The maturation response of stage IV, V, and VI Xenopus oocytes to progesterone stimulation in vitro.

Full-grown Xenopus oocytes, Stage VI (1200-1300 microns), undergo meiotic maturation when exposed to progesterone. Smaller stage IV (800 microns) and stage V (1000 microns) oocytes remain in prophase arrest when exposed to this steroid. The larger stage VI oocytes undergo an intracellular alkalization from 7.2 to 7.6, a six- to eightfold increase in the phosphorylation of the 40 S ribosomal protein S-6, and a two- to threefold increase in total protein synthesis when exposed to progesterone. It was found that 800- to 1000-microns oocytes do not undergo these physiological changes when exposed to progesterone. This lack of response could explain the failure of small oocytes to undergo germinal vesicle breakdown (GVBD). However, when stage IV and V oocytes were artificially alkalized to a pHi of 7.6 by the weak bases, trimethylamine, procaine, or methylamine, S-6 phosphorylation was stimulated four- to sixfold and protein synthesis was stimulated two- to threefold, but they still did not undergo GVBD. Stage IV and V oocytes are able to amplify MPF injected into their cytoplasm and undergo GVBD. Thus, 800- to 1000-microns oocytes appear to contain a store of inactive MPF in their cytoplasm. It seems that an additional physiological parameter(s), that is unique to steroid-treated stage VI oocytes, is responsible for activating this MPF which induces GVBD.