Apoptosis in the developing tooth: association with an embryonic signaling center and suppression by EGF and FGF-4.

Apoptosis was localized in developing mouse teeth from initiation of morphogenesis to completion of cusp formation by using modified TUNEL method for serial sections and Nile Blue staining for whole mounts. Apoptosis was first detected at bud stage (E12-E13) in the central cells of the invaginating dental epithelium suggesting involvement of cell death in epithelial budding morphogenesis. During cusp development, apoptotic cells were located in the enamel knots, which are transient clusters of dental epithelial cells proposed to act as signaling centers directing the morphogenesis of tooth cusps. Apoptosis was also detected in other restricted epithelial cell populations including the dental lamina, ameloblasts, as well as stratum intermedium and stellate reticulum cells suggesting that the removal of these epithelial cells occurs by apoptosis. Apoptotic cells, presumably osteoclasts, were also located on the surfaces of the developing alveolar bone. When dissected E13 dental epithelium or mesenchyme were cultured in isolation, apoptotic cells were abundant throughout the tissues, whereas when cultured together, apoptosis was inhibited in both tissues close to their interface indicating that epithelial-mesenchymal tissue interactions prevent apoptosis. Epidermal growth factor (EGF) and fibroblast growth factor-4 (FGF-4) inhibited apoptosis in the dental mesenchyme when applied locally using agarose or heparin-coated acrylic beads, suggesting involvement of these or related growth factors in the prevention of apoptosis in dental tissues in vivo. The spatially and temporally restricted distribution patterns of apoptotic cells suggest multiple roles for programmed cell death in dental development. Of particular interest is the removal of the enamel knots by apoptosis which may terminate their tasks as regulators of the patterning of the tooth cusps. The apical ectodermal ridge (AER) of the limb bud has similar signaling characteristics as the enamel knot, and it also undergoes apoptosis. Hence, apoptosis may be a general mechanism for the silencing of embryonic signaling centers.