Ultrastructure of spermatogenesis and mature spermatozoon of the freshwater planarian Schmidtea mediterranea (Platyhelminthes, Paludicola).

The aim of this study was to characterize for the first time spermatogenesis and spermiogenesis in the freshwater planarian Schmidtea mediterranea using both light and electron microscopy. Starting from the border towards the testis lumen we found types I and II spermatogonia, clusters of primary and secondary spermatocytes, spermatids and free spermatozoa. Light microscope observations show that type I spermatogonia have a large and pale nucleus whereas type II spermatogonia are significantly smaller than the one of type I, and show a darker and central bulky nucleus. At the ultrastructure level, both type I and type II spermatogonia are characterized by a wide nucleus with scanty cytoplasm containing free ribosomes, mitochondria and a dense chromatoid body whereas endoplasmic reticulum and Golgi complex were not observed. The cytoplasm of primary and secondary spermatocytes displays numerous free ribosomes and many endoplasmic reticulum cisternae and Golgi complexes, suggesting that the development of these organelles during spermatogenesis might contribute to the synthesis of hormones and proteins such as testosterone, transcription factors and tubulin. Mature spermatozoa structure closely resembles those of other freshwater triclads with a nucleus, a single fused mitochondrion, a row of cortical microtubules and a pair of flagella conforming to the 9+'1' microtubule pattern described for other Platyhelminthes.