Cytostatic factor (CSF) activity in cytosols extracted from Xenopus laevis eggs

Cytostatic factor (CSF), found in the cytoplasm of unfertilized eggs of amphibians, causes metaphase arrest when microinjected into cleaving blastomeres. Although CSF from Rana pipiens eggs has been extracted and characterized, little is known about CSF extracted from eggs of other species. We investigated the conditions required to preserve CSF activity in cytosols extracted from Xenopus laevis eggs and found that it was necessary to expose the eggs to CO2 prior to extraction and that the extraction buffer must contain sodium beta-glycerophosphate. CSF activity disappeared after 24 h of storage at 2 degrees C. Cytological examination showed that the arrested blastomeres injected with cytosols had been arrested at metaphase and contained a spindle lacking polar asters, in which highly condensed chromosomes were embedded.