Several approaches have been adopted to study cancer stem cells from several tissues including solid tumors. To study cancer
stem cells, there are some basic concepts of stem cells biology that researchers have to considerer, for example “selfrenewal
capacity” [1, 2], however, some scientists take this functional characteristic for granted in their experiments obviating
the main feature of stem cells. In this issue, we provided an overview of several topics of cancer stem cells from solid tumors,
and how researchers have been studying them. In general, surface proteins have been used to identify and isolate cancer stem
cells from several tissues. Moreover, it is interesting to discuss the role of CD24 [3], CD44 [4], CD49 [5], Lgr5 [6], CD133 [7]
and others proteins that have been used as common stem cell markers. Most of these cancer stem cell markers have a relevant
influence to maintain the stemness of cancer stem cells, such as ALDH, an enzyme that is involved in aldehydes-derived drug
resistance, nevertheless ALDH also has a role to maintain stemness [8] by an unknown mechanism. So far, cancer stem cells
study requires knowing exactly how these cells work, considering their phenotype and their basic functional roles in tumor
growth. Also, microenvironment and cellular niche influence the cancer stem cells behavior by cell signaling pathways including
cell adhesion molecule interactions such as CD49, an integrin that interacts with tumor microenvironment ligands promoting
cell moving, migration and also metastasis [9].
Likewise, the role of sex hormones in Hormone-regulated tumors, like breast cancer has been poorly studied and the results
are still controversial, due to in part, by the difference in sex hormone concentration tested, estrogen and progesterone receptor
expression, as well as the experimental models that have been used [10]. Further, some authors reviewed the relevant function
of cancer stem cells in metastatic disease. Finally, a targeted approach is also used in cancer stem cell field by pointing out the
self-renewal and drug resistance mechanisms to eliminate these high tumorigenic cells [11]. In summary, this thematic issue
can give to the reader a critic overview about cancer stem cells in some solid tumors.