63proliferation (Ghiani et al. 1999 ; MacFarlane and Sontheimer 2000a; Chittajallu
et al. 2002 ) and IA channels are essential for proliferation of multipotent human
neural stem cells (Schaarschmidt et al. 2009 ). Thus, K+ channels in midline RG may
be part of epigenetic mechanisms that regulate proliferation. In addition, IA has been
implied in the differentiation of oligodendrocyte precursors (Sontheimer et al. 1989 )
and rat spinal cord astrocytes (MacFarlane and Sontheimer 2000b). Thus, another
possibility is that K+ currents participate in the transition from RG to post-mitotic
spinal cells.
A minority of midline RG have ICa strong enough to sustain a low threshold
spike, a phenotype described in some floor plate cells (Frischknecht and Randall
1998 ). Ca2+ electrogenesis plays a central role during development by regulating
events such as neural induction (Webb et al. 2005 ) and neuronal differentiation
(Spitzer et al. 2004 ). RG displaying ICa could be precursors showing the first signs
of differentiation into CSFcNs (Marichal et al. 2009 ).
5.6 Spinal Stem Cells Outside Their Niche
Until now, we described the features of the different components of the ependymal
stem cell region in their normal endogenous microenvironment. However, more
than two decades ago, the properties showed by ependymal cells isolated and plated
in different cultures media provided the first evidences of their stem-cell
characteristics.
Weiss et al. ( 1996 ) developed an in vitro protocol for primary cultures of cells
taken from the adult spinal cord and found the presence of growth factor-responsive
stem cells. These cells generated spheres with self-renewal/expansion properties
and multipotency giving rise to neurons, astrocytes, and oligodendrocytes. After
this study, Frisen’s group also tested whether ependymal cells specifically showed
stem cells properties in vitro. For this, they labeled ependymal cells by injecting
1,1′-Dioctadecyl-3,3,3′,3′-Tetramethylindocarbocyanine Perchlorate (DiI)in the
lateral ventricles of the brain (Johansson et al. 1999 ). Neurospheres derived from
DiI-labeled primary cells expressed neuronal, astrocytic and oligodendrocyte
molecular markers.
More recently, the same group generated two transgenic mouse lines expressing
tamoxifen-dependent Cre recombinase (CreER) under the control of FoxJ1 (HFH4)
or nestin regulatory sequences (Meletis et al. 2008 ). These mice allowed to study
the fate map of the spinal cord ependymal cells (Meletis et al. 2008 ). FoxJ1 is a
transcription factor involved in the formation of motile cilia active in the spinal cord
only in cells that line the CC. Primary cultures showed that the recombined neuro-
spheres from both nestin-CreER and FoxJ1-CreER mice could be serially passaged
and were multipotent and differentiated into neurons, astrocytes, and oligodendro-
cytes. Despite being an excellent strategy to make the linage tracing of ependymal
cells, the general expression of these genes in the cells surrounding the CC (excluding
the CSFcNs) does not allow to discriminate between the heterogeneous populations
5 Spinal Cord Stem Cells In Their Microenvironment: The Ependyma as a Stem Cell...