Additionally, the spheroids can be implanted in vivo and used in animal models or within specialized techniques such as the dorsal skin fold window chamber, creating an in vitro/in vivo assay. For decades, it has been postulated that growing tumor cell-only spheroids and implanting them in animals lead to a much higher tumor take with far fewer total cells inoculated. This was hypothesized to be due to the creation of a microenvironment in the spheroid as well as possibly engaging or enriching for cancer stem cell phenotype. Although the optimal conditions vary when co-culturing different cell types with EC, when using tumor-EC spheroids, as few as two spheroids per implantation are sufficient to generate tumor growth and metastasis [164]. This approach may be valuable in studying various treatments or aspects of tumor progression thought to be dependent on vascularization and/or stimulation of metastasis or aggressiveness via tumor cells engaging EC.