Sperm incorporation in Xenopus laevis: characterisation of morphological events and the role of microfilaments.

Scanning and transmission electron microscopy were used to determine the morphological changes in the egg plasma membrane associated with sperm binding, fusion and incorporation in Xenopus laevis. Sperm incorporation in Xenopus is rapid, occurring within 3-5 min following addition of sperm. Images have been obtained of both early sperm-egg interactions and fertilisation bodies. Additionally, two drugs that specifically alter F-actin dynamics, latrunculin and jasplakinolide, were used to determine whether sperm incorporation is a microfilament-dependent process. Jasplakinolide did not prevent sperm incorporation, cortical granule exocytosis or cortical contraction, suggesting these events can occur without depolymerisation of existing, stabilised filaments. Latrunculin A, which competes with thymosin beta4 in ooplasm for binding actin monomer, did not inhibit cortical granule exocytosis, but blocked cortical contraction in 100% of eggs at a concentration of 5 microM. Although a single penetrating sperm was found on an egg pretreated in latrunculin, fertilisation bodies were never observed. At < 5 microM latrunculin, many eggs did undergo cortical contraction with some exhibiting severe distortions of the plasma membrane and abnormal accumulations of pigment granules. Preincubation of eggs in jasplakinolide before latrunculin mitigated both these effects to some degree. However, eggs incubated in latrunculin either prior to or after insemination never progressed through first cleavage.