The mammary glands, or breasts, are a distinguishing feature of mammals. During embryologic development, growth and development of breast tissue occur in both sexes. Multiple glands develop along paired epidermal thickenings, called mammary ridges (milk lines), which extend from the developing axilla to the developing inguinal region. In humans, normally only one group of cells develops into a breast on each side. An extra breast (polymastia) or nipple (polythelia) may occur as an inheritable condition in about 1% of the female population. These relatively rare conditions may also occur in males.
In males, little additional development of the mammary glands normally occurs in postnatal life, and the glands remain rudimentary. In females, the mammary glands undergo further development under hormonal influence. They are also influenced by changes in ovarian hormone levels during each menstrual cycle. The actual initiation of milk secretion is induced by prolactin secreted by the adenohy-pophysis. The ejection of the milk from the breast is stimulated by oxytocin released from the neurohypophysis. With the change in the hormonal environment at menopause, the glandular component of the breast regresses or involutes and is replaced by fat and connective tissue.
Mammary glands are modified apocrine sweat glands that develop under the influence of sex hormones
The inactive adult mammary gland is composed of 15 to 20 irregular lobes of branched tubuloalveolar glands (Fig.
22.31). The lobes, separated by fibrous bands of connective tissue, radiate from the mammary papilla, or nipple, and are further subdivided into numerous lobules. Some of the fibrous bands, called suspensory or Cooper's ligaments, connect with the dermis. Abundant adipose tissue is present in the dense connective tissue of the interlobular spaces. The intralobular connective tissue is much less dense and contains little fat.
The epidermis of the adult nipple and areola is highly pigmented and somewhat wrinkled and has long dermal papillae invading into its deep surface (Fig. 22.32). It is covered by keratinized stratified squamous epithelium. The pigmentation of the nipple increases at puberty, and the nipple becomes more prominent. During pregnancy, the areola becomes larger and the degree of pigmentation increases further. Deep to the areola and nipple, bundles of smooth muscle fibers are arranged radially and circumfer-entially in the dense connective tissue and longitudinally along the lactiferous ducts. These muscle fibers allow the nipple to become erect in response to various stimuli.
The areola contains sebaceous glands, sweat glands, and modified mammary glands (glands of Montgomery). These glands, have a structure intermediate between sweat glands and true mammary glands, and produce small elevations on the surface of the areola. Numerous sensory nerve endings are present in the nipple; the areola contains fewer sensory nerve endings. The tubu-loalveolar glands, derived from modified sweat glands in the epidermis, lie in the subcutaneous tissue. Each gland
lobules of tubuloalveolar glands
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