Glial-defined rhombomere boundaries in developing Xenopus hindbrain

The vertebrate central nervous system is characterized by regional specialization, which arises during early development and contributes to patterning the emerging central nervous system (CNS). In the hindbrain, rhombomeres demarcate nonoverlapping regions of the CNS that give rise to distinct neural structures. The cellular structures that define boundaries between adjacent rhombomeres are as yet unclear. However, in certain species the boundary regions between discrete CNS regions appear to be defined by specialized glial cells. Here, we show that in developing Xenopus, DM gamma, a membrane protein of the proteolipid protein family, is expressed in a subset of radial glia. During development, DM gamma transcripts are first expressed in presumptive glial cells throughout the hindbrain, but later become confined to the ventricular zone at rhombomere centers, whereas the protein is exclusively expressed in radial glial cell processes that occupy the rhombomere boundary regions. Likewise, early in development vimentin and glial fibrillary acidic protein are extensively coexpressed in hindbrain radial glia but subsequently define distinct rhombomere domains: vimentin remains localized in radial glia at the rhombomere boundary regions, whereas expression of glial fibrillary acidic protein becomes restricted to the centers. Moreover, radial glial processes at the boundary region are distinguishable from those at the center region; the processes of the boundary region radial glia extend upward in a fan-shaped arrangement and are encircled by the processes from the center glia. These data suggest that an early event in determining rhombomere topology is the specification of both morphologically and biochemically distinct subsets of radial glia.