Developmental toxicity of dioxin to mouse embryonic teeth in vitro: arrest of tooth morphogenesis involves stimulation of apoptotic program in the dental epithelium.

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can arrest molar tooth development in rats after in utero and lactational exposure, and that the sensitive stage is temporally restricted. To define the stage in which TCDD is able to arrest tooth development and the cellular background of the effect, mouse embryonic molar tooth explants including various early developmental stages from initiation to late cap stage were exposed to TCDD in organ culture. TCDD did not inhibit morphogenesis of the first molar teeth including the early bud-staged E12 first molars, but the teeth were smaller than in control cultures. Accordingly, the second molars underwent morphogenesis in the presence of TCDD when explanted at E15 when they were at the bud stage. TCDD arrested their development when explanted at E14 when they had not yet reached the early bud stage. Immunohistochemical localization of incorporated bromodeoxyuridine in cultured E14 teeth showed that TCDD did not affect cell proliferation. Localization of apoptosis by terminal deoxynucleotidyl transferase (TdT)-mediated nick end labeling (TUNEL) method revealed that TCDD enhanced apoptosis of dental epithelial cells, especially in the dental lamina of both the first and second molars, and in the inner dental epithelium at the cusp tips of the first molars. Thus, TCDD can arrest tooth development in vitro if the exposure starts at the initiation stage, whereas exposure at later stages leads to smaller tooth size and deformation of cuspal morphology. TCDD interferes with tooth development by stimulating apoptosis in those cells of the dental epithelium, which are predetermined to undergo apoptosis during normal development.