Hypobranchial Muscles Research Articles

Developmental Evolution of Hypaxial Muscles: Insights From Cyclostomes and Chondrichthyans.

Jawed vertebrates possess two distinct groups of muscles in the trunk (epaxial and hypaxial muscles) primarily defined by the pattern of motor innervation from the spinal cord. Of these, the hypaxial group includes muscles with highly differentiated morphology and function, such as the muscles associated with paired limbs, shoulder girdles and tongue/infrahyoid (hypobranchial) muscles. Here we summarize the latest findings on the evolutionary mechanisms underlying the morphological variety of hypaxial musculature, with special reference to the molecular insights obtained from several living species that diverged early in vertebrate evolution. Lampreys, extant jawless vertebrates, lack many of derived traits characteristic of the gnathostomes, such as jaws, paired fins and epaxial/hypaxial distinction of the trunk skeletal musculatures. However, these animals possess the primitive form of the hypobranchial muscle. Of the gnathostomes, the elasmobranchs exhibit developmental mode of hypaxial muscles that is not identical to that of other gnathostomes in that the muscle primordia relocate as coherent cell aggregates. Comparison of expression of developmental genes, including Lbx genes, has delineated the temporal order of differentiation of various skeletal muscles, such as the hypobranchial, posterior pharyngeal and cucullaris (trapezius) muscles. We have proposed that the sequential addition of distal muscles, associated with expression of duplicated Lbx genes, promoted the elaboration of skeletal musculature. These analyses have revealed the framework of an evolutionary pathway that gave rise to the morphological complexity and diversity of vertebrate body patterns.

Spinal sensory neurons project onto the hindbrain to stabilize posture and enhance locomotor speed

In the spinal cord, cerebrospinal fluid-contacting neurons (CSF-cNs) are GABAergic interoceptive sensory neurons that detect spinal curvature via a functional coupling with the Reissner fiber. This mechanosensory system has recently been found to be involved in spine morphogenesis and postural control but the underlying mechanisms are not fully understood. In zebrafish, CSF-cNs project an ascending and ipsilateral axon reaching two to six segments away. Rostralmost CSF-cNs send their axons ipsilaterally into the hindbrain, a brain region containing motor nuclei and reticulospinal neurons (RSNs), which send descending motor commands to spinal circuits. Until now, the synaptic connectivity of CSF-cNs has only been investigated in the spinal cord, where they synapse onto motor neurons and premotor excitatory interneurons. The identity of CSF-cN targets in the hindbrain and the behavioral relevance of these sensory projections from the spinal cord to the hindbrain are unknown. Here, we provide anatomical and molecular evidence that rostralmost CSF-cNs synapse onto the axons of large RSNs including Mauthner cells and V2a neurons. Functional anatomy and optogenetically assisted mapping reveal that rostral CSF-cNs also synapse onto the soma and dendrites of cranial motor neurons innervating hypobranchial muscles. During acousto-vestibular evoked escape responses, ablation of rostralmost CSF-cNs results in a weaker escape response with a decreased C-bend amplitude, lower speed, and deficient postural control. Our study demonstrates that spinal sensory feedback enhances speed and stabilizes posture, and reveals a novel spinal gating mechanism acting on the output of descending commands sent from the hindbrain to the spinal cord.

Novel developmental bases for the evolution of hypobranchial muscles in vertebrates

BackgroundVertebrates are characterized by possession of hypobranchial muscles (HBMs). Cyclostomes, or modern jawless vertebrates, possess a rudimentary and superficial HBM lateral to the pharynx, whereas the HBM in jawed vertebrates is internalized and anteroposteriorly specified. Precursor cells of the HBM, marked by expression of Lbx1, originate from somites and undergo extensive migration before becoming innervated by the hypoglossal nerve. How the complex form of HBM arose in evolution is relevant to the establishment of the vertebrate body plan, but despite having long been assumed to be similar to that of limb muscles, modification of developmental mechanisms of HBM remains enigmatic.ResultsHere we characterize the expression of Lbx genes in lamprey and hagfish (cyclostomes) and catshark (gnathostome; jawed vertebrates). We show that the expression patterns of the single cyclostome Lbx homologue, Lbx-A, do not resemble the somitic expression of mammalian Lbx1. Disruption of Lbx-A revealed that LjLbx-A is required for the formation of both HBM and body wall muscles, likely due to the insufficient extension of precursor cells rather than to hindered muscle differentiation. Both homologues of Lbx in the catshark were expressed in the somitic muscle primordia, unlike in amniotes. During catshark embryogenesis, Lbx2 is expressed in the caudal HBM as well as in the abdominal rectus muscle, similar to lamprey Lbx-A, whereas Lbx1 marks the rostral HBM and pectoral fin muscle.ConclusionsWe conclude that the vertebrate HBM primarily emerged as a specialized somatic muscle to cover the pharynx, and the anterior internalized HBM of the gnathostomes is likely a novelty added rostral to the cyclostome-like HBM, for which duplication and functionalization of Lbx genes would have been a prerequisite.

