Towards defining an 'origin'-The case for the mammalian acrosome

The acrosome is an organelle unique to sperm cells, thought to be indispensable for fertilization. In globozoospermia the main defect is characterized by the absent or severely malformed acrosome. Males with severe globozoospermia, to have the ability to father their own children, are committed to one of the most advanced and expensive method of assisted conception: in vitro fertilization with intracytoplasmic sperm injection (ICSI). However, rates of successful ICSI fertilization remain still very poor because globozoospermic spermatozoa have not the ability to trigger oocyte activation. It is emerging that the acrosome is not only fundamental for the release of lytic content to penetrate oocyte's coats; the strategies of oocyte penetration and activation by mammalian spermatozoa uncover less conventional and unexplored roles for acrosomal components. Discovered as cellular organelle in the early 1920s [1], the nature of the acrosome and its biogenesis are still matter of debate. In this review, I discuss the distinct classifications proposed in the years for the acrosome, providing also a glance on the intimate relationship that exists between acrosomal constituents and function. Emphasis is particularly devoted to the recent work on the acrosome biogenesis.