The in‐vitro block to development and initiation of transcription in early equine embryos

Coculture of early cleavage‐stage equine embryos with other cell types, primarily oviductal epithelial cells, can support development to blastocysts in vitro. Embryos cultured in complex medium without cellular support develop at a significantly poorer rate and appear to undergo a developmental block in vitro. In 2 experiments, 1–8‐cell embryos (n = 15) were cultured in Ham's F12: Dulbecco's MEM for 5 days or until morphological degeneration occurred. Embryos were assessed for stage of development and quality score on a daily basis. At the end of culture, embryos were stained to determine the number of nuclei present or were fixed and sectioned for evaluation by light microscopy. Additional embryos (n = 8) collected on Days 2–7 after ovulation were labelled with [³H]uridine and incorporation was determined by autoradiography. Of 15 embryos cultured without cellular support, 3 reached the morula stage and the remaining 12 embryos cleaved to the 8–16‐cell stage before undergoing morphological degeneration. [³H]uridine was incorporated into embryos at the 8‐cell stage or later, but no incorporation was detected in embryos at the 4‐cell stage. These results indicate that early equine embryos appear to undergo a developmental block in vitro at the 8–16‐cell stage, relief of which is temporally associated with the onset of embryonic transcription.