The growth of human preimplantation embryos in vitro.

The parameters of the growth of human embryos in vitro between fertilization and the blastocyst are described. The data on the cleavage of embryos were subjected to statistical analysis. All the pronucleate eggs examined had two pronuclei. There was no evidence of any oocyte with three or more pronuclei, except in one examined 3 days after insemination, which may have been degenerating. Most eggs which were unfertilized remained in metaphase of the second meiotic division, with an extruded polar body. Spermatozoa were found embedded deeply in the zona pellucida of unfertilized and pronucleate eggs. A total of 417 embryos cleaved in vitro. Many were in the two-cell stage by 28 hours, four-cell stage at 43 hours, and eight-cell stage at 54 hours. Cleavage patterns were regular in the great majority. Compaction began at the 16 cell stage; the blastocyst began to form between 4 and 5 days after fertilization. Forty-two embryos were examined sufficiently often to make a statistical analysis of their growth in vitro. The mean doubling time of the cells was 15.82 hours approximately. Estimates of the mean midstage times for each stage of division were calculated. The results are discussed in relation to the cure of infertility by replanting human embryos, to the possibility of chromosomal and other disorders arising in the embryos, and to potential scientific studies on early human development.