Cleavage rates of diploid and tetraploid mouse embryos during the preimplantation period

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Abstract
Despite the fact that spontaneous tetraploidy is a rare phenomenon in mice, such embryos may be produced experimentally by a variety of means, though only a very limited degree of postimplantation development has been achieved. Despite this apparent limitation, much data on the rate of development of preimplantation tetraploid embryos has been published. However, the findings from these studies has often been conflicting. In the light of the recent successful achievement of advanced postimplantation tetraploid development in our laboratory, we decided it was an opportune time to re-evaluate the preimplantation development of these embryos in as near to optimal conditions as we could achieve. Three groups were studied, namely 1) control (diploid) embryos developing in vivo, 2) control (diploid) embryos that had been isolated at the 2-cell stage, briefly retained in culture, then transferred to the oviducts of pseudopregnant recipients, and 3) tetraploid embryos produced by electrofusion of blastomeres at the 2-cell stage, then transferred to the oviducts of pseudopregnant recipients. Embryos were isolated from females from each group at specific times after the HCG injection to induce ovulation. The total cell number of each embryo was established and the log mean values were plotted against time. From the gradients of the lines it was possible to establish that there was a significant difference between the cell doubling time of the transferred controls (group 2) compared to the in vivo controls (group 1) with cell doubling times of 15.86 ± 1.45 h and 10.27 ± 0.24 h, respectively. However, there was no significant difference between the cell doubling time of the tetraploids and the transferred controls (group 2), with the tetraploids having a cell doubling time of 14.87 ± 0.54 h. The results therefore show that under matched experimental conditions, tetraploid mouse embryos develop during the preimplantation period at the same rate as diploids despite their reduced cell number. This finding suggests that the various hypotheses that have been proposed to explain the poor postimplantation development of tetraploids, related principally to their supposed poorer proliferative activity than controls, does not appear to be applicable at least in relation to their preimplantation development.