Substrates and routes of migration of early generated neurons in the developing rat thalamus

We investigated the substrates supporting neuronal migration, and its routes, during early thalamic development in the rat. Neurons and axonal and glial fibres were identified in embryos with single and double immunohistochemistry; dynamic data were obtained with cell tracers in short-term organotypic cultured slices. The earliest thalamic neurons, originating from the ventricular neuroepithelium between embryonic days 13 and 15, include those of the reticular thalamic nucleus. At this developmental stage, calretinin, calbindin or gamma-aminobutyric acid immunostaining revealed both radially and nonradially orientated neurons in the region of reticular thalamic migration, between the dorsal and ventral thalamic primordia. In cultured slices, injections of fluorescent dyes in the neuroepithelium labelled neurons in a migratory stream along radial glia in the same zone. Some labelled fusiform cells departed from this radial trajectory along orthogonal routes within the dorsal thalamus. Confocal microscopy revealed nonradially orientated neurons in close apposition with a fibre system parallel to the lateral thalamic surface. These fibres expressed axonal markers, including the intermediate filament protein alpha-internexin and a polysialylated form of neuronal cell adhesion molecule. Active migration of nonradially orientated neurons along neuronal substrates was confirmed in living cultured slices. In addition, in vitro and ex vivo experiments revealed neurons migrating tangentially in association with glial fibres. These results provide novel evidence that: (i) early generated thalamic neurons follow nonradial routes in addition to glia-linked radial migration; and (ii), nonradially migrating thalamic neurons move along both glial and axonal substrates, which could represent a distinctive feature of thalamic development.