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

BackgroundThe 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.ResultsIn 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.ConclusionsWe 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.

Function of the hypobranchial muscles and hyoidiomandibular ligament during suction capture and bite processing in white-spotted bamboo sharks, Chiloscyllium plagiosum.

Suction feeding in teleost fish is a power-dependent behavior, requiring rapid and forceful expansion of the orobranchial cavity by the hypobranchial and trunk muscles. To increase power production for expansion, many species employ in-series tendons and catch mechanisms to store and release elastic strain energy. Suction feeding sharks such as Chiloscyllium plagiosum lack large in-series tendons on the hypobranchials, yet two of the hypobranchials, the coracohyoideus and coracoarcualis (CH and CA; hyoid depressors), are arranged in-series, and run deep and parallel to a third muscle, the coracomandibularis (CM, jaw depressor). The arrangement of the CH and CA suggests that C. plagiosum is using the CH muscle rather than a tendon to store and release elastic strain energy. Here we describe the anatomy of the feeding apparatus, and present data on hyoid and jaw kinematics and fascicle shortening in the CM, CH and CA quantified using sonomicrometry, with muscle activity and buccal pressure recorded simultaneously. Results from prey capture show that prior to jaw and hyoid depression the CH is actively lengthened by shortening of the in-series CA. The active lengthening of the CH and pre-activation of the CH and CA suggest that the CH is functioning to store and release elastic energy during prey capture. Catch mechanisms are proposed involving a dynamic moment arm and four-bar linkage between the hyoidiomandibular ligament (LHML), jaws and ceratohyals that is influenced by the CM. Furthermore, the LHML may be temporarily disengaged during behaviors such as bite processing to release linkage constraints.

On the peculiar morphology and development of the hypoglossal, glossopharyngeal and vagus nerves and hypobranchial muscles in the hagfish.

IntroductionThe vertebrate body is characterized by its dual segmental organization: pharyngeal arches in the head and somites in the trunk. Muscular and nervous system morphologies are also organized following these metameric patterns, with distinct differences between head and trunk; branchiomeric nerves innervating pharyngeal arches are superficial to spinal nerves innervating somite derivatives. Hypobranchial muscles originate from rostral somites and occupy the “neck” at the head-trunk interface. Hypobranchial muscles, unlike ventral trunk muscles in the lateral body wall, develop from myocytes that migrate ventrally to occupy a space that is ventrolateral to the pharynx and unassociated with coelomic cavities. Occipitospinal nerves innervating these muscles also extend ventrally, thereby crossing the vagus nerve laterally.ResultsIn hagfishes, the basic morphological pattern of vertebrates is obliterated by the extreme caudal shift of the posterior part of the pharynx. The vagus nerve is found unusually medially, and occipitospinal nerves remain unfasciculated, appearing as metameric spinal nerves as in the posterior trunk region. Moreover, the hagfish exhibits an undifferentiated body plan, with the hypobranchial muscles not well dissociated from the abaxial muscles in the trunk. Comparative embryological observation showed that this hagfish-specific morphology is established by secondary modification of the common vertebrate embryonic pattern, and the hypobranchial muscle homologue can be found in the rostral part of the oblique muscle with pars decussata.ConclusionThe morphological pattern of the hagfish represents an extreme case of heterotopy that led to the formation of the typical hypoglossal nerve, and can be regarded as an autapomorphic trait of the hagfish lineage.

