Extracellular coats on the surface of Strongylocentrotus purpuratus eggs: stereo electron microscopy of quick-frozen and deep-etched specimens

Sea urchin (Strongylocentrotus purpuratus) eggs were fixed, quick-frozen, deep-etched, and rotary-replicated, and the three-dimensional structure of the external surface of the egg visualized using stereo electron microscopy. The cell surface is coated with three layers of filaments: the sheetlike vitelline layer adhering closely to the plasma membrane, a second layer of oblique fibrils extending from microvillar tips to the vitelline layer below, and a third, outermost layer of horizontal filaments coursing in bundles over the microvillar tips. After fertilization, the newly elevated vitelline envelope is transformed into a three-layered structure, the central layer being a tightly knit network of fine filaments decorated on each side with a loose network of thicker fibrils. Subsequently, the envelope becomes coated with 'paracrystalline' protein released from the cortical granules, and microvillar casts are reshaped into angular, jagged peaks having two to five sides. The final structure of the fertilization envelope consists of a thick central layer of compact fibrillar material that is coated on each side with thin plates of paracrystalline protein.