Assessment of developmental retardation and abnormality of in vivo produced preimplantation embryos in rat.

In most mammals studied, a substantial numbers of preimplantation embryos are believed to be lost in vivo. In vitro, embryos develop slowly and lose viability. Hence, there is a need to assess the extent and cause of embryonic loss both in vivo and in vitro. In this study, we assessed the quality of in vivo produced ovulation products/embryos, recovered on days 1-5 pregnancy, from naturally bred wistar rats. From day 1 pregnant rats (n = 24), 226 ovulation products were recovered which included 52% (117) unfertilized oocytes and empty zonae with/without cell debris (UFO-EZ:CD) and 48% (109) 1-cells. Flushings of day 2 rats (n = 27) contained 229 ovulation products, consisting of 70% (160) 2-cells and 30% (69) UFO-EZ:CD. Flushings of day 3 rats (n = 27) had 23% (56) 2-cells, 6% (15) 3-cells, 23% (57) 4-cells, 1% (2) 5-7 cells, 2% (5) 8-cells and 45% (112) UFO-EZ:CD, total being 247. Flushings of day 4 rats (n = 28) had 193 ovulation products comprising of one morula, 45% (86) 8-cells, 5% (9) 5-7-cells and the rest were 4-cells (2), 3-cells (2), 2-cells (1) and 48% (92) UFO-EZ:CD. Day 5 flushings (n = 27) had 202 ovulation products which included 13% (27) morulae, 17% (34) early, 36% (73) mid and 2% (5) late blastocysts; additionally, 4-cells (1), 8-cells (2) and 30% (60) UFO-EZ:CD were also recovered. On day 4, embryos (8-cells) migrated from the oviduct to the uterus. When pregnant rats (n = 25) were allowed to term, only 15 females (60%) delivered pups (128) with variable litter size (2-12). These results indicate that 56% (619/1097) of recovered rat preimplantation embryos are of expected developmental age with a mixture of asynchronously cleaving embryos. The remaining 44% (478) is comprised of 38% (417) UFO-EZ:CD and 6% (61) abnormal and developmentally retarded embryos, which are unlikely to produce viable pups at term.