Sesamoids in tetrapods: the origin of new skeletal morphologies

Abstract

Along with supernumerary bones, sesamoids, defined as any organizedintratendinous/intraligamentous structure, including those composed of fibrocartilage, adjacent to an articulation or joint, have been frequently considered as enigmaticstructures associated with the joints of the skeletal system of vertebrates. This review allows us to propose a dynamic model to account for part of skeletal phenotypic diversity: dDuring evolution, sesamoids can become displaced, attaching to anddetaching from the long bone epiphyses and diaphysis. Epiphyses, apophyses anddetached sesamoids are able to transform into each other, contributing to thephenotypic variability of the tetrapod skeleton. This dynamic model is a newparadigm to delineate the contribution of sesamoids to skeletal diversity. Herein, wefirst present a historical approach to the study of sesamoids, discussing the geneticversus epigenetic theories of their genesis and growth. Second, we construct adynamic model. Third, we present a summary of literature on sesamoids of the main groups of tetrapods, including veterinary and human clinical contributions, which are the best- studied aspects of sesamoids in recent decades. Finally, we discuss the identity of certain structures that have been labelled as sesamoids despite insufficient formal testing of homology. We also propose a new definition to help the identification of sesamoids in general. This review is particularly timely, given the recent increasing interest and research activity into the developmental biology and mechanics of sesamoids. With this updated and integrative discussion, we hope to pave the way to improve the understanding of sesamoid biology and evolution