The mammary gland develops from the mammary crest, which is of ectodermal origin. The mammary crest (milk line) extends from the axilla to the inguinal area on the developing trunk (see Fig. 9.1 in Chapter 9) but disappears in humans except in the pectoral area. During prenatal life the primary mammary bud is formed. From the deeper surface layers of the bud, 15-20 solid cellular cords, the secondary epithelial buds, proliferate into the mesenchyme. These subdivide and develop into a system of lactiferous ducts. Parallel with the development of the cellular cords, the surface cells in the primary bud desquamate and form a central depression which, by the end of prenatal life, is everted through proliferation of the underlying dermis and forms the nipple.
The female breast undergoes physiological changes with age, starting with an infantile form, in which the areola and papilla of the nipple are visible but flat. In the next stage, during early adolescence, a slight prominence of the nipple area ("pouting") can be observed, which is followed by primary development of breast tissue with continued prominence of the areola. During puberty the epithelial and fibrous tissues of the mammary gland proliferate under the influence of estrogens, and the stroma, fat, and connective tissues develop, while the ducts grow. In the sexually developed female breast the nipple is integrated into the rounded shape of the adult breast.
The mammary gland consists of 15-20 lobes that radiate from the nipple. The ducts of the lobes unite and form a single excretory duct, which ends in the nipple. The size of the breast varies with age, nutritional state, and the number of pregnancies, as well as with ethnic and genetic background. It is difficult to establish a normal size. An average adult female breast size of 400 mL (range 120-600 mL) has been calculated for nonpregnant northern European women of normal weight using a Plexiglas cylinder to measure breast volume.
Landmarks The female breast can be measured in a vertical diameter (length), measuring lateral to the breast along the trunk wall from the lower insertion along the base of the breast to the point of upper insertion, where a slight curve away from the thorax wall can be found (Fig. 10.17).
The prominence or protrusion of the breast is measured from the middle of the base of the breast on the chest wall to the point of elevation of the areola insertion (Fig. 10.17). Since the size of the nipple varies, it is not included in the measurement.
Figure 10.17 Measurement of breast size.
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