Ameloblast apoptosis and IGF-1 receptor expression in the continuously erupting rat incisor model

Enamel-producing cells (ameloblasts) pass through several phenotypic and fu nctional stages during enamel formation. In the transition between secretor y and maturation stages, about one quarter of the ameloblasts suddenly unde rgo apoptosis. We have studied this phenomenon using the continuously erupt ing rat incisor model. A special feature of this model is that all stages o f ameloblast differentiation are presented within a single longitudinal sec tion of the developing tooth. This permits investigation of the temporal se quence of gene and growth factor receptor expression during ameloblast diff erentiation and apoptosis. We describe the light and electron microscopic m orphology of ameloblast apoptosis and the pattern of insulin-like growth fa ctor-1 receptor expression by ameloblasts in the continuously erupting rat incisor model. In the developing rat incisor, ameloblast apoptosis is assoc iated with downregulated expression of the insulin-like growth factor-1 rec eptor. These data are consistent with the hypothesis that ameloblasts are " hard wired" for apoptosis and that insulin-like growth factor-1 receptor ex pression is required to block the default apoptotic pathway. Possible mecha nisms of insulin-like growth factor-1 inhibition of ameloblast apoptosis ar e presented. The rat incisor model may be useful in studies of physiologica l apoptosis as it presents apoptosis in a predictable pattern in adult tiss ues.