Ontogeny of architectural complexity in embryonic quail visceral arch muscles

Differential growth of the adductor muscles, eyeball, and brain in the chick Gallus gallus with comments on the fossil record of stem-group birds

The avian head is unique among living reptiles in its combination of relatively large brain and eyes, coupled with relatively small adductor jaw muscles. These derived proportions lend themselves to a trade-off hypothesis, wherein adductor size was reduced over evolutionary time as a means (or as a consequence) of neurosensory expansion. In this study, we examine this evolutionary hypothesis through the lens of development by describing the jaw-adductor anatomy of developing chickens, Gallus gallus, and comparing the volumetric expansion of these developing muscles with growth trajectories of the brain and eye. Under the trade-off hypothesis, we predicted that the jaw muscles would grow with negative allometry relative to brain and eyes, and that osteological signatures of a relatively large adductor system, as found in most nonavian dinosaurs, would be differentially expressed in younger chicks. Results did not meet these expectations, at least not generally, with muscle growth exhibiting positive allometry relative to that of brain and eye. We propose three, nonmutually exclusive explanations: (1) these systems do not compete for space, (2) these systems competed for space in the evolutionary past, and growth of the jaw muscles was truncated early in development (paedomorphosis), and (3) trade-offs in developmental investment in these systems are limited temporally to the perinatal period. These explanations are considered in light of the fossil record, and most notably the skull of the stem bird Ichthyornis, which exhibits an interesting combination of plesiomorphically large adductor chamber and apomorphically large brain.

Sequence of chondrocranial development in basal anurans-Let's make a cranium

Background

The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established.

Results

We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I.

Conclusions

In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

Micro-CT guided illustration of the head anatomy of penguins (Aves: Sphenisciformes: Spheniscidae)

The illustration is an important tool to aid in the description and understanding of anatomy, and penguins (Aves: Sphenisciformes: Spheniscidae) are an important clade in environmental monitoring, paleontology, and other research fields. Traditionally, anatomic illustration has been informed by dissection. More recently, micro-computed tomography (micro-CT) has proven to be a powerful tool for three-dimensional anatomic imaging, although larger specimens are more challenging to image due to increased X-ray attenuation. Here, we used traditional dissection and micro-CT to illustrate the skulls of Aptenodytes patagonicus, Eudyptula minor, and Pygoscelis papua, and the extracranial soft tissue of E. minor. Micro-CT prevented the loss of orientation, disarticulation, and distortion of bones that might result from cleaning and drying skulls, while immobilization was achieved by freezing the specimens before imaging. All bony elements in the head were accurately depicted. Fixing, dehydrating, and diffusion staining with iodine (diceCT) enabled the identification of muscles and other large nonmineralized structures, but specimen preparation precluded the ability to show smaller nerves and vessels. The results presented here provide a guide for anatomic studies of penguins and our summary of sample preparation and imaging techniques are applicable for studies of other similarly sized biological specimens.

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.

Cxcr4 and Sdf-1 are critically involved in the formation of facial and non-somitic neck muscles

The present study shows that the CXCR4/SDF-1 axis regulates the migration of second branchial arch-derived muscles as well as non-somitic neck muscles. Cxcr4 is expressed by skeletal muscle progenitor cells in the second branchial arch (BA2). Muscles derived from the second branchial arch, but not from the first, fail to form in Cxcr4 mutants at embryonic days E13.5 and E14.5. Cxcr4 is also required for the development of non-somitic neck muscles. In Cxcr4 mutants, non-somitic neck muscle development is severely perturbed. In vivo experiments in chicken by means of loss-of-function approach based on the application of beads loaded with the CXCR4 inhibitor AMD3100 into the cranial paraxial mesoderm resulted in decreased expression of Tbx1 in the BA2. Furthermore, disrupting this chemokine signal at a later stage by implanting these beads into the BA2 caused a reduction in MyoR, Myf5 and MyoD expression. In contrast, gain-of-function experiments based on the implantation of SDF-1 beads into BA2 resulted in an attraction of myogenic progenitor cells, which was reflected in an expansion of the expression domain of these myogenic markers towards the SDF-1 source. Thus, Cxcr4 is required for the formation of the BA2 derived muscles and non-somitic neck muscles.

