Depletion of maternal SMC2 causes abnormal nuclear morphology
in zygotes
Considering the nucleolar maturation defects observed in SMC2-deficient
oocytes and pronuclear stage arrest of SMC2-/+embryos, we next examined whether reorganization of the maternal and
paternal pronuclei was affected after fertilization.
Abnormal pronuclear morphology was observed in SMC2 −/
+ 1-cell embryos (Fig. 4A). Chromosomes were not properly separated
between maternal pronuclei and PB2. The chromosomes were still connected
by thin threads of decondensed chromatin. We also observed that numerous
abnormal micronuclei existed in fertilized eggs. We also measured the
sizes of male and female pronuclei. Maternal and paternal pronuclei were
distinguished by their position in the fertilized eggs. The female
pronucleus was closer to the second polar body (PB2) than the male
pronucleus. First, we examined the pronuclear morphology of the
SMC2+/+ and SMC2-/+ embryos, and
quantified their relative diameter ratio. 1-cell SMC2−/+ embryos showed smaller male pronuclei and larger
female pronuclei compared to the control 1-cell embryos (Fig. 4B). We
calculated the size ratio of female and male pronuclei diameters (Fig.
4C). Strikingly, female pronuclei, which usually are smaller than male
pronuclei, were larger than male pronuclei in
SMC2-/+ zygotes, contrary to the
SMC2-/+ embryos. In SMC2 +/ + 1-cell
embryos, the nucleus ratio was close to 0.7, but in
SMC2−/ + embryos, in addition to the abnormal female
nucleus morphology, the ratio deviated up to 1.4. Thus, deletion of
maternal SMC2 leads to abnormal female pronuclear organization.