Role of hemocytes in invertebrate adult neurogenesis and brain repair

The repair of lesions of the central nervous system (CNS) varies widely throughout the animal kingdom. At the level of neuronal replacement lie the major differences in CNS regeneration. At one extreme are the amniote vertebrates (reptile, avian and mammalian groups), which have very limited capacity for neuronal replacement, and therefore for neural regeneration; at the other extreme, animals such as planarians (flatworms) and colonial tunicates can repair their entire CNS after major injuries. These differences can be attributed to the abundance of multipotent and/or pluripotent stem cells and/or undifferentiated precursors among the general cell population. In this review we discuss recent advancements in knowledge of regeneration of the CNS of invertebrates. We focus on ascidians, which are a sister group of vertebrates, but we also address other invertebrate groups. Because neurogenesis is central to the events that allow regeneration of the adult CNS, we address this issue focusing on crustaceans, which have provided a paradigm to study the mechanisms underlying this phenomenon. The attraction of hemocytes toward a neurogenic niche and respecification of these cells toward a neural fate has been strongly suggested. Based on recent and emerging research, we suggest that cells of the blood lineage are not only associated with the roles that are generally attributed to them, but are the cells that either signal other cell types to differentiate into neural cells, or even eventually themselves transdifferentiate into neural cells.