Akrosomiin

Akrosomiin, commonly referred to in English as the acrosome, is a cap-like organelle that covers the anterior portion of the spermatozoon’s nucleus. It originates during spermiogenesis from the Golgi-derived acrosomal vesicle and, upon maturation, forms a distinct acrosomal cap that overlays the front of the nucleus.

Anatomy: The acrosome consists of two membranes—the outer acrosomal membrane and the inner acrosomal membrane—separated by the subacrosomal space. It contains hydrolytic enzymes and other proteins essential for fertilization, including acrosin, hyaluronidase, and neuraminidase.

Biogenesis: In the developing spermatid, the Golgi apparatus packages enzymes into proacrosomal vesicles that coalesce to form the acrosomal vesicle. This vesicle then attaches to the nuclear envelope, producing the mature acrosome at the anterior pole, while the nucleus elongates and condenses.

Function: During fertilization, the acrosome undergoes an exocytotic event known as the acrosome reaction in response to contact with the zona pellucida of the oocyte. Released enzymes digest components of the zona pellucida, enabling sperm to reach and fuse with the oocyte plasma membrane. The reaction is tightly regulated and calcium-dependent.

Clinical relevance: Defects in acrosome formation or function can contribute to male infertility. Globozoospermia is a condition characterized by sperm with malformed or absent acrosomes, impeding fertilization. In assisted reproduction, sperm with defective acrosomes may be bypassed by intracytoplasmic sperm injection (ICSI).