CONNECTIONAL TOPOGRAPHY IN THE ZEBRAFISH OLFACTORY SYSTEM - RANDOM POSITIONS BUT REGULAR SPACING OF SENSORY NEURONS PROJECTING TO AN INDIVIDUAL GLOMERULUS

It is unknown how neuronal connections are specified in the olfactory system. To define rules of connectivity in this system, we investigate d whether the projection of sensory neurons from the olfactory epithel ium to the olfactory bulb is topographically ordered. By backtracing w ith 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), we find that neurons projecting into a single identified glome rulus are widely dispersed over the olfactory epithelium. Their positi ons in the sensory surface do not predict their glomerulus specificity and are probably random. A statistical analysis reveals that neurons connected to the same glomerulus are spaced at distances of several ce ll diameters from each other. The convergence of projections to one po int in the target area from neurons that are widely and evenly distrib uted in the sensory surface constitutes an unusual type of connectiona l topography that contrasts with the precise topological (neighborhood -preserving) maps found in other sensory systems. It may maximize the probability to detect odorants that activate a single glomerular unit.