Nerve fibres in relation to gingiva, tooth germs and teeth in the polyphyodont cichlid Tilapia mariae

Sammanfattning: In the late 19th century Retzius observed the presence of nerve fibres in goldfish tooth pulps. However, later studies on the relationship between the dentition and its innervation in non-mammalian vertebrates and its use in experimental research are rare. This thesis presents data on how nerve fibres are related to gingiva, tooth germsand teeth in the lower jaw of a polyphyodont teleost- the cichlid Tilapia mariae. It provides some new aspects on the structure and function of gingival and dental axons. Paper I shows that the complexity of the nodal-paranodal organization in trigeminal alveolar branch (TAB) nerve fibres decreases with decreasing fibre size, like in mammalian nerve fibres. The exceptionally thin myelinated TAB fibres of T. mariae (axonal diameters down to 0. 3 µm) exhibit nodes of Ranvier. The internodal lengths of these fibres varies between 35-50 µm. Theoretical calculations suggest that these extremely thin fibres may be capable of saltatory conduction. Paper II shows that the normal turnover time of an individual tooth (i.e. from eruption to shedding)is about 100 days. Following unilateral neurectomy of the TAB, tooth turnover stops on the denervated side of the jaw. The arrest in tooth turnover is due to a cessation of tooth germ formation. Papers III and IV show that gingival and dental domains are extensively innervated by nerve fibres exhibiting neurofllarnent-, calcitonin generelatedpeptide-, substance P-, tyrosine hydroxylase-, neuropeptide Y -,choline acteyl transferase- or vasoactive intestinal polypeptide-like itnmunoreactivity in a pattern similar to the mammalian counterpart. The rich innervation of odontogenic tissue components in T. mariae by sensory as well as autonomic axons is compatible with the results obtttined in paper II that axons may be involved in odontogenesis. Paper V shows that gingiva and tooth pulps in T. mariae are innervated by trigeminal ganglion (TG) neurons. Like in the rat, TG neurons in T. mariae differ in size and neuropeptide content depending on whether they project to gingiva or tooth pulps.While gingival neurons are exclusively small (perikaryal diameter [Pd]< 20 µm), pulpal neurons may be small or large (Pd;o: 20 µm). Hence, this seems to be an evolutionary old pattern. Taken together, the present results show that the lower jaw dentition and its trigeminal branch in T. mariae is a useful experimental system that may be used for future studies on e. g. the functional properties of the exceptionally thin TAB fibres or the molecular mechanisms behind the neuronal influence on tooth germ formation.