial collateralization are responsible for maintaining tissue viability during reconstruction or recontouring surgery and allow for invasive resection, dismantling, and rebuilding to be performed safely and consistently with low risk of tissue necrosis, providing the tissues are respected intraoperatively. Venous drainage mirrors the arterial supply and drains to the axilla. major muscle generates blood supply from the internal mammary perforators, thoracoacromial artery, intercostal perforators, and the lateral thoracic artery; however, its main pedicle is the thoracoacromial artery. Knowledge of the vascular anatomy of this region is particularly important in reconstructive surgery, where a range of flaps are available. Not only is the thoracoacromial pedicle important for transferring the pectoralis major to the head and neck region, it can also be a useful recipient vessel site for free tissue transfer such as clavicular reconstruction with a fibula flap or in a vessel depleted neck for salvage head and neck reconstruction.1–4

The dermatomal distribution of the breast is T3–T6 (not T1–T5) and is mainly from the anteromedial and anterolateral branches of the thoracic intercostal nerves. Nipple-areolar sensation is most commonly provided by the fourth anterolateral intercostal nerve, although it is supplied by the third or fifth nerves in some cases. Fig. 49.1 Vascular supply to the breast. (From Hall-Findlay EJ. Aesthetic Breast Surgery: Concepts & Techniques. St Louis: Quality Medical Publishing, 2011.) Subclavian artery Thoracoacromial artery Axillary artery Internal thoracic artery Lateral thoracic artery Medial intercostal perforators Thoracodorsal artery Internal thoracic perforating branches Anterolateral intercostal perforators Second superficial branch of internal mammary artery Rib Deep layer of enveloping breast fascia Superficial layer of enveloping breast fascia Duct Gland Fifth superficial branch of internal mammary artery Internal intercostal muscle External intercostal muscle Fourth deep branches of internal mammary vein and artery Cooper’s ligaments Wuringer’s septum Deep fascia Pectoralis minor muscle Pectoralis major muscle Supraclavicular nerves from the cervical plexus (C3 and C4) contribute to innervation of the upper lateral portion, but not the medial portion of the breast. Breast sensitivity is apparently related to breast size, with studies showing increased sensitivity in females with smaller breasts. Sensation has been shown to improve after breast reduction surgery in patients with initially large breasts (Fig. 49.2).1–5 Fig. 49.2 Innervation to the breast. Anterolateral intercostal nerves Anteromedial intercostal nerves Intercostobrachial nerve Cervical plexus

The pectoralis minor is an important landmark during axillary dissection and brachial plexus exploration. The lateral margin divides the superficial from the deep axillary nodes. This muscle may be divided during axillary dissection to access the deep nodes in the apex of the axilla. The axillary nodes are numbered I–III according to pectoralis minor. Level I is the inferior, lateral level, below the lower edge of the pectoralis minor muscle. Level II is located deep to the pectoralis minor and level III is located superior and medial to pectoralis minor. The pectoralis minor originates from the anterolateral surfaces of third to sixth ribs and inserts into the coracoid process of the scapula. It serves to draw the scapula downward and forward. Its main blood supply is the pectoral branch of thoracoacromial artery, with contribution from the lateral thoracic artery, and a direct branch of the axillary artery. In breast cancer, all three levels are not always removed and there remains some debate about this as there may be no advantage in removing all three levels. In contrast, most surgeons performing a dissection for skin cancer would remove all three levels.1–3

Würinger et al1 undertook anatomic dissections and arterial injection studies on female breasts. They found that there is a connective tissue suspensory system of the breast which acts as an internal brassiere. Within the horizontal part of the suspensory system, they identified a septum passing from the pectoral fascia along the fifth rib which is now referred to as Würinger’s septum. Within this septum is the neurovascular supply to the nipple-areola complex (NAC). The anatomic nature of the septum means that laterally based breast pedicles are reliable and have a high chance of maintaining sensation following breast surgery. The septum does not carry the motor nerve supply to pectoralis major, nor does it divide the arterial supply of the breast into four zones. It was O’Dey et al2 who performed an injection study of the breast to map the arterial distribution. They identified four distinct arterial zones, the largest of which were the internal mammary (zone 1) and lateral thoracic (zone 2). This work assessed the vascularity of different breast pedicles in terms of the NAC blood supply and concluded that lateral or medial based pedicles should provide the optimal blood supply to the NAC.