Cephalic muscles of Cyclostomes (hagfishes and lampreys) and Chondrichthyes (sharks, rays and holocephalans): comparative anatomy and early evolution of the vertebrate head muscles

Living vertebrate diversity comprises hagfishes and lampreys (Cyclostomata), elasmobranchs and holocephalans (Chondrichthyes), and bony fish which include tetrapods (Osteichthyes). Based on dissections and an extensive comparative analysis, we provide an updated overview of the anatomy, homologies and evolution of cyclostome and chondrichthyan cephalic muscles, with osteichthyans as primary comparative taxa. The analysis also infers plesiomorphic conditions for vertebrates and gnathostomes. We follow a uniform myological terminology for the Gnathostomata to demonstrate that the last common ancestor of extant vertebrates probably had a single intermandibularis and other mandibular muscles (labial muscles), some constrictores hyoidei and branchiales, and epibranchial and hypobranchial muscle sheets. The division of the cucullaris into levatores arcuum branchialium and protractor pectoralis is an osteichthyan synapomorphy and reflects an evolutionary trend towards a greater separation between the head and pectoral girdle that culminated in the formation of the tetrapod neck. Hence, this paper addresses a long-standing, central issue regarding vertebrate comparative anatomy. It thus provides a valuable basis for future evolutionary, developmental and functional studies of vertebrates and/or of specific vertebrate subgroups/model organisms. © 2014 The Linnean Society of London

Morphological and taxonomic revision of species of Squatina from the Southwestern Atlantic Ocean (Chondrichthyes: Squatiniformes: Squatinidae).

The morphology and taxonomy of species of Squatina from the southwestern Atlantic Ocean are revised. Species previously considered valid, Squatina argentina (Marini, 1930), Squatina guggenheim Marini, 1936 and Squatina occulta Vooren and da Silva, 1991, are investigated and described in detail, including a morphometric and meristic study of specimens from their recorded range. The taxonomic status of the doubtful nominal species Squatina punctata Marini, 1936 was also evaluated. This species was previously considered a junior synonym of S. argentina, a junior synonym of S. guggenheim, or a senior synonym of S. occulta. Although there is much morphological similarity between Squatina species, significant differences in dermal denticle patterns, dorsal coloration, tooth formula, and size at maturity are reported, enabling the recognition of S. argentina, S. guggenheim and S. occulta as valid species, and relegating S. punctata to the synonymy of S. guggenheim. Differences in skeletal morphology between valid species are described and considered supportive of the taxonomic hypothesis, corroborating a previous study of neurocrania. Additionally, an unidentified specimen is reported, as Squatina sp., from the continental shelf of Bahia state, Brazil, recognized by having more vertebral centra and a conspicuous dermal denticle morphology on interspiracular region, features not present in other South America angelshark species. A report on the only known syntype of Squatina dumeril Le Sueur, 1818 is presented, describing features that are still preserved and designating it as lectotype. Lateral-line sensory canals, skeleton, and cranial and hypobranchial muscles for the three valid species of Squatina from the southwestern Atlantic, as well as the brain and cranial nerves of S. guggenheim, are described and illustrated.

Cranial muscles in amphibians: development, novelties and the role of cranial neural crest cells

Our research on the evolution of the vertebrate head focuses on understanding the developmental origins of morphological novelties. Using a broad comparative approach in amphibians, and comparisons with the well-studied quail-chicken system, we investigate how evolutionarily conserved or variable different aspects of head development are. Here we review research on the often overlooked development of cranial muscles, and on its dependence on cranial cartilage development. In general, cranial muscle cell migration and the spatiotemporal pattern of cranial muscle formation appears to be very conserved among the few species of vertebrates that have been studied. However, fate-mapping of somites in the Mexican axolotl revealed differences in the specific formation of hypobranchial muscles (tongue muscles) in comparison to the chicken. The proper development of cranial muscles has been shown to be strongly dependent on the mostly neural crest-derived cartilage elements in the larval head of amphibians. For example, a morpholino-based knock-down of the transcription factor FoxN3 in Xenopus laevis has drastic indirect effects on cranial muscle patterning, although the direct function of the gene is mostly connected to neural crest development. Furthermore, extirpation of single migratory streams of cranial neural crest cells in combination with fate-mapping in a frog shows that individual cranial muscles and their neural crest-derived connective tissue attachments originate from the same visceral arch, even when the muscles attach to skeletal components that are derived from a different arch. The same pattern has also been found in the chicken embryo, the only other species that has been thoroughly investigated, and thus might be a conserved pattern in vertebrates that reflects the fundamental nature of a mechanism that keeps the segmental order of the head in place despite drastic changes in adult anatomy. There is a need for detailed comparative fate-mapping of pre-otic paraxial mesoderm in amphibians, to determine developmental causes underlying the complicated changes in cranial muscle development and architecture within amphibians, and in particular how the novel mouth apparatus in frog tadpoles evolved. This will also form a foundation for further research into the molecular mechanisms that regulate rostral head morphogenesis. Our empirical studies are discussed within a theoretical framework concerned with the evolutionary origin and developmental basis of novel anatomical structures in general. We argue that a common developmental origin is not a fool-proof guide to homology, and that a view that sees only structures without homologs as novel is too restricted, because novelties must be produced by changes in the same framework of developmental processes. At the level of developmental processes and mechanisms, novel structures are therefore likely to have homologs, and we need to develop a hierarchical concept of novelty that takes this into account.

