CYTOSTATIC ACTIVITY DEVELOPS DURING MEIOSIS-I IN OOCYTES OF LT SV MICE/

Oocytes of wild-type mice are ovulated as the secondary oocytes arrest ed at metaphase of the second meiotic division. Their fertilization or parthenogenetic activation triggers the completion of the second meio tic division followed by the first embryonic interphase. Oocytes of th e LT/Sv strain of mice are ovulated either at the first meiotic metaph ase (M I) as primary oocytes or in the second meiotic metaphase (M II) as secondary oocytes. We show here that during in vitro maturation a high proportion of LT/Sv oocytes progresses normally only until metaph ase I. In these oocytes MAP kinase activates shortly after histone H1 kinase (MPE) activation and germinal vesicle breakdown. However, MAP k inase activation is slightly earlier than in oocytes from wild-type F1 (CBA/H x C57Bl/10) mice. The first meiotic spindle of these oocytes f orms similarly to wild-type oocytes. During aging, however, it increas es in size and finally degenerates. In those oocytes which do not rema in in metaphase I the extrusion of first polar bodies is highly delaye d and starts about 15 h after germinal vesicle breakdown. Most of the oocytes enter interphase directly after first polar body extrusion. Fu sion between metaphase I LT/Sv oocytes and wild-type mitotic one-cell embryos results in prolonged M-phase arrest of hybrids in a proportion similar to control LT/Sv oocytes and control hybrids made by fusion o f two M I LT/Sv oocytes. This indicates that LT/Sv oocytes develop cyt ostatic factor during metaphase I. Eventually, anaphase occurs spontan eously and the hybrids extrude the polar body and form pronuclei in a proportion similar as in controls. In hybrids between LT/Sv metaphase I oocytes and wild-type metaphase II oocytes (which contain cytostatic factor) anaphase I proceeds at the time observed in control LT/Sv ooc ytes and hybrids between two M I LT/Sv oocytes, and is followed by the parthenogenetic activation and formation of interphase nuclei. Also t he great majority of hybrids between M I and M II wild-type oocytes un dergoes the anaphase but further arrests in a subsequent M-phase. Thes e observations suggest that an internally triggered anaphase I occurs despite the presence of the cytostatic activity both in LT/Sv and wild -type M I oocytes. Anaphase I triggering mechanism must therefore eith er inactivate or override the CSF activity. The comparison between spo ntaneous and induced activation of metaphase I LT/Sv oocytes shows tha t mechanisms involved in anaphase I triggering are altered in these oo cytes. Thus, the prolongation of metaphase I in LT/Sv oocytes seems to be determined by delayed anaphase I triggering and not provoked direc tly by the cytostatic activity. (C) 1998 Academic Press.