**Acknowledgements**

for angiogenesis through VEGF and hypoxia/HIF-1 [115-120]. In perinecrotic niches, GSCs are generated by hypoxia through HIF-1. Really, in the niches there can be a complicated rela‐ tionship among different cell types, such as macrophages, pericytes, astrocytes, *etc.* with a multiple signalling [54,121,122]. In our experience, perinecrotic GSCs could be the remnants of GSCs that populated hyper-proliferating areas before the development of circumscribed necroses within them; this would take place because of the imbalance between the high proliferation capacity of tumor cells and the low one of endothelial cells [2,123]. GSCs, either as NS or AC, are heterogeneous as for stemness properties, clonogenicity and tumorigenicity and they have been regarded as at the top of a cell hierarchy for some molecular signs [49,50]. Stemness among tumor cells could be distributed in a spectrum with a *crescendo* from quiescent highly differentiated cells, where it is nil, to those in which it reaches the highest degree of expression. Stemness would be regulated by the microenvironment [53] and it could be the feature of a functional status rather than of a subset of cells [124,125]. As it is lost during differentiation of normal cytogenesis beyond the stage of progenitors, in the opposite way it

is gained by dedifferentiating tumor cells when they reach the stage of progenitors.

sites of the tumor [51,55].

82 Tumors of the Central Nervous System – Primary and Secondary

the proliferating zone of GBM [127].

Can they be detected by MRI or other procedures *in vivo*?

The heterogeneity of GBM, before discussed, conditions different genetic assets of the cells in the different clones; going from the samples of the most malignant areas of the tumor to those of tumor periphery, the potential of generating NS or AC decreases. The conclusion is that stem cells are kept as such by microenvironments and these are realized in the most malignant

**6. Location of GSCs in the tumor and their detection by neuro-imaging**

If GSCs were considered as a subset of special cells, they should be located somewhere in the tumor and therefore their search *in vivo* could be justified. According to the hypothe‐ sis that they represent a functional status, they should appear in the tumor when and where, as the consequence of the transformation process, tumor cells reach the threshold of stemness. In some experience, NS would be generated from whatever part of GBM [126], whereas in some other experiences [48], different subsets of GSCs arise from regions of GBM with different malignancy potential, showing different tumorigenic potential and genetic abnormalities, even though originating from the same ancestor cell. Since GSCs reside in niches, their distribution in the tumor should follow that of niches which in turn with their microenvironments develop where malignancy occurs [52]. On the other hand, it has been observed that GSCs occur in the hypoxic area between the central necrosis and

Until today, the only mean to detect GSCs is to apply the neurosphere assay to the surgical samples removed from different parts of the tumor. Their recognition can be therefore achieved only after surgery. It would be highly useful to know in advance where in tumor GSCs are located or generated in order to try to annihilate them without surgery and to cure the patient.

Animal *in vivo* imaging techniques have been applied to some stem cell populations – hema‐ topoietic and leukemic stem cells – but the application to solid tissues has been limited [41]. This work was supported by Grant n. 4011 SD/cv 2011-0438 from Compagnia di San Paolo, Turin, Italy.
