76. Development of The Mammary Glands

  1. The mammary glands (breasts) are derived from 2 thickened strips of epidermal ectoderm, the primitive mammary ridges or milk lines, which appear during week 6. The ridges extend from the axillae to the inguinal regions, but rapidly regress except in the thorax
    1. THE MAMMARY BUDS that persist in the thoracic region penetrate the underlying mesenchyme and give rise to several secondary buds which develop into lactiferous ducts and their branches. These are canalized by the end of the prenatal life
      1. The fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchym The lactiferous ducts form the small ducts and alveoli
      2. Only the main ducts are found at birth, and the gland remains undeveloped until puberty
    2. DURING THE LATE FETAL PERIOD, the epidermis, where the gland originated, becomes depressed to form a shallow mammary pit (epithelial pit) on which the ducts open
      1. The lactiferous ducts at first open onto this epithelial pit which is formed by the original mammary line
    3. THE NIPPLE itself forms during the perinatal period due to proliferation of the mesenchyme under the areola (circular area of skin around the nipple) in the area of the mammary pit. The nipple is often depressed and poorly formed during infancy
    4. THE MAMMARY GLANDS of both newborn males and females are often enlarged and may secrete "witches' milk" or colostrum, as a result of maternal hormones passing into the fetal circulation by way of the placenta
    5. AT PUBERTY, the female mammary glands enlarge rapidly as a result of the development of fat and connective tissue. The duct system also grows, stimulated by the estrogen and progesterone of the ovary
      1. The glandular tissue remains completely undeveloped until pregnancy when the intralobular ducts rapidly develop, form buds, and become alveoli
      2. The male glands undergo little postnatal development
  2. Malformations of the mammary gland
    1. ABSENCE OF THE GLAND (AMASTIA) AND/OR NIPPLE (ATHELIA) is rare; may occur bilaterally or unilaterally, and is due to failure of development or complete disappearance of the mammary ridge(s). Also can be due to failure of the mammary bud to form.
    2. SUPERNUMERARY BREASTS (POLYMASTIA) AND NIPPLES (POLYTHELIA) are seen in about 1% of the female population and are usually inherited. They generally are found below the normal breast, but less commonly are seen in the axilla or abdominal area, developing along the mammary ridges
      1. Polythelia is uncommon, also may be seen in males
      2. In most cases, a single extra nipple or breast is seen, but in 30% of cases, 2 extra nipples or breasts are found
      3. Accessory breasts may have normal tissue and even function during lactation
    3. INVERTED NIPPLES: the nipple fails to develop normally and evert after birth. Probably due to a failure of the underlying mesenchyme to proliferate and push the nipple out
      1. Also may be caused by retraction of the nipple as a result of the presence of a fast-growing tumor in the gland

development of the mammary glands: image #1