The human blastocyst: cell number, death and allocation during late preimplantation development in vitro

The development of 181 surplus human embryos, including both normally and abnormally fertilized, was observed from day 2 to day 5, 6 or 7 in vitro. 63/149 (42%) normally fertilized embryos reached the blastocyst stage on day 5 or 6. Total, trophectoderm (TE) and inner cell mass (ICM) cell numbers were analyzed by differential labelling of the nuclei with polynucleotide-specific fluorochromes. The TE nuclei were labelled with one fluorochrome during immunosurgical lysis, before fixing the embryo and labelling both sets of nuclei with a second fluorochrome (Handyside and Hunter, 1984, 1986). Newly expanded normally fertilized blastocysts on day 5 had a total of 58.3 +/- 8.1 cells, which increased to 84.4 +/- 5.7 and 125.5 +/- 19 on days 6 and 7, respectively. The numbers of TE cells were similar on days 5 and 6 (37.9 +/- 6.0 and 40.3 +/- 5.0, respectively) and then doubled on day 7 (80.6 +/- 15.2). In contrast, ICM cell numbers doubled between days 5 and 6 (20.4 +/- 4.0 and 41.9 +/- 5.0, respectively) and remained virtually unchanged on day 7 (45.6 +/- 10.2). There was widespread cell death in both the TE and ICM as evidenced by fragmenting nuclei, which increased substantially by day 7. These results are compared with the numbers of cells in morphologically abnormal blastocysts and blastocysts derived from abnormally fertilized embryos. The nuclei of arrested embryos were also examined. The number of TE and ICM cells allocated in normally fertilized blastocysts appears to be similar to the numbers allocated in the mouse. Unlike the mouse, however, the proportion of ICM cells remains higher, despite cell death in both lineages.