CALCIUM BUFFER INJECTIONS INHIBIT CYTOKINESIS IN XENOPUS EGGS

A slow cortical wave of high calcium accompanies the elongation of cle avage furrows in medaka fish eggs as well as in Xenopus eggs. We explo red the role of such waves by injecting calcium buffers into Xenopus e ggs at various times before and during first and second cleavage. Inje ction earlier than about 15 minutes before first cleavage normally sta rts delays it for hours. Injection between about 15 minutes and a few minutes before cleavage normally starts allows a (short) furrow to for m on time but usually yields an eccentric one. This forms away from th e injection side, often as far off-center as the egg's equator, and th en regresses. Injection soon after it starts quickly arrests elongatio n of the furrow and eventually induces its regression; while injection a bit later likewise soon arrests elongation but allows delocalized f urrow deepening to continue. The dependence of these inhibitory action s upon the dissociation constants and final cytosolic concentrations o f the injected buffers indicates that they act as shuttle buffers to s uppress needed zones of high calcium in the micromolar range. We concl ude that the high calcium that is found within these furrows is needed to induce them, to extend them and even to maintain them.Moreover, wh ile short, eccentric furrows often form as far off center as the equat or, they somehow always form along a meridian through the animal pole. This seems difficult to explain by the orthodox, diastral model. Rath er, it suggests that the cleavage furrows in Xenopus - and perhaps in animal cells quite generally-are directly induced by a diastema or tel ophase disc rather than by the asters.