Practice question

Vitelline envelope is thin fibrous extracellular matrix immediately surrounding unfertilized sea urchin egg plasma membrane, visible as distinct layer in electron micrographs. Biochemical purification reveals it comprises complex network of sulfated glycoproteins, including 350 kDa EBR1 receptor complex that mediates species-specific sperm binding via bindin interaction. Glycoproteins provide both structural scaffold maintaining envelope integrity and sperm recognition sites determining selectivity. Later cortical granule secretions enzymatically modify this envelope into hardened fertilization envelope blocking polyspermy. Nucleic acids and lipids represent minor contaminants; major functional constituents are heavily glycosylated proteins acting as receptor platform.