TOPOGRAPHIC ORGANIZATION OF EMBRYONIC MOTOR-NEURONS DEFINED BY EXPRESSION OF LIM HOMEOBOX GENES

Motor neurons located at different positions in the embryonic spinal c ord innervate distinct targets in the periphery, establishing a topogr aphic neural map. The topographic organization of motor projections de pends on the generation of subclasses of motor neurons that select spe cific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasse s of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to th e formation of distinct motor axon pathways and before motor columns a ppear. Our results suggest that LIM homeobox genes contribute to the g eneration of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby in itiating the topographic organization of motor projections.