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 different shapes and lengths of their dendrites. Although a number of authors considered his results during the following decades, only a division into two types withstood time: type I neurons had one long and several short processes, whereas type II neurons were characterized by several long processes. Some further structural features were discussed but substantial progress was not made until the late 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 enteric neurons represent no more than a vegetative, postganglionic relay station between the central nervous system and the periphery, and to methodological difficulties. With the application of modern neuroanatomical techniques it was realized that the enteric nervous system contains a considerable number of neuronal subpopulations. The search for morphological correlates of the chemical diversity of enteric neurons was done mainly in the pig and the guinea-pig. In the pig, additional structural features such as axonal projection, distribution of neurons within ganglia, within different plexuses and along the length of the gut, blood supply etc. were included as criteria for further refining neuronal classification. Most of our knowledge about functional features of enteric neurons, e.g. chemical coding, neuronal connectivity, electrophysiological behaviour, was derived from studies in the guinea-pig small intestine. In light of interspecies differences, comparison of findings from different species is mandatory. The search for morphological 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.