Cell cycle analysis of facial mesenchyme in the chick embryo. I. Labelled mitoses and continuous labelling studies.

Cell cycle parameters were analysed in mesenchyme of the maxillary process and the roof of the stomodeum in the chick embryo from stages 19 through 28. The generation times at stages 24-26 were determined by pulse labelling of embryos with [3H] thymidine, followed by labelled mitosis counts and construction and analysis of percent-labelled mitosis curves employing computer-assisted curve-fitting techniques. The median generation time was approximately 10.6 h in the maxillary process, and 16 h in the roof of the stomodeum; corresponding values for mean generation times were approximately 12.0 and 18.2 h, respectively. Median values for transit times of G1, S, and G2 were 2.0, 5.4, and 2.5 h in the maxillary process and 5.2, 6.7, and 2.7 h in the roof of the stomodeum. The distribution of generation times of cells in the roof of the stomodeum, however, appeared to be more heterogeneous than those of cells in the maxillary process. The percentage of cells which continue to cycle rapidly (i.e. the 'growth fraction') was determined by repeated-labelling experiments with [3H]thymidine in chick embryos from stages 19 through 28. Cumulative labelling of mesenchymal cells in both the maxillary process and roof of the stomodeum approached 100% at stage 19 but dropped markedly from stage 19 to 25 declining to approximately 60-70% in the maxillary process, and to 30% in the roof of the stomodeum. The decline in cell proliferation rates for these regions, determined in previous studies with labelling indices, appears to be a result of the removal of cells from rapidly cycling cell populations into subpopulations which are cycling more slowly and possibly into subpopulations which have become quiescent; the difference in growth rates between these regions could be attributed to the time of appearance and the size of these emerging slow cycling or quiescent subpopulations.