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Paired ectodermal primitive mammary streaks (milk streaks) form along the ventral surface of the embryo during the fifth week of gestation. These extend from the axilla to the inguinal region. The mammary ridges arise in the thorax from these streaks and there is progressive differentiation into breast parenchyma during fetal development. The other portions of the mammary streaks normally regress; incomplete involution of embryonic breast tissue can result in accessory or ectopic breast tissue anywhere along the primitive mammary streaks.
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The prepubertal breast is a rudimentary organ that consists of simple branched ducts surrounded by a connective tissue stroma. With the onset of puberty, the ducts elongate, divide, and form terminal duct lobular units. Lobular differentiation begins in the peripheral regions of the breast and progresses centrally. At the end of pubertal development, the breast consists of a ductal system lined by epithelial cells and ending in terminal duct lobular units. Normal breast development is classified into a 5-stage Tanner grading system.1
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Tanner stage 1 is prepubertal. Sonography of the prepubertal breast may show thin ill-defined retroareolar tissue that is slightly hypoechoic or hyperechoic with respect to adjacent subcutaneous fat (Figure 9-1).
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At the beginning of puberty, the breast bud develops; this is Tanner stage 2. Initially, there is a subareolar lump that is appreciable by palpation only. Enlargement of the breast bud produces a single mound of tissue that includes breast tissue and the nipple. The breast bud is demonstrated sonographically as a retroareolar nodule that is hypoechoic relative to adjacent fat and contains low-level echoes. Often, there is a central star-shaped or linear hypoechoic area that correlates histologically with simple branched ducts.
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During Tanner stage 3, enlargement and elevation of the entire breast occur. As with stage 2, the breast tissue and nipple form a single mound of tissue. Development of glandular tissue occurs during this stage. The glandular tissue is relatively hyperechoic on sonography. A central retroareolar spider-shaped hypoechoic region is often visible, representing the ductal system.
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Tanner stage 4 is transient and does not occur in all individuals. A secondary mound develops, with the nipple and areola projecting anterior to the breast tissue. Sonography shows a prominent hypoechoic nodule in the retroareolar region, surrounded by hyperechoic periareolar fibroglandular tissue.
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Tanner stage 5 indicates development of a mature breast contour, with regression of the areola to form a smooth contour with the remainder of the breast tissue. The hypoechoic central nodule visible in the earlier Tanner stages is no longer visible with sonography. The breast predominantly consists ...