Okadaic acid induces premature chromosome condensation reflecting the cell cycle progression in one‐cell stage mouse embryos

Abstract

Haploid parthenogenetic embryos as well as fertilized mouse eggs were treated in vitro with 1–10 μM okadaic acid (OA) at the one‐cell stage. Cytogenetic analysis detected that OA induces nuclear envelope breakdown (NEBD) and premature condensation of interphase chromosomes in pronuclei as well as in 2nd polar body (PB) nuclei. G1‐, S‐, and G2‐type prematurely condensed chromosomes (PCC) were found in pronuclei of embryos of different age, which reflects their progression through the first cell cycle. In nuclei from 2nd PBs only G1‐ and S‐type PCC were observed. Using the types of PCC as a criterion of different phases of the cell cycle, it was possible to estimate that in haploid parthenogenetic embryos G1‐phase lasts until 5.5 hr post activation (hpa), S‐phase takes from 4.5 to 9.5 hpa, and from 8.5 hpa G2‐phase had started. Second PBs were found to be in G1‐phase until 6.5 hpa and S‐phase started in some as early as 5.5 hpa, but in most not before 7.5 hpa. Treatment with OA visualizes G1‐chromosomes in pronuclei as well as in 2nd PBs, and it is easy to count the number of these chromosomes and recognize a T6 marker chromosome. The possibility to apply cytogenetic analysis of G1‐chromosomes from 2nd PBs for a more accurate detection of maternal meiotic nondisjunction is discussed.