Evolution and development of the vertebrate neck

Muscles of the vertebrate neck include the cucullaris and hypobranchials. Although a functional neck first evolved in the lobe-finned fishes (Sarcopterygii) with the separation of the pectoral/shoulder girdle from the skull, the neck muscles themselves have a much earlier origin among the vertebrates. For example, lampreys possess hypobranchial muscles, and may also possess the cucullaris. Recent research in chick has established that these two muscles groups have different origins, the hypobranchial muscles having a somitic origin but the cucullaris muscle deriving from anterior lateral plate mesoderm associated with somites 1-3. Additionally, the cucullaris utilizes genetic pathways more similar to the head than the trunk musculature. Although the latter results are from experiments in the chick, cucullaris homologues occur in a variety of more basal vertebrates such as the sharks and zebrafish. Data are urgently needed from these taxa to determine whether the cucullaris in these groups also derives from lateral plate mesoderm or from the anterior somites, and whether the former or the latter represent the basal vertebrate condition. Other lateral plate mesoderm derivatives include the appendicular skeleton (fins, limbs and supporting girdles). If the cucullaris is a definitive lateral plate-derived structure it may have evolved in conjunction with the shoulder/limb skeleton in vertebrates and thereby provided a greater degree of flexibility to the heads of predatory vertebrates.

Expression and interaction of muscle-related genes in the lamprey imply the evolutionary scenario for vertebrate skeletal muscle, in association with the acquisition of the neck and fins

Gnathostomes (jawed vertebrates) possess skeletal muscles with unique functional and developmental features that are absent from cyclostomes—i.e., lamprey and hagfish. These gnathostome-specific traits include the epaxial and hypaxial division of myotomes, paired fin/limb muscles, shoulder girdle muscles, and the muscle associated with the tongue and the neck. Many of these muscles are derived from several rostral somites, specifically from their hypaxial myotomic domains. However, it has not been clarified how the complicated morphology of these muscles was acquired in the evolution of vertebrates. Here we describe the expression of lamprey homologs of transcription factor genes, including a myogenic regulatory factor of the Myod family (MRF), Pax3/7, Lbx, and Zic, which play important roles in the development of ep-/hypaxial somitic muscles in gnathostomes, and show that the ventral portion of lamprey somites is comparable to the ventral dermomyotome in gnathostomes. The supra- and infraoptic muscles, derived from the two anterior somites in the lamprey, are molecularly specified before their extensive invasion into the head region. Of these, the infraoptic myotomes are suggested to represent the cucullaris homologue in the lamprey based on their topographical position in the embryonic pattern. Slightly caudal myotomes in the lamprey give rise to the hypobranchial muscle, the developmental homologue of the gnathostome hypobranchial musculature. The dorsal moieties of the lamprey somites express a Z ic gene, which in teleosts specifies the epaxial identities of the somites. These evidences suggest that, although the myotomes in the ancestral jawless vertebrates do not exhibit ep-/hypaxial distinction at the morphological level, their dorsoventral specification would have already been present at gene regulatory levels, prior to the cyclostome–gnathostome divergence, which may have functioned as the key innovation to establish the ep-/hypaxial distinction in gnathostomes.

