Journal of Anatomy最新文献

Cover image: Face and leading edges of the pinnae of the Tadarida brasiliensis specimen used in the study by R. Carter, ‘A computational model of pinna tubercle aerodynamics in the fast-flying bat, Tadarida brasiliensis’, this issue.

Cartilage thickness in mammalian joints increases with higher body masses. Contradicting previous studies found this increase to be positive allometric or negative allometric. Since approaches like computational modelling of animal locomotion, muscle moment arms, and joint dynamics rely on estimates of joint spacing (JS), it is important to accurately estimate an animal's cartilage thickness based on body mass. Here, we measured ex vivo internal joint distances (IJDs) or bone-to-bone distances on CT scans of fresh cadaveric forelimbs in a sample of small- to medium-sized mammals. IJDs were measured in the humero-ulnar and humero-radial joint. We find IJDs to scale isometrically in both joints across the entire sample, with positive allometric tendencies only within a subsample of cursorial species and only in the humero-ulnar articulation. The previously reported positive allometry can be linked to a cursorial sampling bias, and negative allometry results from a size constraint, acting on larger mammals than sampled here. Additionally, the IJDs were not affected by limb poses (i.e., flexed to extended, supinated, pronated). In rats and guinea pigs of varying sizes, we observed intraspecific isometric scaling with slight positive trends. This suggests that theoretically greater absolute forces-resulting from increased body mass with similar posture-only marginally contribute to relatively thicker cartilage in small- to medium-sized mammals. Further, we conducted a "range of motion" (ROM) analysis in the humero-ulnar joint of rats, guinea pigs, and maras, thus species of increasing body mass and level of cursoriality. ROM was assessed with varying estimated JS. Differences in the results are most biologically reasonable when allowing all six degrees of freedom (DOF, three rotational and three translational). Mobility decreases with increasing body mass and level of cursoriality, facilitated by increased restriction of movement to a single axis of flexion and extension. Such a trend persisted regardless of whether JS thresholds were estimated using intraspecific or interspecific regression models. The results suggest that variations in elbow mobility are less influenced by applied JS than by the morphological characteristics of the bones forming the joint. This observation has implications for future comparative studies of mammalian elbow function. Still, more research is needed to separate body mass, degree of cursoriality, or locomotor type as factors for elbow mobility.

We propose for the first time a direct comparison between brain and endocast characteristics-the number, position, length and proportion of sulci as revealed by the evidence on the brain and their corresponding marks on the endocast that is the internal surface of the skull-in the same living individuals. Using a tailored MRI imaging methodology developed to overcome the limitations inherent to medical imaging on healthy subjects, we compare 3D models of the brain and of the endocast obtained from the same cohort of 75 volunteers. These data were quantified in terms of observation frequency, proportion of dimension for the same structures on the endocast compared to the brain and variation among the analysed sample. We show that identifiable marks on the endocast are often short, discontinuous, and primarily located in the lower regions (particularly in the inferior frontal and temporal lobes areas). This observation contradicts previous practices of drawing long, straight marks on fossil endocasts. An unexpected result of the study is the discovery of endocranial marks unrelated to cerebral sulci, called MNAS (for endocranial Marks Not Associated with Sulci). These marks represent approximately 12% of the depressions observed and pose a challenge for the interpretation of fossil endocasts. Finally, we propose with this work a new standardised approach to studying fossil endocasts, which combines precise guidelines for sulci variation description as well as for inter-individual comparisons, and cross-validation by several researchers. The development of a new approach based on a large quantity of anatomical information presented in the present study, offers new perspectives for reconstructing the anatomy of fossil hominin's brain in a more objective framework, and will have a lasting impact for the study of the evolution of the hominin brain, particularly functional lateralisation and cognitive abilities.

Palaeoanthropology is highly dependent on anatomical knowledge. Anatomical descriptions and comparisons have long formed the basis of the discipline, allowing researchers to draw inferences about our ancestors' behaviour, biology, and locomotion, as well as to establish phylogenetic relationships between extinct hominins and define taxa within the fossil record. The integration of virtual anatomy methods, relying on high-resolution imaging, 3D modelling, and computational simulations, has revolutionised the field, enabling palaeoanthropologists to quantify anatomical variation, simulate biological processes, and reconstruct morphology in fossil material with unprecedented precision. In a field limited not only by the sole preservation of hard mineralised tissues such as bone and teeth, but also by their fragmentary condition and frequent geological or taphonomic distortion, the introduction of methods that allow analytical procedures to be shared, repeated, and performed non-destructively has greatly enhanced anatomical studies of extinct hominins. Moreover, despite the lack of direct evidence for soft tissues, the adaptive properties of bone, shaped by the muscles, tendons, and organs surrounding it and the forces they exert, now allow palaeoanthropologists to reconstruct muscle attachment sites, brain morphology, and other soft tissue-related structures. Researchers can also simulate bone responses to activities such as bipedal locomotion or tool use through static and dynamic modelling that enables them to virtually construct 'what-if' scenarios, effectively bringing to life some of the most iconic specimens in human evolution. Despite these advances, the field still has a long way to go. The recent introduction of AI algorithms to automate specific preprocessing phases (e.g., segmentation), alongside the ongoing need for methodological integration and broader access to technological resources, highlights the importance of developing a more open, replicable, and globally accessible toolkit for palaeoanthropologists worldwide.

