acrosome A thin-walled vesicle that forms a cap on the head of a spermatozoon. On contact with an ovum it bursts, releasing powerful hydrolytic enzymes which cause a localized softening of the vitelline membrane, thus facilitating fertilization.

acrosome A membranous sac at or near the front of a sperm that assists in penetration of the egg. The acrosome contains enzymes, which are released when the sperm contacts the egg prior to fertilization. The enzymes break down the outer layers of the egg to permit entry of the sperm. In some invertebrate sperms the acrosome contains actin filaments, which elongate to help gain entrance to the egg.

acrosome (ak-rŏ-sohm) n. the caplike structure on the front end of a spermatozoon.