123cells and terminally differentiated cells were considered lower in hierarchy (Nguyen
et al. 2012 ; Dick 2009 ; Cabrera et al. 2015 ; Kreso and Dick John 2014 ).
The existence of GSCs still remains a point of intense contention within scien-
tific circles due to (i) the relatively small number of these cells within patient GBM
samples, (ii) its uncertain cell of origin; and (iii) lack of a universal identification
marker specific for GSC immunophenotype, as recent studies show that not all
GSCs are CD133 + .
7.2.1 Identification and Isolation of GSCs Using Cell Specific
Markers
A high variability in cell surface marker expression in observed within patient GBM
samples and between samples from different patients due to the highly heteroge-
neous nature of the disease (MDM et al. 2010 ). Although initial studies had identi-
fied cell populations enriched for CD133 cell surface marker as GSCs, further
reports identified GBM cells which did not show enrichment in their CD133 expres-
sion but formed neurospheres in culture and tumorigenesis in xenograft assays.
Cellular heterogeneity within biopsy samples and inefficient antibody labeling dur-
ing FACS could explain for the lack of expression of CD133 in these cells (Son
et al. 2009 ; Ogden et al. 2008 ; Shackleton et al. 2009 ; Nishide et al. 2009 ; Kelly
et al. 2009 ). GBMs consist of various clones expressing a diverse genetic profile and
it is hypothesized that GSCs from one region of a tumor could express different cell
surface markers when compared to GSCs in another region of the tumor. Therefore,
the location within tumors from where GSCs are enriched may also play a vital role
in there stem cell marker expression.
CD133+ GBM cells have been shown to be resistant to radiation and chemo-
therapy due to activation of cell cycle checkpoint pathways, enhancement of DNA
repair and aberrant cell survival mechanisms. Gene expression profiling of treat-
ment resistant GBM cells showed them to be enriched for CD133 expression (Bao
et al. 2006a; Murat et al. 2008b). Tumors derived from CD133+ GBM cells have
also been reported to be highly vascularized due to promotion of angiogenesis
through secretion of VEGF and SDF-1. These tumors also show greater invasion
into normal brain tissue when compared to other GBM cell lines (Bao et al. 2006b).
Additionally, the stage of a patient’s disease could also impact the cell surface
marker expression of GSCs thereby showing variable results when comparing dif-
ferent patient samples. The particular therapy regiment along with its duration could
also potentially impact cell surface marker expression within GBMs of patients.
GSCs have also been reported to show high cellular plasticity. Recent reports have
shown that not only can GSCs differentiate into progenitor and terminal cells, but
their daughter cells can also de-differentiate back into GSCs depending on microen-
vironmental cues and adaptation to treatment (Lee et al. 2016 ; Safa et al. 2015 ).
Terminal differentiated GBM cells could therefore express markers of different
lineages simultaneously and have no counterparts in normal physiological lineages
7 Glioblastoma Stem Cells and Their Microenvironment