Cell fate and timing in the evolution of neural crest and mesoderm development in the head region of amphibians and lungfishes

AbstractOur research on the evolution of head development focuses on understanding the developmental origins of morphological innovations and involves asking questions like: How flexible (or conserved) are cell fates, patterns of cell migration or the timing of developmental events (heterochrony)? How do timing changes, or changes in life history affect head development and growth? Our ‘model system’ is a comparison between lungfishes and representatives from all three extant groups of amphibians. Within anuran amphibians, major changes in life history such as the repeated evolution of larval specializations (e.g. carnivory), or indeed the loss of a free‐swimming larva, allows us to test for developmental constraints. Cell migration and cell fate are conserved in cranial neural crest cells in all vertebrates studied so far. Patterning and developmental anatomy of cranial neural crest and head mesoderm cells are conserved within amphibians and even between birds, mammals and amphibians. However, the specific formation of hypobranchial muscles from ventral somitic processes shows variation within tetrapods. The evolution of carnivorous larvae in terminal taxa is correlated with changes in both pattern and timing of head skeletal and muscle development. Sequence‐heterochronic changes are correlated with feeding mode in terminal taxa and with phylogenetic relatedness in basal branches of the phylogeny. Eye muscles seem to form a developmental module that can evolve relatively independently from other head muscles, at least in terms of timing of muscle differentiation.

From fish to modern humans--comparative anatomy, homologies and evolution of the head and neck musculature.

In a recent paper Diogo (2008) reported the results of the first part of an investigation of the comparative anatomy, homologies and evolution of the head and neck muscles of osteichthyans (bony fish + tetrapods). That report mainly focused on actinopterygian fish, but also compared these fish with certain non-mammalian sarcopterygians. The present paper focuses mainly on sarcopterygians, and particularly on how the head and neck muscles have evolved during the transitions from sarcopterygian fish and non-mammalian tetrapods to monotreme and therian mammals, including modern humans. The data obtained from our dissections of the head and neck muscles of representative members of sarcopterygian fish, amphibians, reptiles, monotremes and therian mammals, such as rodents, tree-shrews, colugos and primates, including modern humans, are compared with the information available in the literature. Our observations and comparisons indicate that the number of mandibular and true branchial muscles (sensu this work) present in modern humans is smaller than that found in mammals such as tree-shrews, rats and monotremes, as well as in reptiles such as lizards. Regarding the pharyngeal musculature, there is an increase in the number of muscles at the time of the evolutionary transition leading to therian mammals, but there was no significant increase during the transition leading to the emergence of higher primates and modern humans. The number of hypobranchial muscles is relatively constant within the therian mammals we examined, although in this case modern humans have more muscles than other mammals. The number of laryngeal and facial muscles in modern humans is greater than that found in most other therian taxa. Interestingly, modern humans possess peculiar laryngeal and facial muscles that are not present in the majority of the other mammalian taxa; this seems to corroborate the crucial role played by vocal communication and by facial expressions in primate and especially in human evolution. It is hoped that by compiling, in one paper, data about the head and neck muscles of a wide range of sarcopterygians, the present work could be useful to comparative anatomists, evolutionary biologists and functional morphologists and to researchers working in other fields such as developmental biology, genetics and/or evolutionary developmental biology.

Development of mandibular, hyoid and hypobranchial muscles in the zebrafish: homologies and evolution of these muscles within bony fishes and tetrapods

