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locations (Fig. 5.1b). It is pertinent to discuss here the proposed presence of a fourth
kind of cell termed “tanycytes” (Horstmann 1954 ). These cells, firstly described in
the brain of cartilaginous fishes (Horstmann 1954 ), have the aspect of RG and are
particularly abundant in the floor and ventrolateral walls of the third ventricle of
mammals. Taking into account their location and other morphological details, they
are usually subdivided into alpha and beta subtypes (Goodman and Hajihosseini
2015 ). Even though some authors have used the same term to identify a particular
kind of ependymal cell in the spinal cord of mice (Meletis et al. 2008 ), cells with a
morphological or molecular phenotype of tanycytes in the rat or turtle spinal cord
has not been reported (Trujillo-Cenóz et al. 2007 ; Marichal et al. 2012 ). It is worth
noting that the CC is far from being an empty channel. In addition to the cilia and
shorter microvilli projections, the CC contains cellular material of diverse origin. In
the particular case of mice, the bulbous terminals of the numerous CSFcNs appear
as the most conspicuous well organized structures within the CC lumen. Together
with these structurally complex cell compartments, there are also numerous cell
projections arising from the ependymocytes apical membranes. In rodents, TEM
studies show that intermingled with motile cilia there are vesicles of dissimilar sizes
and shapes which seem to be floating freely in the CSF. In other cases, membrane
extrusions attached by a thin process to their parent cells can be observed. This data
strongly suggest that the release of cell membranes into the CSF is a common mech-
anism to introduce different kinds of molecules into the CSF. Marzesco et al. ( 2005 )
have proposed that membrane bounded particles are the carriers within the CC of
the stem cell marker prominin 1. In other vertebrates like turtles, the CC contains
together with vesicular material, a clearly visible Reissner’s ribbon occupying most
of the channel lumen. Nevertheless, in rodents the condensation of Reissner’s rib-
bon glycoproteins only becomes apparent within the phylum terminalis. At this
level, condensation of the proteins takes the form of the so-called massa caudalis
(Molina et al. 2001 ).
As already mentioned, the basal pole of the ependimocytes also bear short and
long processes that invade adjacent neuropile areas vanishing the apparent boundar-
ies between the ependymal layer and the surrounding spinal tissue. The longest
processes reach the pial surface whereas the shorter ones contact neighbor capillar-
ies or end intermingled with glial and neuronal processes composing of adjacent
neuropile zones. As found in other well recognized stem cell niches, progenitor
cells are related to their neighbors at the level of their apical poles by zonnula
occludens and gap junctions (Russo et al. 2008 ; Marichal et al. 2012 ).
Besides structural differences, several neural stem/progenitor cell markers such
as Sox2, CD15, CD133, nestin, vimentin, BLBP and GFAP are expressed by sub-
populations of cells lining the CC (Meletis et al. 2008 ; Hamilton et al. 2009 ; Hugnot
and Franzen 2011 ). As in the brain (Spassky et al. 2005 ), most cells in the lateral
domains of the CC express the ependymal cell marker S100β, with a sub-population
that co-express the RG marker 3CB2 or vimentin and had a basal process projecting
away from the CC, suggesting a progenitor cell nature (Pinto and Götz 2007 ).
N. Marichal et al.