Paraspermatogenesis of Cerithioidean snails: Retention of an acrosome and nuclear remnant

Paraspermatozoa of Cerithioidea are oligopyrenic with a central nuclear remnant surrounded by glycoprotein bodies and an anterior acrosome complex. Posteriorly, the parasperm have one long and several small flagella. Biosynthesis of proteins begins in the rough endoplasmic reticulum (RER), early in paraspermatogenesis, prior to the degradation of the nucleus. The breakdown of the nucleus has features characteristic of apoptosis but a nuclear remnant remains that is composed of intact DNA. The acrosome complex of parasperm is Golgi‐derived, forming posteriorly and migrating anteriorly along the plasma membrane to the apex of the nuclear remnant as the paraspermatid matures. This mechanism of acrosome formation is similar to that in euspermiogenesis in neomenioid aplacophorans and neritid snails and is plesiomorphic to mollusks. However, eusperm acrosomes of Cerithioidea form differently. In the paraspermatid, small, dense granules secreted by the Golgi body fuse to form larger granular vesicles, many of which are released by exocytosis into the seminal fluid. These granular vesicles stain red with acridine orange at pH 7 under 489‐nm light, indicating that they are lysosomes. The retention of a nuclear remnant and development of an acrosome complex in the parasperm of cerithioideans, as well as the secretion of lysosomes into the seminal fluid, suggests novel functions for these unusual germ cells. J. Morphol. © 2005 Wiley‐Liss, Inc.