Abstract. ERK-type MAP kinase activity is required for normal first meiotic (MI) metaphase spindle dynamics and first polar body formation at the MI/MII transition, and for MII arrest until egg activation. MEK and MAPK, however, remain active until meiosis is completed and pronuclei form, but whether MEK/MAPK activity affects MII spindle function during egg activation has been unknown. Polarized light microscopy revealed that the MII spindle rapidly (within approximately 15 min) lost birefringence upon treatment of the egg with U0126, indicating decreased organization at the molecular level upon MEK inhibition. In contrast, birefringence rapidly increased when MPF was inhibited with roscovitine, and this was similar to the increased birefringence previously shown after fertilization or parthenogenetic activation with Sr 2+ . Confocal microscopy indicated that many spindles in U0126-activated eggs had failed to rotate or were dissociated from the egg cortex. Subsequently, abnormally-located midbodies were evident in U0126-induced parthenogenotes. Thus, MEK/MAPK activity is required to maintain the ordered structure of the MII spindle and for normal spindle dynamics during second polar body formation. Key words: Actin, Cytostatic factor, Meiosis, Mouse, M-phase promoting factor, Parthenogenesis (J. Reprod. Dev. 55: [30][31][32][33][34][35][36][37][38] 2009) uring meiotic maturation of the oocyte, the spindle undergoes a series of precise movements unique to meiosis. In the mammalian oocyte, the first meiotic metaphase (MI) spindle first forms centrally, then moves parallel to its long axis until it contacts and attaches to the oocyte cortex, permitting highly asymmetric cytokinesis leading to extrusion of the first polar body [1]. The second meiotic metaphase MII spindle then forms and lies parallel to the surface, remaining in this orientation until fertilization [2], when it rotates to once again become perpendicular to the surface, permitting second polar body emission [3][4][5]. Similar spindle rotations and cortical attachment appear to be a conserved feature of oocyte meiosis, such as in Xenopus, C. elegans,.Meiosis is also uniquely characterized by prolonged arrests at specific points in the cell cycle. Active maintenance of MII arrest is controlled by the meiosis-specific cytostatic factor (CSF), which requires constitutive activation of the mitogen-activated protein kinase (MAPK) pathway in the MII oocyte. It is well established that meiotic MAPK activation is controlled by an oocyte-specific protein kinase, MOS, which activates the MAP kinase kinase, MEK, which in turn activates ERK-type MAPKs [9] and downstream effectors that maintain M-phase promoting factor (MPF) activity [10].Oocytes of mos -/-female mice, in which the MOS/MEK/MAPK pathway is inactive, not only fail to arrest in MII, but also produce abnormally large first polar bodies when the MI spindle fails to migrate to the cortex and becomes abnormally elongated before cytokinesis [1,[11][12][13][14]. MOS activity is required for normal micr...