Muscle development in the shark Scyliorhinus canicula: implications for the evolution of the gnathostome head and paired appendage musculature

BACKGROUND

The origin of jawed vertebrates was marked by profound reconfigurations of the skeleton and muscles of the head and by the acquisition of two sets of paired appendages. Extant cartilaginous fish retained numerous plesiomorphic characters of jawed vertebrates, which include several aspects of their musculature. Therefore, myogenic studies on sharks are essential in yielding clues on the developmental processes involved in the origin of the muscular anatomy.

RESULTS

Here we provide a detailed description of the development of specific muscular units integrating the cephalic and appendicular musculature of the shark model, Scyliorhinus canicula. In addition, we analyze the muscle development across gnathostomes by comparing the developmental onset of muscle groups in distinct taxa. Our data reveal that appendicular myogenesis occurs earlier in the pectoral than in the pelvic appendages. Additionally, the pectoral musculature includes muscles that have their primordial developmental origin in the head. This culminates in a tight muscular connection between the pectoral girdle and the cranium, which founds no parallel in the pelvic fins. Moreover, we identified a lateral to ventral pattern of formation of the cephalic muscles, that has been equally documented in osteichthyans but, in contrast with these gnathostomes, the hyoid muscles develop earlier than mandibular muscle in S. canicula.

CONCLUSION

Our analyses reveal considerable differences in the formation of the pectoral and pelvic musculatures in S. canicula, reinforcing the idea that head tissues have contributed to the formation of the pectoral appendages in the common ancestor of extant gnathostomes. In addition, temporal differences in the formation of some cranial muscles between chondrichthyans and osteichthyans might support the hypothesis that the similarity between the musculature of the mandibular arch and of the other pharyngeal arches represents a derived feature of jawed vertebrates.

New insight into the anatomy of the hyolingual apparatus of Alligator mississippiensis and implications for reconstructing feeding in extinct archosaurs

Anatomical studies of the cranium of crocodilians motivated by an interest in its function in feeding largely focused on bite force, the jaw apparatus and associated muscles innervated by the trigeminal nerve. However, the ossified and cartilaginous elements of the hyoid and the associated hyolingual muscles, innervated by the facial, hypoglossal and glossopharyngeal nerves, received much less attention. Crocodilians are known to retain what are ancestrally the 'Rhythmic Hyobranchial Behaviors' such as buccal oscillation, but show diminished freedom and movement for the hyobranchial apparatus and the tongue in food transport and manipulation. Feeding among crocodilians, generally on larger prey items than other reptilian outgroups, involves passive transport of the food within the mouth. The tongue in extant crocodilians is firmly attached to the buccal floor and shows little movement during feeding. Here, we present a detailed anatomical description of the myology of the hyolingual apparatus of Alligator mississippiensis, utilizing contrast-enhanced micro-computed tomography and dissection. We construct the first three-dimensional (3D) description of hyolingual myology in Alligator mississippiensis and discuss the detailed implications of these data for our understanding of hyolingual muscle homology across Reptilia. These anatomical data and an evaluation of the fossil record of hyoid structures also shed light on the evolution of feeding in Reptilia. Simplification of the hyoid occurs early in the evolution of archosaurs. A hyoid with only one pair of ceratobranchials and a weakly ossified or cartilaginous midline basihyal is ancestral to Archosauriformes. The comparison with non-archosaurian reptilian outgroup demonstrates that loss of the second set of ceratobranchials as well as reduced ossification in basihyal occurred prior to the origin of crown-clade archosaurs, crocodilians and birds. Early modification in feeding ecology appears to characterize the early evolution of the clade. Hyoid simplification has been linked to ingestion of large prey items, and this shift in hyoid-related feeding ecology may occur in early archosauriform evolution. A second transformation in hyoid morphology occurs within the crocodilian stem lineage after the split from birds. In Crocodyliformes, deflections in the ceratobrachials become more pronounced. The morphology of the hyoid in Archosauriformes indicates that aspects of the hyolingual apparatus in extant crocodilians are derived, including a strong deflection near the midpoint of the ceratobranchials, and their condition should not be treated as ancestral for Archosauria.

