A cytogenetic study of early embryonic development in an animal model

Abstract

Several cytogenetic studies of human spontaneous abortions have reported 50 percent to contain chromosomal anomalies. Earlier losses due to chromosomal abnormalities or other causes which are not recognized as abortions remain unknown. Due to extreme difficulty in collecting material from man, there is a definite need for a good animal model. Chromosome defects have been observed in preimplantation blastocysts and embryos of other mammalian species including mouse, rat, hamster, rabbit and pig. The present study was designed to cytogenetically analyze early embryonic development by recovering preimplantation blastocysts and by collecting embryos during the implantation period in pigs. Fifty-eight animals were cytogenetically analyzed at 10, 11, 12, 13, 16, 17, 18 and 19 days gestation. Using direct chromosome preparation, 338 blastocysts from thirty-one animals were analyzed. A variety of ploidies were observed in the same blastocyst ranging from 2N to 21N. Forty blastocysts from seven animals were cultured for 24 hours. A variety of ploidies were observed in the same blastocyst ranging from 2N to large numbers of "high-order" polyploids greater than 32N. Fifty percent of the preimp1antation blastocysts in the direct preparation were polyploid mosaics, while 9S percent of the blastocysts after 24-hour culture were polyploid mosaics. This mosaicism was most likely due to chromosome preparation of trophoblast giant cells. Five cases of chromosomally abnormal blastocysts included three triploids, one haploid, and one translocation mosaic. The frequency of blastocysts demonstrating chromosome abnormalities (1.48 percent) most likely represented an underestimate of spontaneously occurring abnormalities. Tae polyploid mosaics were not included in the abnormal estimate. Using direct chromosome preparation, 192 embryos from twenty animals were analyzed. Polyploidy was insignificant and only four chromosomally abnormal embryos were observed. All four were monosomic mosaics and represented 2.08 percent of embryos studied. The pooled estimate of mortality for blastocysts was 12.8 percent and that for embryos was 31.8 percent. These data suggest that a significant number of conceptuses were lost during the implantation process. The cytogenetic data failed to support the hypothesis that the increased mortality was due to chromosomally abnormal blastocysts failing to survive implantation. However, the frequency of abnormal blastocysts was most likely an underestimate due to the mixed population of trophoblast giant cells and embryonic disc cells in the blastocyst preparations. A direct comparison to anomalies observed in human spontaneous abortions would not be appropriate. However, the value of swine as an animal model for early embryonic development remains to be fully explored.