SEGMENTAL AND NEURONAL ARCHITECTURE OF THE HINDBRAIN OF KROX-20 MOUSEMUTANTS

The vertebrate hindbrain is transiently segmented during its early dev elopment with the formation of reiterated bulges, the rhombomeres (r), The Krox-20 gene, which encodes a zinc finger transcription factor, h as been shown previously to be implicated in the maintenance of r3 and r5 (Schneider-Maunoury, S., Topilko, P., Seitanidou, T., Levi, G., Co hen-Tannoudji, M., Pournin, S., Babinet, C. and Charnay, P. (1993) Cel l 75, 1199-1214; Swiatek, P. J. and Gridley, T (1993) Genes Dev. 7, 20 71-2084, However, it was not clear from these analyses how extensive t he deletion of r3 and r5 was and whether the overall segmentation and internal architecture of the hindbrain was affected, We have now reinv estigated these issues by analysis of rhombomere boundaries, using bot h morphological and molecular markers, and of the fate of specific mot or neuron populations, using retrograde and anterograde carbocyanine d ye tracing, We conclude that r3 and r5 and their derivatives are compl etely eliminated in Krox-20(-/-) embryos while overall hindbrain segme ntation is maintained, In addition, we show that the disappearance of these territories has important consequences for even-numbered rhombom eres as well, in particular on axonal navigation: (i) a population of r6 motoneurons, presumably normally fated to join the glossopharyngeal nerve, has its axons misrouted toward the facial exit point in r4; (i i) the trigeminal motor axons are also misrouted, presumably because o f the proximity of the trigeminal and facial exit points, They fascicu late with facial axons outside the neural tube and enter the second br anchial arch instead of the first arch, This navigational error could explain the disappearance, at around 17.5 dpc, of the trigeminal motor nucleus in Krox-20(-/-) embryos by inadequate supply of essential, po ssibly arch-specific survival factors.