Development of the Avian Dorsal Thalamus: Patterns and Gradients of Neurogenesis

The dorsal thalamus is a region of the diencephalon that relays sensory and motor information between areas of the brain stem and the telencephalon. Although a dorsal thalamic region is recognized in all vertebrates and believed to be homologous, little is known about how the regions within it evolved and whether some or all regions within the dorsal thalamus are homologous among different vertebrate species. To characterize the gradients and patterns of neurogenesis of the avian dorsal thalamus, a single application of a low dose of bromodeoxyuridine (BrdU) was delivered to each chick between embryonic day (E)3 and E8 (stages 21 and 34), and chicks were followed up to E8 or E10 (stage 34 or 36). Comparisons of anti-BrdU labeling patterns across the different injection days suggest that nearly all dorsal thalamic neurons are born early in chick embryogenesis, between E3 and E8. Furthermore, neurons in the lateral, dorsal, and posterior parts of the dorsal thalamus are generally born earlier than those in the medial, ventral, and anterior parts. Analyses of the birth dates for nine regions show that the general pattern of neurogenesis in the avian dorsal resembles that of homologous regions within the rodent thalamus, with the exception of the auditory region, the nucleus ovoidalis, which is born later than the mammalian auditory medial geniculate nucleus. The similar pattern of neurogenesis in birds and mammals may represent a highly conserved developmental pattern that was present in the common ancestor of living birds and mammals, or may represent independently derived states. Additional studies in reptiles and amphibians are needed to distinguish between these evolutionary histories.