Asters play only a dispensable role in the induction of the cleavage furrow in the blastomeres of early Xenopus embryos

Abstract

Kuroda et al . (2001) of our laboratory have previously revealed that exposure of early Xenopus embryos to 150 m M urethane results in complete suppression of formation of the asters and the cleavage furrow, as well as significant reduction of the size of the spindle in the blastomeres, allowing only 1 or 2 cycles of mitosis but not cytokinesis. In the course of closer examination of the effect of urethane on the cleavage of blastomeres of early Xenopus embryos, we unexpectedly discovered that exposure of early Xenopus embryos to 75 m M urethane did not prevent cell division at all, though asters were not detected in the blastomeres. Instead, they contained a spindle that appeared rather normal. They also formed the diastema, a thin yolk-free structure, which is considered to play an essential role in the induction of the cleavage furrow. Essentially the same results were obtained in the exposure of embryos to vinblastine, a well-known microtubule inhibitor: exposure of embryos to 20 µg/mL vinblastine resulted in complete suppression of cleavage of the blastomeres, where formation of both the spindle and asters were perfectly suppressed. By contrast, exposure of embryos to 5 µg/mL vinblastine did not prevent cleavage in the blastomeres though asters were not detected, whereas the rather normal spindle was formed. Thus, there was a close correlation between the formation of the normal spindle, not asters, and that of the cell division furrow and the diastema in the blastomeres of early Xenopus embryos. We suggest that while the spindle plays an essential role, asters are likely to play only a dispensable role in the induction of the cleavage furrow in even very large cells like the blastomeres of early Xenopus embryos.

Dependence of the timing system regulating the onset of gastrulation on cytoplasmic, but not nuclear, activities in the Xenopus embryo

“…Eggs were exposed to urethane at a final concentration of 150 mM. These concentrations are sufficient for the suppression of cell division in Xenopus embryos (Hara et al 1980;Shinagawa 1983;Huntchison et al 1989;Newport & Dasso 1989;Zhang & Masui 1992;Clute & Masui 1997;Flament et al 2000;Kuroda et al 2001;Sakaida et al 2004).…”

“…Urethane is a local anesthetic which has an ability to stabilize microfilaments and, conversely, destabilize microtubules in both amphibian and echinoderm embryos (Hylander & Summers 1981;Rappaport & Rappaport 1984;Begg et al 1996;Kuroda et al 2001;Sakaida et al 2004). However, this ability does not seem to be primarily responsible for the delay in the onset of gastrulation observed here, because vinblastine, which is also a strong inhibitor of microtubules, did not delay the onset of gastrulation.…”