Ultrastructure of Neoblasts in Turbellarian Geocentrophora wagini Timoshkin, 1984 (Lecithoepitheliata: Plathelminthes)

Acoelomorpha and Plathelminthes have a unique system of stem cells (neoblasts), which is believed to represent a common proliferative compartment of somatic and germ line cells. Meanwhile, on the electron-microscopic level, these cells are not studied in most taxa of Turbellaria. In this study, I describe the ultrastructure of neoblasts in Geocentrophora wagini Timoshkin, 1984 (Lecithoepitheliata, Plathelminthes), an endemic turbellarian from Lake Baikal. The neoblast-like cells showed a high nucleus/cytoplasm ratio. The cytoplasm revealed the features of undifferentiated cells. Particularly, there were free ribosomes and mitochondria, while other organelles were rare and did not occur in all the cells studied. Based on their cytoplasmic and nuclear organization, three main types of parenchymal neoblasts have been distinguished. In type 1, the nuclei had a complex, highly branched configuration. The peripheral heterochromatin was not developed. A large loose structure of fibrous nature and a tiny Golgi apparatus with several secretory granules could be observed in the type 1 neoblast cytoplasm. The nuclei of the type 2 neoblasts had much simpler outlines than those of type 1, despite some processes or invaginations. The poor development of peripheral condensed chromatin was observed and distribution density of the heterochromatin clumps tended to be slightly higher, as compared to type 1. A cluster of small dense granules or a little loose body occasionally could be seen in the proximity to nuclear membrane. Type 3 neoblasts had the most compact nuclei and their heterochromatin was seen as large, irregular clumps of extremely high electron density. Many of these clumps were connected with each other and with the nuclear membrane. The scarce cytoplasm contained only mitochondria and ribosomes. Undifferentiated cells in the gastrodermis were similar to the type 2 neoblasts in the parenchyma. For the first time outside Tricladida special structures were found in the neoblast cytoplasm. These loose fibrous bodies and clusters of granules are likely to be functionally identical to the planarian chromatoid bodies. The obtained results contribute to the comparative morphology of the stem cells in flatworms and basic Bilateria and confirm the heterogeneity of the proliferative compartment in Turbellaria.