Morphological classifications of enteric neurons - 100 years after Dogiel

The first differentiation of enteric neurons into three morphological types was done by the russian histologist A. S. Dogiel on the basis of the diffe rent shapes and lengths of their dendrites. Although a number of au thors c onsidered his results during the following decades, only a division into tw o types withstood time: type I neurons had one long and several short proce sses, whereas type II neurons were characterized by several long processes. Some further structural features were disscussed but substantial progress was not made until the 1970s. This stagnation was due to some inaccuracies in Dogiel's descriptions, to the fact that most histologists in this field followed the reticular concept of the nervous system, to the idea that ente ric neurons represent no more than a vegetative, postganglionic relay stati on between the central nervous system and the periphery, and to methodologi cal difficulties. With the application of modern neuroanatomical techniques it was realized that the enteric nervous system contains a considerable nu mber of neuronal subpopulations. The search for morphological correlates of the chemical diversity of enteric neurons was done mainly in the pig and t he guinea-pig. In the pig, additional structural features such as axonal pr ojection, distribution of neurons within ganglia, within different plexuses and along the length of the gut, blood supply etc. were included as criter ia for further refining neuronal classification. Most of our knowledge abou t functional features of enteric neurons, e.g. chemical coding, neuronal co nnectivity, electrophysiological behaviour, was derived from studies in the guinea-pig small intestine. In light of interspecies differences, comparis on of findings from different species is mandatory. The search for morpholo gical and functional peculiarities of human enteric neuronal circuitry has to consider all methodological and conceptual advances made within the past 100 years since the pioneering work of Dogiel.