CYTOSKELETAL ORGANIZATION IN THE OOCYTE, ZYGOTE, AND EARLY CLEAVING EMBRYO OF THE STRIPE-FACED DUNNART (SMINTHOPSIS-MACROURA)

Ovulation occurs in Sminthopsis macroura approximately 160 hr after ad ministration of 1.3 IU PMSG, and yields significantly more oocytes tha n does spontaneous ovulation (P = 0.001). Germinal vesicle (GV)-stage oocytes have a thin cortical rim of microfilaments, which is disrupted by exposure to cytochalasin D. After GV breakdown, the first meiotic spindle forms subcortically and parallel to the oolemma. It rotates du ring anaphase and telophase to extrude the first polar body. This rota tion is associated with a local cortical concentration of microfilamen ts, which is extruded in the first polar body. The second meiotic spin dle is orthogonal to the surface, and extrusion of the second polar bo dy is not associated with obvious local changes in cortical actin, res ulting in a polar body containing little polymerized actin. The sites of second polar body emission and sperm entry are always in the half o f the oocyte opposite the concentrating yolk mass, and are within 60 d egrees of each other in most oocytes. During the concentration and ecc entric movement of the yolk, microfilaments condense around it. During yolk expulsion, these microfilaments become continuous with those loc ated subcortically. During early cleavage, the cytocortex of the zygot e, but not of the extruded yolk mass, stains heavily for polymerised a ctin. Multiple sites of pericentriolar material are detectable in the cytoplasm of some secondary unfertilized oocytes which, in the presenc e of taxol, generate large cytasters and pseudospindle structures. Aft er fertilization, a large aster is formed in association with the sper m entry point and serves as the center of an extensive cytoplasmic net work of microtubules which surrounds but does not enter the yolk mass. Taxol treatment generates small cytasters within this meshwork and pr omotes selective stabilization of some periyolk microtubules opposite to the sperm aster. (C) 1995 Wiley-Liss, Inc.