A reliable parameter to standardize the scoring of stem cell spheres

Sphere formation assay is widely used in selection and enrichment of normal stem cells or cancer stem cells (CSCs), also known as tumor initiating cells (TICs), based on their ability to grow in serum-free suspension culture for clonal proliferation. However, there is no standardized parameter to accurately score the spheres, which should be reflected by both the number and size of the spheres. Here we define a novel parameter, designated as Standardized Sphere Score (SSS), which is expressed by the total volume of selected spheres divided by the number of cells initially plated. SSS was validated in quantification of both tumor spheres from cancer cell lines and embryonic bodies (EB) from mouse embryonic stem cells with high sensitivity and reproducibility.