Reproductive Technology

Large Offspring or Large Placenta Syndrome? Morphometric Analysis of Late Gestation Bovine Placentomes from Somatic Nuclear Transfer Pregnancies Complicated by Hydrallantois

Abstract
Somatic nuclear transfer (NT) in cattle is often complicated by fetal oversize (Large Offspring Syndrome), hydrallantois and placentomegaly in late gestation. The aims of this work were to obtain data on the placentome structure in NT recipient cows with hydrallantois (NTH) and to relate these with fetal and placental weights to better understand the abnormalities observed in NTH pregnancies during the third trimester. Pregnant cows were slaughtered between 180 and 280 days of gestation. The fetuses were weighed and the placentomes numbered and weighed. Placentomes were examined by histology and stereology. Macroscopic data showed that placental overgrowth preceded fetal overgrowth and the fetal/total placentome weight ratio was lower in NTH versus controls after Day 220. This suggests that placental overgrowth is due to placental default rather than fetal overgrowth, as shown also by stereological analysis showing primary dergulation of the growth of cotyledonary tissues. Observed alterations like thinning of the maternal epithelium within placentomes and increased trophoblastic surface could be secondary adaptations. Thus, placental growth deregulations would be due to modifications of the expression of placental factors. Various evidences of placental deficiency were observed, suggesting that some fetal abnormalities observed in NTH calves, such as enlarged heart, enlarged umbilical cord, abdominal ascites, are consequences of placental dysfunction. Therefore, the term 'Large Offspring Syndrome' might be better replaced by 'Large Placenta Syndrome for this syndrome which affects on average 50% of late gestation NT pregnancies. No conclusion can be drawn from this work on apparently normal pregnancies.

Key words: Assisted Reproductive Technology • Pregnancy • Conceptus • Developmental biology • Placenta

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