Enigmatic central canal contacting cells: immature neurons in "standby mode"?

The region that surrounds the central canal of the spinal cord derives from the neural tube and retains a substantial degree of plasticity. In turtles, this region is a neurogenic niche where newborn neurons coexist with precursors, a fact that may be related with the endogenous repair capabilities of low vertebrates. Immunohistochemical evidence suggests that the ependyma of the mammalian spinal cord may contain cells with similar properties, but their actual nature remains unsolved. Here, we combined immunohistochemistry for cell-specific markers with patch-clamp recordings to test the hypothesis that the ependyma of neonatal rats contains immature neurons similar to those in low vertebrates. We found that a subclass of cells expressed HuC/D neuronal proteins, doublecortin, and PSA-NCAM (polysialylated neural cell adhesion molecule) but did not express NeuN (anti-neuronal nuclei). These immature neurons displayed electrophysiological properties ranging from slow Ca(2+)-mediated responses to fast repetitive Na(+) spikes, suggesting different stages of maturation. These cells originated in the embryo, because we found colocalization of neuronal markers with 5-bromo-2'-deoxyuridine when injected during embryonic day 7-17 but not in postnatal day 0-5. Our findings represent the first evidence that the ependyma of the rat spinal cord contains cells with molecular and functional features similar to immature neurons in adult neurogenic niches. The fact that these cells retain the expression of molecules that participate in migration and neuronal differentiation raises the possibility that the ependyma of the rat spinal cord is a reservoir of immature neurons in "standby mode," which under some circumstances (e.g., injury) may complete their maturation to integrate spinal circuits.