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single primary cilium (9 + 0) projecting into the ventricle lumen. These cells form
tube–like structures ensheathing the so called A-type cells when migrating towards
the olfactory bulb (Lois et al. 1996 ). Recent fine structural studies performed in
mice (Cebrián-Silla et al. 2017 ), indicated that some B cells show envelope-limited
chromatin sheets, a rare specialization of the nuclear membrane initially described
in blood neutrophils (Davies and Small 1968 ). These unusual nuclear compartments
have been associated with the isolation of telomeres and epigenetic modifications.
When dealing with other commonly accepted adult neurogenic region like the hip-
pocampus, the unequivocal identification of a cell type as a “primary precursor” is
less clear. According to Kempermann et al. ( 2008 ) several cell types “are involved
in the course of adult hippocampal neurogenesis”. The common feature is that pre-
cursor cells found in the dentate gyrus are of astroglial nature (Seri et al. 2001 ).
However, not all neural progenitors share these anatomical features, such as inter-
mediate progenitors in the SVZ of the developing cortex which have multipolar
processes that do not contact the ventricle or pial surface (Kriegstein and Alvarez-
Buylla 2009 ). In fact, progenitors are heterogeneous cells expressing different mol-
ecules that seem related with their lineage potential (Pinto and Götz 2007 ). For
example, expression of brain lipid binding protein (BLBP) seems to determine RG
as bi-potent or multipotent progenitors (Pinto and Götz 2007 ; Hartfuss et al. 2001 ;
Anthony et al. 2007 ). The transcription factor Pax6 is a major regulator of the sub-
population of neurogenic RG (Götz et al. 1998 ; Heins et al. 2002 ; Bel-Vialar et al.
2007 ) and progenitors in the adult mammalian brain (Kohwi et al. 2005 ; Maekawa
et al. 2005 ). In the developing spinal cord Pax6 interacts in a combinatorial manner
with other transcription factors such as Olig2, Nkx2.2 and Sox9 to control neuro-
genesis and gliogenesis (Heins et al. 2002 ; Lee and Pfaff 2001 ; Rowitch 2004 ;
Nacher et al. 2005 ; Guillemot 2007 ; Sugimori et al. 2007 ). The importance of the
combination of key transcription factors in determining the biology of progenitors
is highlighted by the possibility of reprogramming somatic cells to pluripotent stem
cells with just a handful of factors (Yamanaka 2012 ).
5.4 Progenitor Cells in the Spinal Cord: The CC as a Stem
Cell Niche
The identity of stem cells in the adult spinal cord has been difficult to establish and
remains controversial. Although Horner et al. ( 2000 ) described proliferating cells in
the grey and white matter of the rat spinal cord, other studies showed that the vast
majority of stem cells resides within the ependyma (Mothe and Tator 2005 ; Sabourin
et al. 2009 ). In most text-books the ependyma is depicted as a layer of ephitelial
cells (Peters et al. 1991 ). Nevertheless, this oversimplified view has been challenged
by both classical (Ramón y Cajal 1909 ) and recent studies. Both in mammals and
non-mammalian vertebrates, the ependymal region is a complex structure consisting
of diverse kinds of cells arranged in lateral and dorsal-ventral domains (Schnapp
et al. 2005 ; Trujillo-Cenóz et al. 2007 ; Marichal et al. 2012 ). In murine, the cell
N. Marichal et al.