During meiosis, the cytostatic factor (CSF) activity stabilizes the activit
y of the M-phase promoting factor (MPF) in metaphase II arrested vertebrate
oocytes. Upon oocyte activation, the inactivation of both MPF and CSF enab
les the entry into the first embryonic mitotic cell cycle. Using a biologic
al assay based on cell-fusion (hybrid between a parthenogenetically activat
ed egg entering the first mitotic division and an activated oocyte), we obs
erved that in activated mouse oocytes a first drop in CSF activity is detec
table as early as 20 min post-activation. This suggests that CSF is inactiv
ated upon MPF inactivation. However, CSF activity increases again to reach
a maximum 60 min postactivation and gradually disappears during the followi
ng 40 min. Thus, in activated mouse oocytes (undergoing the transition to i
nterphase) CSF activity fluctuates before definitive inactivation. We found
that hybrids arrested in M-phase, thus containing CSF activity after oocyt
e activation, have activated forms of MAP kinases while hybrids in interpha
se have inactive forms of these enzymes. We postulate that CSF inactivation
in mouse oocytes proceeds in two steps. The initial inactivation of CSF, r
equired for MPF inactivation, is transient and does not require MAP kinase
inactivation. The final inactivation of CSF, required for normal embryonic
cell cycle progression, is dependent upon the inactivation of MAP kinases.
(C) 1999 Editions scientifiques et medicales Elsevier SAS.