The role of Grg4 in tectal laminar formation.

Mature chick optic tecta consist of 16 laminae and receive retinal fiber projections in a precise retinotopic manner. Retinal axons arborize in laminae a-f of the SGFS, but do not cross the border between lamina f and g. In order to elucidate molecular mechanisms of tectal laminar formation, we first looked at the migration of tectal postmitotic cells. We found that the migration pattern of postmitotic cells changes around E5 and that late migratory cells intervened laminae that were formed by early migratory cells. The coincident appearance of Grg4 expression in the tectal ventricular layer and the change in migration pattern suggested an important role for Grg4. Clonal misexpression of Grg4 resulted in cells migrating to laminae h-j of the SGFS. Massive misexpression of Grg4 resulted in disruption of laminae that were formed by early migratory cells, in particular lamina g of the SGFS. Application of Grg4 morpholino antisense oligonucleotide or the misexpression of a dominant-negative form of Grg4 exerted the opposite effect. We concluded that Grg4 may direct tectal postmitotic cells to follow a late migratory pathway.