Breast


Core procedures

  • Mastectomy: simple; skin-sparing; nipple-sparing, skin-sparing

  • Wide local excision of breast lesion

  • Wire-localized wide local excision

  • Sentinel lymph node biopsy

  • Microdochectomy

Development

Developmentally, the breast is a modified and highly specialized apocrine gland, composed of glandular tissue and ducts making up the breast parenchyma, supported by a connective tissue framework, adipose tissue and vasculature, which make up the breast stroma. Breast development begins in utero and continues in stages during infancy, puberty, pregnancy and lactation. It involves complex epithelial–mesenchymal interaction, driven by signalling, growth factors and hormonal pathways. During development, ingrowths of ectoderm into the mesenchyme become the breast parenchyma, and the mesenchyme becomes the stroma.

Embryology and prenatal development

Our understanding of prenatal breast development has been largely drawn from mammalian studies, particularly in mice. There is variation in stages of human fetal breast development at particular gestational ages between individuals. Prenatal breast development is identical in males and females, and usually begins around the fifth to sixth weeks in utero . Firstly, two ventral ectodermal ridges, around 2–4 cell layers thick – the ‘mammary bands’ or ‘milk streaks’ – develop from the axillae to the inguinal regions bilaterally. Next, in the sixth to seventh weeks, the ‘mammary crest’ develops. This is a disc-shaped thickening, 4–6 cells wide, on the mammary band, at the fourth intercostal space on the anterior thorax, indicating the future nipple position. The third stage involves proliferation and invagination of the mammary crests, penetrating downwards as diverticuli into the underlying mesenchyme, forming the primary breast buds ( Fig. 43.1A ). Around the same time, the remaining mammary bands start to involute.

Fig. 43.1, Early development of breast epithelium. Abbreviation: i.u., intrauterine.

From weeks 12–20, secondary epithelial outgrowths from the deep surface of the primary breast buds arise, penetrating further into the mesenchyme ( Fig. 43.1B ). These mammary sprouts, or primitive ducts, have a slender stalk with a bulbous end. They undergo progressive elongation, branching and canalization until term, when around 15–20 lobes of glandular breast tissue, each connected to a lactiferous duct, have formed ( Fig. 43.1C ). The epithelial cells lining them are arranged in two layers of cuboidal cells: the innermost luminal layer is secretory in function, and the basal layer differentiates into myo­epithelial cells.

Formation of the primary breast bud and subsequent secondary epithelial outgrowths are regulated by hormonal and growth factor signalling from the lipid-rich extracellular matrix of the surrounding adipose tissue in the mesoderm in the first and second trimesters. In the third trimester, canalization of the ducts occurs under the influence of placental oestrogens entering the fetal circulation. Throughout the second and third trimesters, the mesenchyme differentiates into the supporting breast stroma, containing adipocytes, collagens, fibroblasts, smooth muscle and nerve cells. The stroma is also rich in immune cells such as macrophages and eosinophils.

The nipple is formed in the third trimester, at the site of initial epidermal downgrowth: the mammary pit ( Fig. 43.1D ). The overlying epidermal cells differentiate into nipple skin, with associated Montgomery's tubercle apocrine glands on the areola. The mesenchyme proliferates and differentiates to circular smooth muscle fibres arranged both longitudinally and circularly, and the nipple and areola become pigmented.

Infancy

In most infants born at term there are palpable breast nodules. Following the withdrawal of maternal oestrogens at parturition, fetal prolactin is produced from the pituitary gland, causing transient production of colostrum, which can continue for 3–4 weeks. From around 3–4 months after birth, the breasts undergo involution, similar to changes seen in the postmenopausal breast, and remain quiescent until puberty.

Puberty

In females, pubertal breast development begins between the ages of 9 and 13 years, at an average of 11 years, and takes between 3 and 5 years to complete. At a histological level, changes occur in both the breast parenchyma and the stroma. In the parenchyma, ductal elongation and branching morphogenesis occur. Cells are arranged in an outer myoepithelial layer and an inner luminal layer, which is further divided into ductal luminal epithelial cells lining the ducts, and alveolar luminal cells, capable of secretory function. More alveoli arise with each menstrual cycle but the number only increases significantly with pregnancy. In the stroma, there is an increase in size and elasticity of the connective tissues, with enhanced vascularity and adiposity.

Pubertal mammary branching is dependent on increased levels of circulating oestradiol, exerting its effect through oestrogen receptors, predominantly ERα and, to a lesser extent, ERβ, in both the parenchyma and the stroma. Breast development is also dependent on growth hormone, insulin-like growth factor-1 and prolactin.

At a macroscopic level, the changes seen in the breast at puberty have been described in five stages, originally by Tanner ( Fig. 43.2 ; Table 43.1 ).

Fig. 43.2, Pre- and postpubertal development and structure of the female breast, demonstrating changes in the contour of the breast.

TABLE 43.1
Tanner stages of female pubertal breast development
Stage Description
I Pre-adolescent, elevation of the nipple only
II Breast bud stage, glandular subareolar tissue develops, nipple and breast project from the chest wall as a single mound
III Breast and areola further enlarge as a single mound, areola pigments and increases in diameter; glandular breast tissue proliferates and enlarges
IV Nipple and areola further enlarge and pigment, and project as a distinct mound above the level of the main breast mound
V Mature stage, development of smooth contour to the breast with no projection of the areola and nipple above the breast

Developmental Anomalies

Hyperplastic and accessory breast abnormalities

Polythelia is the presence of an accessory nipple and occurs in up to 5% of the adult population. Polythelia may be found at any site along the line of the embryological mammary band, most commonly in the thoracic, axillary or abdominal regions ( Fig. 43.3 ).

