Spermiogenesis and spermatozoon ultrastructure of the davaineid cestode Raillietina micracantha (Fuhrmann, 1909)

Miquel, J., Torres, J., Foronda, P. and Feliu, C. 2010. Spermiogenesis and spermatozoon ultrastructure of the davaineid cestode Raillietina micracantha. — Acta Zoologica (Stockholm) 91 : 212–221 The spermiogenesis and the ultrastructural organization of the spermatozoon of the davaineid cestode Raillietina micracantha are described by means of transmission electron microscopy. Spermiogenesis begins with the formation of a zone of differentiation containing two centrioles. One of the centrioles develops a free flagellum that later fuses with a cytoplasmic extension. The nucleus migrates along the spermatid body after the proximodistal fusion of the flagellum and the cytoplasmic extension. During advanced stages of spermiogenesis a periaxonemal sheath and intracytoplasmic walls appear in the spermatids. Spermiogenesis finishes with the appearance of two helicoidal crested bodies at the base of spermatids and, finally, the narrowing of the ring of arched membranes detaches the fully formed spermatozoon. The mature spermatozoon of R. micracantha is a long and filiform cell, tapered at both ends, which lacks mitochondria. It exhibits two crested bodies of different lengths, one axoneme of the 9 + ‘1’ pattern of trepaxonematan Platyhelminthes, twisted cortical microtubules, a periaxonemal sheath, intracytoplasmic walls, granules of glycogen and a spiralled nucleus. The anterior extremity of the spermatozoon is characterized by the presence of an electron‐dense apical cone and two spiralled crested bodies while the posterior extremity of the male gamete exhibits only the axoneme and an electron‐dense posterior tip.