Study of the thymocyte cell cycle by bivariate analysis of incorporated bromodeoxyuridine and DNA content

The in vivo cell cycle of normal murine thymocytes was studied by bivariate analysis of bromodeoxyuridine and total DNA content in the 24 h following a single injection of the thymidine analogue. Bromodeoxyuridine incorporation was strictly limited to cells in S phase and 98% of S phase cells were labeled, demonstrating high efficiency and specificity. Cell-cycle parameters were determined by measuring the DNA content of bromodeoxyuridine-labeled cells, related to their distribution in the different phases. The changes of this distribution as a function of time reflected the progression of the cells along the cell cycle. The duration of total cycle, S phase, and both G2/M and G1 was 10 h, 6.5 h and 1.5 h, respectively. All thymocytes labeled in S phase entered G2/M, divided and returned to the G0/G1. Seventy percent of them remained in the resting state, and the other 30% re-entered a second S phase. Cell-cycle parameters of isolated CD4-CD8- cells were also determined. No evidence of cell loss during S or G2/M phase was found, suggesting that intrathymic cell death is not directly linked to the proliferative phases of differentiation.