61Indeed, many S100β+/3CB2+/vimentin + cells express PCNA –indicating that they
are cycling cells- with few undergoing division as indicated by pH3 expression
(Eisch and Mandyam 2007 ). The expression of nestin -a marker of neuroepithelial
cells and RG (Pinto and Götz 2007 )- defines a second domain of heterogeneous
cells contacting the poles of the CC (Fig. 5.1a). In adult mice, nestin is expressed
preferentially on cells contacting the dorsal pole of the CC (Hamilton et al. 2009 ).
Although the perikarya of these cells in neonatal rats lay at various distances from
the CC, their centrosomes are always located in apical endfeet some of which bear
a single cilium with a 9 + 0 organization, a structural signature of neural stem cells
(Alvarez-Buylla et al. 2001 ; Kriegstein and Alvarez-Buylla 2009 ). The fact that pH3
nuclei belonging to nestin + cells are always found close to the CC lumen supports
the idea that RG nuclei in the post-natal spinal cord move apically to divide as
described for neurogenic RG in the embryo (Kriegstein and Alvarez-Buylla 2009 ).
Progenitors in adult neurogenic niches express GFAP in addition to nestin (Ma et al.
2008 ). However, the ependyma in the rat lacked GFAP immunoreactivity, in con-
trast with GFAP-GFP transgenic mice which bear GFAP+ cells contacting the dor-
sal pole (Sabourin et al. 2009 ). The discrepancy between data obtained in rats and
mice may be species specific or age related (neonatal versus adult). Another feature
suggesting that midline domains may not be identical in their potential is the fact
that cells contacting the ventral but not the dorsal pole express the astrocyte and
ependymal cell marker S100β (Marichal et al. 2012 ). In line with this, GFAP
RG-like cells located on the dorsal pole of the ependyma of adult mice have been
proposed to be the only cells capable to generate several passages of neurospheres
producing astrocytes, oligodendrocytes and neurons (Sabourin et al. 2009 , dis-
cussed in more detail below). Another possibility is that nestin+/S100β + cells on
the ventral pole may be a transitional stage between RG and ependymal cells as
described during brain development (Spassky et al. 2005 ).
The occurrence of a well-developed actin skeleton completes the structural and
molecular architecture of the ependymal region. 3D reconstructions using
phalloidin- conjugated fluorophores have revealed that the apical portions of the
ependymal cells are surrounded by a dense network of actin fibers usually associ-
ated with the zonula adherens (Fig. 5.1c). The same technical approach discloses a
dense actin network close to the inner layer of the plasma membrane of all ependy-
mal cells. The use of genetically modified mice whose CSFcNs express GFP com-
bined with actin staining have demonstrated that the apical dendrites of these nerve
cells are surrounded by the actin rich apical segments of ependymocytes (Fig. 5.1 c,
unpublished). Experimental studies have revealed that intraventricular perfusion of
cytochalasin B produce a pronounced disarrangement of actin microfilaments in
ependymocytes. In turn, they became rounded up and loosened from their neighbors
(Mestres and Garfia 1980 ). It is reasonable to speculate that migration of the epen-
dymal cells induced by different kind of injuries should be preceded by disorganiza-
tion of the normal actin network.
5 Spinal Cord Stem Cells In Their Microenvironment: The Ependyma as a Stem Cell...