Asymmetric division in mouse oocytes: with or without Mos

In both vertebrates and invertebrates, meiotic divisions in oocytes are typ ically asymmetric, resulting in the formation of a large oocyte and small p olar bodies. The size difference between the daughter cells is usually a co nsequence of asymmetric positioning of the spindle before cytokinesis, Spin dle movements are often related to interactions between the cell cortex and the spindle asters [1,2], The spindles of mammalian oocytes are, however, typically devoid of astral microtubules, which normally connect the spindle to the cortex, suggesting that another mechanism is responsible for the un equal divisions in these oocytes, We observed the formation of the first po lar body in wild-type oocytes and oocytes derived from c-Mos knockout mice [3], In wild-type oocytes, the meiotic spindle formed in the centre of the cell and migrated to the cortex just before polar-body extrusion. The spind le did not elongate during anaphase, in mos(-/-) oocytes, the spindle forme d centrally but did not migrate, although an asymmetric division still took place. In these oocytes, the spindle elongated during anaphase and the pol e closest to the cortex moved while the other remained in place. Thus, a co mpensation mechanism exists in mouse oocytes and formation of the first pol ar body can be achieved in two ways: either after migration of the spindle to the cortex in wild-type oocytes, or after elongation, without migration, of the first meiotic spindle in mos(-/-) oocytes.