The spikes protruding into the perivitelline space during fertilization are composed of actin filaments.

Mature eggs of A. aranciacus were microinjected with Alexa Fluor 488-phalloidin (50 µM, pipette concentration) prior to fertilization. (A) Transmission photomicrograph of the fertilized egg 10 min after insemination. (B) The corresponding confocal image of Alexa Fluor 488 phalloidin-stained F-actin. In addition to the strong staining of the actin bundles associated with the penetrating sperm (arrow), fluorescent phalloidin disclosed numerous spikes in the perivitelline space (arrowheads). Abbreviation: FE (fertilization envelope), PV (perivitelline space), PM (plasma membrane). Scale bar, 50 µm.