The sea urchin is one of the several model organisms that has been used to decipher the basic cellular and molecular biology of animal fertilization. More is known about fertilization in sea urchins than is known about fertilization in most vertebrates. Sperm follow a decapeptide gradient in sea water towards the region of higher concentration at jelly coat of recently spawned fully mature ova. Such a channel might become activated by speract-induced hyperpolarization and lead to flagellar beating. Hydrolytic enzymes from the acrosomal vesicle begin to digest constituents of the jelly coat.
Alkaline cytoplasm promotes the loss of profilactin blocking protein from ends of actin monomers.
Explosive polymerization of actin generates microfilaments that extend a membrane covered acrosomal process from the bottom of the vacant acrosomal vesicle. The acrosomal process projects through the remains of the jelly coat towards the vitelline membrane of the egg. On its carboxyl terminus it has a short cytoplasmic domain, followed by a single membrane spanning domain. Resolution of this issue will await further sequence data. The membrane of this spermatozoan fuses with that of the fully mature ovum.