INTRODUCTION Several neuronal populations are generated in the dorsal rhombencephalon, at the level of the rhombic lip. They migrate following distinct dorsoventral paths identified as the pontine, olivary and superficial migratory streams. They settle in the ventral neural tube and contribute neurons to several precerebellar nuclei. Therefore, the precebellar system permits the study of the cellular and molecular mechanisms involved in the control of cell migration. We have focused on two populations of precerebellar neurons generated in the rhombic lip of rhombomeres 7/8 (or 8) (Cambronero and Puelles, 2000) and destined to form the inferior olive (ION), the lateral reticular (LRN) and the external cuneatus (ECN) nuclei. These neurons circumnavigate at different depths around the medulla following two routes, both orthogonal to the anteroposterior axis (Harkmark, 1954; Altman and Bayer, 1987a; Altman and Bayer, 1987b; Bourrat and Sotelo, 1988; Bourrat and Sotelo, 1990; Bourrat et al., 1989; Tan and Le Douarin, 1991; Ambrosiani et al., 1996). The olivary migration follows a deep parenchymal pathway; the ION neurons stop migrating ipsilaterally and aggregate near the floor plate, whereas their axons cross the floor plate and travel towards the contralateral hemicerebellum where they project. The LRN and ECN neurons, which are generated slightly later, gather under the pial surface forming a superficial migration, which crosses the ventral midline to continue dorsalward on the contralateral side. The floor plate is a complex ventral midline structure involved in neural tube patterning and in the guidance of both growth cone and cell body migration. The floor plate is the source of several short- and long-range chemoattractants and chemorepellents that are likely to govern both the dorsoventral orientation of the olivary and superficial migrations and their different behaviors at the ventral midline. In addition, it could provide olivary neurons with a stop signal preventing them 297 Development 129, 297-306 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV2801 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 SUMMARY Multiple influences on the migration of precerebellar neurons in the caudal medulla I. de Diego 1, * ,‡ , K. Kyriakopoulou 1,2,‡ , D. Karagogeos 2 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@wotan.ens.fr) Accepted 9 October 2001