Fertilization cone formation in starfish oocytes: the role of the egg cortex actin microfilaments in sperm incorporation.

The process of sperm incorporation into starfish (Asterias amurensis) oocytes was examined by electron and fluorescence microscopy. The fertilization cone began to form at the place where the acrosomal process fused with the egg surface and developed into an inverted conical mass containing a small amount of electron-dense cytoplasm. Microfilaments, which stained with NBD-phallacidin, were detected in the fertilization cone. Microvillar protrusions from the fully grown fertilization cone engulfed the sperm head outside the fertilization membrane. The sperm organelles were incorporated into the egg cortex with the absorption of the protrusions. Cytochalasin B inhibited sperm incorporation, fertilization cone formation, and actin filament organization. It is suggested that the development and reduction of the fertilization cone, which depend on the functioning of microfilaments, are necessary for sperm incorporation in starfish.