Apoptosis-related factors (Fas receptor, Fas ligand, FADD) in early tooth development of the field vole ( Microtus agrestis)

Abstract

Fas (CD95/APO-1) belongs to the TNF receptor (TNFR) family. Fas ligand binding followed by Fas-receptor oligomerisation leads to formation of a death-inducing signal complex starting with recruitment of the Fas-adapter protein (FADD). Components of this initiation complex (Fas, Fas-L, FADD) were correlated with apoptotic cells, detected by specific DNA fragmentation and morphological criteria. Apoptotic cells can be detected throughout the embryonic development of molar teeth. Restricted temporospatial distribution suggests several important roles for apoptosis in tooth morphogenesis. However, the mechanisms employed in dental apoptosis remain unclear. Frontal sections of the field vole at stage 13.5-15.5 of embryonic development were exploited to investigate and correlate location of Fas, Fas-ligand, FADD molecules and apoptosis in developing first molars by immunohistochemistry. During these stages the primary enamel knot appears and is gradually terminated by apoptosis. Initially, apoptotic cells were demonstrated in the most superficial layer of the dental lamina. The number of TUNEL-positive cells expanded from late bud to cap stages. Restricted areas of apoptotic cells were found in the stalk and primary enamel knot. Fas, Fas-L and FADD were co-localised, particularly in the primary enamel knot, and the stalk, correlating with the occurrence of apoptosis in these areas. Fas-L, however, was also found in proliferating parts of the developing tooth germ, such as in the cervical loops. Interestingly, FADD molecules were also observed in areas, where Fas protein was not detected. According to the immunohistochemical data, Fas-mediated signalling may have a triggering or enhancing role in dental apoptosis. This remains to be functionally confirmed.