The surface events of fertilization: the movements of the spermatozoon through the sea urchin egg surface and the roles of the surface layers.

The sea urchin egg surface at fertilization has been examined with the scanning electron microscope to reveal the movements of the spermatozoon from the exterior, through the surface layers, and into the egg cytoplasm. The layers that the spermatozoon encounter have been studied to determine their physical and chemical natures and their role in early development. By studying the outside of whole eggs and the inner face of surfaces isolated shortly after fertilization, it has been possible to compile data on the movements of the spermatozoon through the egg surface. The spermatozoon initially contacts the egg with the elongated acrosomal process. The vitelline sheet, the outermost layer of the egg, separates slightly next to the attached spermatozoon. As membrane fusion between the gametes occurs, the plasma membrane from the egg engulfs the spermhead, the cortical granules start to discharge their contents, and a spreading surface deformation, concommitant with a distortion of the fibrous cortex, is initiated. A cluster of elongate microville surround the perpendicularly fusing spermatozoon. These microvilli interidigitate as the spermatozoon is forced to lie upon the egg surface between the plasma membrane and the matrix of cortical fibers. The spermatozoon then rotates additionally to enter the egg cytoplasm with the posterior end first; it has rotated 180 degrees through the cell surface. Finally, it detaches into the egg cytoplasm, leaving a scar in the cortex through which it penetrated. The egg cortex, previously unobserved by electron microscopy, is revealed to be composed of 50-200 nm fibers. At fertilization they are uniformly organized but during later development this order is lost. The cortex is from 0.2-0.5 micronm thick and is a contractile structure. The role of the outer surface in releasing the cell from the metabolic constraints of the unfertilized egg is shown, and the apparent differences in the mobilities of the membranes derived from the sperm and from the egg are demonstrated. The relation of these layers to the movements of the spermatozoon, to the activation of the egg, to the block to polyspermy, and to each other are discussed.