213also been used for the identification and characterization of CSCs. The orosphere
culture system is also efficient in separating CSCs from oral cancer cell lines or
tumors. Finally, the gold standard quantitative xenotransplantation assay that assess
the in vivo tumorigenicity and self-renewing potential of putative CSCs is finally
performed for functional characterization of isolated CSCs (Lin et al. 2011 ).
11.4 Tumor Microenvironment
Now-a-days tumors are considered as an organ with distinct tumor vasculature in
which the cancer cells are accompanied by a protumor microenvironment (Egeblad
et al. 2010 ). Rudolf Virchow was the first person to demonstrate the presence of
leucocytes in tumor tissues and proposed that non-cancerous tissue elements came
from the field of inflammation might affect tumorigenesis (Balkwill and Mantovani
2001 ). TME comprises (1) aberrantly proliferating cancer cells, (2) cancer stem
cells (3) extracellular matrix (ECM) (4) infiltrating immune cells (neurophill, eosin-
ophil, basophil, B and T lymphocytes, mast cells, natural killer cells), antigen pre-
senting cells (APC) (macrophages, dendritic cells) and Tumor associated
macrophages (TAMs) (5) angiogenic endothelial cells and their precursors includ-
ing pericytes and (6) Stromal cells like fibroblast cells, myofibroblasts and cancer
associated fibroblasts (CAFs) (Hanahan and Coussens 2012 ; Friedl and Alexander
2011 ). TME shelters cancer cells and provides protection against various genetic
and epigenetic insults (Hovinga et al. 2010 ; Folkins et al. 2007 ; Tlsty and Coussens
2006 ; Yun 2008 ). Tumor niches are discrete and dynamic domains ensuring precise
functional characteristics that helps in forming a suitable habitat for certain cells
with specific fates. Likely, CSC niche provides a special microhabitat for survival,
maintenance of stemness and post-therapy tumor re-propagation. Moreover, spe-
cific stimuli from the microenvironment prop up the CSCs to maintain its exclusive
properties (Fuchs et al. 2004 ; Xie and Li 2007 ; Morrison and Spradling 2008 ).
Though century ago, it has been established that TME plays a crucial role in the
tumor progression and metastasis, till date it is not clear whether CSCs direct the
TME modulation, generate their own “micro-niches”, or exploit the pre-existing
environment in its favor (Paget 1889 ). However, the existence of a supportive peri-
vascular niche is evidenced in HNSCC and majority of the CSCs are located within
a 100 μm-radius of blood vessels in primary tumors (Krishnamurthy et al. 2010 ).
11.4.1 Extracellular Matrix
Extracellular matrix is a highly complex and dynamic array of interacting proteins
that are being regularly synthesized, processed and assembled during cell homeo-
stasis and adhesion, migration, wound repair and tumor development and progres-
sion. Regardless of once viewed only as an architectural support, the ECM is now
11 Oral Cancer Stem Cells Microenvironment