Ultrasound evaluation of fetal and placental development in clone and control bovine pregnancies
Résumé
Since the first success in sheep, the production of viable cloned offspring by somatic cell nuclear transfer (SCNT) in various mammals has increased significantly. The incidence of pregnancy failure and foetal death, however, is still very high, whatever the species. In cattle, most gestation losses are initially due to early foetal losses during the pre-implantation period and thereafter to abnormal placentation associated withan overgrowth of the foetus known as the Abnormal Offspring Syndrome (AOS). As a consequence, the International Embryo Transfer Society (ETS) recommends in the IETS manual that regular ultrasound (US) evaluation of the SCNT foetuses are performed throughout gestation in order to terminate pregnancy if foetal and/or maternal survival are at risk. Although US criteria such as qualitative evaluation of the placentomes, foetal movement, and foetal fluids have been proposed, there is a need to establish new quantitative criteria that could be used for the diagnosis of AOS. The objective of this study was to evaluate the use of new, quantitative parameters, for the assessment of foeto-placental development in AI and SCNT bovine pregnancies.
Twenty-two heifers of 4 different breeds were used as SCNT recipients and 11 Holstein heifers were used as AI controls (C). All SCNT foetuses were produced as previously published in the laboratory, using the same fibroblast donor. Animals were scanned every two weeks from 150 days of pregnancy, using a Voluson-i (GE Medical Systems) equipped with a transabdominal multifrequency probe (2.2-6.5 MHz). For each examination, 7 placentomes were classified from 0 to 3 according to echogenicity, general appearance and degree of oedema. Intercoastal distances were measured in a coronal view just behind the heart. Doppler velocimetry indices, pulsatility index (IP) and resistance index (IR) of the foetal umbilical artery were obtained.
So far, 26 heifers have either delivered (N= 10 SCNT and 7 C) or pregnancies have been terminated due to AOS (N=9 SCNT). Preliminary results indicate that Inter-coastal space is increasing with gestational age (p<0.0001) and consistently larger in SCNT vs C (p<0.0001) at all stages of pregnancy in the third trimester. The mean placentome classification (for 7 placentomes/examination) is significantly higher (less normal) in SCNT compared to C but does not vary according to gestational age (p<0.0001). In contrast, the umbilical cord pulsatility index (PI) and the resistance index (IR) tend to decrease with pregnancy (p<0.01) but are not different between SCNT and C.
These preliminary data indicate that there are significant differences between SCNT and C pregnancies and further analyses are on-going taking into account pregnancy outcomes and the last on-going pregnancies.