## 3 Circulating Tumor Cells--Relevance to Skin Cancers
### From CTCs to Other Important Components Obtained via Liquid Biopsy  
As liquid biopsy has been refined, researchers have turned their attention to other components isolated from peripheral blood samples, which might be of interest for the metastatic process (see Table 2). Thereby, another recently described component obtained from liquid biopsy samples are \"tumor educated platelets\" (TEPs). TEPs are thought to play central roles in the systemic and local response to tumor growth. These subtypes of platelets have altered RNA profiles [51]. One distinct feature is the capacity of TEPs to be directly and indirectly modulated by tumor cells and the tumor microenvironment. The main pathway remains direct RNA ingestion, with subsequent queue-specific splice events that confer specific RNA signatures according to external tumoral stimuli [52]. With studies current in their infancy, the promising results obtained in the field of sarcoma diagnosis [53] indicate that the interactions between platelets and skin tumor cells could present an interesting potential for diagnosis and monitor also for MM or NMSC. Exosomes, playing important roles in cell crosstalk [54], are also of interest when considering the detection of tumors and tumor-related alterations. This type of extracellular \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **CTCs** & **TEPs** & **Exosomes** & **ctDNA** & **miRNAsAs** \\ \hline \multirow{4}{*}{**Advantages**} & Morphological & Abundant; & Potential & & \\  & characterization; & Relatively simple & correlation with & & \\  & Functional analysis; & isolation & tumor & & \\  & Possibility of cellular & technique; & drug-resistance & isolation technique; & \\  & RNA analysis; & Possibility of & and metastasis; & Useful in tumor & Stable in \\  & Interest/correlation & & tumor RNA & tumor DNA, RNA & monitoring and & peripheral blood \\  & prediction, prognosis & & analysis & analysis & & \\ \hline \multirow{4}{*}{**Limitations**} & Extreme rarity; & No standardized & No effective & & & \\  & Fragility; & approach; & enrichment & & & \\  & Heterogeneity & Time consuming & & method; & & \\ \cline{1-1}  & & & Heterogeneity & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: CTCs and other relevant particles obtained via liquid biopsy. vesicles has an endocytic origin and a variable size between 30 and 100 nm [55]. Similar to CTCs, exosomes are highly heterogeneous; several detection and isolation procedures mainly based on surface membrane markers from the group of CD molecules (CD63, CD9, and CD81) have been described [56]. Continuous improvements in isolation techniques are essential for a better characterization of exosomes; the now available physical methods, such as density gradient centrifugation and ultracentrifugation, are limited in separating a high purity sample [57]. The roles of exosomes in MM are just beginning to be determined. One important discovery is the presence of melanoma-derived exosomes in MM patients, which promote the growth and metastasis of melanoma. Macrophages are one of the main targets, melanoma-derived exosomes inducing a mixed M1 and M2 polarization, towards a protumor macrophage activation phenotype [58]. Other immune cells are also modulated by melanoma-derived exosomes, with CD8+ T lymphocytes being inhibited mainly by exosomes derived from the B16F0 melanoma cell line [59]. Regarding natural-killer cells that are associated with anti-tumoral immune response, exosomes derived from the NK-92 cell line seem to exert cytotoxic effects against melanoma [60], suggesting that the development of similar nano-vesicles could be a potential therapy direction. One of the smallest compounds obtained via liquid biopsy, but relevant for the molecular profiling of skin cancers are circulating cell-free DNA (ccfDNA) and miRNAs. ccfDNA, although proposed as potential skin cancer biomarkers, do not present the necessary sensitivity and specificity, with only the circulating DNA derived directly from the tumor (ctDNA) being of interest for further analysis.