acrosome

The acrosome is a cap-like organelle that overlays the anterior portion of the sperm head. It forms during spermiogenesis from Golgi-derived proacrosomal vesicles and, after maturation, plays a crucial role in fertilization by enabling the sperm to penetrate the oocyte’s protective layers.

Structure and contents: The acrosome consists of an outer acrosomal membrane and an inner acrosomal membrane that enclose the acrosomal matrix. It covers a portion of the anterior nucleus, forming a cap. The contents include hydrolytic enzymes such as acrosin and hyaluronidase, along with other proteases and esterases, which facilitate passage through the zona pellucida.

Function and mechanism: Upon contact with the zona pellucida, a regulated exocytosis called the acrosome reaction is triggered, largely in response to calcium influx. The acrosomal membranes fuse with the sperm plasma membrane, releasing the contents of the acrosome. The released enzymes digest components of the zona pellucida, creating a path for the sperm to reach the oocyte. The acrosome reaction is coordinated with capacitation, a maturation process that occurs in the female reproductive tract.

Clinical relevance: Defects in acrosome formation or function can impair fertilization and contribute to male infertility. Some conditions, such as globozoospermia, involve abnormal acrosomes or failure to complete the acrosome reaction. The acrosome is a conserved structure across many species, reflecting its essential role in the fertilization process.