BackgroundDuring vertebrate head evolution, muscle changes accompanied radical modification of the skeleton. Recent studies have suggested that muscles and their innervation evolve less rapidly than cartilage. The freshwater teleostean zebrafish (Danio rerio) is the most studied actinopterygian model organism, and is sometimes taken to represent osteichthyans as a whole, which include bony fishes and tetrapods. Most work concerning zebrafish cranial muscles has focused on larval stages. We set out to describe the later development of zebrafish head muscles and compare muscle homologies across the Osteichthyes.ResultsWe describe one new muscle and show that the number of mandibular, hyoid and hypobranchial muscles found in four day-old zebrafish larvae is similar to that found in the adult. However, the overall configuration and/or the number of divisions of these muscles change during development. For example, the undivided adductor mandibulae of early larvae gives rise to the adductor mandibulae sections A0, A1-OST, A2 and Aω, and the protractor hyoideus becomes divided into dorsal and ventral portions in adults. There is not always a correspondence between the ontogeny of these muscles in the zebrafish and their evolution within the Osteichthyes. All of the 13 mandibular, hyoid and hypobranchial muscles present in the adult zebrafish are found in at least some other living teleosts, and all except the protractor hyoideus are found in at least some extant non-teleost actinopterygians. Of these muscles, about a quarter (intermandibularis anterior, adductor mandibulae, sternohyoideus) are found in at least some living tetrapods, and a further quarter (levator arcus palatini, adductor arcus palatini, adductor operculi) in at least some extant sarcopterygian fish.ConclusionAlthough the zebrafish occupies a rather derived phylogenetic position within actinopterygians and even within teleosts, with respect to the mandibular, hyoid and hypobranchial muscles it seems justified to consider it an appropriate representative of these two groups. Among these muscles, the three with clear homologues in tetrapods and the further three identified in sarcopterygian fish are particularly appropriate for comparisons of results between the actinopterygian zebrafish and the sarcopterygians.

Comparative anatomy, homologies and evolution of mandibular, hyoid and hypobranchial muscles of bony fish and tetrapods: a new insight

AbstractThe Osteichthyes, including bony fishes and tetrapods, is a highly speciose group of vertebrates, comprising more than 42000 living species. The anatomy of osteichthyans has been the subject of numerous comparative studies, but these mainly concern osteological structures; much less attention has been paid to muscles. In fact, the most detailed and comprehensive myological comparative analyses that were actually based on a direct observation of representatives of various major osteichthyan groups were provided various decades by authors such as Luther, Kesteven and principally Edgeworth. The present work provides an updated discussion of the homologies and evolution of the osteichthyan mandibular, hyoid and hypobranchial muscles, based on the author's own analyses and on a survey of the literature, both old and recent. The risks of discussing muscle homologies on the basis of a single line of evidence, even when it concerns innervation or development, is emphasized. It is stressed than only by taking into consideration various lines of evidence (e.g. developmental biology, comparative anatomy, functional morphology, paleontology, molecular biology, experimental embryology, innervation and/or phylogeny) it is possible to establish well-grounded hypotheses of muscle homology.

Muscular derivatives of the cranialmost somites revealed by long‐term fate mapping in the Mexican axolotl (Ambystoma mexicanum)

The fate of single somites has not been analyzed from a comparative perspective with modern cell-marking methods. Most of what we know is based on work using quail-chick chimeras. Consequently, to what degree cell fate has been conserved despite the anatomical differences among vertebrates is unknown. We have analyzed the cell fate of the cranialmost somites, with the focus on somite two, in the Mexican axolotl (Ambystoma mexicanum). Somite cells were marked by injection of dextran-fluorescein and detected using immunofluorescence after 2 months of development in paraffin sections. Our data confirm and extend earlier studies based on classical histology in salamanders. We show that somite two contributes to different muscles, skeletal elements, and connective tissues of the head and cranial trunk region. Cells from somites two and three migrate latero-ventrally and contribute to the hypobranchial muscles mm. geniohyoideus and rectus cervicis. We provide evidence that the specific formation of the hypobranchial musculature from ventral processes of the somites might be variable in different classes of vertebrates. We further demonstrate that mm. cucullaris and dilatator laryngis, which were earlier thought to have a branchial origin, arise from somitic material in a manner very similar to the findings in quail-chick chimeras. Our findings indicate that the pattern of somitic derivatives is highly conserved within tetrapods.

Evolutionary perspectives from development of mesodermal components in the lamprey

Lampreys, a jawless vertebrate species, lack not only jaws but also several other organs, including ventral migratory muscles shared by gnathostomes. In the lamprey embryo, the mesoderm consists primarily of unsegmented head mesoderm, segmented somites, and yet uncharacterized lateral plate mesoderm, as in gnathostomes. Although the adult lamprey possesses segmented myotomes in the head, the head mesoderm of this animal is primarily unsegmented, similar to that in gnathostomes. In the trunk, the large part of lamprey somites is destined to form myotomes, and the Pax3/7 gene expression domain in the lateral part of somites is suggested to represent a dermomyotome homologue. Lamprey myotomes are not segregated by a horizontal myoseptum, which has been regarded as consistent with the apparent absence of a migratory population of hypaxial muscles shared by gnathostomes. However, recent analysis suggests that lampreys have established the gene regulatory cascade necessary for the ventrally migrating myoblasts, which functions in part during the development of the primordial hypobranchial muscle. There have also been new insights on the developmental cascade of lamprey cartilages, in which the Sox family of transcription factors plays major roles, as in gnathostomes. Thus, mesoderm development in lampreys may represent the ancestral state of gene regulatory mechanisms required for the evolution of the complex and diverse body plan of gnathostomes.