Vertebrate neurogenic placode development: historical highlights that have shaped our current understanding

With the flood of published research encountered today, it is important to occasionally reflect upon how we arrived at our current understanding in a particular scientific discipline, thereby positioning new discoveries into proper context with long-established models. This historical review highlights some of the important scientific contributions in the field of neurogenic placode development. By viewing cumulatively the rich historical data, we can more fully appreciate and apply what has been accomplished. Early descriptive work in fish and experimental approaches in amphibians and chick yielded important conceptual models of placode induction and cellular differentiation. Current efforts to discover genes and their molecular functions continue to expand our understanding of the placodes. Carefully considering the body of work may improve current models and help focus modern experimental design.

Cranial muscle development in frogs with different developmental modes: direct development versus biphasic development

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana(Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved.

Development of the trigeminal motor neurons in parrots: implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest-derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology.

A role for FoxN3 in the development of cranial cartilages and muscles in Xenopus laevis (Amphibia: Anura: Pipidae) with special emphasis on the novel rostral cartilages

The origin of morphological novelties is a controversial topic in evolutionary developmental biology. The heads of anuran larvae have several unique structures, including the supra- and infrarostral cartilages, the specialised structure of the gill basket (used for filtration), and novel cranial muscle arrangements. FoxN3, a member of the forkhead/winged helix family of transcription factors, has been implicated as important for normal craniofacial development in the pipid anuran Xenopus laevis. We have investigated the effects of functional knockdown of FoxN3 (using antisense oligonucleotide morpholino) on the development of the larval head skeleton and the associated cranial muscles in X. laevis. Our data complement earlier studies and provide a more complete account of the requirement of FoxN3 in chondrocranium development. In addition, we analyse the effects of FoxN3 knockdown on cranial muscle development. We show that FoxN3 knockdown primarily affects the novel skeletal structures unique to anuran larvae, i.e. the rostralia or the fine structure of the gill apparatus. The articulation between the infrarostral and Meckel's cartilage is malformed and the filigreed processes of the gill basket do not develop. Because these features do not develop after FoxN3 knockdown, the head morphology resembles that in the less specialised larvae of salamanders. Furthermore, the development of all cartilages derived from the neural crest is delayed and cranial muscle fibre development incomplete. The cartilage precursors initially condense in their proper position but later differentiate incompletely; several visceral arch muscles start to differentiate at their origin but fail to extend toward their insertion. Our findings indicate that FoxN3 is essential for the development of novel cartilages such as the infrarostral and other cranial tissues derived from the neural crest and, indirectly, also for muscle morphogenesis.

Cadherin-7 and cadherin-6B differentially regulate the growth, branching and guidance of cranial motor axons

Cadherin-7 (Cad7) and cadherin-6B (Cad6B) are expressed in early and late phases of cranial motoneuron development, respectively. Cad7 is expressed by cranial motoneurons soon after they are generated, as well as in the environment through which their axons extend. By contrast, Cad6B is expressed by mature cranial motoneurons. We demonstrate in chick that these cadherins play distinct roles in cranial motor axon morphology, branching and projection. Using in vitro approaches, we show that Cad7 enhances motor axon outgrowth, suppresses the formation of multiple axons and restricts interstitial branching, thus promoting the development of a single unbranched axon characteristic of differentiating motoneurons. Conversely, Cad6B in vitro promotes motor axon branching, a characteristic of mature motoneurons. In vivo gain- and loss-of-function experiments for these cadherins yielded phenotypes consistent with this interpretation. In particular, a loss of cadherin-mediated interactions in vivo led to dysregulation of the cranial motoneuron normal branching programme and caused axon navigation defects. We also demonstrate that Cad6B functions via the phosphatidylinositol 3-kinase pathway. Together, these data show that Cad7 and Cad6B differentially regulate cranial motoneuron growth, branching and axon guidance.

