Acrosome-like structures in the spermatozoa of teleost fishes

The spermatozoa of teleost fishes lack an acrosome. However, peculiar structures have been described at the front of the nucleus in some species, and these take various forms. One or more vesicles may be placed at the front of the spermatid nucleus and, in some cases, may be retained in the spermatozoon. The anterior nuclear envelope may exhibit densification or intermembraneous differentiation. Stacks of membranes may be apposed to the nuclear envelope. No acrosome has been unambiguously identified to date in the spermatozoon of any teleost. The acrosome has been lost in the Teleostei during evolution, though some species have retained traces of it at the front of the germ cells. Thus, vestiges of acrosomal vesicles remain in Gambusia affinis, Lepadogaster lepadogaster, and Oncorhynchus mykiss. There is also a subacrosomal type of differentiation of the nuclear envelope in some spermatozoa. Similar structures have been described in the Porifera and Cnidaria, groups of primitive Metazoa in which the acros...