Pattern formation in the molar dentition of the mouse.

The developing molar tissues of 10 to 16 day mouse embryos have been dissected from the lower jaws and cultured for periods of up to four weeks in the anterior eye chambers of homologous adult hosts. Grafts of 10 and 11 day presumptive molar tissues developed two complete tooth crowns identifiable as M1 and M2. Grafts of 12 day (dental lamina) and later stages produced similar results but in many cases all three molar teeth were developed, in their normal sequence and with normal crown shapes and proportions. From 12 days and probably earlier the complete molar dentition is represented by a discrete group of cells whose later growth and differentiation are already determined. The fact that individual primordia for the second and third molars are not yet present at this stage suggests that neither their appearance nor their normal differentiation in the jaws requires a morphogenetic gradient field, an extrinsic control which has been proposed and is often assumed to exist. The results are consistent with developmental theories which propose that gradations of shape and size in the individual sequentially initiated elements of a series are expressions of intrinsic time-dependent alterations in the growing cell population which forms them.