Developmental origins of species-specific muscle pattern

Vertebrate jaw muscle anatomy is conspicuously diverse but developmental processes that generate such variation remain relatively obscure. To identify mechanisms that produce species-specific jaw muscle pattern we conducted transplant experiments using Japanese quail and White Pekin duck, which exhibit considerably different jaw morphologies in association with their particular modes of feeding. Previous work indicates that cranial muscle formation requires interactions with adjacent skeletal and muscular connective tissues, which arise from neural crest mesenchyme. We transplanted neural crest mesenchyme from quail to duck embryos, to test if quail donor-derived skeletal and muscular connective tissues could confer species-specific identity to duck host jaw muscles. Our results show that duck host jaw muscles acquire quail-like shape and attachment sites due to the presence of quail donor neural crest-derived skeletal and muscular connective tissues. Further, we find that these species-specific transformations are preceded by spatiotemporal changes in expression of genes within skeletal and muscular connective tissues including Sox9, Runx2, Scx, and Tcf4, but not by alterations to histogenic or molecular programs underlying muscle differentiation or specification. Thus, neural crest mesenchyme plays an essential role in generating species-specific jaw muscle pattern and in promoting structural and functional integration of the musculoskeletal system during evolution.

Relationship between neural crest cells and cranial mesoderm during head muscle development

Background

In vertebrates, the skeletal elements of the jaw, together with the connective tissues and tendons, originate from neural crest cells, while the associated muscles derive mainly from cranial mesoderm. Previous studies have shown that neural crest cells migrate in close association with cranial mesoderm and then circumscribe but do not penetrate the core of muscle precursor cells of the branchial arches at early stages of development, thus defining a sharp boundary between neural crest cells and mesodermal muscle progenitor cells. Tendons constitute one of the neural crest derivatives likely to interact with muscle formation. However, head tendon formation has not been studied, nor have tendon and muscle interactions in the head.

Methodology/Principal Findings

Reinvestigation of the relationship between cranial neural crest cells and muscle precursor cells during development of the first branchial arch, using quail/chick chimeras and molecular markers revealed several novel features concerning the interface between neural crest cells and mesoderm. We observed that neural crest cells migrate into the cephalic mesoderm containing myogenic precursor cells, leading to the presence of neural crest cells inside the mesodermal core of the first branchial arch. We have also established that all the forming tendons associated with branchiomeric and eye muscles are of neural crest origin and express the Scleraxis marker in chick and mouse embryos. Moreover, analysis of Scleraxis expression in the absence of branchiomeric muscles in Tbx1^−/− mutant mice, showed that muscles are not necessary for the initiation of tendon formation but are required for further tendon development.

Conclusions/Significance

This results show that neural crest cells and muscle progenitor cells are more extensively mixed than previously believed during arch development. In addition, our results show that interactions between muscles and tendons during craniofacial development are similar to those observed in the limb, despite the distinct embryological origin of these cell types in the head.

Mandibular arch muscle identity is regulated by a conserved molecular process during vertebrate development

Vertebrate head muscles exhibit a highly conserved pattern of innervation and skeletal connectivity and yet it is unclear whether the molecular basis of their development is likewise conserved. Using the highly conserved expression of Engrailed 2 (En2) as a marker of identity in the dorsal mandibular muscles of zebrafish, we have investigated the molecular signals and tissues required for patterning these muscles. We show that muscle En2 expression is not dependent on signals from the adjacent neural tube, pharyngeal endoderm or axial mesoderm and that early identity of head muscles does not require bone morphogenetic pathway, Notch or Hedgehog (Hh) signalling. However, constrictor dorsalis En2 expression is completely lost after a loss of fibroblast growth factor (Fgf) signalling and we show that is true throughout head muscle development. These results suggest that head muscle identity is dependent on Fgf signalling. Data from experiments performed in chick suggest a similar regulation of En2 genes by Fgf signalling revealing a conserved mechanism for specifying head muscle identity. We present evidence that another key gene important in the development of mouse head muscles, Tbx1, is also critical for specification of mandibular arch muscle identity and that this is independent of Fgf signalling. These data imply that dorsal mandibular arch muscle identity in fish, chick and mouse is specified by a highly conserved molecular process despite differing functions of these muscles in different lineages.

