Ultrastructural studies of sperm morphology and sperm—egg interaction in the decapod Callinectes sapidus

Functional and morphological aspects of a brachyuran sperm were examined by light and electron microscopy and by cytochemical methods. The spherical acrosomal region has a PAS-positive acrosomal vesicle and a PAS-negative acrosomal tubule. This region is partially bounded by a lamellated membranous central region and is embedded in a nuclear cup with only its apical cap exposed to the sperm surface. The Feulgen-positive nuclear cup has eight radial arms. These attach to the egg chorion and bring the apical cap and chorion into direct contact. Subsequently the acrosome reaction everts the acrosomal region which penetrates the chorion, forming a cup-shaped depression in the egg plasma membrane. The acrosomal tubule is in direct contact with this membrane and may function in gamete fusion and nuclear entry into the egg cytoplasm. The arms-chorion attachment may involve “attachment substances.” Penetration of the chorion may involve lysins from the sperm.