Evolution and development of the vertebrate neck

Protein-protein interaction network module changes associated with the vertebrate fin-to-limb transition

Evolutionary phenotypic transitions, such as the fin-to-limb transition in vertebrates, result from modifications in related proteins and their interactions, often in response to changing environment. Identifying these alterations in protein networks is crucial for a more comprehensive understanding of these transitions. However, previous research has not attempted to compare protein-protein interaction (PPI) networks associated with evolutionary transitions, and most experimental studies concentrate on a limited set of proteins. Therefore, the goal of this work was to develop a network-based platform for investigating the fin-to-limb transition using PPI networks. Quality-enhanced protein networks, constructed by integrating PPI networks with anatomy ontology data, were leveraged to compare protein modules for paired fins (pectoral fin and pelvic fin) of fishes (zebrafish) to those of the paired limbs (forelimb and hindlimb) of mammals (mouse). This also included prediction of novel protein candidates and their validation by enrichment and homology analyses. Hub proteins such as shh and bmp4, which are crucial for module stability, were identified, and their changing roles throughout the transition were examined. Proteins with preserved roles during the fin-to-limb transition were more likely to be hub proteins. This study also addressed hypotheses regarding the role of non-preserved proteins associated with the transition.

Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle

The origin of vertebrate paired appendages is one of the most investigated and debated examples of evolutionary novelty1-7. Paired appendages are widely considered as key innovations that enabled new opportunities for controlled swimming and gill ventilation and were prerequisites for the eventual transition from water to land. The past 150 years of debate8-10 has been shaped by two contentious theories4,5: the ventrolateral fin-fold hypothesis9,10 and the archipterygium hypothesis8. The latter proposes that fins and girdles evolved from an ancestral gill arch. Although studies in animal development have revived interest in this idea11-13, it is apparently unsupported by fossil evidence. Here we present palaeontological support for a pharyngeal basis for the vertebrate shoulder girdle. We use computed tomography scanning to reveal details of the braincase of Kolymaspis sibirica14, an Early Devonian placoderm fish from Siberia, that suggests a pharyngeal component of the shoulder. We combine these findings with refreshed comparative anatomy of placoderms and jawless outgroups to place the origin of the shoulder girdle on the sixth branchial arch. These findings provide a novel framework for understanding the origin of the pectoral girdle. Our evidence clarifies the location of the presumptive head-trunk interface in jawless fishes and explains the constraint on branchial arch number in gnathostomes15. The results revive a key aspect of the archipterygium hypothesis and help reconcile it with the ventrolateral fin-fold model.

Cortical Integration of Vestibular and Visual Cues for Navigation, Visual Processing, and Perception

Despite increasing evidence of its involvement in several key functions of the cerebral cortex, the vestibular sense rarely enters our consciousness. Indeed, the extent to which these internal signals are incorporated within cortical sensory representation and how they might be relied upon for sensory-driven decision-making, during, for example, spatial navigation, is yet to be understood. Recent novel experimental approaches in rodents have probed both the physiological and behavioral significance of vestibular signals and indicate that their widespread integration with vision improves both the cortical representation and perceptual accuracy of self-motion and orientation. Here, we summarize these recent findings with a focus on cortical circuits involved in visual perception and spatial navigation and highlight the major remaining knowledge gaps. We suggest that vestibulo-visual integration reflects a process of constant updating regarding the status of self-motion, and access to such information by the cortex is used for sensory perception and predictions that may be implemented for rapid, navigation-related decision-making.

Lateral thinking in syndromic congenital cardiovascular disease

Syndromic birth defects are rare diseases that can present with seemingly pleiotropic comorbidities. Prime examples are rare congenital heart and cardiovascular anomalies that can be accompanied by forelimb defects, kidney disorders and more. Whether such multi-organ defects share a developmental link remains a key question with relevance to the diagnosis, therapeutic intervention and long-term care of affected patients. The heart, endothelial and blood lineages develop together from the lateral plate mesoderm (LPM), which also harbors the progenitor cells for limb connective tissue, kidneys, mesothelia and smooth muscle. This developmental plasticity of the LPM, which founds on multi-lineage progenitor cells and shared transcription factor expression across different descendant lineages, has the potential to explain the seemingly disparate syndromic defects in rare congenital diseases. Combining patient genome-sequencing data with model organism studies has already provided a wealth of insights into complex LPM-associated birth defects, such as heart-hand syndromes. Here, we summarize developmental and known disease-causing mechanisms in early LPM patterning, address how defects in these processes drive multi-organ comorbidities, and outline how several cardiovascular and hematopoietic birth defects with complex comorbidities may be LPM-associated diseases. We also discuss strategies to integrate patient sequencing, data-aggregating resources and model organism studies to mechanistically decode congenital defects, including potentially LPM-associated orphan diseases. Eventually, linking complex congenital phenotypes to a common LPM origin provides a framework to discover developmental mechanisms and to anticipate comorbidities in congenital diseases affecting the cardiovascular system and beyond.

Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte

The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft anatomy. Skeletal transformations can be examined directly by studying fossil stem gnathostomes; however, preservation of soft anatomy is rare. We describe the only known example of a three-dimensionally mineralized heart, thick-walled stomach, and bilobed liver from arthrodire placoderms, stem gnathostomes from the Late Devonian Gogo Formation in Western Australia. The application of synchrotron and neutron microtomography to this material shows evidence of a flat S-shaped heart, which is well separated from the liver and other abdominal organs, and the absence of lungs. Arthrodires thus show the earliest phylogenetic evidence for repositioning of the gnathostome heart associated with the evolution of the complex neck region in jawed vertebrates.

Development and evolution of the vestibular apparatuses of the inner ear

The vertebrate inner ear is a labyrinthine sensory organ responsible for perceiving sound and body motion. While a great deal of research has been invested in understanding the auditory system, a growing body of work has begun to delineate the complex developmental program behind the apparatuses of the inner ear involved with vestibular function. These animal studies have helped identify genes involved in inner ear development and model syndromes known to include vestibular dysfunction, paving the way for generating treatments for people suffering from these disorders. This review will provide an overview of known inner ear anatomy and function and summarize the exciting discoveries behind inner ear development and the evolution of its vestibular apparatuses.

Evolution and medicine - The central role of anatomy

In medicine, there is an increasing number of publications that deal with or at least consider an evolutionary background. In zoology or comparative anatomy, work on evolutionary developments is taking on an ever-greater role in parallel. The pre-clinical (or pre-medical) phase in medical studies would be able to form a bridge between these related and yet so distant subjects but is currently completely evolution-free. This means that there is no consideration of the evolution of the healthy human being as a prerequisite for a systematic study of the evolutionary background in medicine. In this work the view is expressed that anatomy should be given a central, framework-giving and integrating role, which should urgently be actively pursued.

The CXCR4/SDF-1 Axis in the Development of Facial Expression and Non-somitic Neck Muscles

Trunk and head muscles originate from distinct embryonic regions: while the trunk muscles derive from the paraxial mesoderm that becomes segmented into somites, the majority of head muscles develops from the unsegmented cranial paraxial mesoderm. Differences in the molecular control of trunk versus head and neck muscles have been discovered about 25 years ago; interestingly, differences in satellite cell subpopulations were also described more recently. Specifically, the satellite cells of the facial expression muscles share properties with heart muscle. In adult vertebrates, neck muscles span the transition zone between head and trunk. Mastication and facial expression muscles derive from the mesodermal progenitor cells that are located in the first and second branchial arches, respectively. The cucullaris muscle (non-somitic neck muscle) originates from the posterior-most branchial arches. Like other subclasses within the chemokines and chemokine receptors, CXCR4 and SDF-1 play essential roles in the migration of cells within a number of various tissues during development. CXCR4 as receptor together with its ligand SDF-1 have mainly been described to regulate the migration of the trunk muscle progenitor cells. This review first underlines our recent understanding of the development of the facial expression (second arch-derived) muscles, focusing on new insights into the migration event and how this embryonic process is different from the development of mastication (first arch-derived) muscles. Other muscles associated with the head, such as non-somitic neck muscles derived from muscle progenitor cells located in the posterior branchial arches, are also in the focus of this review. Implications on human muscle dystrophies affecting the muscles of face and neck are also discussed.

Is the plantaris muscle the most undefined human skeletal muscle?

The plantaris muscle is located in the posterior aspect of the superficial compartment of the lower leg, running from the lateral condyle of the femur to the calcaneal tuberosity. Classically, it is characterized by a small and fusiform muscle belly, which then changes into a long slender tendon. From the evolutionary point of view, the muscle is considered vestigial. However, it has recently been suspected of being a highly specialized sensory muscle because of its high density of muscle spindles. It has a noticeable tendency to vary in respect of both origin and insertion. Researchers have published many reports on the potential clinical significance of the muscle belly and tendon, including mid-portion Achilles tendinopathy, ‘tennis leg syndrome’, and popliteal artery entrapment syndrome. The right knee joint area was subjected to classical anatomical dissection, during which an atypical plantaris muscle was found and examined in detail. Accurate morphometric measurements were made. The muscle belly was assessed as bifurcated. Morphologically, superior and inferior parts were presented. There was a tendinous connection (named band A) with the iliotibial tract and an additional insertion (named band B) to the semimembranosus tendon. Both bands A and B presented very broad fan-shaped attachments. The human plantaris muscle is of considerable interest and has frequent morphological variations in its proximal part. Its specific characteristics can cause clinical problems and lead to confusion in diagnosis. More studies are needed to define its actual features and functions.

