CELL-MIGRATION IN THE DEVELOPING CHICK DIENCEPHALON

We previously reported that retrovirally marked clones in the mature c hick diencephalon were widely dispersed in the mediolateral, dorsovent ral and rostrocaudal planes, The current study was undertaken to defin e the migration routes that led to the dispersion, Embryos were infect ed between stages 10 and 14 with a retroviral stock encoding alkaline phosphatase and a library of molecular tags, Embryos were harvested 2. 5-5.5 days later and the brains were fixed and serially sectioned. Sib ling relationships were determined following PCR amplification and seq uencing of the molecular tag, On embryonic day 4, all clones were orga nized in radial columns spanning the neuroepithelium, which was compos ed primarily of a ventricular zone at this age, No tangential migratio n was seen in the ventricular zone. On embryonic day 5, most clones re mained radial with many cells located in the ventricular zone; however , a few clones had cells migrating perpendicular to the radial column, in either a rostrocaudal or dorsoventral direction, The tangential mi gration began just beyond the basal limit of the ventricular zone. On embryonic days 6 and 7, many clones had cells migrating perpendicular to the radial column, which spanned from the ventricular to the pial s urface, The migrating cells appeared to be aligned along axes that wer e perpendicular to the radial column, Using a combination of DiI traci ng, immunohistochemistry and electron microscopy, we have determined t hat axonal tracts are present and are aligned with the migrating cells , suggesting that they support the non-radial cell migration, These da ta indicate that migration along pathways independent of radial glia o ccur outside of the ventricular zone in more than 50% of the clones in the chick diencephalon.