Mammary Glands

The first indication of mammary glands is found in the form of a bandlike thickening of the epidermis, the mammary line or mammary ridge. In a 7-week embryo, this line extends on each side of the body from the base of the fore-limb to the region of the hindlimb (Fig. 18.4C). Although the major part of the mammary line disappears shortly after it forms, a small portion in the thoracic region persists and penetrates the underlying mesenchyme (Fig. 18.4A). Here it forms 16 to 24 sprouts, which in turn give rise to small, solid buds. By the end of prenatal life, the epithelial sprouts are canalized and form the lactiferous ducts, and the buds form small ducts and alveoli of the gland. Initially, the lactiferous ducts open into a small epithelial pit (Fig. 18.4B). Shortly after birth, this pit is transformed into the nipple by proliferation of the underlying mesenchyme.

Mammary Line
Figure 18.4 A and B. Sections through the developing mammary gland at the third and eighth months, respectively. C. Positions of accessory nipples (blue line, mammary line).

CLINICAL CORRELATES Mammary Gland Abnormalities

Polythelia is a condition where accessory nipples have formed due to the persistence of fragments of the mammary line (Fig. 18.4C). Accessory nipples may develop anywhere along the original mammary line, but usually appear in the axillary region.

Polymastia occurs when a remnant of the mammary line develops into a complete breast.

Inverted nipple is a condition in which the lactiferous ducts open into the original epithelial pit that has failed to evert.

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