The evolution of the pectoral extrinsic appendicular and infrahyoid musculature in theropods and its functional and behavioral importance

Birds have lost and modified the musculature joining the pectoral girdle to the skull and hyoid, called the pectoral extrinsic appendicular and infrahyoid musculature. These muscles include the levator scapulae, sternomandibularis, sternohyoideus, episternocleidomastoideus, trapezius, and omohyoideus. As non-avian theropod dinosaurs are the closest relatives to birds, it is worth investigating what conditions they may have exhibited to learn when and how these muscles were lost or modified. Using extant phylogenetic bracketing, osteological correlates and non-osteological influences of these muscles are identified and discussed. Compsognathids and basal Maniraptoriformes were found to have been the likeliest transition points of a derived avian condition of losing or modifying these muscles. Increasing needs to control the feather tracts of the neck and shoulder, for insulation, display, or tightening/readjustment of the skin after dynamic neck movements may have been the selective force that drove some of these muscles to be modified into dermo-osseous muscles. The loss and modification of shoulder protractors created a more immobile girdle that would later be advantageous for flight in birds. The loss of the infrahyoid muscles freed the hyolarynx, trachea, and esophagus which may have aided in vocal tract filtering.

Reappraisal of the Silurian placoderm Silurolepis and insights into the dermal neck joint evolution

Silurolepis platydorsalis, a Silurian jawed vertebrate originally identified as an antiarch, is here redescribed as a maxillate placoderm close to Qilinyu and is anteroposteriorly reversed as opposed to the original description. The cuboid trunk shield possesses three longitudinal cristae, obstanic grooves on the trunk shield and three median dorsal plates, all uniquely shared with Qilinyu. Further preparation reveals the morphology of the dermal neck joint, with slot-shaped articular fossae on the trunk shield similar to Qilinyu and antiarchs. However, new tomographic data reveal that Qilinyu uniquely bears a dual articulation between the skull roof and trunk shield, which does not fit into the traditional 'ginglymoid' and 'reverse ginglymoid' categories. An extended comparison in early jawed vertebrates confirms that a sliding-type dermal neck joint is widely distributed and other types are elaborated in different lineages by developing various laminae. Nine new characters related to the dermal neck joint are proposed for a new phylogenetic analysis, in which Silurolepis forms a clade with Qilinyu. The current phylogenetic framework conflicts with the parsimonious evolution of dermal neck joints in suggesting that the shared trunk shield characters between antiarchs and Qilinyu are independently acquired, and the sliding-type joint in Entelognathus is reversely evolved from the dual articulation in Qilinyu.

The Spine: A Strong, Stable, and Flexible Structure with Biomimetics Potential

From its first appearance in early vertebrates, the spine evolved the function of protecting the spinal cord, avoiding excessive straining during body motion. Its stiffness and strength provided the basis for the development of the axial skeleton as the mechanical support of later animals, especially those which moved to the terrestrial environment where gravity loads are not alleviated by the buoyant force of water. In tetrapods, the functions of the spine can be summarized as follows: protecting the spinal cord; supporting the weight of the body, transmitting it to the ground through the limbs; allowing the motion of the trunk, through to its flexibility; providing robust origins and insertions to the muscles of trunk and limbs. This narrative review provides a brief perspective on the development of the spine in vertebrates, first from an evolutionary, and then from an embryological point of view. The paper describes functions and the shape of the spine throughout the whole evolution of vertebrates and vertebrate embryos, from primordial jawless fish to extant animals such as birds and humans, highlighting its fundamental features such as strength, stability, and flexibility, which gives it huge potential as a basis for bio-inspired technologies.