Bilateral quadrification of the anterior digastric muscles with variations of the median accessory digastric muscles

During dissection of the submental region, the anterior bellies of the right and left digastric muscles were found to have four separate insertions. Two median accessory digastric muscles were located medially to anterior bellies of the digastrics and inferiorly to the mylohyoid and deep in the platysma. The four accessory muscles of the anterior bellies of the digastric muscles originated from the digastric fossa and inserted into the hyoid bone. Two median accessory digastric muscles were located between the anterior bellies of the digastric muscle and inserted into the hyoid bone as well. These muscle fibers formed a muscular floor for the oral cavity similar to the second mylohyoid muscle. When the muscle heads were analyzed, the anterior belly of the digastric muscle appeared to have six heads. These six heads were united by an intermediate rounded tendon, which was attached to the hyoid bone. This finding of a bilateral quadrification of the anterior digastric muscles with variations in the median accessory digastric muscles has not previously been reported. Anatomic variations of the anterior bellies of the digastric muscles can be easily confused on computed tomographic scans and magnetic resonance imaging. The possible occurrence of such anomalies should be kept in mind during surgical procedures involving the submental region.

The cause of the difference in the submental region: aberrant muscle bundles of the anterior belly of the digastric muscle

The aberrant bundles' presence in the anterior belly of the digastric muscle is important in terms of causing asymmetry in the submental region, getting confused with some pathologic cases, radiologic examination, and aesthetic facial surgery. To provide data, aberrant bundles in the submental region were investigated in 30 cadaver heads. During the dissection of the submental region, origin, insertion, shape, and bilaterality of the anterior bellies of the digastric muscles and the aberrant bundles were investigated. The 20 heads with aberrant bundles were classified into two types based on the muscle arrangement: digastric fossa type and crossover type. The aberrant bundles, which did not cross the median line, were classified as being of the digastric fossa type, whereas those that crossed the line were of the crossover type. Fifteen of the heads contained bundles of the unilateral type and five heads contained the crossover type. In three heads, digastric fossa and crossover types coexisted. In this study, a wide range for incidence in the submental region was observed of variations. Some cases were not described in the classification of the previous studies of this muscle. It is also possible that the incidences may vary as a result of the ethnic differences of the populations studied. Bilaterality was frequently observed in this study. Anatomic variations of the anterior bellies of the digastric muscle can easily be confused with the pathologic conditions in ultrasonography, computed tomography, and magnetic resonance imaging; therefore, it is necessary to recognize that variants of the anterior belly of the digastric muscle occur to avoid confusion when diagnosis shows abnormal lesions in the floor of the mouth and submental region. Additionally, the possible occurrence of such anomalies should be remembered during the surgical procedures involving the submental region.

Archosaur adductor chamber evolution: Integration of musculoskeletal and topological criteria in jaw muscle homology

The homologies of jaw muscles among archosaurs and other sauropsids have been unclear, confounding interpretation of adductor chamber morphology and evolution. Relevant topological patterns of muscles, nerves, and blood vessels were compared across a large sample of extant archosaurs (birds and crocodylians) and outgroups (e.g., lepidosaurs and turtles) to test the utility of positional criteria, such as the relative position of the trigeminal divisions, as predictors of jaw muscle homology. Anatomical structures were visualized using dissection, sectioning, computed tomography (CT), and vascular injection. Data gathered provide a new and robust view of jaw muscle homology and introduce the first synthesized nomenclature of sauropsid musculature using multiple lines of evidence. Despite the great divergences in cephalic morphology among birds, crocodylians, and outgroups, several key sensory nerves (e.g., n. anguli oris, n. supraorbitalis, n. caudalis) and arteries proved useful for muscle identification, and vice versa. Extant crocodylians exhibit an apomorphic neuromuscular pattern counter to the trigeminal topological paradigm: the maxillary nerve runs medial, rather than lateral to M. pseudotemporalis superficialis. Alternative hypotheses of homology necessitate less parsimonious interpretations of changes in topology. Sensory branches to the rictus, external acoustic meatus, supraorbital region, and other cephalic regions suggest conservative dermatomes among reptiles. Different avian clades exhibit shifts in some muscle positions, but maintain the plesiomorphic, diapsid soft-tissue topological pattern. Positional data suggest M. intramandibularis is merely the distal portion of M. pseudotemporalis separated by an intramuscular fibrocartilaginous sesamoid. These adductor chamber patterns indicate multiple topological criteria are necessary for interpretations of soft-tissue homology and warrant further investigation into character congruence and developmental connectivity.

