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

Feats of supercontractile strength: functional convergence of supercontracting muscle properties among hyoid musculature in chameleons

The structure of sarcomeres imposes limits to the capacity of striated muscle to change length and produce force, with z-disc and myosin filament interactions constraining shortening. Conversely, supercontracting muscles, hitherto only known among vertebrates in the tongue retractor muscle (m. hyoglossus) of chameleons, have perforated z-discs that allow myosin filaments to extend through them into adjacent sarcomeres, permitting continued shortening and force development. Additional hyolingual muscles in chameleons undergo extreme length changes during feeding as well and may benefit from supercontractile properties. We compared length-tension relationship data and transmission electron microscopy images from four chameleon muscles to test for the presence of additional supercontracting muscle. We document the second known example of a supercontracting muscle among vertebrates (the m. sternohyoideus superficialis) and show that the m. sternohyoideus profundus exhibits functional convergence with supercontracting muscles by increasing the range of muscle lengths over which it can exert force through the exploitation of sarcomere length non-uniformity across its muscle fibres. Additionally, we show that chameleon supercontracting muscles may share common contractile and structural properties due to a common origin from occipital somites. These results provide important insights into the developmental and evolutionary patterns associated with supercontracting muscle and extreme muscle elongation.

Co-option of neck muscles supported the vertebrate water-to-land transition

A major event in vertebrate evolution was the separation of the skull from the pectoral girdle and the acquisition of a functional neck, transitions that required profound developmental rearrangements of the musculoskeletal system. The neck is a hallmark of the tetrapod body plan and allows for complex head movements on land. While head and trunk muscles arise from distinct embryonic mesoderm populations, the origins of neck muscles remain elusive. Here, we combine comparative embryology and anatomy to reconstruct the mesodermal contribution to neck evolution. We demonstrate that head/trunk-connecting muscle groups have conserved mesodermal origins in fishes and tetrapods and that the neck evolved from muscle groups present in fishes. We propose that expansions of mesodermal populations into head and trunk domains during embryonic development underpinned the emergence and adaptation of the tetrapod neck. Our results provide evidence for the exaptation of archetypal muscle groups in ancestral fishes, which were co-opted to acquire novel functions adapted to a terrestrial lifestyle.

Cell-cell interaction determines cell fate of mesoderm-derived cell in tongue development through Hh signaling

Dysfunction of primary cilia leads to genetic disorder, ciliopathies, which shows various malformations in many vital organs such as brain. Multiple tongue deformities including cleft, hamartoma, and ankyloglossia are also seen in ciliopathies, which yield difficulties in fundamental functions such as mastication and vocalization. Here, we found these tongue anomalies in mice with mutation of ciliary protein. Abnormal cranial neural crest-derived cells (CNCC) failed to evoke Hh signal for differentiation of mesoderm-derived cells into myoblasts, which resulted in abnormal differentiation of mesoderm-derived cells into adipocytes. The ectopic adipose subsequently arrested tongue swelling formation. Ankyloglossia was caused by aberrant cell migration due to lack of non-canonical Wnt signaling. In addition to ciliopathies, these tongue anomalies are often observed as non-familial condition in human. We found that these tongue deformities could be reproduced in wild-type mice by simple mechanical manipulations to disturb cellular processes which were disrupted in mutant mice. Our results provide hints for possible future treatment in ciliopathies.

Retinoic acid signalling regulates branchiomeric neck muscle development at the head/trunk interface

Skeletal muscles of the head and trunk originate in distinct lineages with divergent regulatory programmes converging on activation of myogenic determination factors. Branchiomeric head and neck muscles share a common origin with cardiac progenitor cells in cardiopharyngeal mesoderm (CPM). The retinoic acid (RA) signalling pathway is required during a defined early time window for normal deployment of cells from posterior CPM to the heart. Here, we show that blocking RA signalling in the early mouse embryo also results in selective loss of the trapezius neck muscle, without affecting other skeletal muscles. RA signalling is required for robust expression of myogenic determination factors in posterior CPM and subsequent expansion of the trapezius primordium. Lineage-specific activation of a dominant-negative RA receptor reveals that trapezius development is not regulated by direct RA signalling to myogenic progenitor cells in CPM, or through neural crest cells, but indirectly through the somitic lineage, closely apposed with posterior CPM in the early embryo. These findings suggest that trapezius development is dependent on precise spatiotemporal interactions between cranial and somitic mesoderm at the head/trunk interface.