Unique morphogenetic signatures define mammalian neck muscles and associated connective tissues

In vertebrates, head and trunk muscles develop from different mesodermal populations and are regulated by distinct genetic networks. Neck muscles at the head-trunk interface remain poorly defined due to their complex morphogenesis and dual mesodermal origins. Here, we use genetically modified mice to establish a 3D model that integrates regulatory genes, cell populations and morphogenetic events that define this transition zone. We show that the evolutionary conserved cucullaris-derived muscles originate from posterior cardiopharyngeal mesoderm, not lateral plate mesoderm, and we define new boundaries for neural crest and mesodermal contributions to neck connective tissue. Furthermore, lineage studies and functional analysis of Tbx1- and Pax3-null mice reveal a unique developmental program for somitic neck muscles that is distinct from that of somitic trunk muscles. Our findings unveil the embryological and developmental requirements underlying tetrapod neck myogenesis and provide a blueprint to investigate how muscle subsets are selectively affected in some human myopathies.

Problems in Fish-to-Tetrapod Transition: Genetic Expeditions Into Old Specimens

The fish-to-tetrapod transition is one of the fundamental problems in evolutionary biology. A significant amount of paleontological data has revealed the morphological trajectories of skeletons, such as those of the skull, vertebrae, and appendages in vertebrate history. Shifts in bone differentiation, from dermal to endochondral bones, are key to explaining skeletal transformations during the transition from water to land. However, the genetic underpinnings underlying the evolution of dermal and endochondral bones are largely missing. Recent genetic approaches utilizing model organisms—zebrafish, frogs, chickens, and mice—reveal the molecular mechanisms underlying vertebrate skeletal development and provide new insights for how the skeletal system has evolved. Currently, our experimental horizons to test evolutionary hypotheses are being expanded to non-model organisms with state-of-the-art techniques in molecular biology and imaging. An integration of functional genomics, developmental genetics, and high-resolution CT scanning into evolutionary inquiries allows us to reevaluate our understanding of old specimens. Here, we summarize the current perspectives in genetic programs underlying the development and evolution of the dermal skull roof, shoulder girdle, and appendages. The ratio shifts of dermal and endochondral bones, and its underlying mechanisms, during the fish-to-tetrapod transition are particularly emphasized. Recent studies have suggested the novel cell origins of dermal bones, and the interchangeability between dermal and endochondral bones, obscuring the ontogenetic distinction of these two types of bones. Assimilation of ontogenetic knowledge of dermal and endochondral bones from different structures demands revisions of the prevalent consensus in the evolutionary mechanisms of vertebrate skeletal shifts.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Three-dimensional reconstruction of the cranial and anterior spinal nerves in early tadpoles of Xenopus laevis (Pipidae, Anura)

Xenopus laevis is one of the most widely used model organism in neurobiology. It is therefore surprising, that no detailed and complete description of the cranial nerves exists for this species. Using classical histological sectioning in combination with fluorescent whole mount antibody staining and micro-computed tomography we prepared a detailed innervation map and a freely-rotatable three-dimensional (3D) model of the cranial nerves and anterior-most spinal nerves of early X. laevis tadpoles. Our results confirm earlier descriptions of the pre-otic cranial nerves and present the first detailed description of the post-otic cranial nerves. Tracing the innervation, we found two previously undescribed head muscles (the processo-articularis and diaphragmatico-branchialis muscles) in X. laevis. Data on the cranial nerve morphology of tadpoles are scarce, and only one other species (Discoglossus pictus) has been described in great detail. A comparison of Xenopus and Discoglossus reveals a relatively conserved pattern of the post-otic and a more variable morphology of the pre-otic cranial nerves. Furthermore, the innervation map and the 3D models presented here can serve as an easily accessible basis to identify alterations of the innervation produced by experimental studies such as genetic gain- and loss of function experiments.

Development of hypobranchial muscles with special reference to the evolution of the vertebrate neck

BACKGROUND

The extant vertebrates include cyclostomes (lamprey and hagfish) and crown gnathostomes (jawed vertebrates), but there are various anatomical disparities between these two groups. Conspicuous in the gnathostomes is the neck, which occupies the interfacial domain between the head and trunk, including the occipital part of the cranium, the shoulder girdle, and the cucullaris and hypobranchial muscles (HBMs). Of these, HBMs originate from occipital somites to form the ventral pharyngeal and neck musculature in gnathostomes. Cyclostomes also have HBMs on the ventral pharynx, but lack the other neck elements, including the occipital region, the pectoral girdle, and cucullaris muscles. These anatomical differences raise questions about the evolution of the neck in vertebrates.

RESULTS

In this study, we observed developing HBMs as a basis for comparison between the two groups and show that the arrangement of the head-trunk interface in gnathostomes is distinct from that of lampreys. Our comparative analyses reveal that, although HBM precursors initially pass through the lateral side of the pericardium in both groups, the relative positions of the pericardium withrespect to the pharyngeal arches differ between the two, resulting in diverse trajectories of HBMs in gnathostomes and lampreys.