Fig. 43.3, Milk lines.

Polymastia is the presence of accessory or ectopic breast tissue, occurring in 2–6% of adults. The axilla is the site of accessory breast tissue in 60–70% and may be bilateral. Other reported sites include the thoraco-abdominal wall, down to the level of the groin, the vulva and subscapular regions. Ectopic tissue can undergo physiological fluctuation with the menstrual cycle and often enlarges in pregnancy, with patients often only first realising its presence during a pregnancy. If the accessory breast has an associated nipple, it may function during lactation post-partum.

Adolescent macromastia and juvenile breast hypertrophy occur when the breasts become excessively large, the former at a steady rate throughout puberty, and the latter at a dramatic rate over 6–12 months. Both can result in marked asymmetry and psychological upset. The aetiology is idiopathic, although one theory proposes breast hypersensitivity to normal levels of circulating oestrogens. Breast development is complete by the age of 20, and at this stage reduction mammoplasty may be considered in some patients with macromastia.

Hypoplastic breast abnormalities

Hypoplastic breast abnormalities can be unilateral or bilateral, and if bilateral, may be symmetrical or asymmetrical. An absence of any stage of breast development by the age of 14 years warrants concern. Hypoplasia is underdevelopment of the breast. Amastia is the congenital absence of breast, nipple and areolar tissue. Amazia is the absence of breast tissue with the presence of a nipple and may be congenital or acquired . The most common acquired reason is iatrogenic injury by biopsy of a pubertal or infant breast, in effect excising most of the breast bud and resulting in marked deformity. It is crucial that such intervention is always avoided. Other acquired reasons include childhood radiation for intrathoracic disease, thoracostomy tube placement or injuries such as severe burns. Athelia is the absence of a nipple; however, this is more common on accessory breast tissue.

Poland's syndrome is a congenital condition characterized by unilateral hypoplastic or absent breast tissue, with hypoplastic or absent pectoral muscles, combined with shortening of the ipsilateral upper limb and digits. There is wide variation in clinical presentation, with some patients having further chest wall anomalies, including deformed or absent ribs.

Tuberous breast deformity is an anomaly characterized by hypo­plasia of the lower pole of the breast, in either the lower medial or the lower lateral quadrant, or in both. Reconstruction with augmentation and/or mastopexy can be challenging due a smaller skin envelope and higher-sitting inframammary fold on the affected side.

Pregnancy

After puberty, the breast undergoes a degree of involution until pregnancy, when massive remodelling occurs. At pregnancy, there is further duct and lobule formation, with secondary and tertiary ductal branching, and increased numbers of secretory acini. Alveoli increase dramatically in number and individually increase in size, with reduction in the stromal adipose tissue ( Fig. 43.4 ). Macroscopically, there is significant breast enlargement, dilation of superficial veins and increased pigmentation of the nipple/areolar complex. Pregnant breast development relies primarily on progesterone and prolactin, both crucial for ductal side branching and development of alveoli, in addition to oestrogens, lactogen and chorionic gonadotrophin.

Fig. 43.4, Changes in the breast during lactation.

Lactation

At parturition there is a sudden drop in placental lactogen and ovarian sex steroids, which, throughout pregnancy, have inhibited the lactogenic effect of prolactin on breast epithelium. Lactation usually begins at 1–4 days post-partum and can continue for 3 years or so, dependent on prolactin and oxytocin secretions from the anterior and posterior pituitary, respectively, in response to infant suckling. When the breast is no longer lactating, involution of the glandular tissue occurs due to apoptosis of epithelial cells. Atrophy of the breast parenchyma is accompanied by increased deposition of adipocytes, returning the breast to a pre-pregnant state, ready for subsequent pregnancies.

Menopause

At the menopause, there is a decrease in overall volume of the breast, with involution of the glandular breast tissue. There is increased deposition of fatty tissue and regression of connective tissues in the stroma.

Male breast

The male breast remains rudimentary from infancy and does not form lobules or alveoli. However, it can hypertrophy, resulting in gynaecomastia. This occurs most often at puberty, when boys will often notice a small amount of breast tissue behind the nipple. Occasionally, this will develop further either unilaterally or bilaterally. In later life, typically between the ages of 50 and 80 years, gynaecomastia may also occur. Often the aetiology is obscure but commonly recognized causes include drugs such as histamine H2-receptor antagonists, spironolactone, and antiandrogens used in prostatic carcinoma. Recreational drugs, including marijuana, alcohol and anabolic steroids, are also identified as causes.

Surgical surface anatomy

In the adult female, the base of the breast extends superiorly to inferiorly from the second to the sixth rib, and medially to laterally from the sternal edge to the mid-axillary line. The superolateral quadrant extends towards the axilla along the inferior and lateral border of pectoralis major ( Fig. 43.5 ). A small portion may extend through the clavipectoral fascia into the axilla itself, as the axillary tail of Spence. The breast lies upon a deep fascia, with two-thirds of it overlying pectoralis major as the pectoral fascia, and the remaining one-third lying laterally and inferiorly as fascia over serratus anterior. Inferiorly, this deep fascia is continuous, overlying the upper portion of external oblique and its aponeurosis. Ligamentous fibrous bands, known as Cooper's ligaments, run through the breast. They connect the deep fascial layer to the superficial fascia or fibrous tissue of the dermis, forming an interlobular fascial framework within the breast. The retro­mammary space lies between the breast tissue and the deep pectoral fascia, and allows for a degree of movement of the breast on the chest wall. It contains lymphatics and small vessels.

Fig. 43.5, The structure of the breast.

The breast is composed of skin, subcutaneous tissue and breast tissue, which is further divided into glandular epithelial tissue and stroma.

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