Multiple embryonic sources converge to form the pectoral girdle skeleton in zebrafish

The morphological transformation of the pectoral/shoulder girdle is fundamental to the water-to-land transition in vertebrate evolution. Although previous studies have resolved the embryonic origins of tetrapod shoulder girdles, those of fish pectoral girdles remain uncharacterized, creating a gap in the understanding of girdle transformation mechanisms from fish to tetrapods. Here, we identify the embryonic origins of the zebrafish pectoral girdle, including the cleithrum as an ancestral girdle element lost in extant tetrapods. Our combinatorial approach of photoconversion and genetic lineage tracing demonstrates that cleithrum development combines four adjoining embryonic populations. A comparison of these pectoral girdle progenitors with extinct and extant vertebrates highlights that cleithrum loss, indispensable for neck evolution, is associated with the disappearance of its unique developmental environment at the head/trunk interface. Overall, our study establishes an embryological framework for pectoral/shoulder girdle formation and provides evolutionary trajectories from their origin in water to diversification on land.

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.

Thyroid and endostyle development in cyclostomes provides new insights into the evolutionary history of vertebrates

Background

The endostyle is an epithelial exocrine gland found in non-vertebrate chordates (amphioxi and tunicates) and the larvae of modern lampreys. It is generally considered to be an evolutionary precursor of the thyroid gland of vertebrates. Transformation of the endostyle into the thyroid gland during the metamorphosis of lampreys is thus deemed to be a recapitulation of a past event in vertebrate evolution. In 1906, Stockard reported that the thyroid gland in hagfish, the sister cyclostome group of lampreys, develops through an endostyle-like primordium, strongly supporting the plesiomorphy of the lamprey endostyle. However, the findings in hagfish thyroid development were solely based on this single study, and these have not been confirmed by modern molecular, genetic, and morphological data pertaining to hagfish thyroid development over the last century.

Results

Here, we showed that the thyroid gland of hagfish undergoes direct development from the ventrorostral pharyngeal endoderm, where the previously described endostyle-like primordium was not found. The developmental pattern of the hagfish thyroid, including histological features and regulatory gene expression profiles, closely resembles that found in modern jawed vertebrates (gnathostomes). Meanwhile, as opposed to gnathostomes but similar to non-vertebrate chordates, lamprey and hagfish share a broad expression domain of Nkx2-1/2-4, a key regulatory gene, in the pharyngeal epithelium during early developmental stages.

Conclusions

Based on the direct development of the thyroid gland both in hagfish and gnathostomes, and the shared expression profile of thyroid-related transcription factors in the cyclostomes, we challenge the plesiomorphic status of the lamprey endostyle and propose an alternative hypothesis where the lamprey endostyle could be obtained secondarily in crown lampreys.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01282-7.

Novel developmental bases for the evolution of hypobranchial muscles in vertebrates

BACKGROUND

Vertebrates 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.

RESULTS

Here 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.

CONCLUSIONS

We 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.

Cardiopharyngeal mesoderm origins of musculoskeletal and connective tissues in the mammalian pharynx

Cardiopharyngeal mesoderm (CPM) gives rise to muscles of the head and heart. Using genetic lineage analysis in mice, we show that CPM develops into a broad range of pharyngeal structures and cell types encompassing musculoskeletal and connective tissues. We demonstrate that CPM contributes to medial pharyngeal skeletal and connective tissues associated with both branchiomeric and somite-derived neck muscles. CPM and neural crest cells (NCC) make complementary mediolateral contributions to pharyngeal structures, in a distribution established in the early embryo. We further show that biallelic expression of the CPM regulatory gene Tbx1, haploinsufficient in 22q11.2 deletion syndrome patients, is required for the correct patterning of muscles with CPM-derived connective tissue. Our results suggest that CPM plays a patterning role during muscle development, similar to that of NCC during craniofacial myogenesis. The broad lineage contributions of CPM to pharyngeal structures provide new insights into congenital disorders and evolution of the mammalian pharynx.

