Developmental regulation of activated ERK expression in the spinal cord and dorsal root ganglion of the chick embryo

Mitogen-activated protein kinases (MAPKs) are involved in the intracellular pathways that respond to various extracellular signals. Extracellular signal-regulated kinase (ERK) is a member of MAPKs and has various functions in neural development. However, the in vivo distribution of the activated form of ERK (p-ERK) in the developing nervous system is not well understood. Here, we investigated the expression of p-ERK in the spinal cord and dorsal root ganglion (DRG) of chick embryos. In the spinal cord, p-ERK-positive cells appeared in the ventral ventricular zone on embryonic day 4 (E4). From E6 onward, they appeared in the gray matter and in the white matter, suggesting migration from the ventricular zone. A double labeling method revealed that these p-ERK-positive cells included oligodendrocyte precursors. In the dorsal horn, p-ERK-positive small cells appeared on E6. Subsequently, the positive cells in the dorsal horn increased transiently in number and then decreased markedly by E10. Motoneurons also expressed p-ERK transiently on E7. In the DRG, weak p-ERK immunoreaction appeared in the ventrolateral region on E5. From E6, the immunoreactivity became stronger and by E9 intense p-ERK-positive cells were observed throughout the DRG. These data provide a neuroanatomical framework to begin to examine the in vivo role of ERK in neural development.