Tooth–bone attachment tissue is produced by cells with a mixture of odontoblastic and osteoblastic features in reptiles

Teeth are anchored in the jaw in a highly variable manner across vertebrates. In mammals and crocodiles, the teeth are cushioned inside bony sockets by periodontal ligaments, whereas most squamate reptiles have teeth firmly attached to the jawbone. Here, we analyzed the development of the attachment tissue in the veiled chameleon, a species with firm acrodont tooth attachment, to reveal the cellular processes establishing ankylosis and to determine the cell types contributing to the attachment. The tooth-bearing bones formed pedicles with edges fusing to the dentine via an attachment tissue produced by morphologically distinct cells exhibiting both osteoblastic and odontoblastic features. These cells were RUNX2-positive, suggesting their potential to differentiate into hard-tissue-producing cells. However, in contrast to the osteoblasts of the bony pedicles, tooth-bone interface (TBI) cells expressed elevated levels of Na⁺-/K⁺-ATPase and thus resembled odontoblasts. TBI cells were visible only temporarily, and after tooth-bone fusion they were removed by apoptosis and phagocytosis. Dynamic deposition of the hard matrix continued on both sides of the TBI and during the posthatching stages through the participation of osteoblasts. Overall, our findings demonstrate both odontoblast- and osteoblast-like characteristics of cells producing the attachment tissue at the TBI during development in chameleons, highlighting the existence of a transient intermediate cell population, which we call ankyloblasts.