Muscle precursor cell movements in zebrafish are dynamic and require Six family genes

Muscle precursors need to be correctly positioned during embryonic development for proper body movement. In zebrafish, a subset of hypaxial muscle precursors from the anterior somites undergo long-range migration, moving away from the trunk in three streams to form muscles in distal locations such as the fin. We mapped long-distance muscle precursor migrations with unprecedented resolution using live imaging. We identified conserved genes necessary for normal precursor motility (six1a, six1b, six4a, six4b and met). These genes are required for movement away from somites and later to partition two muscles within the fin bud. During normal development, the middle muscle precursor stream initially populates the fin bud, then the remainder of this stream contributes to the posterior hypaxial muscle. When we block fin bud development by impairing retinoic acid synthesis or Fgfr function, the entire stream contributes to the posterior hypaxial muscle indicating that muscle precursors are not committed to the fin during migration. Our findings demonstrate a conserved muscle precursor motility pathway, identify dynamic cell movements that generate posterior hypaxial and fin muscles, and demonstrate flexibility in muscle precursor fates.

Connecting muscle development, birth defects, and evolution: An essential role for muscle connective tissue

Skeletal muscle powers all movement of the vertebrate body and is distributed in multiple regions that have evolved distinct functions. Axial muscles are ancestral muscles essential for support and locomotion of the whole body. The evolution of the head was accompanied by development of cranial muscles essential for eye movement, feeding, vocalization, and facial expression. With the evolution of paired fins and limbs and their associated muscles, vertebrates gained increased locomotor agility, populated the land, and acquired fine motor skills. Finally, unique muscles with specialized functions have evolved in some groups, and the diaphragm which solely evolved in mammals to increase respiratory capacity is one such example. The function of all these muscles requires their integration with the other components of the musculoskeletal system: muscle connective tissue (MCT), tendons, bones as well as nerves and vasculature. MCT is muscle's closest anatomical and functional partner. Not only is MCT critical in the adult for muscle structure and function, but recently MCT in the embryo has been found to be crucial for muscle development. In this review, we examine the important role of the MCT in axial, head, limb, and diaphragm muscles for regulating normal muscle development, discuss how defects in MCT-muscle interactions during development underlie the etiology of a range of birth defects, and explore how changes in MCT development or communication with muscle may have led to the modification and acquisition of new muscles during vertebrate evolution.

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.

Stepwise participation of HGF/MET signaling in the development of migratory muscle precursors during vertebrate evolution

BACKGROUND

The skeletal musculature of gnathostomes, which is derived from embryonic somites, consists of epaxial and hypaxial portions. Some hypaxial muscles, such as tongue and limb muscles, undergo de-epithelialization and migration during development. Delamination and migration of these myoblasts, or migratory muscle precursors (MMPs), is generally thought to be regulated by hepatocyte growth factor (HGF) and receptor tyrosine kinase (MET) signaling. However, the prevalence of this mechanism and the expression patterns of the genes involved in MMP development across different vertebrate species remain elusive.

RESULTS

We performed a comparative analysis of Hgf and Met gene expression in several vertebrates, including mouse, chicken, dogfish (Scyliorhinus torazame), and lamprey (Lethenteron camtschaticum). While both Hgf and Met were expressed during development in the mouse tongue muscle, and in limb muscle formation in the mouse and chicken, we found no clear evidence for the involvement of HGF/MET signaling in MMP development in shark or lamprey embryos.

CONCLUSIONS

Our results indicate that the expressions and functions of both Hgf and Met genes do not represent shared features of vertebrate MMPs, suggesting a stepwise participation of HGF/MET signaling in MMP development during vertebrate evolution.

The neural crest and evolution of the head/trunk interface in vertebrates

The migration and distribution patterns of neural crest (NC) cells reflect the distinct embryonic environments of the head and trunk: cephalic NC cells migrate predominantly along the dorsolateral pathway to populate the craniofacial and pharyngeal regions, whereas trunk crest cells migrate along the ventrolateral pathways to form the dorsal root ganglia. These two patterns thus reflect the branchiomeric and somitomeric architecture, respectively, of the vertebrate body plan. The so-called vagal NC occupies a postotic, intermediate level between the head and trunk NC. This level of NC gives rise to both trunk- and cephalic-type (circumpharyngeal) NC cells. The anatomical pattern of the amphioxus, a basal chordate, suggests that somites and pharyngeal gills coexist along an extensive length of the body axis, indicating that the embryonic environment is similar to that of vertebrate vagal NC cells and may have been ancestral for vertebrates. The amniote-like condition in which the cephalic and trunk domains are distinctly separated would have been brought about, in part, by anteroposterior reduction of the pharyngeal domain.