The ultrastructure of the sea urchin egg cortex isolated before and after fertilization

The three-dimensional organization of cortices isolated from unfertilized and fertilized Strongylocentrotus purpuratus eggs has been examined by several techniques of light and electron microscopy. It has been found that when moderate shear forces are used, the isolated unfertilized egg cortex, in addition to cortical granules, contains acidic vesicles and an elaborate network of rough endoplasmic reticulum. This network provides a physical link between the cell surface and several kinds of cytoplasmic organelles (mitochondria, yolk granules, acidic vesicles) which are retained as part of the isolated cortex when gentle shear forces are applied. Furthermore a good visualization of actin in the cortex is provided: it is present as short filaments and mostly within the stubby microvilli of the egg. Finally, it has been noted that plaques exist on the inside face of the plasma membrane ready to assemble into typical clathrin coats that prefigure the burst of coated vesicle endocytosis that takes place after fertilization. The cortex isolated soon after fertilization is shown to contain coated pits and a scaffolding of filaments (mostly actin) in which many acidic vesicles are embedded.