Continuity and discontinuity of the radial scaffolding in the forebrain of a lizard embryo

The rostral and caudal boundaries of the diencephalon

Knowledge of nature and features of the boundaries between the main neural regions seems to be essential to understand the rules of brain regionalization. On the light of several current and classical criteria used to define cerebral boundaries, we examine the features of the places recognized as rostral and caudal boundaries in the developing diencephalon and provide new images about the glial features of these boundaries. One demonstrated property of some embryonic boundaries is the prevention of the crossing cells in the early ventricular zone (clonal restriction), while the intermediate zone seems to lack it. Data available so far indicate that the early boundary between diencephalon and mesencephalon (d/m) is a clonal restriction limit, but not between diencephalon and telencephalon (d/t). Later, while diencephalic nuclei form, cellular dispersion does not occur through the alar part of d/m, but it achieves in the corresponding d/t alar portion. The relationship between origin, migration, and cell-type specification of neural cells is being the object of special attention in the telencephalon, where specific cellular fenotipes can migrate to distant regions following non-radial routes. Such is the case of most GABAergic interneurons of avian and mammalian pallium and oligodendrocytes of the forebrain. In this regard, little attention has been devoted to the diencephalon, where this type of migration, specially those through the rostral boundary, has been reported by different authors. We introduce increasing evidence about non-conventional neuronal migration in the developing diencephalon and compare the reported migratory behavior with respect to both boundaries.

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.M feminine. 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.