The midbrain of sauropsides shares a common subdivision pattern defined by embryonic radial glia

In a previous study of the embryonic midbrain radial glia in a lizard, we observed that these cells define boundaries and regional subdivisions in a pattern that largely supports the adult model [C. Diaz, C. Yanes, C.Mª. Trujillo, L. Puelles, Cytoarchitectonic subdivisions in the subtectal midbrain of the lizard Gallotia galloti, J. Neurocytol. 29 (2000) 569-593]. With the goal to check whether the midbrain of chick embryos has a similar pattern, we examined the radial glia distribution in this model using a lipophilic dye (DiI) injected intraventriculary. As in the lizard, chick radial glia distribution and fasciculation defines at least six regional subdivisions in the midbrain, five of which are alar and one basal. Each territory corresponds to a particular cytoarchitectonic area, recognized previously. The rostral mesencephalic limit (m/d) is underlined in the alar zone by a thick band of fasciculated radial glia intercalated between the griseum tectale and the posterior commissure. A fasciculated radial glia band also defines the caudal mesencephalic limit (m/r) across the alar and basal zones. These glial specializations, not described previously in the chick, give physical entity to the limits deduced from gene expression studies and suggested by descriptive cytoarchitectonic analysis. We conclude that the midbrain of sauropsides shares a common subdivision pattern co-defined by radial glia arrangements.