Little is known on the keratinisation and cornification in the alligator tongue. This process has been studied during late and pre-hatching embryonic stages, using histochemical, immunolabelling and electron microscopy. The study revealed that PAS-positive, Alcian blue and Blue Nile Sulphate reactive glycoprotein/glycolipids are produced and accumulated in the corneous layer of the lingual epithelium before hatching. Also, Intermediate Filament Keratins (IFKs) but not Corneous Beta Proteins (CBPs) are synthesised in the tongue epithelium. Alligator IFKs have variable composition and cysteine content, 0.1%-1.1%, and few contain 2.5%-4.9% cysteine, and participate in the formation of a hard-corneous layer. These keratins also contain numerous hydrophobic amino acids, in particular leucine and valine, suggesting that they are partially hydrophobic. The predicted cross-reactivity with the IFK-antibodies here employed suggests that these antibodies can recognise some alligator IFKs. Light and transmission electron microscopy show that the stratified corneous layer is formed by narrow alpha-corneocytes storing peripheral corneous material and a central electron-pale and likely glycolipid core. Numerous membrane coating granules 0.1-0.2 μm in diameter and reactive to silver-methenamine reaction for glycoproteins are accumulated and merge with keratin bundles along the plasma membrane or are extruded among pre-corneous keratinocytes. Corneocytes of 0.1-0.2 μm in thickness pile up on embryonic tongue before hatching, forming the stratum corneum, but they do not accumulate detectable CBPs, the prevalent proteins instead present in alligator scales and claws. Whether CBPs are later produced in juveniles and adult alligator tongue to mechanically strengthen the lingual epithelium is unknown but it is hypothesised that the presence of cysteine-rich IFKs may contribute to the hardness of the tongue corneous layer.

Conventional embryological models propose that the urorectal septum (URS) fuses with the cloacal membrane (CM) to form separate exits for the hindgut and urogenital sinus (UGS). However, previous studies have been re-evaluating this assumption, and the precise morphogenetic mechanisms determining the position of the anal opening remain unclear. To elucidate the spatial and temporal dynamics of cloacal development, we analysed mouse embryos from embryonic day (E)11.5 to E13.5 using serial midsagittal histological sections combined with three-dimensional reconstruction. We focused on the positional relationship between the URS and CM, as well as the internal remodelling events of the cloaca that lead to anal opening formation. Throughout development, the URS enlarged and shifted distally, but consistently remained dorsodistal to the CM without direct fusion. At E13.0, we identified an expanded space at the caudal end of the hindgut, distinct from the hindgut lumen. By E13.25, this space connected to the UGS via a duct-like structure, contributing to the separation of the UGS and hindgut. By E13.5, the CM ruptured and the anal opening emerged precisely at the junction between the hindgut lumen and the expanded space. Our findings demonstrate that the position of the anal opening is predetermined by cloacal internal space remodelling rather than fusion of the URS and CM. This study offers novel insights into normal anorectal development and the aetiology of congenital anorectal malformations.

Salamander evolution featured multiple transitions between water and land that promoted distinct adaptations in limb bones for buoyancy control versus increased load-bearing capacities, respectively. Many extant species spend their entire lives either in water or on land, while others undergo water-land transitions within their lifetime. However, exposure to both environments may impose competing demands that restrict adaptive evolution for a particular habitat. Using a 3D morphological dataset of 133 species spanning the phylogenetic and ecological breadth of salamanders, we find that the external and internal morphology of limb bones is evolutionarily decoupled, which increases the evolvability of limb bones in response to diverse mechanical demands. Terrestrial salamanders have stiffer bones with greater resistance to fracture, while aquatic species have denser bones that are hypothesized to aid in buoyancy regulation. We uncover a functional trade-off between stiffness and density that promotes stiff yet lightweight bones in terrestrial lineages. Released from terrestrial constraints, aquatic paedomorphs have disparate external morphologies, whereas terrestrial direct developers consistently share a rod-like bone shape. Aquatic and terrestrial multiphasic taxa show less morphological divergence than monophasic species living in comparable habitats but are not constrained by their complex life cycle. Multiphasic species have distinct external limb bone shapes that have evolved as fast or faster than monophasic species. Taken together, we propose that the trade-offs imposed by different habitats and complex life cycles increase limb bone diversity by promoting alternate evolutionary pathways.

