Consequences Of Altered Placental Development For Fetal Growth

Placental weight is a powerful determinant of fetal growth in all domestic mammals, especially during late gestation. This has been demonstrated in sheep by natural reduction of placental growth due to increased litter size, environmental heat stress, or overfeeding of adolescent ewes, and by surgical removal of uterine caruncles before mating (Fig. 3).[9] These manipulations of placental growth have been used to identify mechanisms by which fetal growth is constrained by placental functional capacity as determined by vascular growth[13] and the development of specific nutrient transport systems.[9]

In vitro manipulation of bovine and ovine embryos has caused increased incidence of excessive fetal growth and of perinatal mortality and morbidity. Underlying mechanisms are complex and vary with the type of manipulation. However, in vitro culture of cleavage-stage embryos is a common element in most cases of large-offspring syndrome. Evidence for the involvement of abnormal placental development in this syndrome was recently reported.[14] Bovine embryos produced in vitro were more predisposed to later embryonic and early fetal mortality. During midpregnancy (days 90 and 180), increased fetal size was associated with fewer, larger placentomes with considerably greater total surface area. These observations are consistent with, but do not necessarily prove, the direct involvement of increased placental growth in large-offspring syndrome. Also, not all cases of large offspring derived from embryos manipulated in vitro have been associated with altered placental morphology.

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