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Involvement of a urethane‐sensitive system in timing the onset of gastrulation in Xenopus laevis embryos

This paper describes success in delaying the onset of gastrulation in Xenopus laevis embryos without damage to their subsequent development by temporarily arresting cleavage with urethane. Exposure of X. laevis embryos to 150 mM urethane before gastrulation resulted in cleavage arrest and its removal led to cleavage resumption. During cleavage arrest, cyclic activities including nuclear replication and the M‐phase‐promoting factor cycle continued, although their duration was lengthened to nearly 1.8‐fold that of the controls. Because of a 30‐min time lag from removal of urethane to resumption of cleavage, as well as the retardation of cyclic activities during cleavage arrest, the development of embryos after a 60‐min exposure to urethane lagged two cell cycles behind that of control embryos. Here, the two cell cycle delay is equivalent to 50 min at 22–23°C. The start of gastrulation in exposed embryos was accordingly delayed about 50 min, although the delay in mid‐blastula transition was as little as 20–25 min. Consistent results were obtained in embryos exposed to urethane for 90 or 120 min and those exposed to procaine or NH4Cl for 60 min. Although these results imply that delay in the start of gastrulation in exposed embryos is ascribed simply to delay in their development raised by cleavage arrest, at the same time they suggest that the onset of gastrulation is timed by systems sensitive to urethane, procaine and NH4Cl in X. laevis embryos.