Organization of the mouse dorsal thalamus based on topology, calretinin immnunostaining, and gene expression

To better understand the organization and evolution of the dorsal thalamus, we have made a first approach to analyze the possible histogenetic compartments of the mammalian dorsal thalamus using mouse embryos. For that, we have analyzed the expression of the proneural gene Math4a and the protein calretinin. Our results suggest the existence of rostrodorsal, caudoventral, and ventral compartments in the embryonic dorsal thalamus of the mouse, which partly parallel the dorsoventral histogenetic tiers postulated in the dorsal thalamus of sauropsids. The rostrodorsal compartment of the mouse dorsal thalamus is characterized by expression of Math4a, and it appears to include sensory and motor thalamic nuclei projecting to the dorsal pallium (isocortex). This compartment appears equivalent to the lemnothalamus proposed by Butler in tetrapods based on hodological grounds. The caudoventral and ventral compartments of the mouse dorsal thalamus lack expression of Math4a in the mantle, but they are characterized by several populations of calretinin-immunorective neurons that show projections to the claustroamygdaloid region in the ventrolateral pallium. More studies will be needed to analyze if the compartments proposed in this study represent true histogenetic units, and to find homologous developmental fields in all vertebrates.