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Hormonal Effects on Mammary Cytology

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

The mammary epithelial cycle of an adult can be divided into four morphological stages of development and regression. Adult development of the epithelium begins with cellular proliferation producing the major outline of the mammary gland microanatomy in the form of small ducts, terminal end buds, and finally alveoli. Proliferation begins during puberty and is completed during pregnancy. The second stage of adult development is the morphological differentiation of the epithelia of terminal ducts and alveoli into secretory cells. This change occurs during late pregnancy and extends to immediately after parturition. The third stage of the adult mammary epithelial cycle begins when cellular activity is directed toward milk synthesis and celgrowth diminishes. It starts shortly after parturition and predominates through lactation. The fourth stage, involution, is the regression of secretory cells which results in atrophy of the gland. Involution starts early in lactation and becomes most pronounced after extended lactation or cessation of suckling or milking.