Analysis of cellular composition of co-culture spheroids.

3D spheroids and in particular co-culture spheroids reflect the natural organization of cells in tissues much better than 2D cell cultures as indicated by differences in cellular phyisology. However, most methods to analyze cells were established for 2D cultures and cannot easily be applied to spheroids. This study has aimed to demonstrate the possibility of quantification of the cellular composition of co-culture spheroids without previous dissociation into single cells. Prior to the generation of the spheroids, human endothelial cells, osteoblasts and fibroblasts were stained with fluoresent dyes for living cells. Co-culture spheroids of defined stoichiometric compositions were generated by the liquid overlay technique, cultivated for one, three or six days, respectively, and afterwards snap-frozen in liquid nitrogen. Cryo-sections of co-culture spheroids were analyzed by fluorescence microscopy and a newly established semi-automatic measuring routine. In order to compare the results, spheroids of one group were dissociated and the cellular composition was quantified by FACS-analysis. Staining efficiencies were higher than 95% as quantified in preliminary experiments with 2D cultures. Depending on the staining procedure, variations from uniform to punctate signals were detected. The size of all co-culture spheroids decreased over time and snap-freezing did not lead to shrinkage of the spheroids. We were able to detect organizational patterns of different cell types within the spheroids. It was possible to determine the cellular composition by quantitative microscopic analyses of cryo-sections as it could be confirmed by flow cytometric analyses. Depending on the experimental requirements, a combination of both methods might lead to valuable synergy.