Experimental analysis of polar body formation in starfish eggs

Abstract

AbstractIn polar body formation, a protuberance develops on the oocyte surface and subsequently separates from it by a process resembling normal cytokinesis. Protuberance formation has usually been attributed to controlled cytoplasmic outflow associated with local surface weakening. This investigation was designed to ascertain by physical manipulation more information concerning the physical mechanisms that accomplish the form changes. In unoperated Pisaster giganteus and Asterias vulgaris oocytes, unsymmetrical polar body formation was associated with failure of the mitotic apparatus to assume its normal orientation perpendicular to the oocyte surface. Needles inserted perpendicular to the plane of division straddling the mitotic apparatus can block separation of the polar body depending on the distance between them. Consequences of moving the mitotic apparatus with a needle were consistent with those in oocytes in which the mitotic apparatus was spontaneously maloriented. Shifting all or part of the mitotic apparatus resulted in polar body formation activity if the apparatus was close enough to the surface. Shifting at interphase did not affect the surface until the next meiotic cycle. Shifting during meiotic activity resulted in rapid local protuberance formation at the new site (av. 1.74 min, n=21). Increasing internal hydrostatic pressure by flattening the oocyte did not enhance cytoplasmic flow into the normal protuberance and usually caused it to recede rapidly. When oocytes were immersed in 50% seawater, the osmotic swelling did not enhance the flow of cytoplasm into the protuberance and the point of rupture showed no tendency to be localized in the polar body area. There did not appear to be a cytochalasin B resistant form change during starfish polar body formation. Extensive tears in the oocyte surface, which prompted massive cytoplasmic outflow, had no effect on the form of the nearby protuberance. Experiments indicated clear differences in response to hydrostatic pressure between normal early stages of polar body formation and similarly sized excrescences formed on the oocyte by mechanical or chemical weakening of a restricted part of the surface. They suggest that the protuberance of normal polar body formation may not result from local surface weakening.