Akrosomi

Akrosomi, commonly called acrosomes, are cap-like organelles that cover the anterior portion of the sperm head in many animals. They originate during spermiogenesis from Golgi-derived proacrosomal vesicles and mature as part of the sperm's head. The acrosome contains an inner and outer acrosomal membrane and a dense enzyme-rich layer known as the acrosomal matrix. Enzymes such as hyaluronidase, acrosin, and neuraminidase are stored within the acrosome and are released during the acrosomal reaction to facilitate zona pellucida penetration.

Function: At fertilization, contact with the oocyte's zona pellucida triggers the acrosomal reaction, exocytosis of the acrosomal contents, and digestion of extracellular matrix components. This creates a path through the zona and allows the sperm to reach and fuse with the oocyte plasma membrane.

Development and structure: The acrosome is formed by Golgi-derived vesicles that fuse to form the acrosomal cap over the anterior nucleus. The acroplaxome and manchette contribute to anchoring and shaping the developing sperm head.

Clinical relevance: Defects in acrosome formation or function can impair fertilization, contributing to male infertility. Globozoospermia is a notable condition characterized by a spermatozoon lacking a functional acrosome.

Evolution and variation: The acrosome is a conserved feature of many vertebrates and some invertebrates; while the exact content and triggered pathways can vary, its role in penetrating the egg's protective layer is a common theme.