Axon guidance during establishment of electroreceptor innervation in the catfish Kryptopterus.

The principles were studied, according to which the characteristic electroreceptor distribution pattern consisting of organ rows and clusters on the tail of the catfish Kryptopterus spec. is formed. As each electroreceptor is induced by its future afferent ('electrosensory') nerve fibre at the site where the outgrowing fibre reaches the epidermis, it is the fibre navigation which controls the organ distribution. Three navigation principles of the outgrowing electrosensory fibres were found. (i) The electrosensory fibre courses are bound to the tail segmentation. Nerve displacement experiments suggest that the fibres are guided by the intersegmental connective tissue sheaths, i.e. the myosepta and myocommata. (ii) The individual fibres have no specificity for a certain route or target area on the tail, but can grow in any direction and into any tail area and induce organs there. This is indicated by experiments with nerve elimination and nerve deflection. (iii) An outgrowing fibre's only orientation is towards a nearby 'free site'; i.e. it aims for a place in clusters with fewer organs than their actual capacity allows. The capacity increases continuously with the specimen's age, so that free sites progressively develop. So, it depends on chance which outgrowing fibre occupies which 'free site'; a free site is targeted by whatever outgrowing fibre happens to be the nearest. The mechanisms of development of the somatotopic projection of the electrosensory fibres to the central nervous system (CNS) are discussed.