Central nervous system regeneration in ascidians: cell migration and differentiation

Adult ascidians have the capacity to regenerate the central nervous system (CNS) and are therefore excellent models for studies on neuroregeneration. The possibility that undifferentiated blood cells are involved in adult neuroregeneration merits investigation. We analyzed the migration, circulation, and role of hemocytes of the ascidian Styela plicata in neuroregeneration. Hemocytes were removed and incubated with superparamagnetic iron oxide nanoparticles (SPION), and these SPION-labeled hemocytes were injected back into the animals (autologous transplant), followed by neurodegeneration with the neurotoxin 3-acetylpyridine (3AP). Magnetic resonance imaging showed that 1, 5, and 10 days after injury, hemocytes migrated to the intestinal region, siphons, and CNS. Immunohistochemistry revealed that the hemocytes that migrated to the CNS were putative stem cells (P-element-induced wimpy testis + or PIWI + cells). In the cortex of the neural ganglion, migrated hemocytes started to lose their PIWI labeling 5 days after injury, and 10 days later started to show beta-III tubulin labeling. In the neural gland, however, the hemocytes remained undifferentiated during the entire experimental period. Transmission electron microscopy revealed regions in the neural gland with characteristics of neurogenic niches, not previously reported in ascidians. These results showed that migration of hemocytes to the hematopoietic tissue and to the 3AP-neurodegenerated region is central to the complex mechanism of neuroregeneration.