Congenital Breast Anomalies

Embryology

Breast development begins at approximately the 5th to 6th embryonic week. It proceeds from a single ectodermal bud and begins developing along the ventral side of the embryo as paired longitudinal bands of thickened ectoderm called milk lines (mammary ridges). The latter extend from axilla to groin. The majority of the mammary ridge involutes by the 9th week of gestation except for a focus of epithelial cells at the fourth intercostal space that subsequently form a single pair of mammary buds. Failure of complete involution results in polythelia/polymastia along the mammary ridge. The primary buds form the secondary buds as ectodermal cells begin to invade the underlying mesenchyme by the 12th week of gestation. While the epithelial cells grow downward as mammary ducts, terminating in lobular buds, the mesenchymal cells differentiate into smooth muscle of the nipple and areola. This specialized epithelium differentiates into 15–20 branches that first consist of solid epithelial columns and then, by the 20–24th week of gestation, develop inner lumens and eventually canalize to form the lactiferous ducts and their branches. These branched epithelial tissues are stimulated to canalize by the placental hormones that enter the fetal circulation during the third trimester. The ends of these branches differentiate into lobulo‐alveolar structures that contain colostrum , which is a yellow, sticky, and serous fluid. This colostral milk, stimulated by the placental hormones, can be expressed for 4–7 days after birth in both male and female newborns (“witch’s milk”). Sebaceous glands, sweat glands, and specialized apocrine glands (Montgomery glands) develop in the second trimester. During the final weeks of gestation, the mass of the mammary gland increases consistently and the nipple–areola complex (NAC) develops and becomes pigmented. With the withdrawal of placental hormones after birth, the secretion of colostrum stops and the breast begins to involute. The supporting structure of the breast, composed of connective tissue and fat, originates from the mesenchyme that invades and surrounds the epithelial system. Failure of some of these stages in breast development to take place can result in congenital abnormalities of the breast.

After birth, circulating maternal estrogens may cause breast enlargement. Thereafter, maternal estrogens are metabolized resulting in involution of breast tissue and a period of quiescence during childhood. At puberty, hormonal flux results in breast growth or thelarche , with estrogens affecting ductal and stromal tissue proliferation and progesterone causing alveolar budding and lobular growth. Thelarche occurs at an average of 11 years of age (range, 8–15). The Tanner Scale describes breast development as proceeding through five stages, with growth generally reaching completion at 16–18 years of age.

Polythelia and Polymastia

Polythelia or presence of supernumerary nipples or nipple–areola complexes are often noted at birth, with the majority located along the embryonic milk line ( Fig. 37.1 ). These form due to failure of regression of the mammary ridge. However, uncommon locations include the back, thigh, neck, and face. The condition usually occurs sporadically, but may be familial, and has a reported incidence as high as 5.6%. ^, The supernumerary nipple is most commonly located in the inframammary region. There is little evidence to support an association with other congenital anomalies or syndromes nor is there any correlation with gender or predilection for the right or left side, however, it has been suggested that the prevalence is higher on the left side and in males. The clinical significance of supernumerary nipples is the possibility of growth and development of similar pathologies as a normal breast, including breast neoplasia. Traditionally, the management of polythelia has been observation. However, pigmented lesions along the embryonic lines are routinely excised before puberty in order to avoid wider tissue excision secondary to glandular growth, which may be necessary after the onset of puberty in girls. ^, If multiple nipple–areola complexes are present on the breast proper, magnetic resonance imaging may be required to determine the complex associated with glandular/ductal tissue. Changes to the pigmented lesion should be treated as for any melanocytic nevus, with early excision and histopathological assessment. An oncologist should evaluate nipple growth that occurs at a time other than childhood, puberty, or pregnancy.

Polymastia occurs due to failure of the mammary ridge to completely regress in utero, leading to the development of breast tissue following hormonal stimulation. Ectopic breast tissue may or may not have an overlying NAC, but it is not uncommon for the nipple and areola to be absent or rudimentary. Polymastia has an even lower incidence than polythelia (1–2%), and also occurs along the embryonic milk line. Unlike polythelia, polymastia may only be noticed during puberty, pregnancy or lactation. A common location is the axilla and unusual locations include the vulva, lower extremities and dorsal trunk. Like polythelia, it is usually sporadic, but can have familial inheritance. Unlike polythelia, polymastia can be associated with other congenital anomalies, particularly congenital renal anomalies. Treatment of polymastia is variable and depends on the size and location of the supernumerary breast gland and nipple.

In view of the fact that ectopic breast tissue is subject to the same pathology as normal breast parenchyma, complete resection is recommended. Simple mastectomy is the option of choice in patients who present with a third distinct breast mound; however, disruption of the inframammary fold and soft tissue envelope of the remaining breast should be prevented when possible. In cases where the accessory breast is adjoined to the native breast, tissue-sparing techniques with skin de-epithelialization and accessory nipple excision can usually restore the mound to a normal appearance and location. Corrective surgery for polymastia should be performed when breast development is complete and final breast tissue volumes have been achieved. It is often difficult to predict the form and position of the accessory breast mound or nipple and early excision may ultimately compromise the eventual outcome. Delaying excision until the final shape and position of the native breast tissue is complete will prevent eventual deformities from inadvertent iatrogenic insult to the developing breast.