71 DEVELOPMENTAL DELAY OF PRE-IMPLANTATION OVINE IN VITRO CULTURED AND SOMATIC CELL NUCLEAR TRANSFER EMBRYOS

Abstract

Despite advances in the production of somatic cell nuclear transfer (SCNT) embryos, significant embryo losses are persistent, particularly around implantation. Malformations of the placenta and in a variety of organs are seen, and have been linked to deviant epigenetic reprogramming. The aim of the present study was to examine the formation of germ layers, which are prerequisites for formation of the embryo proper and placenta, in invivo-derived (in vivo), partly in vitro-cultured (IVC), and SCNT ovine embryos. Embryos were derived as follows: In vivo embryos (n = 27) were flushed from the uterus on Days 7, 9, 11, and 13. For IVC embryos (n = 22) in vivo zygotes were flushed, followed by culture in the presence of 20% human serum, transfer to the uterus on Day 6, and flushing as in vivo embryos. SCNT embryos (n = 41) were produced by fusion of serum starved granulosa cells with enucleated oocytes, followed by activation, culture in SOF, transfer to the uterus on Day 6, and flushing as described for in vivo embryos. Recovered embryos were processed for light microscopy (LM) and transmission electron microscopy (TEM), and paraffin sections were immunohistochemically labelled for the germ layers: alpha-1-fetoprotein for potential endoderm, cytokeratin-8 for potential ectoderm, and vimentin for potential mesoderm. A consistent delay of the IVC and particularly the SCNT embryos was noted throughout all time points: On Days 7 and 9, differentiation of the inner cell mass into hypoblast and epiblast was evident in 7 out of 12 in vivo embryos, whereas this phenomenon was less prominent or absent in 9 out of 13 IVC and 13 out of 15 SCNT embryos. Furthermore, 6 of the IVC and 12 of the SCNT embryos lacked an identifiable embryonic disc. On Day 11, half of the in vivo embryos had initiated gastrulation, evidenced by localization of endoderm and mesoderm precursor cells between the hypoblast and the epiblast. This feature was noted in only a single IVC and in none of the SCNT embryos. On Day 13, all in vivo embryos had completed gastrulation including the formation of somatic and visceral mesoderm. This feature was noted in only 1 out of 3 IVC and in none of the SCNT embryos. Likewise, amniotic folds were seen in one third of the in vivo embryos at this stage, but not observed in any IVC or SCNT embryos. The immunohistochemical markers displayed the same cell lineage localization in all three groups of embryos, but a developmental delay in the IVC and in particular the SCNT embryos was evident. In conclusion, ovine IVC and SCNT embryos develop at a slower rate than in vivo embryos at least up until Day 13 of gestation.