Role of placental function in mediating conceptus growth and survival2

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Abstract
Conceptus mortality is a significant factor limiting reproductive efficiency of livestock. In both singlet- (i.e., cattle) and litter- (i.e., pigs and sheep) bearing species, investigations of conceptus mortality have focused on the period immediately preceding and throughout the attachment phase, around the time of maternal recognition of pregnancy. Recently, data have emerged leading to the suggestion that conceptus loss later in gestation is also significant and that variation in placental size and function may play a very important part in determining whether a conceptus survives. In the pig, the number of conceptuses present after the initial period of loss that survive to term appears to be influenced by the total amount of placental mass present, such that litters containing individuals with relatively small placentae have a greater potential for a large litter size when compared with litters containing similar numbers of individuals with relatively large placentae. In ruminants, recent evidence supports the time of placental development and initial vascularization (between d 28 and 40) as a second period of significant loss, particularly in situations involving manipulation (ovulation synchronization for timed AI in cattle and out-of-season breeding in sheep). In the pig, not only does placental size vary, but efficiency (as measured by the ratio of fetal weight to placental weight) can also vary as much as threefold within a litter, leading to the suggestion that selection for small, very efficient placentae may provide a mechanism for increasing litter size. In ruminants, there are obvious cases where placental growth has been markedly altered (i.e., large offspring syndrome or heat stress) and a subsequent deviation from “normal” placental efficiency occurs. Less information is available on normal variation in placental size and efficiency; however, the timing of the secondary period of loss supports a role for events during placental development and vascularization being critical to survival and potentially contributing to the observed loss. Copyright © 2002. American Society of Animal Science . ©2002 American Society of Animal Science

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