Cytophotometric measurement of the cellular DNA content of [³H]thymidine-labelled spheroids. Demonstration that some non-labelled cells have S and G₂ DNA content

Spheroids from the V₂79-171b and MCa-11 cells lines were incubated continuously for 24 hr in [³H]thymidine for labelling of the outer cells of the viable rim. The spheroids were dispersed into single cells, and the DNA content of photomapped cells was measured by absorption cytophotometry. Autoradiographs were then prepared from which we ascertained cellular labelling. For spheroids of both cell lines, we found a larger proportion of cells with a G₀/G₁ DNA content among the non-labelled inner spheroid cells than among the labelled outer cells (P < 0.001). This block of non-labelled spheroid cells in G₀/G₁ was not a cell cycle perturbation caused by the isotope for the MCa-11 spheroids. Approximately 8% of non labelled MCa-11 spheroid cells had S/G₂ DNA content, suggesting that non-cycling cells in spheroids may be blocked in S and G₂ as well as in the G₀/G₁ phase of the cell cycle.