Evolution of the muscular system in tetrapod limbs

Reconstructing dinosaur locomotion

Dinosaur locomotor biomechanics are of major interest. Locomotion of an animal affects many, if not most, aspects of life reconstruction, including behaviour, performance, ecology and appearance. Yet locomotion is one aspect of non-avian dinosaurs that we cannot directly observe. To shed light on how dinosaurs moved, we must draw from multiple sources of evidence. Extant taxa provide the basic principles of locomotion, bracket soft-tissue reconstructions and provide validation data for methods and hypotheses applied to dinosaurs. The skeletal evidence itself can be used directly to reconstruct posture, range of motion and mass (segment and whole-body). Building on skeletal reconstructions, musculoskeletal models inform muscle function and form the basis of simulations to test hypotheses of locomotor performance. Finally, fossilized footprints are our only direct record of motion and can provide important snapshots of extinct animals, shedding light on speed, gait and posture. Building confident reconstructions of dinosaur locomotion requires evidence from all four sources of information. This review explores recent work in these areas, with a methodological focus.

Spatial Transcriptomics Analysis: Maternal Obesity Impairs Myogenic Cell Migration and Differentiation during Embryonic Limb Development

Limb muscle is responsible for physical activities and myogenic cell migration during embryogenesis is indispensable for limb muscle formation. Maternal obesity (MO) impairs prenatal skeletal muscle development, but the effects of MO on myogenic cell migration remain to be examined. C57BL/6 mice embryos were collected at E13.5. The GeoMx DSP platform was used to customize five regions along myogenic cell migration routes (myotome, dorsal/ventral limb, limb stroma, limb tip), and data were analyzed by GeomxTools 3.6.0. A total of 2224 genes were down-regulated in the MO group. The GO enrichment analysis showed that MO inhibited migration-related biological processes. The signaling pathways guiding myogenic migration such as hepatocyte growth factor signaling, fibroblast growth factor signaling, Wnt signaling and GTPase signaling were down-regulated in the MO E13.5 limb tip. Correspondingly, the expression levels of genes involved in myogenic cell migration, such as Pax3, Gab1, Pxn, Tln2 and Arpc, were decreased in the MO group, especially in the dorsal and ventral sides of the limb. Additionally, myogenic differentiation-related genes were down-regulated in the MO limb. MO impedes myogenic cell migration and differentiation in the embryonic limb, providing an explanation for the impairment of fetal muscle development and offspring muscle function due to MO.

An arch worth revisiting: a study on the feline humeral supracondylar foramen and its evolutionary significance

The supracondylar foramen with a (seemingly) osseous peripheral arch noticed on the medio-distal feline humeri had remained disputed among anatomists. Some scholars have argued in favor of homology between this foramen and the supracondyloid foramen formed in presence of the ligament of Struthers in humans. Other theories include its presence as a retinaculum holding the median nerve and brachial artery to their anatomical position in a flexed elbow. Unfortunately, these theories lack investigative rigor. The emergence of non-invasive imaging modalities, such as micro-computed tomography, has enabled researchers to inspect the internal anatomy of bones without dismantling them. Thus, a micro-computed tomographic investigation was conducted on three feline (Felis catus) humeri specimens while the internal anatomy of the supracondylar foramina was examined. Unlike the humerus, the thin peripheral arch of the feline supracondylar foramen failed to elicit any osseous trabeculae or foci of calcification. While adhering to the humeral periosteum at its origin, the non-osseous arch, typical of a muscular tendon, attaches into the bony saddle related to the medial humeral epicondyle suggestive of a tendon or aponeurotic extension of a (vestigial) brachial muscle, with the coracobrachialis longus emerging to be the most likely candidate.

Ca2+ as a coordinator of skeletal muscle differentiation, fusion and contraction

Muscle regeneration is essential for vertebrate muscle homeostasis and recovery after injury. During regeneration, muscle stem cells differentiate into myocytes, which then fuse with pre-existing muscle fibres. Hence, differentiation, fusion and contraction must be tightly regulated during regeneration to avoid the disastrous consequences of premature fusion of myocytes to actively contracting fibres. Cytosolic calcium (Ca²⁺ ), which is coupled to both induction of myogenic differentiation and contraction, has more recently been implicated in the regulation of myocyte-to-myotube fusion. In this viewpoint, we propose that Ca²⁺ -mediated coordination of differentiation, fusion and contraction is a feature selected in the amniotes to facilitate muscle regeneration.

Ontogenetic development of the shoulder joint muscles in frogs (Amphibia: Anura) assessed by digital dissection with implications for interspecific muscle homologies and nomenclature

Previous myological studies show inconsistencies with regard to the identification and naming of the shoulder joint muscles in frogs and toads (Amphibia: Anura). Those inconsistencies were revealed and resolved by assessing the ontogenetic development, innervation, and adult morphology of selected anuran species representing ancient lineages and two major neobatrachian groups. To do so, digital dissections of volumes acquired by histological serial sectioning, episcopic microtomy, and contrast-enhanced micro-computed tomography scanning were performed and three-dimensional reconstructions were derived. Muscle units crossing the shoulder joint were defined, their ontogenetic development was described, their homology across species was established, and a consistent nomenclature was suggested. The mm. anconaeus, dorsalis scapulae, latissimus dorsi, and the group of scapulohumeralis muscles were ontogenetically derived from the dorsal pre-muscle mass present in all tetrapods. The ventral pre-muscle mass gave rise to the mm. cleidohumeralis, episternohumeralis, supracoracoideus, coracoradialis, subcoracoscapularis, coracobrachialis, and pectoralis. The results indicate that the mm. anconaeus, dorsalis scapulae, latissimus dorsi, coracoradialis, and the portionis sternalis and abdominalis of the m. pectoralis have consistently been recognized and denoted in previous studies, whereas the names for the muscle units commonly denoted as m. coraco-brachialis longus and as parts of the m. deltoideus are misleading with regard to the ontogenetic origin of these muscles. The mm. scapulohumeralis profundus anterior and posterior, although present, have been overlooked in some studies. The mm. cleidohumeralis, supracoracoideus, and coracobrachialis are present with two parts or portions in some species, these portions have previously not always been recognized and assigned correctly.

The zebrafish HGF receptor met controls migration of myogenic progenitor cells in appendicular development

The hepatocyte growth factor receptor C-met plays an important role in cellular migration, which is crucial for many developmental processes as well as for cancer cell metastasis. C-met has been linked to the development of mammalian appendicular muscle, which are derived from migrating muscle progenitor cells (MMPs) from within the somite. Mammalian limbs are homologous to the teleost pectoral and pelvic fins. In this study we used Crispr/Cas9 to mutate the zebrafish met gene and found that the MMP derived musculature of the paired appendages was severely affected. The mutation resulted in a reduced muscle fibre number, in particular in the pectoral abductor, and in a disturbed pectoral fin function. Other MMP derived muscles, such as the sternohyoid muscle and posterior hypaxial muscle were also affected in met mutants. This indicates that the role of met in MMP function and appendicular myogenesis is conserved within vertebrates.