Degradation of an extracellular matrix: sea urchin hatching enzyme removes cortical granule-derived proteins from the fertilization envelope

The sea urchin fertilization envelope is an extracellular matrix assembled at fertilization to prevent polyspermy and protect the embryo during early development. During hatching, the embryo secretes a proteolytic hatching enzyme which dissolves the fertilization envelope, allowing a ciliated blastula to swim free. In this study we examined ultrastructural changes in the fertilization envelope during degradation of this matrix by hatching enzyme. The completed fertilization envelope is a trilaminar structure consisting of a dense, central layer of filaments sandwiched between surface coats of paracrystalline material. Hatching enzyme disassembles this matrix by degrading the paracrystalline layers and removing macromolecules from the central layer leaving behind a thin matrix of loosely woven fibers.