CONCLUSIONS

We suggest that a heterotopic rearrangement of early embryonic components, including the pericardium and pharyngeal arches, may have played a fundamental role in establishing the gnathostome HBMs, which would also have served as the basis for neck formation in the jawed vertebrate lineage.

Musculoskeletal networks reveal topological disparity in mammalian neck evolution

BACKGROUND

The increase in locomotor and metabolic performance during mammalian evolution was accompanied by the limitation of the number of cervical vertebrae to only seven. In turn, nuchal muscles underwent a reorganization while forelimb muscles expanded into the neck region. As variation in the cervical spine is low, the variation in the arrangement of the neck muscles and their attachment sites (i.e., the variability of the neck's musculoskeletal organization) is thus proposed to be an important source of neck disparity across mammals. Anatomical network analysis provides a novel framework to study the organization of the anatomical arrangement, or connectivity pattern, of the bones and muscles that constitute the mammalian neck in an evolutionary context.

RESULTS

Neck organization in mammals is characterized by a combination of conserved and highly variable network properties. We uncovered a conserved regionalization of the musculoskeletal organization of the neck into upper, mid and lower cervical modules. In contrast, there is a varying degree of complexity or specialization and of the integration of the pectoral elements. The musculoskeletal organization of the monotreme neck is distinctively different from that of therian mammals.

CONCLUSIONS

Our findings reveal that the limited number of vertebrae in the mammalian neck does not result in a low musculoskeletal disparity when examined in an evolutionary context. However, this disparity evolved late in mammalian history in parallel with the radiation of certain lineages (e.g., cetartiodactyls, xenarthrans). Disparity is further facilitated by the enhanced incorporation of forelimb muscles into the neck and their variability in attachment sites.

Motor neurons with limb-innervating character in the cervical spinal cord are sculpted by apoptosis based on the Hox code in chick embryo

In the developing chick embryo, a certain population of motor neurons (MNs) in the non-limb-innervating cervical spinal cord undergoes apoptosis between embryonic days 4 and 5. However, the characteristics of these apoptotic MNs remain undefined. Here, by examining the spatiotemporal profiles of apoptosis and MN subtype marker expression in normal or apoptosis-inhibited chick embryos, we found that this apoptotic population is distinguishable by Foxp1 expression. When apoptosis was inhibited, the Foxp1⁺ MNs survived and showed characteristics of lateral motor column (LMC) neurons, which are of a limb-innervating subtype, suggesting that cervical Foxp1⁺ MNs are the rostral continuation of the LMC. Knockdown and misexpression of Foxp1 did not affect apoptosis progression, but revealed the role of Foxp1 in conferring LMC identity on the cervical MNs. Furthermore, ectopic expression of Hox genes that are normally expressed in the brachial region prevented apoptosis, and directed Foxp1⁺ MNs to LMC neurons at the cervical level. These results indicate that apoptosis in the cervical spinal cord plays a role in sculpting Foxp1⁺ MNs committed to LMC neurons, depending on the Hox expression pattern.

The development of the cucullaris muscle and the branchial musculature in the Longnose Gar, (Lepisosteus osseus, Lepisosteiformes, Actinopterygii) and its implications for the evolution and development of the head/trunk interface in vertebrates

The vertebrate head/trunk interface is the region of the body where the different developmental programs of the head and trunk come in contact. Many anatomical structures that develop in this transition zone differ from similar structures in the head or the trunk. This is best exemplified by the cucullaris/trapezius muscle, spanning the head/trunk interface by connecting the head to the pectoral girdle. The source of this muscle has been claimed to be either the unsegmented head mesoderm or the somites of the trunk. However most recent data on the development of the cucullaris muscle are derived from tetrapods and information from actinopterygian taxa is scarce. We used classical histology in combination with fluorescent whole-mount antibody staining and micro-computed tomography to investigate the developmental pattern of the cucullaris and the branchial muscles in a basal actinopterygian, the Longnose gar (Lepisosteus osseus). Our results show (1) that the cucullaris has been misidentified in earlier studies on its development in Lepisosteus. (2) Cucullaris development is delayed compared to other head and trunk muscles. (3) This developmental pattern of the cucullaris is similar to that reported from some tetrapod taxa. (4) That the retractor dorsalis muscle of L. osseus shows a delayed developmental pattern similar to the cucullaris. Our data are in agreement with an explanatory scenario for the cucullaris development in tetrapods, suggesting that these mechanisms are conserved throughout the Osteichthyes. Furthermore the developmental pattern of the retractor dorsalis, also spanning the head/trunk interface, seems to be controlled by similar mechanisms.

