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

This study examines the properties of the timer that regulates the onset of gastrulation in the Xenopus embryo. Pre-gastrulation embryos were exposed to aphidicolin, vinblastine, 6-dimethylaminopurine (6-DMAP) or urethane. Embryos exposed to aphidicolin or vinblastine for 0.

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

Kuroda et al. (2001) of our laboratory have previously revealed that exposure of early Xenopus embryos to 150 mm 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 mm urethane did not prevent cell division at all, though asters were not detected in the blastomeres.

Timing system for the start of gastrulation in the Xenopus embryo.

This study examined which component of the egg, the nucleus or cytoplasm, is involved in the timing of the start of gastrulation in the Xenopus embryo, and when it starts to measure time. First, nuclei of cells of 256-cell stage embryos were transplanted to enucleated eggs 60 min after activation. These eggs showed first cleavage 20-30 min later than control eggs fertilized at the same time as the activation of recipient eggs, and started gastrulation 25-35 min later than control embryos (depending on the delay in the first cleavage).

Production of hyperdorsal larvae by exposing uncleaved Xenopus eggs to a centrifugal force directed from the animal pole to the vegetal pole.

Exposure of uncleaved Xenopus eggs to a centrifugal force directed from the animal pole to the vegetal pole produces larvae with enhanced dorsal structures, which resemble 'hyperdorso-anterior' larvae produced by D O-treatment at 0.3 normalized time (NT). Optimal conditions are 70 g for 6 min at 20% of the first cell cycle (0.

Relative Timing of Stiffening with Various Combinations of Nucleate and Enucleated Egg Fragments of Xenopus laevis: (egg stiffening/Xenopus laevis/cell cycle/timing system).

Previously, I found that in Xenopus eggs, the surface contraction wave (SCW) can arise at a point close to a male, a female or a zygote nucleus or a top portion of the egg (9). This finding suggested that all types of nucleus and cytoplasm of a Xenopus egg have the potential to determine the point of initiation of the SCW. Since stiffening is closely associated with the SCW, to determine the factors governing this phenomenon, I compared the times of egg stiffening of pairs of fragments containing different types of nuclei or cytoplasm.

Mechanism of Dorso-Ventral Axis Specification in Nuclear Transplanted Eggs of Xenopus laevis: (dorso-ventral axis/nuclear transplantation/subcortical rotation/gray crescent/Xenopus laevis).

The rotation of subcortical cytoplasm relative to egg surface ("subcortical rotation", 28) is known to specify the dorso-ventral axis of fertilized Xenopus eggs. However, except for brief descriptions (14, 15), no information has been provided on the process of the dorso-ventral axis specification in nuclear transplanted Xenopus eggs. This study shows that (i), among the events that occur in association with dorso-ventral axis specification in fertilized Xenopus eggs, only the subcortical rotation occurs in a normal manner in both nuclear transplanted and non-nuclear transplanted (enucleated) Xenopus eggs, (ii), in these eggs, there is no factor that appears to affect the direction of subcortical rotation as the sperm entry point does in fertilized eggs: neither the site of pricking for activation nor that of injection for nuclear transplantation affects the direction of the subcortical rotation in those eggs and (iii) the subcortical rotation in nuclear transplanted eggs predicts very precisely the direction of their dorso-ventral axis as it does in fertilized eggs.

Marked Alteration at Midblastula Transition in the Effect of Lithium on Formation of the Larval Body Pattern of Xenopus laevis: (midblastula transition/LiCl/pattern formation/half-egg fragment/Xenopus laevis).

Embryos of Xenopus laevis were exposed to 0.4 M LiCl for 5 min at various stages of development. The effect of lithium on the larval body pattern could be detected from the 2-cell to the late gastrula stage, but changed from reduction of posterior structures ("anteriorization") to reduction of anterior structures ("posteriorization") just after the 12th cleavage, the time of midblastula transition (MBT).

Nuclear Involvement in Localization of the Initiation Site of Surface Contraction Waves in Xenopus Eggs: (surface contraction waves/Xenopus laevis/nucleus/colchicine/cleavage).

The initiation site of surface contraction waves (SCWs) was examined in fertilized, parthenogenetically activated and enucleated Xenopus eggs after either rotation through 90° off the vertical axis or injection of colchicine. In enucleated eggs, SCWs always started from a top site of the egg under all conditions examined. In fertilized or activated eggs, SCWs started, depending on the experimental conditions, from either the sperm entry point, the animal pole region located sideward or the top site of the egg.

Close Correlation of the Periodicities of the Cleavage Cycle and Cytoplasmic Cycle in Early Newt Embryos: (newt/cleavage/cytoplasmic cycle).

The correlation between autonomous cyclic activity of the cytoplasm (cytoplasmic cycle) and the cleavage cycle was studied by using animal and vegetal half fragments of newt eggs formed by bisecting the uncleaved eggs after those eggs had been rotated through 90° off the vertical axis so as to alter the allocations of the cytoplasmic content in the two halves. When the bisection was made shortly after the rotation (Early Bisection), the resulting vegetal fragment showed 1.5 times longer intervals in the cytoplasmic cycle than its animal partner when cleavage was prevented by injection of colchicine, while when the bisection was made 30 min after the rotation (Late Bisection), the resulting pair of halves displayed equal intervals in the cytoplasmic cycle.