[Segmentary organisation of the efferents of the vestibular complex in chicken embryos: is this an example of the general case?].
In this paper we present, from the perspective of the embryological segmentary layout of the hindbrain, the topographic layout of the vestibular projection neurons that sustain the vestibulospinal, vestibulo ocular and vestibulocerebellous efferents, in correlation with the classic vestibular nuclei. Four vestibular nuclei are usually described superior, lateral, medial and inferior. These originate in at least nine successive rhombomeric segments or pseudosegments, which suggests the possibility of a more precise analysis of their neuronal populations and of their respective connections and functions. It has recently been observed that the vestibular projection neurons identified for a particular target tend to appear aggregated in discrete accumulations, which have been proved to correlate either with rhombomeric units, where they apparently develop, or with internal subdivisions within them. Each projection has its own particular organisation. Comparing them with the resulting connective mosaic in different species shows that various aspects of this organisation are conserved throughout evolution in vertebrates. It is argued that certain genes that control the development of the rhombomeric units in the brain stem may determine, among other aspects, the specific properties of the different neuronal subpopulations related with their axonal navigation and synaptogenesis. This type of analysis furthers our understanding of how the functional circuitry of a complex system, such as the vestibular system, is generated and is a line of reasoning that in principle can be applied to the whole neural tube.