Embracing change: striated-for-smooth muscle replacement in esophagus development

The esophagus functions to transport food from the oropharyngeal region to the stomach via waves of peristalsis and transient relaxation of the lower esophageal sphincter. The gastrointestinal tract, including the esophagus, is ensheathed by the muscularis externa (ME). However, while the ME of the gastrointestinal tract distal to the esophagus is exclusively smooth muscle, the esophageal ME of many vertebrate species comprises a variable amount of striated muscle. The esophageal ME is initially composed only of smooth muscle, but its developmental maturation involves proximal-to-distal replacement of smooth muscle with striated muscle. This fascinating phenomenon raises two important questions: what is the developmental origin of the striated muscle precursor cells, and what are the cellular and morphogenetic mechanisms underlying the process? Studies addressing these questions have provided controversial answers. In this review, we discuss the development of ideas in this area and recent work that has shed light on these issues. A working model has emerged that should permit deeper understanding of the role of ME development and maturation in esophageal disorders and in the functional and evolutionary underpinnings of the variable degree of esophageal striated myogenesis in vertebrate species.

Developmental origin of the clavicle, and its implications for the evolution of the neck and the paired appendages in vertebrates

In fish, the pectoral appendage is adjacent to the head, but during vertebrate evolution a long neck region emerged via caudal relocation of the pectoral appendage. The pectoral appendage is comprised of endochondral portions, such as the humerus and the scapula, and a dermal portion, such as the clavicle, that contributes to the shoulder girdle. In the search for clues to the mechanism of the caudal relocation of the pectoral appendage, the cell lineage of the rostral lateral plate mesoderm was analyzed in chickens. It was found that, despite the long neck region in chickens, the origin of the clavicle attached to the head mesoderm ranged between 1 and 14 somite levels. Because the pectoral limb bud and the endochondral pectoral appendage developed on 15-20 and 15-24 somite levels, respectively, the clavicle-forming region corresponds to the embryonic neck, which suggests that the relocation would have been executed by the expansion of the source of the clavicle. The rostral portion of the clavicle-forming region overlaps the source of the cucullaris muscle, embraces the pharyngeal arches caudally, and can be experimentally replaced with the head mesoderm to form the cucullaris muscle, which implies that the mesodermal portion could have been the head mesoderm and that the clavicle would have developed at the head/trunk boundary. The link between the head mesoderm and the presumptive clavicle appears to have been the developmental constraint needed to create the evolutionarily conserved musculoskeletal connectivities characterizing the gnathostome neck. In this sense, the dermal girdle of the ganathostomes would represent the wall of the branchial chamber into which the endochondral pectoral appendage appears to have attached since its appearance in evolution.

On the vagal cardiac nerves, with special reference to the early evolution of the head-trunk interface

The vagus nerve, or the tenth cranial nerve, innervates the heart in addition to other visceral organs, including the posterior visceral arches. In amniotes, the anterior and posterior cardiac branches arise from the branchial and intestinal portions of the vagus nerve to innervate the arterial and venous poles of the heart, respectively. The evolution of this innervation pattern has yet to be elucidated, due mainly to the lack of morphological data on the vagus in basal vertebrates. To investigate this topic, we observed the vagus nerves of the lamprey (Lethenteron japonicum), elephant shark (Callorhinchus milii), and mouse (Mus musculus), focusing on the embryonic patterns of the vagal branches in the venous pole. In the lamprey, no vagus branch was found in the venous pole throughout development, whereas the arterial pole was innervated by a branch from the branchial portion. In contrast, the vagus innervated the arterial and venous poles in the mouse and elephant shark. Based on the morphological patterns of these branches, the venous vagal branches of the mouse and elephant shark appear to belong to the intestinal part of the vagus, implying that the cardiac nerve pattern is conserved among crown gnathostomes. Furthermore, we found a topographical shift of the structures adjacent to the venous pole (i.e., the hypoglossal nerve and pronephros) between the extant gnathostomes and lamprey. Phylogenetically, the lamprey morphology is likely to be the ancestral condition for vertebrates, suggesting that the evolution of the venous branch occurred early in the gnathostome lineage, in parallel with the remodeling of the head-trunk interfacial domain during the acquisition of the neck. J. Morphol. 277:1146-1158, 2016. © 2016 Wiley Periodicals, Inc.

