Is the Enamel Knot Present in Reptiles?
|Year of publication||2019|
|MU Faculty or unit|
|Description||In mammals, odontogenesis is regulated by transient signaling centers known as enamel knots (EKs). However, the developmental mechanisms contributing to the formation of superficial tooth structures in reptiles are not fully understood. The aim of our study is therefore to elucidate whether signaling centers similar to enamel knots appear also during reptilian tooth development. Here, we have selected the veiled chameleon as a model species with several unique features including: heterogeneous tooth shape along the jaw, a monophyodont dentition and teeth that are firmly ankylosed to the underlying bone. The crown of the chameleon tooth possesses one central and two lateral cusps with the central cusp further split into two ridges. In early embryos, a morphological structure resembling the mammalian enamel knot was observed which expressed SHH. This structure housed large numbers of apoptotic bodies and no PCNA-positive cells, similar to the mammalian EK. Based on these findings, we propose that this structure may act as an organizer of tooth formation in the chameleon tooth. Later in development, a deep ridge appears at the tip of the developing central cusp. An associated cluster of cells was observed forming in the center of the developing enamel organ, resembling a mammalian secondary enamel knot. 3D-analysis of the shape of the nuclei in this cluster of cells revealed a distinct arrangement of the cells in this region, associated with ultrastructural changes that would lead to asymmetrical enamel deposition. We suggest that such asymmetric deposition would then initiate the formation of two enamel ridges separated by a groove with low production of enamel, thereby creating the final ridge pattern. Whether there is a link between the early cluster of SHH-cells and the later cells contributing to the ridge is recently under investigation.|