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5.5 Membrane Properties of Cells in the CC Stem Cell Niche
The membrane properties of cells lining the CC has been characterized both in the
spinal cord of turtles and rats (Russo et al. 2004 , 2008 ; Marichal et al. 2009 , 2012).
Both in the turtle and neonatal rats, the cells in the lateral domains of the CC had
electrophysiological properties similar to those of progenitors during cortical devel-
opment: (a) low input resistances, (b) passive responses, (c) hyperpolarized resting
membrane potentials and (d) extensive gap junction coupling via Cx43 (Bittman
et al. 1997 ; Noctor et al. 2002 ). The cluster of electrically and metabolically cou-
pled ependymal cells in the lateral aspects of the CC in turtles matches the expres-
sion of BLBP and the transcription factor Pax6 and Nkx6.1, suggesting these cells
are multipotent progenitors (Pinto and Götz 2007 ). In line with this electrophysio-
logical and molecular signature, these progenitors have a higher rate of proliferation
to those located in dorsal or ventral domains (Russo et al. 2008 ) and can generate
both glial cells and neurons (Fernández et al. 2002 ). Whereas progenitors in the
lateral domains of rodents share some properties of those in non-mammalian verte-
brates such as the predominance of passive membrane properties, extensive cou-
pling via Cx43 and higher rates of proliferation than midline domains, they lack the
expression of BLBP and Pax6, a fact that may be related to their inability to gener-
ate new neurons in the post-natal life (Marichal et al. 2009 ).
In contrast to cells on lateral domains, RG in dorsal and ventral domains are not
electrically coupled via Cx43 and function as individual units, both in reptiles
(Russo et al. 2008 ) and mammals (Marichal et al. 2012 ). Unlike neurogenic RG in
the developing cortex (Noctor et al. 2002 ), cells in the midline domains of the CC
have active membrane properties. In turtles, a subset of cells in the midline with
the morphological phenotype of RG display a conductance that is active at resting
membrane potentials and deactivates slowly when the membrane is hyperpolarized
(Reali et al. 2011 ). The mechanisms and functional relevance of this voltage-gated
conductance remains to be explored. Similarly, RG in the post-natal spinal cord of
rodents had complex electrophysiological phenotypes displaying various combi-
nations of a delayed rectifier (IKD), A-type (IA) and/or calcium currents. The pres-
ence of IKD is a common feature among adult progenitors since it has been reported
in hippocampal nestin+ type 2 cells (Filippov et al. 2003 ) and GFAP+ cells in the
SVZ (Liu et al. 2006 ). Although IA was not found in the adult SVZ (Liu et al.
2006 ), progenitors from the embryonic (Smith et al. 2008 ) and neonatal (Stewart
et al. 1999 ) SVZ as well as human stem cells (Schaarschmidt et al. 2009 ) express
IA. The phenotype of midline RG with conspicuous IKD and IA is remarkably simi-
lar to that of oligodendrocyte progenitors (Chittajallu et al. 2004 ), raising the pos-
sibility they are bipolar precursors committed to the oligodendrocyte lineage
(Levine et al. 2001 ). It remains to be explored whether these cells can differentiate
in oligodendrocyte progenitor cells with a multipolar morphology and expression
of NG2 or PDGFRα.
The complex repertoire of K+ currents may regulate fundamental properties of
ependymal progenitor-like cells. IKD channels are major regulators of cell
N. Marichal et al.