Pax-6 is required for thalamocortical pathway formation in fetal rats.

Pax-6, a transcription regulatory factor, has been demonstrated to play important roles in eye, nose, and brain development by analyzing mice, rats, and humans with a Pax-6 gene mutation. We examined the role of Pax-6 with special attention to the formation of efferent and afferent pathways of the cerebral cortex by using the rat Small eye (rSey2), which has a mutation in the Pax-6 gene. In rSey2/rSey2 fetuses, cortical efferent axons develop with normal trajectory, at least within the cortical anlage, when examined with immunohistochemistry of the neuronal cell adhesion molecule TAG-1 and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) labeling from the cortical surface. A remarkable disorder was found in the trajectory of dorsal thalamic axons by immunostaining of the neurofilament and the neural cell adhesion molecule L1 and DiI labeling from the dorsal thalamus. In normal rat fetuses, dorsal thalamic axons curved laterally in the ventral thalamus without invading a Pax-6-immunoreactive cell cluster in the ventral part of the ventral thalamus. These axons then coursed up to the cortical anlage, passing just dorsal to another Pax-6-immunoreactive cell cluster in the amygdaloid region. In contrast, in rSey2/rSey2 fetuses, dorsal thalamic axons extended downward to converge in the ventrolateral corner of the ventral thalamus and fanned out in the amygdaloid region without reaching the cortical anlage. These results suggest that Pax-6-expressing cell clusters along the thalamocortical pathway (ventral part of the ventral thalamus and amygdala) are responsible for the determination of the axonal pathfinding of the thalamocortical pathway.

Expression of receptor tyrosine kinase RYK in developing rat central nervous system.

Receptor tyrosine kinase RYK is a mammalian homologue of Drosophila Lio, which is involved in learning and memory and in axon guidance. We cloned a rat ryk gene and characterized its expression pattern in the central nervous system. Northern blot analysis of the whole brain revealed that the RYK mRNA was abundant during the period from 13 to 18 embryonic days (E13-18) and it decreased by E20. In the postnatal brain, the RYK signal was higher in postnatal one week (P1W) cerebrum and in P2W cerebellum than in later stages. In situ hybridization revealed that RYK was expressed throughout the central nervous system, mainly in the ventricular zone on E11 and E13. On E18 and E20, the remarkable level of RYK mRNA was detected in the ventricular zone as well as in the cortical plate of the forebrain. These two regions overlapped the immunoreactive areas of nestin and MAP2, a neural stem cell marker and a mature neural marker, respectively. Moreover, the double-labeling analysis showed that the same cells expressed both RYK and nestin in the ventricular zone. In the postnatal brain, RYK was predominantly expressed in neurons of various regions. These observations suggest that RYK plays a contributory role as a multifunctional molecule in the differentiation and maturation of neuronal cells in the central nervous system.