"Dendroarchitectonics": From Santiago Ramón y Cajal to Enrique Ramón-Moliner or vice versa?

Here, we review the morphological taxonomy of neurons proposed by Enrique Ramón-Moliner in the vertebrate central nervous system based on "dendroarchitectonics" and compare these findings with Santiago Ramón y Cajal's work. Ramón-Moliner distinguished three main groups of nerve cells situated on a spectrum of dendritic configuration in the mammalian central nervous system with decreasing degree of morphological specialization, i.e., idiodendritic, allodendritic, and isodendritic neurons. Leptodendritic neurons would be an even more primitive type, and lophodendritic nerve cells would develop into pyramidal neurons. Using two developmental lines (i.e., telencephalic and rhombencephalic trends), Ramón-Moliner reconstructed the probable course of events in the phylogenetic history that led to the dendroarchitectonic families. While an increasing morphological specialization is associated with the projected phylogenetic development as an abstract "whole," phylogenetically "primitive neurons" such as the reticular formation may be present in later phylogenetic stages, and vice versa, phylogenetical "new arrivals," such as the cortical pyramidal cell, may be found early in phylogeny. Thus, Ramón-Moliner adopted the notion of an in-parallel neuronal development during phylogeny and ontogeny. In contrast, Cajal argued earlier in favor of the idea that ontogeny recapitulates phylogeny, focusing on the pyramidal neuron. In ontogeny, the early developmental features show a higher degree of similarity than the comparison of their adult forms. These results corroborate the rejection of the interpretative framework of ontogeny as a simple, speedy repetition of the phylogeny. Understanding morphological findings with the change in their interpretation and the historic underpinnings provide a framework for refined scientific hypotheses.