Evolution of the head-trunk interface in tetrapod vertebrates

Vertebrate neck musculature spans the transition zone between head and trunk. The extent to which the cucullaris muscle is a cranial muscle allied with the gill levators of anamniotes or is instead a trunk muscle is an ongoing debate. Novel computed tomography datasets reveal broad conservation of the cucullaris in gnathostomes, including coelacanth and caecilian, two sarcopterygians previously thought to lack it. In chicken, lateral plate mesoderm (LPM) adjacent to occipital somites is a recently identified embryonic source of cervical musculature. We fate-map this mesoderm in the axolotl (Ambystoma mexicanum), which retains external gills, and demonstrate its contribution to posterior gill-levator muscles and the cucullaris. Accordingly, LPM adjacent to the occipital somites should be regarded as posterior cranial mesoderm. The axial position of the head-trunk border in axolotl is congruent between LPM and somitic mesoderm, unlike in chicken and possibly other amniotes.

The lateral plate mesoderm: a novel source of skeletal muscle

It has been established in the last century that the skeletal muscle cells of vertebrates originate from the paraxial mesoderm. However, recently the lateral plate mesoderm has been identified as a novel source of the skeletal muscle. The branchiomeric muscles, such as masticatory and facial muscles, receive muscle progenitor cells from both the cranial paraxial mesoderm and lateral plate mesoderm. At the occipital level, the lateral plate mesoderm is the sole source of the muscle progenitors of the dorsolateral neck muscle, such as trapezius and sternocleidomastoideus in mammals and cucullaris in birds. The lateral plate mesoderm requires a longer time for generating skeletal muscle cells than the somites. The myogenesis of the lateral plate is determined early, but not cell autonomously and requires local signals. Lateral plate myogenesis is regulated by mechanisms controlling the cranial myogenesis. The connective tissue of the lateral plate-derived muscle is formed by the cranial neural crest. Although the cranial neural crest cells do not control the early myogenesis, they regulate the patterning of the branchiomeric muscles and the cucullaris muscle. Although satellite cells derived from the cranial lateral plate show distinct properties from those of the trunk, they can respond to local signals and generate myofibers for injured muscles in the limbs. In this review, we key feature in detail the muscle forming properties of the lateral plate mesoderm and propose models of how the myogenic fate may have arisen.

Evolutionary and developmental understanding of the spinal accessory nerve

The vertebrate spinal accessory nerve (SAN) innervates the cucullaris muscle, the major muscle of the neck, and is recognized as a synapomorphy that defines living jawed vertebrates. Morphologically, the cucullaris muscle exists between the branchiomeric series of muscles innervated by special visceral efferent neurons and the rostral somitic muscles innervated by general somatic efferent neurons. The category to which the SAN belongs to both developmentally and evolutionarily has long been controversial. To clarify this, we assessed the innervation and cytoarchitecture of the spinal nerve plexus in the lamprey and reviewed studies of SAN in various species of vertebrates and their embryos. We then reconstructed an evolutionary sequence in which phylogenetic changes in developmental neuronal patterning led towards the gnathostome-specific SAN. We hypothesize that the SAN arose as part of a lamprey-like spinal nerve plexus that innervates the cyclostome-type infraoptic muscle, a candidate cucullaris precursor.

How the pterosaur got its wings

Throughout the evolutionary history of life, only three vertebrate lineages took to the air by acquiring a body plan suitable for powered flight: birds, bats, and pterosaurs. Because pterosaurs were the earliest vertebrate lineage capable of powered flight and included the largest volant animal in the history of the earth, understanding how they evolved their flight apparatus, the wing, is an important issue in evolutionary biology. Herein, I speculate on the potential basis of pterosaur wing evolution using recent advances in the developmental biology of flying and non-flying vertebrates. The most significant morphological features of pterosaur wings are: (i) a disproportionately elongated fourth finger, and (ii) a wing membrane called the brachiopatagium, which stretches from the posterior surface of the arm and elongated fourth finger to the anterior surface of the leg. At limb-forming stages of pterosaur embryos, the zone of polarizing activity (ZPA) cells, from which the fourth finger eventually differentiates, could up-regulate, restrict, and prolong expression of 5'-located Homeobox D (Hoxd) genes (e.g. Hoxd11, Hoxd12, and Hoxd13) around the ZPA through pterosaur-specific exploitation of sonic hedgehog (SHH) signalling. 5'Hoxd genes could then influence downstream bone morphogenetic protein (BMP) signalling to facilitate chondrocyte proliferation in long bones. Potential expression of Fgf10 and Tbx3 in the primordium of the brachiopatagium formed posterior to the forelimb bud might also facilitate elongation of the phalanges of the fourth finger. To establish the flight-adapted musculoskeletal morphology shared by all volant vertebrates, pterosaurs probably underwent regulatory changes in the expression of genes controlling forelimb and pectoral girdle musculoskeletal development (e.g. Tbx5), as well as certain changes in the mode of cell-cell interactions between muscular and connective tissues in the early phase of their evolution. Developmental data now accumulating for extant vertebrate taxa could be helpful in understanding the cellular and molecular mechanisms of body-plan evolution in extinct vertebrates as well as extant vertebrates with unique morphology whose embryonic materials are hard to obtain.

