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

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Abstract
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 inS phase entered G2/M, divided and returned to the G0/G1. Seventy percent of them remained in the resting state, and the other 30% reentered 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.