Sesamoids in tetrapods: the origin of new skeletal morphologies

Abstract

Along with supernumerary bones, sesamoids, defined as any organized intratendinous/intraligamentous structure, including those composed of fibrocartilage, adjacent to an articulation or joint, have been frequently considered as enigmatic structures 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: during evolution, sesamoids can become displaced, attaching to and detaching from the long bone epiphyses and diaphysis. Epiphyses, apophyses and detached sesamoids are able to transform into each other, contributing to the phenotypic variability of the tetrapod skeleton. This dynamic model is a new paradigm to delineate the contribution of sesamoids to skeletal diversity. Herein, we first present a historical approach to the study of sesamoids, discussing the genetic versus epigenetic theories of their genesis and growth. Second, we construct a dynamic 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.

Peer Review #1 of "Evolution of the patella and patelloid in marsupial mammals (v0.1)"

“…There are interesting broader stakes for evolutionary developmental biology and biomechanics. For example, marsupial patellae are relevant to the concept of "traction epiphyses", that 'new' bones may originate by disconnection from or be 'lost' by fusion to long bones (see reviews in Vickaryous and Olsen, 2007;Abdala et al, 2019). Whether ossified or fibrocartilaginous, the patella of marsupials and other mammals might have originated as a traction epiphysis from the femur (Eyal et al, 2015(Eyal et al, , 2017 or the tibia.…”

Peer Review #1 of "Delimiting the boundaries of sesamoid identities under the network theory framework (v0.2)"

“…Thus, the loose connectivity pattern characterizing sesamoids seems to be a consequence of their delay in ontogeny and phylogeny. The facultative expression of many sesamoids in the phenotype as a response to continuous mechanical stress (i.e., epigenetic influence) (Abdala & Ponssa, 2012;Abdala et al, 2019) could be, at least in part, a consequence of the low burden of these skeletal pieces. Perturbations on sesamoids development is unlikely to be accompanied by systemic consequences for an organism, as could be the case of perturbations on the development of canonical elements.…”

“…The following definitions serve as examples: "Sesamoids are nodules of cartilage or bone formed in tendons or ligaments, especially where a tendon passes over an angulation of the skeleton" (Hall, 2005); "Sesamoids are periarticular skeletal elements, which initially form in juxtaposition to or independently of bones and joints. They are commonly related to tendons and ligaments (...)" (Abdala et al, 2019). Moreover, beyond those definitions, Jerez, Mangione & Abdala (2010) categorized sesamoids into four types, also based on their specific relationship to surrounding tissues: (i) embedded sesamoid (ES) (surrounded in all their surfaces by tendinous tissue); (ii) inter-osseous sesamoid (loosely attached to the closest ligaments); (iii) glide sesamoid (GS) (associated to tendons, but not surrounded by them and not fixed to them); and (iv) supporting sesamoid (serving as muscle attachment areas to the corresponding bones).…”

“…While the pelvic girdle and hindlimbs take on a significant role of propulsion, the pectoral girdle and forelimbs are mostly related to landing (Emerson, 1979;Nauwelaerts & Aerts, 2006;Astley & Roberts, 2014). The skeletal pieces are affected by the high mechanical load of jumping locomotion, which could affect the genesis and development of sesamoids pieces (Abdala, Vera & Ponssa, 2017, Abdala et al, 2019. In this group, sesamoids are present mostly in the joints of the limbs and in the sacral vertebrae (Hoyos, 2003;Ponssa, Goldberg & Abdala, 2010;Abdala et al, 2019).…”

Peer Review #2 of "Evolution of the patella and patelloid in marsupial mammals (v0.1)"

“…One such diverse anatomical feature is the patellar sesamoid ("kneecap" or simply patella). Sesamoids are skeletal elements found in connective tissues near joints (Vickaryous & Olson, 2007;Abdala et al, 2019), that can modify the forces acting across it (e.g. acting as levers; Alexander & Dimery, 1987;Fatima et al, 2019) and protect periarticular structures.…”