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A 100-Year Review: Mammary development and lactation.

What is old is new again-and with respect to the study of the mammary development and function in dairy animals, the expression resonates. Many of the mammary and milk production questions raised in the early years of the Journal of Dairy Science apply today. To be sure, scientists have filled in many details regarding, for example, identification of hormones and growth factors important in the control of mammary growth, the onset of copious milk production at calving, and maintenance of lactation. Early years focused on identification and subsequent availability of classic mammogenic, lactogenic, and galactopoietic hormones (e.g., steroids, prolactin, and growth hormone). The advent of sensitive assays to measure concentrations of these hormones and, subsequently, myriad growth factors in blood, milk, and tissues, allowed creation of multiple hypotheses to explain mammary cell proliferation and regulation of function. It is also apparent that we understand many of the fundamentals of milk removal, milking frequency, milking management, and milk ejection for successful lactation. However, some questions remain. Are the principles that were identified when cows produced markedly less milk still valid for the high-producing cows of today and the future? What mechanism(s) explain the positive effects of early increased milking frequency on subsequent milk production? Can the persistency of lactation be improved (secretory cell number vs. secretory cell function) or does early management "program" future mammary development or productivity (epigenetics, immune responsiveness, other)? The explosion of tools and techniques (Southern and Northern blots, PCR, and the "-omics" revolution) has driven an almost overwhelming evaluation of cellular and molecular functions in the mammary gland and other tissues. One key may be the discovery of a "Rosetta stone" that will allow understanding of this mass of detailed information on gene expression, cell signaling, and so on. Many scientists can now better appreciate the difficulty of the dairy farmer seeking to process DHIA or Dairy Comp 305 data, milking data, weights, feeding reports, pedometer readings, or genomic evaluations to manage their operations.