Evolutionarily conserved morphogenetic movements at the vertebrate head-trunk interface coordinate the transport and assembly of hypopharyngeal structures

The vertebrate head–trunk interface (occipital region) has been heavily remodelled during evolution, and its development is still poorly understood. In extant jawed vertebrates, this region provides muscle precursors for the throat and tongue (hypopharyngeal/hypobranchial/hypoglossal muscle precursors, HMP) that take a stereotype path rostrally along the pharynx and are thought to reach their target sites via active migration. Yet, this projection pattern emerged in jawless vertebrates before the evolution of migratory muscle precursors. This suggests that a so far elusive, more basic transport mechanism must have existed and may still be traceable today.

Here we show for the first time that all occipital tissues participate in well-conserved cell movements. These cell movements are spearheaded by the occipital lateral mesoderm and ectoderm that split into two streams. The rostrally directed stream projects along the floor of the pharynx and reaches as far rostrally as the floor of the mandibular arch and outflow tract of the heart. Notably, this stream leads and engulfs the later emerging HMP, neural crest cells and hypoglossal nerve. When we (i) attempted to redirect hypobranchial/hypoglossal muscle precursors towards various attractants, (ii) placed non-migratory muscle precursors into the occipital environment or (iii) molecularly or (iv) genetically rendered muscle precursors non-migratory, they still followed the trajectory set by the occipital lateral mesoderm and ectoderm. Thus, we have discovered evolutionarily conserved morphogenetic movements, driven by the occipital lateral mesoderm and ectoderm, that ensure cell transport and organ assembly at the head–trunk interface.

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At the vertebrate head–trunk interface, all tissues engage in stereotype cell movements.

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A ventrally–rostrally directed stream of cells leads along the floor of the pharynx to the developing jaw and outflow tract of the heart.

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The cell movements are spearheaded by the lateral mesoderm and surface ectoderm; muscle precursors for throat and tongue muscles (hypopharyngeal muscles); neural crest cells and outgrowing axons of the hypoglossal nerve follow.

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Hypopharyngeal muscle precursors follow the trajectory set by the lateral mesoderm and ectoderm, even when challenged with ectopic attractants or when rendered non-migratory.

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The newly discovered cell movements are the likely ground state for cell transport and organ assembly at the head–trunk interface before actively migrating muscle precursors evolved in “bony” (osteichthyan) vertebrates.

Cranial muscle development in the model organism ambystoma mexicanum: implications for tetrapod and vertebrate comparative and evolutionary morphology and notes on ontogeny and phylogeny

There is still confusion about the homology of several cranial muscles in salamanders with those of other vertebrates. This is true, in part, because of the fact that many muscles present in early ontogeny of amphibians disappear during development and specifically during metamorphosis. Resolving this confusion is important for the understanding of the comparative and evolutionary morphology of vertebrates and tetrapods because amphibians are the phylogenetically most plesiomorphic tetrapods, concerning for example their myology, and include two often used model organisms, Xenopus laevis (anuran) and Ambystoma mexicanum (urodele). Here we provide the first detailed report of the cranial muscle development in axolotl from early ontogenetic stages to the adult stage. We describe different and complementary types of general muscle morphogenetic gradients in the head: from anterior to posterior, from lateral to medial, and from origin to insertion. Furthermore, even during the development of neotenic salamanders such as axolotls, various larval muscles become indistinct, contradicting the commonly accepted view that during ontogeny the tendency is mostly toward the differentiation of muscles. We provide an updated comparison between these muscles and the muscles of other vertebrates, a discussion of the homologies and evolution, and show that the order in which the muscles appear during axolotl ontogeny is in general similar to their appearance in phylogeny (e.g. differentiation of adductor mandibulae muscles from one anlage to four muscles), with only a few remarkable exceptions, as for example the dilatator laryngis that appears evolutionary later but in the development before the intermandibularis.