Sharks Have Bone and Other Runx2-dependent Skeletal Features

Introduction: Many inherited or trauma-induced conditions, such as cleidocranial dysplasia or non-union fractures, result from inadequate bone formation [1]. Perhaps providing insight into such clinical maladies, bone formation was lost during evolution. Chondrichthyes, fish whose internal skeleton is entirely cartilaginous, evolved from an ancestral vertebrate population that possessed bone. What happened to the ability to make bone during Chondrichthyan evolution? Perhaps there were alterations to the genetic system underlying bone formation in these fish. Recent studies in mice have identified a transcription factor, Runx2 (CBFA1), which plays multiple roles during skeletal tissue formation. Using genetic techniques, mice were created that lacked the function of Runx2, and they did not develop bone anywhere in their bodies [2]. Interestingly, these mice also had several other skeletal defects, including absence of tooth formation, and lack of chondrocyte hypertrophy and mineralization [3, 4]. If sharks were defective in this gene or in the same genetic pathway, then their skeletons would appear similar to Runx2(-/-) mice. We tested the hypothesis that members of the Chondrichthyan lineage phenocopy mice lacking functional Runx2. Histological, histochemical, and immunohistochemical analyses of the entire embryonic skeleton of the swell shark, Cephaloscyllium ventriosum, refuted our hypothesis, however. Here, we describe the presence in shark of all skeletal attributes known to be under the control of Runx2 in mice. Most strikingly, we found abundant bone formation in the shark vertebrae. In addition, teeth formed and many cartilages displayed evidence of chondrocyte hypertrophy and mineralization.