Ultrastructure of the sperm and spermatogenesis and spermiogenesis of Dina lineata (Hirudinea, Erpobdellidae).

The mature sperm of Dina lineata is of the modified type. The sperm are 48 micron long and 0.3 micron wide. The sperm are filiform and helicoidal cells with a distinct head, a midpiece, and a tail. There are two distinct regions in the head: the acrosome and the posterior acrosome, each with its own characteristic morphology. The midpiece is the mitochondrial region and has a single mitochondrion. Two distinct portions can be observed in the tail: the axonematic region and the terminal piece. In the process of spermatogenesis the early spermatogonia divide to form a poliplast of 512 spermatic cells. In the spermiogenesis the following sequential stages can be distinguished: elongation of the flagellum; reciprocal migration of mitochondria and Golgi complex; condensation of chromatin and formation of the posterior acrosome; spiralization of nuclear and mitochondrial regions; and, finally, formation of the anterior acrosome. The extreme morphological complexity of the Dina spermatozoon is related to the peculiar hypodermal fertilization which characterizes the erpobdellid family. Correlation between sperm morphology and fertilization biology in the Annelida is revised.