Spermatogenesis of Paromalostomum fusculum (Platy helminthes, Macrostomida, Dolichomacrostomidae)

Summary

Spermatocytes (nuclei with synaptonemal complexes) have already well developed Golgi complexes that produce large numbers of dense granules. In spermatids (or late spermatocytes?) the granules are densely packed and the chromatin condenses showing a small dense protrusion (or protrusions?). Two centrioles, arranged at an angle to each other, are formed close to the nucleus, and peripheral microtubules are close to them. The microtubules extend into an outgrowing cytoplasmic process, with the centrioles located in it. The nucleus elongates, its chromatin showing a characteristic fibrillar-reticular configuration, and several long mitochondria surround it in parts of the cell arranged in several rows separated by intracellular “septa”. Parts of the nucleus are strongly coiled. The peripheral microtubules are arranged in two rows of (usually) four on opposite sides of the sperm. They extend along the distal part of the sperm which is filled with many dense granules, but not into the proximal part containing only nucleus and mitochondria. A long dense rod arranged in a screw-like fashion extends in the distal part of the sperm and microtubules decrease in number at the distal tip. At the proximal tip, only the nucleus is present. Mature sperm from the sperm duct differ from those in the testis mainly in the more condensed nucleus. This is the first time that flagellar elements (centrioles) have been clearly identified in macrostomid spermatid by electronmicroscopy.

Key words:

• Platyhelminthes
• Macrostomida
• Paromalostomum
• sperm
• spermatogenesis
• ultra-structure
• phylogeny