An assemblage of distinct cell types constitutes
most solid tumors. Both the parenchyma and stroma of
tumors contain distinct cell types and subtypes that
collectively enable tumor growth and progression.
Notably, the immune inflammatory cells present in tumors
can include both tumor-promoting as well as tumor-killing
subclasses.
(Lower) The distinctive microenvironments of tumors. The
multiple stromal cell types create a succession of tumor
microenvironments that change as tumors invade normal
tissue and thereafter seed and colonize distant tissues.
The abundance, histologic organization, and phenotypic
characteristics of the stromal cell types, as well as of the
extracellular matrix (hatched background), evolve during
progression, thereby enabling primary, invasive, and then
metastatic growth. The surrounding normal cells of the
primary and metastatic sites, shown only schematically,
likely also affect the character of the various neoplastic
microenvironments. (Not shown are the premalignant
stages in tumorigenesis, which also have distinctive
microenvironments that are created by the abundance
and characteristics of the assembled cells.)