Development of surface polarity in mouse eggs

Investigations were carried out to determine what effects components of the cytoskeletal system and meiotic spindle have on the development and maintenance of surface polarity in mouse ova. The surface of the mature egg possessed numerous microvilli except for a region (microvillus-free area) adjacent to the meiotic spindle. In contrast, the surface of the immature oocyte was covered uniformly with a dense population of microvilli. When cultured in vitro immature oocytes spontaneously underwent maturation; a meiotic spindle formed in the center of the ovum which then moved to the cortex. Coincident with the cortical localization of the meiotic spindle was the formation of a microvillus-free area and subjacent layer of microfilaments. A microvillus-free area did not form when meiotic maturation was inhibited with dibutyryl cyclic AMP or chloroquine. If immature oocytes were incubated in cytochalasin B a meiotic spindle developed, but it did not become localized to the egg cortex and a microvillus-free area failed to form. Oocytes incubated in colchicine underwent germinal vesicle breakdown and chromosome condensation, a meiotic spindle did not form but the chromosomes became localized to the ovum cortex where a microvillus-free area developed. These results and observations of mature ova treated with cytochalasin B or colchicine indicate that mechanisms involving the movement of the meiotic spindle to the oocyte cortex and development and maintenance of surface polarity are cytochalasin B sensitive. Cortical localization of meiotic chromosomes brings about the formation of a microvillus-free area.