Abstract
Sesamoid identity has long been the focus of debate, and how they are linked to other elements of the skeleton has often been considered relevant to their definition. A driving hypothesis of our work was that sesamoids’ nature relies deeply on their connections, and thus we propose an explicit network framework to investigate this subject in Leptodactylus latinasus (Anura: Leptodactylidae). Through the dissection of L. latinasus’ skeleton, we modeled its anatomical network where skeletal elements were considered nodes while joints, muscles, tendons, and aponeurosis were considered links. The skeletal elements were categorized into canonical skeletal pieces, embedded sesamoids, and glide sesamoids. We inquired about the general network characterization and we have explored further into sesamoid connectivity behavior. We found that the network is structured in a modular hierarchical organization, with five modules on the first level and two modules on the second one. The modules reflect a functional, rather than a topological proximity clustering of the skeleton. The 25 sesamoid pieces are members of four of the first-level modules. Node parameters (centrality indicators) showed that: (i) sesamoids are, in general terms, peripheral elements of the skeleton, loosely connected to the canonical bone structures; (ii) embedded sesamoids are not significantly distinguishable from canonical skeletal elements; and (iii) glide sesamoids exhibit the lowest centrality values and strongly differ from both canonical skeletal elements and embedded sesamoids. The loose connectivity pattern of sesamoids, especially glides, could be related to their evolvability, which in turn seems to be reflected in their morphological variation and facultative expression. Based on the connectivity differences among skeletal categories found in our study, an open question remains: can embedded and glide sesamoids be defined under the same criteria? This study presents a new approach to the study of sesamoid identity and to the knowledge of their morphological evolution.
Highlights
Sesamoids have been intriguing scientists since the beginning of the last century (Abdala et al, 2019)
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
We presented a new approach to the study of sesamoid identity and we hope to contribute to the current research on their morphological evolution
Summary
Sesamoids have been intriguing scientists since the beginning of the last century (Abdala et al, 2019). 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). Beyond those definitions, Jerez, Mangione & Abdala (2010) categorized sesamoids into four types, 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)
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