Peripheral nerve glia as multipotent progenitors in craniofacial development

Sammanfattning: Craniofacial development is complex. Numerous populations of progenitor cells coordinate activities to produce an array of highly integrated tissues inside the developing head. However, it is not clear how some key multipotent progenitors continue to exist in late developing head compartments. The general hypothesis of this thesis is focused around the idea of an embryonic infrastructure represented by peripheral nerves that serves as a niche for glial multipotent neural crest-like cells. The nerve-adjacent glial cells can change their fate and be recruited in a targeted way to produce tissues at remote destinations during fast growth, development and regeneration. Results presented in this thesis explain how the nerves contribute pulp cells and matrix-producing cells of odontoblast lineage to the developing and growing tooth. Glial cells as an unexpected progenitor source give rise to almost half of all pulp cells and odontoblasts in the growing incisor. Furthermore, lineage tracing with colour-coding of individual recombination events allowed us to discover new aspects of tooth development and coordination between pulp cell lineage and odontoblasts. Another important component of the craniofacial compartment, the parasympathetic nervous system that targets glands in the head, is crucial for "rest-and-digest" or "feed and breed" activities especially during eating, salivation and lacrimation. Importantly, neurons of the autonomic parasympathetic nervous system are located very close to or inside the tissues they innervate and appear late in embryonic development. The discrepancy in developmental timing raised new questions: how do early neural crest-derived progenitors of parasympathetic neurons reach their destinations, and how do they acquire neuronal properties in situ? Furthermore, what is the nature of those progenitor cells? Our results clearly demonstrate that cells of glial origin located in the peripheral nervous system possess multipotency and gives rise to parasympathetic neurons during later developmental stages. Peripheral glial cells arrive to late-developing tissues on the pioneer presynaptic nerve fibres. Subsequently, some glial cells change fate, navigate for short distances and then convert into neurons and satellite cells of parasympathetic ganglia. Our conclusions redraw a fundamental principle on how the peripheral nervous system develops and provide a new type of logic, where both the cellular elements, as well as, the wiring are solved by a simple deposition of the postsynaptic elements from the presynaptic. During our work we used a wide spectrum of approaches including advanced genetic tracing with multicolor reporters, analysis of numerous mouse mutants, in vitro cell cultures and 3D imaging of developing embryos. We have applied both genetic and surgical ablation techniques to the peripheral nerves and investigated targeted recruitment of glia from the nerves in each case. Peripheral glia represents a novel amenable source of multipotent progenitor cells with putative regenerative potential that in the future might be applied for the treatment of congenital craniofacial pathologies, trauma cases or used for aesthetic body treatments.