Abstract
Abstract *Background* Repeated research findings over the last 4 decades show that involution of mammary glands in dairy cows did not regress to same extend as that noticed in other mammalian species.*Methodology/Principal Findings*We took an advantage of a rare event in the normal modern dairy farming: A cow that was false-positively identified as being pregnant was "dried up" (i.e., induced into involution) conventionally about 60 before her expected parturition. This cow was culled, and samples of her mammary gland tissue were examined for gross histology. In this study we demonstrate for the first time that modern dairy cow may undergo extensive obliteration of the lobular-alveolar structure, as expected in involution stage II. *Conclusions/Significance* We conclude that lack of histological evidence for the appearance of involution stage II in the vast majority of modern cow's population is related to the peculiar modern dairy husbandry, in which dairy cows are induced into involution still pregnant. Because retardation of involution stage II in pregnant mammals is most likely a general physiological phenomena, it might occurs in other mammals, particularly in lactating humans. Thus, based on basic comparative physiology considerations, we suggest that concurrent lactation and pregnancy should be considered as an independent risk factor for breast cancer.
Highlights
Mammary glands of adult's mammals undergo cycles of avolution, lactation, and involution in coordination with the reproductive cycle [1]
Repeated research findings over the last 4 decades show that involution of mammary glands in dairy cows did not regress to same extend as that noticed in other mammalian species
We conclude that lack of histological evidence for the appearance of involution stage II in the vast majority of modern cow's population is related to the peculiar modern dairy husbandry, in which dairy cows are induced into involution still pregnant
Summary
Mammary glands of adult's mammals undergo cycles of avolution (proliferation), lactation, and involution (regression) in coordination with the reproductive cycle [1]. Because of intensive selection for high milk yield and increase in milk yield persistency during the last 4 to 6 decades [7], dairy cows are dried off while producing considerable amounts of milk: 20, 40 and sometimes even 50 L day-1. Such a practice results in the accumulation of milk in the udder, which leads to udder engorgement, milk leakage, increased risk to acquire bacterial infection [2], and frequently causes noticeable agony to the cow, which might scream loudly for several days [8]. Repeated research findings over the last 4 decades show that involution of mammary glands in dairy cows did not regress to same extend as that noticed in other mammalian species
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