Regulation of cell cycle by cytoplasmic components in the amphibian eggs.

Injections of extracts of Xenopus egg cytoplasm induce precocious cleavage furrows in activated Pleurodeles eggs. This cleavage timing system (CTS) activity is retained in two distinct fractions separated by gel filtration. Acceleration of the first cell cycle of the Pleurodeles egg is directly related to the concentration of the injected extracts. However, the maximum advance of the cleavage cycle does not exceed 25 to 30% of the control cycle. Preliminary characterization of these fractions indicates that the CTS effect is abolished after protease but not RNAse treatment, and that the CTS factors are thermostable. The phase of the egg cell cycle sensitive to the CTS factors, which act as a 'clock system mechanism', is discussed.