218 DEVELOPMENT OF EVENLY AND UNEVENLY CLEAVED TWO-CELL PORCINE EMBRYOS

Abstract

Mammalian eggs are so microlecithal that the embryos would be expected to divide in unison and that each division would lead to 2 equal blastomeres, which are believed to have a greater competence for further development than zygotes with unequal cleavage. However, some studies have shown that uneven blastomere size commonly occurs from the very first division in mammals, and it seems to be concerned with the generation of the first cell lineages of the blastocyst cells: trophectoderm and the inner cell mass (Gueth-Hallonet and Maro 1992 Trends Genet. 8, 274–279). In our study, we produced porcine embryos in vitro (Kikuchi et al. 2002 Biol. Reprod. 66, 1031–1041), and newly formed 2-cell embryos were collected. Based on the timing of the first cleavage (30 or 36 h after insemination), the cleavage pattern (E: equal; U: unequal) and the presence or absence of a second cleavage (+ or –) within the first 2 days of IVC was classified into groups: 30E(–), 30E(+), 30U(–), 30U(+), 36E(–), 36E(+), 36U(–), or 36U(+). There was no difference between the 30E and 30U groups in proportions of the 2-cell stage, which had a nucleus in both blastomeres (99.0 ± 0.8% and 91.4 ± 3.6%, respectively) or between the 36E and 36U groups (98.2 ± 1.1% and 88.0 ± 7.2%, respectively). Comparison of further development between the 30E and 30U groups showed that there was no difference in blastocyst rates (70.7 ± 5.7% and 61.7 ± 7.8%, respectively) and total cell numbers (39.1 ± 2.1 and 31.7 ± 2.3, respectively). Although the blastocyst rate in the 36E group (37.3 ± 6.7%) was significantly higher (P < 0.05) than that of the 36U group (12.0 ± 5.1%), the total cell number was not different (26.3 ± 5.5 and 25.3 ± 5.2, respectively). The timing of the first division, however, had a great influence on further development of the embryos; the 30-h cleaved embryos had a greater rate of blastocyst development (68.2 ± 6.3%) than did the 36-h embryos (28.2 ± 4.8%, P < 0.01 by ANOVA). The cell numbers of blastocysts derived from 30-h cleaved embryos (37.2 ± 2.6) were significantly higher than those of the 36-h embryos (26.2 ± 2.3, P < 0.01) as well. Two-cell embryos that were newly formed at 30 h and underwent the next cleavage within the first 2 days of IVC (30 + group) had a higher blastocyst rate (74.8 ± 7.0%) and greater cell numbers (40.6 ± 2.6) than those not showing a second division during this period (30– group; 46.8 ± 5.0% and 19.9 ± 2.2, respectively). In contrast, for embryos showing the first cleavage at 36 h of insemination, the presence of the next cleavage within 2 days after the first cleavage did not have any effect on embryonic development. These results suggest that the developmental ability of porcine embryos was influenced by the timing and shape of the first cleavage and by the subsequent occurrence of the second cleavage.