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Multiple influences on the migration of precerebellar neurons in the caudal medulla

I. de Diego^1,*,, K. Kyriakopoulou^1,2,, D. Karagogeos² and M. Wassef^1,

¹ CNRS UMR C8542, Régionalisation Nerveuse, niveau 8, Ecole Normale Supérieure 46, rue d’Ulm 75230 Paris Cedex 05, France
² University of Crete Medical School and Institute of Molecular Biology and Biotechnology, PO Box 1527, Heraklion 711 10, Crete, Greece
^* Present address: Departamento de Biología Celular, Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos, 29071 Málaga, España
These authors contributed equally to this work

Author for correspondence (e-mail: wassef{at}wotan.ens.fr)

Accepted 9 October 2001

Neurons destined to form several precerebellar nuclei are generated in the dorsal neuroepithelium (rhombic lip) of caudal hindbrain. They form two ventrally directed migratory streams, which behave differently. While neurons in the superficial migration migrate in a subpial position and cross the midline to settle into the contralateral hindbrain, neurons in the olivary migration travel deeper in the parenchyma and stop ipsilaterally against the floor plate. In the present study, we compared the behavior of the two neuronal populations in an organotypic culture system that preserves several aspects of their in vivo environment. Both migrations occurred in mouse hindbrain explants dissected at E11.5 even when the floor plate was ablated at the onset of the culture period, indicating that they could rely on dorsoventral cues already distributed in the neural tube. Nevertheless, the local constraints necessary for the superficial migration were more specific than for the olivary migration. Distinct chemoattractive and chemorespulsive signal were found to operate on the migrations. The floor plate exhibited a strong chemoattractive influence on both migrations, which deviated from their normal path in the direction of ectopic floor plate fragments. It was also found to produce a short-range stop signal and to induce inferior olive aggregation. The ventral neural tube was also found to inhibit or slow down the migration of olivary neurons. Interestingly, while ectopic sources of netrin were found to influence both migrations, this effect was locally modulated and affected differentially the successive phases of migration. Consistent with this observation, while neurons in the superficial migration expressed the Dcc-netrin receptor, the migrating olivary neurons did not express Dcc before they reached the midline. Our observations provide a clearer picture of the hierarchy of environmental cues that influence the morphogenesis of these precerebellar nuclei.

Key words: Precerebellar neuron, Tangential migration, Inferior olive, Floor plate, Netrin, Mouse

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