Is the human oocyte plasma membrane polarized?

Using scanning electron microscopy, we have shown that the plasma membrane of the human metaphase II oocyte is organized in evenly spaced, short microvilli of 1-3 microns in length. In contrast to other mammals studied to date, there is no microvillus-free area overlying the metaphase spindle and there were no other indications of polarization at this level of organization. Functional polarity of the plasma membrane, studied using localized microsurgery of the zona pellucida followed by insemination, suggests that sperm fusion and entry in the human may occur anywhere over the oocyte surface. Aged oocytes and those exposed to acidic Tyrode's solution had surfaces which were not homogeneously covered by microvilli. Oocytes exposed to a sucrose solution and subzonally injected with spermatozoa showed evidence of partial cortical granule exocytosis.