Spermatogenesis and sperm ultrastructure of the squid, Todarodes pacificus (Cephalopoda: Ommastrephidae)

This study sought to characterize spermatogenesis and mature sperm within the spermatophore of Todarodes pacificus. The process of spermatogenesis in this species can be divided into four types: spermatogonium, spermatocyte, spermatid, and sperm. In spermatogonia, the nucleus occupies most of the cell, and a synaptonemal complex is observed in primary spermatocytes. The karyoplasm of secondary spermatocytes exhibits a clear distinction between euchromatin and heterochromatin. Spermatids can be further divided into early, mid, and late during spermiogenesis. In early spermatids, the karyoplasm appeared granular, and proacrosomal vesicles were found in the apical of nucleus. Mid spermatids featured a fibrillar karyoplasm, with well-developed mitochondria in the midpiece. The nucleus of late spermatids was elongated and rectangular, with a homogenized karyoplasm of increased electron density. A mitochondrial spur was present below the nucleus. In mature sperm within the spermatophore, the nucleus measures approximately 4.5 μM in length, and the karyoplasm has an irregular asterisk- shape with high electron density. The acrosome is blunt-cone shaped and approximately 0.7 μM long, whereas the axoneme of flagellum exhibits a typical 9+2 microtubular structure. One mitochondrial spur was shaped like a long triangular flag, containing numerous mitochondria in its upper region and electron-dense glycogen granules in its lower region.