CONSEQUENCES OF NON-EXTRUSION OF THE FIRST POLAR BODY AND CONTROL OF THE SEQUENTIAL SEGREGATION OF HOMOLOGS AND CHROMATIDS IN MAMMALIAN OOCYTES

Absence of polar body formation, or premature chromatin condensation ( PCC) in human oocytes can cause infertility. We studied in-vitro matur ing mouse oocytes in order to identify risk factors for such condition s, and for the precocious segregation of homologues or chromatids. Tre atment with the actin-binding drug cytochalasin D (10 mu g/ml) arreste d oocytes in metaphase I. Upon exposure to Ca2+-ionophores, anaphase I was triggered in the absence of cytokinesis. Chiasmata resolved and h omologues separated instantaneously. In some oocytes predivision of al l chromatids occurred. Homologues or chromatids never separated even a fter exposure to Ca2+-ionophores when microtubules were depolymerized, although bivalents could eventually decondense. Thus, in meiosis I ch eckpoints exist which ensure that homologue separation only takes plac e when a metaphase I spindle is present but cytokinesis and anaphase p rogression can be uncoupled, Cycloheximide induced a sequential separa tion of homologues in oocytes with intact metaphase I spindle, resulti ng in metaphase II chromosomes and bivalents in individual cells as al so found in some human oocytes of aged females. In oocytes which progr essed to metaphase II but failed to extrude a first polar body, the tw o sets of chromosomes eventually aligned on a common spindle ('diploid ' metaphase II). PCC of one set was never observed, Ageing in vitro of cytochalasin D-blocked metaphase I oocytes had no pronounced effect o n chromosome segregation.