Consequences of non-extrusion of the first polar body and control of the sequential segregation of homologues 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 maturing mouse oocytes in order to identify risk factors for such conditions, and for the precocious segregation of homologues or chromatids. Treatment with the actin-binding drug cytochalasin D (10 micrograms/ml) arrested oocytes in metaphase I. Upon exposure to Ca(2+)-ionophores, anaphase I was triggered in the absence of cytokinesis. Chiasmata resolved and homologues separated instantaneously. In some oocytes predivision of all chromatids occurred. Homologues or chromatids never separated even after exposure to Ca(2+)-ionophores when microtubules were depolymerized, although bivalents could eventually decondense. Thus, in meiosis I checkpoints exist which ensure that homologue separation only takes place when a metaphase I spindle is present but cytokinesis and anaphase progression can be uncoupled. Cycloheximide induced a sequential separation of homologues in oocytes with intact metaphase I spindle, resulting in metaphase II chromosomes and bivalents in individual cells as also found in some human oocytes of aged females. In oocytes which progressed to metaphase II but failed to extrude a first polar body, the two 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 on chromosome segregation.