Partial activation of surf clam oocytes by hypertonic treatment.

Abstract

In prophase I‐arrested surf clam oocytes, fertilization is the normal trigger for resumption of meiosis, first evidenced by germinal vesicle breakdown (GVBD). Various artificial agents are able to induce GVBD and most of them require the presence of external Ca2+ to be efficient. One exception to this rule is the reported possibility of inducing GVBD by an hypertonic treatment, using high concentrations of glycerol, in the complete absence of external Ca2+. We have investigated the processes underling glycerol‐induced activation and found that, under this condition, GVBD shows very slow kinetics and is not followed by any normal subsequent steps of meiosis, such as formation of metaphase chromosomes or polar body extrusion. Glycerol‐activated oocytes do not show the normal response in protein synthesis but they undergo increased protein phosphorylation which is inhibited by 6‐dimethylaminopurine (6‐DMAP), which also inhibits GVBD. We conclude that the hypertonic treatment by glycerol, in the complete absence of external Ca2+, induces a partial program of activation through increased protein phosphorylation but that the normal full response requires an increased Ca2+ influx as triggered by other well‐known artificial activating agents.

Mechanisms of activation of surf clam (Spisula solidissima) oocytes

“…One notable exception to this rule was reportqd by Dessev and Goldman (1990) who showed that hypertonic treatment led to GVBD in Spisula oocytes, in the absence of external caZ+. We have confirmed this observation but showed that the timing of GVBD, upon hypertonic treatment, was much slower than normal and that oocytes would not proceed further to polar body extrusion nor to the alignment of chromosomes in a normal metaphase, suggesting an absent or deficient meiotic spindle (Coutu et al, 1993). This intriguing phenomenon may be interpreted in different ways, all of which might not be relevant to the physiological process.…”