The nipple is the central projection in the areola. When the nipple is pulled in and points inward instead of out, it is termed nipple inversion or retraction. Nipple inversion can affect one breast or both, and can be congenital or acquired. Acquired nipple inversion can be due to benign or malignant causes. Benign nipple inversion is usually a gradual process, occurring over a few years. When nipple inversion occurs rapidly, the underlying cause can be inflammation, postsurgical changes, or an underlying malignancy .
The evaluation and management of nipple inversion will be reviewed here. Nipple discharge, common breast problems, and the diagnostic evaluation of a suspected breast cancer are discussed elsewhere. (See "Nipple discharge" and "Clinical manifestations and diagnosis of a palpable breast mass" and "Diagnostic evaluation of women with suspected breast cancer".)
The breast and nipple begin to form during the second month of gestation from lines of thickened ectoderm running from the axilla to the groin (figure 1) . The nipple is initially an epidermal pit and eventually everts as the fetus nears gestation. An inverted nipple may represent a failure of this eversion during development . Breast development and Tanner staging are depicted in the figure and are discussed in detail elsewhere (picture 1). (See "Breast development and morphology".)
The color of the nipple and areola is initially imparted by blood vessels coursing near the skin. Melanin is deposited in the basal cells as part of the aging process, thereby darkening the nipple-areolar complex .
The normal nipple projects from the central area of the areola (figure 2 and figure 3). The darkly pigmented areola, located in the center of the breast, contains dense collagen fibers and a thin layer of contractile muscle. While there is significant variation in color, shape, size, and projection of the nipple-areola complex, a study of morphologic characteristics in 300 women reported a mean nipple height of 0.9 cm . Differences in nipple projection can be affected by age, race, weight, and hormonal changes.