aspect of the scapula. Serratus anterior plays an important role in stabilization of the scapula against the chest wall. It receives blood supply through the lateral thoracic artery and branches of the thoracodorsal artery. It is innervated by the long thoracic nerve C6–C7, and when this nerve is damaged a winged scapula will occur. The pectoralis minor is innervated by the medial (not the lateral) pectoral nerve and is so named because it originates from the medial cord of the brachial plexus (C8 and T1). It originates from the third to sixth ribs and inserts onto the coronoid process. Its function is to draw the scapula downward and forward. Blood supply to the pectoralis minor is from the pectoral branch of thoracoacromial, lateral thoracic, and a direct branch from the axillary artery. The pectoralis major originates from the medial clavicle, sternum, second to sixth ribs, external oblique, and rectus abdominis fascia. It inserts on the intertubercular groove of the humerus laterally (not medially). The latissimus dorsi inserts between the pectoralis and teres major muscles. (It is sometimes described as the “lady between two majors.”) The functions of pectoralis major are to adduct and medially rotate the arm. Innervation is from the medial and lateral pectoral nerves. The rectus abdominis flexes the vertebral column and tenses the abdominal wall. Innervation is from the seventh to twelfth intercostals. It originates from the pubic line and inserts on the third to seventh (not the seventh to twelfth) costal cartilages. External oblique originates from the lower anterior and lateral ribs, inserting at the iliac crest and medial abdominal fascial aponeurosis. Although it may assist in flexion of the vertebral column, a key role is compression of abdominal contents. It receives innervation from the seventh to twelfth intercostal nerves (not the fourth to tenth).1–3

The axillary lymph nodes are named according to their relationship with the pectoralis minor. Level 1 is lateral to, level 2 is behind, and level 3 is medial to the pectoralis minor, respectively. The breast has an extensive network of superficial and deep lymphatic drainage. Although most of the breast drains into the axillary nodes, not all areas do so. For example, medial drainage follows the internal mammary perforators and drains to the parasternal nodes. Lymphatic drainage from the outer upper quadrant may pass directly to the subscapular nodes.1,2 Rotter’s nodes are found between the pectoralis major and minor. They are named after the German surgeon, Josef Rotter1 (1857–1924), who described them in the late nineteenth century, and they may be involved in the metastatic spread of breast cancer.

Although there are a number of “typical” measurements for the “normal” breast, there is actually significant variation between individuals. Some of the “typical” measurements are in keeping with the “ideal” or most aesthetic breast appearance. For example, the height of the nipple should ideally be at level with the inframammary fold (IMF), or just above it. This also usually matches with the level of the midhumeral point and a distance of around 19–21 cm from the sternal notch (although this does also depend on the patient’s height and breast size). This information is useful when planning breast reduction and mastopexy techniques, as it provides a guide as to where to resite the nipple-areola complex. The normal breast should have a teardrop shape when viewed from the lateral aspect and this is due to greater fullness in the lower pole. The breast is tethered at the IMF so naturally a degree of glandular ptosis occurs where the breast soft tissues fall to sit just below this line. The nipple should still remain above the IMF level in the ideal breast. Therefore, although a minimal amount of glandular ptosis is normal and aesthetically pleasing, more significant amounts are not. Degrees of true ptosis are described by the relationship of the nipple to the IMF and breast parenchyma. Few women have symmetrical breasts at any age and it is important to identify this before performing breast surgery, as the asymmetry may remain or even be exaggerated after surgery. For example, when the nipple is lateralized on the breast mound, augmentation will make this more obvious. Furthermore, surgery tends to increase patients’ awareness and focus on detailed specifics of their anatomy, and irregularities and asymmetries are more likely to be noticed.1–3

Poland’s syndrome is a disorder where affected individuals are born with missing or incompletely formed elements of the upper limb and chest on one side. It occurs in 1:30,000 births with no specific gender preference. The right side is affected twice as often as the left. The current theory is that there is in utero compression of the subclavian artery which causes ischemia and hypoplastic development of key chest and limb structures. The main findings include absence of the sternal head of pectoralis major muscle, absence of the costal cartilages, hypoplasia of the breast and nipple-areola complex, deficiency of subcutaneous fat and axillary hair, and syndactyly or hypoplasia of the upper limb. Although it can cause a variety of muscle abnormalities such as those listed, a consistent feature is absence of the sternal portion of the pectoralis major.1,2

The medial arm is supplied by the medial brachial and intercostobrachial nerves from T1–T2. The intercostobrachial nerve usually joins with the medial brachial nerve. The medial antebrachial nerve supplies the anterior surfaces of the arm and anteromedial forearm. Transection of any of these can result in paresthesia of the arm; however, the most likely cause is transection of the intercostobrachial nerve. Although a postoperative seroma or hematoma can cause paresthesia in the upper limb, it can also directly irritate the brachial plexus and lead to a combined motor and sensory pathology. Other causes of postoperative upper limb dysfunction include nerve stretch or compression from patient positioning.1,2