Additional Supporting Information may be found in the online version of this article.

SC-12-0290_sm_SupplFigure1.pdf69KSupplementary figure 1. Co-expression of 3CB2 (1) and S100β (2) was observed in some cells on the lateral aspects of the CC (1-3, arrowheads). Single confocal optical planes.
SC-12-0290_sm_SupplFigure2.tif2847KSupplementary figure 2. Cells contacting the poles of the CC have RG morphology. A, Distribution of input resistances and resting membrane potentials in midline cells. B, Typical RG recorded in the dorsal pole of the CC, with a thick apical process reaching the lumen (1, arrow), bearing finger-like protrusions (2, arrowheads) and a thinner distal fiber projecting to the pia (1, arrowhead). C, Other RG showed smooth apical and distal processes (arrows). D, The perikarya of cells within the midline located at different distances from the CC lumen (1- 4). All images are confocal Z-stack projections. In D the ventral pole of the CC is upward.
SC-12-0290_sm_SupplFigure3.pdf101KSupplementary figure 3. Molecular phenotypes of recorded midline RG. A, Biocytinfilled RG (1, arrow) in the ventral pole of the CC with apical and distal thin processes (arrowheads). This cell expressed nestin (2-4, arrows). B, Some midline RG did not react to nestin. The arrowhead points to a nearby nestin+ cell. C, Midline RG were negative for PDGFRα. However, a few PDGFRα+ cells were found close to midline but without a connection with the CC (arrowhead). D, The ependyma was devoid of NG2 immunoreactivity but abundant NG2+ cells outside the ependymal region occurred (arrowheads). A, Single confocal optical planes. B, C, Confocal Z- stack projections.
SC-12-0290_sm_SupplFigure4.tif372KSupplementary figure 4. Differences in proliferative potential between the lateral and the midline domains of the CC. A, Schematic drawing showing the limits of the regions (area shaded in green: midline region, area shaded in red: lateral aspects of the CC) in which PCNA and pH3 nuclei were counted. B, Number of PCNA+ and pH3+ cells per section analyzed in the different domains surrounding the CC (mean ± SEM).
SC-12-0290_sm_SupplFigure5.pdf154KSupplementary figure 5. The ependyma of the mature rat spinal cord. A, Cell recorded in the dorsal pole of the CC in a P15 rat with a thick apical process reaching the lumen (arrow), a round perikarya (open arrowhead) and a thinner distal process projecting to the pia (arrowhead). B, response to a series of voltage steps of the cell shown in A. C, Cell in the dorsal pole of the CC in a P20 rat with a short process contacting the CC lumen (arrow) and a thin basal process (arrowhead). D, The cell bodies of some RG were also found apposed to the CC lumen in a P21 rat. E, Cell recorded on the lateral aspect appeared dye-coupled with neighboring cells in a P20 rat. Notice that clustered cells extended until the dorsal pole of the ependyma (arrowhead). The inset shows the electrical response of the cluster of cells in the main panel. F, Immunocytochemistry for nestin shows many positive cells with long radial processes in the dorsal and ventral poles in a P40 rat. G, Nestin+ cells maintained their cell bodies located at various distances from the CC lumen (arrowheads). H, 3CB2 immunoreactivity was observed both in the poles and lateral aspects of the ependyma. I, PCNA+ nuclei around the CC in a P40 rat. A, D-I, Confocal Z-stack projection. C, Conventional epifluorescence in a living slice.
SC-12-0290_sm_Suppltable1.pdf52KSupplementary Table 1
SC-12-0290_sm_Suppltable2.pdf8KSupplementary Table 2

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