The role of the mesenchyme in mouse neural fold elevation. I. Patterns of mesenchymal cell distribution and proliferation in embryos developing in vitro.

Using the computer-assisted method of smoothed spatial averaging, spatial and temporal patterns of cell distribution and mitotic activity were analyzed in the cranial mesenchyme underlying the mesencephalic neural folds of mouse embryos maintained in roller tube culture. Total cell density increased in central and medial mesenchymal regions after 12 hr in culture, decreased after 18 hr, and showed a further decrease after 24 hr when the neural folds of the embryos had elevated, converged, and were fusing or fused. Mitotic activity, as measured by the ratio of 3H-thymidine-labeled cells to unlabeled cells, was highest in the central mesenchyme at all culture times. Embryos were also cultured in the presence of diazo-oxo-norleucine (DON), which inhibits glycosaminoglycan and glycoprotein synthesis. After 24 hr in culture, neural folds of DON-treated embryos had failed to elevate. Total cell density increased in central and medial regions of the mesenchyme of DON-treated folds at 12 hr but showed no significant decrease in these regions with further culture. Mitotic activity was highest in the central mesenchyme of these treated embryos. These results suggest that cell distribution patterns observed in the cranial mesenchyme during neural fold elevation in normal cultured embryos are not produced by regional differences in mitotic activity. Rather, we propose that cell distribution patterns in the central and medial regions of the mesenchyme result from expansion of a glycosaminoglycan-rich extracellular matrix that disperses cells from these regions and decreases their density. In DON-treated embryos, in which expansion of the mesenchyme is prohibited by the decreased glycosaminoglycan and glycoprotein content of the extracellular matrix, mitotic activity apparently determines these patterns.