Transition from maternal to embryonic control of development in IVM/IVF domestic cat embryos.

The timing of the transition from maternal to zygotic control of development (MZT) and the initiation of transcription was studied in domestic cat embryos to determine if there is a temporal association between these phenomena and the development block observed in cat embryos fertilized in vitro. Embryos were derived from in vitro-matured, in vitro-fertilized (IVM/IVF) oocytes. In Experiment 1, embryos (n = 52) were cultured continuously in the presence of 10 micrograms/ml alpha-amanitin (a transcriptional inhibitor) from 12-hr postinsemination (hpi), and cleavage stage was evaluated every 24 hr. The proportion of embryos cleaving to at least the 5-8 cell stage in the presence of alpha-amanitin (32/52) was similar (P > 0.05) to that of controls cultured without alpha-amanitin (25/50). In contrast, only 7.7% of alpha-amanitin-treated embryos cleaved to the 9-16-cell stage, compared with 38.0% of the controls (P < 0.05), indicating that products of embryonic transcription were required for cleavage beyond the 5-8-cell stage. In Experiment 2, embryos were cultured in the presence of 20 microM 3H-uridine for 12 hr beginning at 24, 36, 48, or 60 hpi and subjected to autoradiography. Embryos of 5-8-cell and 9-16-cell stages (14 of 27 and 8 of 12, respectively) clearly demonstrated nuclear labeling, a finding also confirmed by computer-aided densitometry. It is concluded that embryonic transcription and the MZT occur by the 5-8-cell stage of IVM/IVF domestic cat embryo development and the MZT is not directly related to the partial morula-to-blastocyst developmental block observed in cultured IVF cat embryos.