Quantitative histological analysis of equine embryos at exactly 156 and 168 h after ovulation

Equine embryos were collected at exactly 156 +/- 0.5 (n=8) and 168 +/- 0.5 h (n=11) after ovulation. The embryos were fixed in glutaraldehyde, sectioned serially and observed using light microscopy. In the 156 h group, all embryos were early blastocysts except for one, which was a morula. The morula and one early blastocyst had no capsule. The capsules of the other embryos were thin. The mean +/- SD total number of cells was 275 +/- 105 (range 117-417). The mean +/- SD proportions of mitotic and pycnotic cells were 2.5 +/- 1.2 and 1.1 +/- 1.8%, respectively, and there were no differences between each inner cell mass (ICM) and trophoblast. The mean +/- SD proportion of ICM cells was 36.5 +/- 5.2%. In the 168 h group, there were early, mid- and expanded blastocysts, all of which had a capsule. The mean +/- SD total number of cells was 1093 +/- 666 (range 272-2217). The mean +/- SD proportions of mitotic and pycnotic cells were 3.5 +/- 1.4% and 0.1 +/- 0.03%, respectively, and there were no differences between each ICM and trophoblast. The mean +/- SD proportion of ICM cells was 21.1 +/- 9.7%. In the 168 h group, there was a significant correlation (P < 0.01) between the total number of cells and the diameters and proportions of ICM cells. The 168 h embryos were composed of significantly (P < 0.01) more cells (approximately four times) than were the 156 h embryos but had lower proportions of ICM cells. These results indicate that in equine embryos there is: (i) an individual (perhaps seasonal) variability in the rate of development; (ii) a doubling in the number of cells every 6 h between 156 h and 168 h after ovulation; and (iii) a decrease in the proportion of ICM cells between the early and expanded blastocyst stages of development.