Possible involvement of gradients in guidance of receptor cell axons towards their target position on the olfactory bulb.

Abstract

There is increasing evidence for directional guidance of growing axons by molecular gradients in target tissues. Aside from biochemical studies on gradients and their role, the capability of axons to approach their target position from different aspects of a two-dimensional field is itself an indication for guidance by gradients. According to this criterion, such guidance is expected to be involved not only in map-formation in the visual system but also in targeting of receptor cell axons in the olfactory bulb. In this paper, physico-chemical concepts of visual mapping are adapted to olfactory targeting. In both cases there must be sophisticated processing of graded cues in the growing tip of the axon for growth cone navigation. In visual map formation, a target position is determined by influences of cues depending on the position of axonal origin; in olfactory targeting, however, these influences are expected to be based on properties of the receptor-cell-specific molecules (possibly including the receptor molecule itself), as well as by gene regulation affecting the levels of expression. According to this concept, the main role of molecules expressed in a receptor-cell-type specific manner is not matching specific counterparts on the target tissue, but instead quantitative modulation of growth cone steering for sensing the direction towards the target position.