Coordinated differentiation and morphogenesis in the mammalian tooth. (A) As development progresses, the neural crest-derived mesenchyme of the jaw undergoes stepwise differentiation as it interacts with the jaw epithelium. (B) Concentration of paracrine growth and differentiation factors in the region where morphogenesis and differentiation are occurring in the 14-day embryonic mouse lower molar. The boundary of the tooth epithelium is shown in white. The paracrine factors are being secreted by the enamel knot, a mass of nondividing epithelial cells. (The panel on the left shows that the cells of the enamel knot are not replicating DNA.) Above each in situ hybridization photograph is a serial reconstruction of the area of gene expression. (A after Thesleff et al. 1990 and Thesleff and Sahlberg 1996; B from Jernvall 1995, photographs courtesy of A. Vaahtokari, J. Jernvall, and I. Thesleff.)