La spermiogenèse de Myzostomum sp. (Procoelomata, Myzostomida)

The nucleus of the Myzostomum spermatid contains chromatin and protein granules. The single centriole gives rise to a 9 + 0 type flagellum and then migrates in the cytoplasm of the spermatid. A cytoplasmic canal is thus constituted toward which the mitochondria migrate. The nucleus becomes longer and 16 to 18 microtubules appear on its periphery, forming a rudimentary manchette. The cytoplasmic canal then opens along almost its whole length. The flagellum is thus in an extracellular position. The nuclear envelope breaks open, bringing the nuclear and cytoplasmic elements into direct contact. A nucleocytoplasmic derivative in which the chromatin is peripheral and the protein granules are in a central position is thus formed. The centriole, capped by a vesicle, undergoes a second forward migration which does not end until the distal extremity of the flagellum has gone past the posterior extremity of the nucleocytoplasmic derivative. The free-swimming part of the flagellum now constitutes the anterior portion of the spermatozoon. The vesicle attached to the centriole turns into a rod which protrudes at the front of the gamete. The microtubules of the manchette persist in the spermatozoon where they are arranged against the plasma membrane. The spermatozoon in propelled by two types of motion. The first, which is not very effective, causes a forward movement, with the flagellum in front. We suppose it is due to undulations which go from the centriole toward the extremity of the flagellum. The second, which is rapid and effective, produces a backward movement, with the flagellum behind. It appears to be due to the intervention of the microtubules which deform the nucleocytoplasmic derivative and transmit undulations to the flagellum, going from its distal extremity toward the centriole.