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Anatomy and physiology of the breast
The breast is a defining characteristic of mammals. Its fundamental purpose is to produce milk, but the appearance and sensation provided by breasts give them a huge role in female personal and sexual identity. Surgical and other therapeutic interventions on the breast may have a significant impact on this.
Normal breast development
Embryology
The breast is a modified sweat gland made up of glandular tissue, supporting connective tissue and fat of ectodermal and mesodermal origin. There are no morphological or physiological differences in the breasts between the sexes until puberty. By week 5 of foetal development, parallel ectodermal ridges (milk lines) form along the ventral surface of the embryo from the developing axilla to the inguinal region. By week 9, the distal elements of the paired ectodermal ridges begin to disappear. The ectoderm in the pectoral region invaginates into the surrounding mesoderm. During the third trimester, the ectodermal extensions epithelialise, branch and canalise. At birth, 6–10 ducts reach the nipple lined by a single layer of epithelium, surrounded by a layer of myoepithelial cells, terminating in a blind sac.
Clinical considerations
Withdrawal of maternal hormones at birth results in prolactin secretion and can lead to transient palpable breast tissue and milk production in the neonatal period.
Failure of regression of the ectodermal ridge results in the formation of accessory nipples and/or accessory breast tissue. Complete regression of the ridges leads to congenital absence of the breast (called amastia) and nipple (athelia). Failure of one or both breasts to develop can be congenital or acquired. Congenital causes include Poland’s syndrome, a spectrum of conditions including absence or hypoplasia of the pectoral muscle and breast, deformity of the underlying chest wall and upper limb abnormalities. It is rare, usually partial and more common in men. Acquired causes include trauma (including surgery) and radiation exposure.
Mild asymmetry is common and often just requires reassurance. More marked asymmetry may require surgical intervention including increasing the size of the smaller breast by implant placement with or without tissue expansion and/or lipomodelling or reduction of the larger breast. Muscle or chest wall defects may require pedicled or free flap surgery, potentially combined with implant placement and lipomodelling ( Fig. 1.1 ).
Biopsies should be avoided if possible during breast development as they may damage and distort the ultimate breast.
Puberty
The adult female breast is under the constant influence of autocrine (systemic hormones) and paracrine (growth factors and cytokines) modulators of growth and development. Thelarche marks the onset of adult breast development with ductal elongation, increase in the height of the epithelium and an increase in stromal density. Gonadotrophin releasing hormone (GnRH) from the hypothalamus stimulates production of follicle stimulating hormone (FSH) and luteinising hormone (LH) from the pituitary. These stimulate oestrogen production by the ovaries. This results in the addition of lobular units with terminal end buds forming new, small alveolar buds which branch and divide into ductules. Type 1 lobules consist of a terminal duct and 10–12 associated alveolar buds and these are the characteristic feature of the peri -menarchal breast. After 1–2 years, cyclical ovarian oestrogen and progesterone production results in progression to type 2 lobules. In the adult breast, cyclical changes occur during each menstrual cycle with an increase in the rate of proliferation in the luteal and secretory phase (with variable associated increase in breast size) followed by a wave of apoptosis.
Clinical considerations
Premature breast development (before the age of 8) or precocious puberty should be investigated with input from endocrinology.
Tubular breasts are usually small, widely spaced with a high inframammary fold, narrow breast base and large areola. The condition varies in extent and the cause is not clear. Surgical treatment can be challenging and tends to involve a combination of lipomodelling, tissue expansion, implant placement and areolar reduction ( Fig. 1.2 ).
Accessory nipples are most commonly seen below the breast and above the level of the umbilicus. They rarely cause problems and are often unrecognised by the patient. They may be excised if causing irritation ( Fig. 1.3 ).
Accessory breast tissue is usually seen in the axilla but occasionally below the breast. It often becomes more prominent during pregnancy and lactation. Reassurance is usually all that is required. Surgical excision can be performed, but wound healing, sensitivity, unsightly scars and recurrence can be issues ( Fig. 1.4 ).
Macromastia is defined as breast weight more than 2–3% of total body weight. It can be progressive and may be unilateral. It can be exacerbated by pregnancy. Reduction mammoplasty after adolescence is the treatment of choice. Macromastia may recur if reduction surgery is performed while the breast is still developing or before subsequent pregnancy.
The variation in blood flow and epithelial proliferation in the breast through the menstrual cycle is such that elective magnetic resonance imaging (MRI) scanning of the breast is recommended to be performed in the period between menstruation and ovulation.
Pregnancy and lactation
During early pregnancy, under the influence of oestrogen and prolactin, distal ducts proliferate to form more lobules and more alveoli per lobule forming type 3 lobules. As pregnancy progresses, lobular units differentiate into secretory units (type 4 lobules) with human placental lactogen contributing to the hormonal milieu. Alveoli form acini, epithelial cells fill with fat droplets and colostrum begins to enter the acini. Alveolar epithelial cells increase synthetic pathways to make milk protein, fat and lactose. Initiation of milk production is inhibited by persisting levels of progesterone. Following birth, colostrum containing nutritional elements plus immunoglobulins (providing passive immunity) is replaced by mature milk production after 30–40 hours. Falling progesterone levels after birth, together with prolactin and oxytocin stimulated by suckling, stimulates milk production and let down respectively. After weaning, the breast involutes with a decrease in the size of lobules and number of alveoli to type 3 lobules.
Clinical considerations
A galactocoele is a milk-filled cystic lesion that presents as a well-defined lump during lactation. These require full assessment. Aspiration usually leads the lump to disappear although some recur until weaning and in a few the contents thicken so preventing aspiration, in which case a core biopsy may be required to establish the diagnosis. Recurrent galactocoeles do resolve once breastfeeding stops. Patients having breast needling or core biopsy during pregnancy should be warned that this might result in blood in the milk. After local anaesthetic infiltration of a lactating breast, feeding is discouraged for a few hours to prevent the baby ingesting local anaesthetic in milk.
Fibroadenomas often grow during pregnancy and lactation. New lesions should be assessed as normal. Previously biopsied lesions which have grown during pregnancy can be observed as they will usually shrink again following weaning.
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