Testicular organization and spermatogenesis in Tegula (Chlorostoma) tridentata (Potiez and Michaud, 1838) (Mollusca: Archaeogastropoda: Trochidae).

The basic environment for the generation of spermatozoa in the gastropods is a gametogenic compartment resulting from somatic and germ cells interaction. In arachaeogastropods however, such association has not been characterized. In this study at the light and transmission electron microscopy level, evidences are given that in T. (C.) tridentata, spermatogenesis is centrifugal and occurs around seminiferous tubules surrounding the lumen that is a blood vessel. This basic organization presents somatic cells interacting with germ cells. There are three types of spermatogonia: primary spermatocytes with proacrosomic granules, and spermatids that undergo the following spermiohistogenic events: 1. Nuclear chromatin condensation to form a short cylindrical nucleus with an anterior invagination containing the base of the axial rod and a posterior invagination containing the proximal centriole. 2. Polarization of mitochondria attached to the posterior nuclear membrane to integrate the midpiece. 3. Coalescence of proacrosomic granules in one acrosomic vesicle, polarization of the acrosomic vesicle in the anterior nuclear region and transformation of the acrosomic vesicle in the conical acrosomic complex with a subacrosomic cavity containing the axial rod. 4. Tail formation from the distal centriole. Apparently the supporting cells do not associate closely with the spermatids. Therefore, they would not perform a mechanical role in the generation of spermatozoa shape that is characteristic of the Trochidae and other archaeogastropod families. This spermatozoon is peculiar due to its prominent acrosome larger than the nucleus.