The abdominal musculature is of prime importance for stabilization and flexion movements of the trunk, and of high clinical relevance in hernia surgery. Despite a number of severe congenital malformations involving defective abdominal muscle formation, little is known about abdominal muscle development during embryogenesis. Here, we used the chicken model system to investigate anatomy, morphogenesis, segmental origin, and fiber formation of the abdominal musculature. The specific anatomy of the chicken abdominal musculature was first determined by macroscopic dissections of adult specimens. We then describe the sculpting of the external oblique, internal oblique, transversus, and rectus muscles from a uniform hypaxial muscle blastema on the basis of whole-mount embryos and sections. We show that abdominal muscles arise from somites 24-28, that all abdominal muscles receive muscle cells from multiple somites, and that somites 25 and 26 provide the major source of abdominal muscle cells. We find that the contribution of individual somites to distinct muscle portions is heterogeneous, with a roughly segmental arrangement of muscle fibers in the transversus muscle, and a random mix of fibers of different segmental origin in the rectus muscle. We furthermore show that despite extensive fiber mixing, there is no fusion between fibers of different segments, so that the segmental identity of individual fibers in the abdominal muscle sheets is maintained. We present abdominal muscle formation as a paradigm for the development of large, segment-crossing trunk muscles from segmental myotomal anlagen.

Black bears (Ursus americanus) undergo prolonged inactivity during hibernation without the cortical bone loss typical of other mammals, yet the response of axial bones to this process remains understudied. This study compares midshaft femoral and rib histology in four individuals-three wild and one captive-to assess skeletal responses to hibernation, growth, and stress. Femora exhibit a laminar-plexiform woven-parallel complex (WPC), with parallel-fibered annuli bounded by lines of arrested growth (LAGs). The captive bear displayed an outer circumferential layer (OCL) after the seventh LAG, indicating skeletal maturity. One wild female with mange showed thick annuli and extensive remodeling, suggesting chronic physiological stress. Despite likely not hibernating, the captive bear's femoral histology closely resembled that of wild hibernators, suggesting conserved growth dynamics. Relative Cortical Area was consistently higher in femora than ribs, reflecting greater structural demands. Femoral remodeling was localized around the linea aspera, while ribs were heavily remodeled with enlarged resorption cavities and expanded trabecular networks. These results are consistent with region-specific skeletal responses and possible calcium mobilization during hibernation. Overall, bears appear to employ a region-specific skeletal strategy, maintaining cortical integrity in weight-bearing limbs while remodeling axial elements for calcium mobilization. This dual pattern highlights key insights into mammalian bone plasticity under seasonal metabolic stress.

In most mammals, the astragalus features a single-pulley surface for articulation with the tibia; in contrast, artiodactyls possess pulley structures at both the proximal and distal ends of the astragalus, a condition known as the double-pulley astragalus. This structure has been hypothesized to increase the force exerted during plantarflexion by elongating the lever arm formed by the calcaneus. However, this hypothesis, based on skeletal simulations, has lacked empirical confirmation under ex vivo conditions. To address this, the present study used CT scanning to examine the hindlimbs obtained from necropsies of 15 mammalian species-including artiodactyls, perissodactyls, and a cheetah-and generated three-dimensional (3D) reconstructions of the tarsal joint to measure mobility using the obtained 3D models. Distances between the tuber calcanei and astragalar trochleae, interosseous angles, and ranges of motion (ROMs) were quantified in maximally dorsiflexed and plantarflexed postures. In artiodactyls, the distances between the tuber calcanei and the proximal and distal trochleae of the astragalus varied with joint posture and changed strictly in antiphase, reflecting a shift in the functional locus of rotation within the astragalus. Despite these postural changes, the effective lever length relative to the instantaneous axis of rotation remained nearly constant. In species with a single-trochlea astragalus, including perissodactyls and the cheetah, almost the entire ROM of the tarsal joint was attributable to motion at the crurotarsal joint, and the calcaneus and astragalus were virtually immobile relative to each other. By contrast, artiodactyls exhibited substantial calcaneo-astragalar mobility, with changes in relative orientation reaching up to 90°. Tarsal joint mobility in artiodactyls was distributed between the proximal and distal astragalar joints, with ruminants showing particularly large ROMs at the distal astragalar joint. Clear differences were observed between plesiomorphic and apomorphic taxa: equids and ruminants displayed greater overall tarsal joint mobility than Tapiridae and Suina, respectively. Although Suina did not differ from ruminants in ROM at the proximal astragalar joint, they were markedly restricted in mobility at the distal astragalar joint and in calcaneo-astragalar motion. Llamas, and likely camelids in general, exhibited tarsal joint characteristics closely resembling those of ruminants, a pattern most plausibly interpreted as convergent acquisition of similar joint configurations. These results demonstrate that the double-pulley astragalus is associated with a distinctive pattern of joint partitioning and interosseous mobility within the tarsus, rather than with simple elongation of the calcaneal lever arm, highlighting its anatomical significance in shaping artiodactyl tarsal joint morphology.