3D Microstructural Architecture of Muscle Attachments in Extant and Fossil Vertebrates Revealed by Synchrotron Microtomography

Abstract

BackgroundFirm attachments binding muscles to skeleton are crucial mechanical components of the vertebrate body. These attachments (entheses) are complex three-dimensional structures, containing distinctive arrangements of cells and fibre systems embedded in the bone, which can be modified during ontogeny. Until recently it has only been possible to obtain 2D surface and thin section images of entheses, leaving their 3D histology largely unstudied except by extrapolation from 2D data. Entheses are frequently preserved in fossil bones, but sectioning is inappropriate for rare or unique fossil material.Methodology/Principal FindingsHere we present the first non-destructive 3D investigation, by propagation phase contrast synchrotron microtomography (PPC-SRµCT), of enthesis histology in extant and fossil vertebrates. We are able to identify entheses in the humerus of the salamander Desmognathus from the organization of bone-cell lacunae and extrinsic fibres. Statistical analysis of the lacunae differentiates types of attachments, and the orientation of the fibres, reflect the approximate alignment of the muscle. Similar histological structures, including ontogenetically related pattern changes, are perfectly preserved in two 380 million year old fossil vertebrates, the placoderm Compagopiscis croucheri and the sarcopterygian fish Eusthenopteron foordi.Conclusions/SignificanceWe are able to determine the position of entheses in fossil vertebrates, the approximate orientation of the attached muscles, and aspects of their ontogenetic histories, from PPC-SRµCT data. Sub-micron microtomography thus provides a powerful tool for studying the structure, development, evolution and palaeobiology of muscle attachments.

New Insights in the Ontogeny and Taphonomy of the Devonian Acanthodian Triazeugacanthus affinis From the Miguasha Fossil-Lagerstätte, Eastern Canada

“…Despite the excellent state of preservation of bones at the microstructural [49][50][51], histological (Figure 1h,i) and anatomical levels, our results indicate a pervasive recrystallization of bioapatite in Triazeugacanthus skeletal structures. Modification of the mineral composition of fossil bones (e.g., presence of authigenic calcite) from the Escuminac Formation has been previously reported in the placoderm Bothriolepis canadensis and the osteolepiform Eusthenopteron foordi [36].…”

“…However, such type of replacement modifies the structure of the biological material at a micrometric scale (e.g., presence of carbonate-fluorapatite infillings discordant with the biogenic Retzius striae in mammalian tooth enamel [28]). In the Miguasha Fossil-Lagerstätte, the preservation of the bone microstructures [49][50][51]64] could suggest a more subtle transformation mechanism, consistent with an early diagenetic stage, even though the formation of the "francolite"-type environment of substituted carbonates implies a transformation of apatite down to the atomic scale. Therefore, the respective contributions of early transformation and potential slow modifications [65] in the sedimentological environment of the Escuminac Formation have still to be determined.…”

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins‐to‐limbs transition

“…For example, the study provides an anatomical 'map' for identifying potential muscle attachment sites in fossils that do not leave osteological correlates using methods such as microhistology (e.g. Sanchez et al, 2013). The sequence of muscle differentiation proposed here can be tested by the results of future studies, as well as fossil discoveries that display transitional morphologies.…”

Origin of the unique ventilatory apparatus of turtles

“…2) used by modern turtles to ventilate their lungs was present in E. africanus, we compared its dorsal rib histology based on 3 individuals (eight ribs in total) to that of 47 extinct and extant taxa representing all major amniote clades (Supplementary Data 1). Sharpey's fibres (ShFs), a component of the tendinous insertion of muscle into bone, can indicate the presence/absence of muscular attachments in extinct taxa 4,[24][25][26] . All of the non-chelonian extant taxa examined (N ¼ 36) possess intercostal muscles, and all of them exhibited ShF on both the cranial and caudal sides of the ribs (Fig.…”