EFFECTS OF DEVELOPMENTAL STAGE AND TISSUE-TYPE ON EMBRYO FETAL DNA DISTRIBUTIONS AND 5-FLUOROURACIL-INDUCED CELL-CYCLE PERTURBATIONS

Cell-cycle analysis of nuclei obtained from the circulating erythrobla sts (gestational day [GD] 11-16), livers (GD 14-19), and whole embryos (GD 10-13) or remaining (extrahepatic) tissues (GD 14-16) of rat embr yos/fetuses revealed age- and tissue-dependent variations in the relat ive percentages of cells in the G0/G1, S, and G2/M phases of the cell cycle. With development, the rate of cell proliferation declined resul ting in decreases in the relative percentage of S-phase cells and incr eases in the G0/G1 percentage, while the percentage of G2/M-phase cell s remained relatively constant. Comparing tissue cell-cycle profiles d uring development, erythroblasts exhibited the most rapid age-dependen t decline in S-phase percentage (from 75% at GD 11 to 8% by GD 14), em bryos/extrahepatic tissues exhibited a more gradual reduction (from 55 % at GD 10 to 14% by GD 15), while the hepatic isolates exhibited a re latively constant S-phase percentage of approximately 40% from GD 14 t o GD 18 before decreasing to 23% at GD 19. These age-dependent variati ons suggest that cell-cycle distribution may be useful in staging embr yogenesis and in detecting abnormal development. To determine how thes e developmental and organ-specific cell-cycle variations affect toxic response, we sampled GD 11-13 embryos 6 hr after maternal administrati on of a teratogenic dose of 5-fluorouracil (5-FU), a thymidylate synth etase inhibitor that induces S-phase accumulation. The results indicat e that, on a relative basis, the amount of induced S-phase accumulatio n in erythroblasts and whole embryos 6 hr postdosing increased with de velopment. In contrast, a time course of hepatic cell-cycle distributi ons from GD 14 embryos after maternal dosing revealed that S-phase acc umulations occurred at an earlier time, possibly as a consequence of a higher proliferative rate. These findings suggest that the extent of observed toxicant-induced cell-cycle perturbations depends on gestatio nal age, tissue type, and the time of sampling. (C) 1993 Wiley-Liss, I nc.