Hyobranchial skeleton and hypobranchial muscles of rhipidistians

The hyobranchial skeleton of the porolepiform rhipidistian Laccognathus panderi Gross is described. The double composition of the ceratohyal in crossopterygians is proposed. The urohyal of porolepiforms, like that of Latimeria, consists of cartilaginous axial and membranous peripheral portions. The differences between porolepiforms and osteolepiforms in the structure of the hyobranchial skeleton, particularly, in the shape of the urohyal are attributable to different arrangements of the hypobranchial muscles. Porolepiforms and coelacanths have retained the coracomandibularis muscle inherited from early gnathostomes, whereas the same muscle of osteolepiforms was transformed into the geniohyoideus muscle. This transformation is accounted for by functional changes in the hyobranchial apparatus.

Evolution and developmental patterning of the vertebrate skeletal muscles: Perspectives from the lamprey

The myotome in gnathostome vertebrates, which gives rise to the trunk skeletal muscles, consists of epaxial (dorsal) and hypaxial (ventral) portions, separated by the horizontal myoseptum. The hypaxial portion contains some highly derived musculature that is functionally as well as morphologically well differentiated in all the gnathostome species. In contrast, the trunk muscles of agnathan lampreys lack these distinctions and any semblance of limb muscles. Therefore, the lamprey myotomes probably represent a primitive condition compared with gnathostomes. In this review, we compare the patterns of expression of some muscle-specific genes between the lamprey and gnathostomes. Although the cellular and tissue morphology of lamprey myotomes seems uniform and undifferentiated, some of the muscle-specific genes are expressed in a spatially restricted manner. The lamprey Pax3/7 gene, a cognate of gnathostome Pax3, is expressed only at the lateral edge of the myotomes and in the hypobranchial muscle, which we presume is homologous to the gnathostome hypobranchial muscle. Thus, the emergence of some part of a hypaxial-specific gene regulatory cascade might have evolved before the agnathan/gnathostome divergence, or before the evolutionary separation of epaxial and hypaxial muscles.

Placoderm branchial and hypobranchial muscles and origins in jawed vertebrates

ABSTRACT Muscle morphology in the Placodermi can be inferred by comparison to extant gnathostomes (Chon-drichthyes (Actinopterygii + Sarcopterygii)). Examination of the chondrichthyan hypobranchial musculature suggested a comparable musculature for placoderms, including the coracomandibularis and coracohyoideus, acting separately to depress the mandible and hyoid arch. In sharks, this was associated with the absence of a mandibulohyoid ligament functioning in mandible depression, a characteristic feature of actinopterygians and sarcopterygians. Additionally, the coracobranchiales muscles depress the chondrichthyan ventral gill arches, the clavobranchiales being the corresponding muscles in actinopterygians and sarcopterygians. These muscles and ligaments are not preserved in placoderms, but their attachments can be compared to those of extant gnathostomes. On the placoderm lower jaw, attachment surfaces for the mandibulohyoid ligament are only present in derived taxa and position relative to the jaw joint suggests the ligament functioned poorly (or not at all). This compares to the chondrichthyan condition, but attachment surfaces on the placoderm trunkshield indicate that while some primitive placoderms possessed a coracobranchialis musculature, other derived taxa possessed a clavobranchialis. The Osteostraci are the most closely related agnathans to the jawed vertebrates, and lack muscles involved in active inspiration, the evolution of which was linked to increased activity near the origin of jawed vertebrates. Since overall morphology suggests osteostracans were active animals, absence of these muscles may instead relate to the absence of segmented gill arches and the anterior position of the osteostracan pectoral fin. The latter may affect development of the hypobranchial musculature, questioning homology of these muscles in lampreys and jawed vertebrates.