Relations and interactions between cranial mesoderm and neural crest populations

The embryonic head is populated by two robust mesenchymal populations, paraxial mesoderm and neural crest cells. Although the developmental histories of each are distinct and separate, they quickly establish intimate relations that are variably important for the histogenesis and morphogenesis of musculoskeletal components of the calvaria, midface and branchial regions. This review will focus first on the genesis and organization within nascent mesodermal and crest populations, emphasizing interactions that probably initiate or augment the establishment of lineages within each. The principal goal is an analysis of the interactions between crest and mesoderm populations, from their first contacts through their concerted movements into peripheral domains, particularly the branchial arches, and continuing to stages at which both the differentiation and the integrated three-dimensional assembly of vascular, connective and muscular tissues is evident. Current views on unresolved or contentious issues, including the relevance of head somitomeres, the processes by which crest cells change locations and constancy of cell-cell relations at the crest-mesoderm interface, are addressed.

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells

Fate maps based on quail-chick grafting of avian cephalic neural crest precursors and paraxial mesoderm cells have identified the majority of derivatives from each population but have not unequivocally resolved the precise locations of and population dynamics at the interface between them. The relation between these two mesenchymal tissues is especially critical for the development of skeletal muscles, because crest cells play an essential role in their differentiation and subsequent spatial organization. It is not known whether myogenic mesoderm and skeletogenic neural crest cells establish permanent relations while en route to their final destinations, or later at the sites where musculoskeletal morphogenesis is completed. We applied beta-galactosidase-encoding, replication-incompetent retroviruses to paraxial mesoderm, to crest progenitors, or at the interface between mesodermal and overlying neural crest as both were en route to branchial or periocular regions in chick embryos. With respect to skeletal structures, the results identify the avian neural crest:mesoderm boundary at the junction of the supraorbital and calvarial regions of the frontal bone, lateral to the hypophyseal foramen, and rostral to laryngeal cartilages. Therefore, in the chick embryo, most of the frontal and the entire parietal bone are of mesodermal, not neural crest, origin. Within paraxial mesoderm, the progenitors of each lineage display different behaviors. Chondrogenic cells are relatively stationary and intramembranous osteogenic cells move only in transverse planes around the brain. Angioblasts migrate invasively in all directions. Extraocular muscle precursors form tightly aggregated masses that en masse cross the crest:mesoderm interface to enter periocular territories, while branchial myogenic lineages shift ventrally coincidental with the movements of corresponding neural crest cells. En route to the branchial arches, myogenic mesoderm cells do not maintain constant, nearest-neighbor relations with adjacent, overlying neural crest cells. Thus, progenitors of individual muscles do not establish stable, permanent relations with their connective tissues until both populations reach the sites of their morphogenesis within branchial arches or orbital regions.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Ephrin-As play a rhombomere-specific role in trigeminal motor axon projections in the chick embryo

In this study, we investigate the possible role of ephrin-Eph signaling in trigeminal motor axon projections. We find that EphA receptors are expressed at higher levels by rhombomere 2 (r2) trigeminal motor neurons than by r3 trigeminal motor neurons in the chick embryo. Mapping of rhombomere-specific axon projections shows that r2 and r3 trigeminal motor neurons project to different muscle targets, including the mandibular adductor and the intermandibularis muscles respectively. Ephrin-A5 is expressed in these muscles, especially in some regions of the intermandibularis muscle, and can cause growth cone collapse of both r2 and r3 motor axons in vitro. We demonstrate that in vivo overexpression of ephrin-A5 in the intermandibularis muscle, or overexpression of dominant-negative EphA receptors in trigeminal motor neurons leads to a reduction in branching of r3-derived motor axons specifically. Overexpression of full-length EphA receptors impairs the formation of r3 projections to the intermandibularis muscle. These findings indicate that ephrins and their Eph receptors play a role in trigeminal motor axon topographic mapping and in rhombomere 3-derived projections in particular.

Morphogenesis of parrot jaw muscles: understanding the development of an evolutionary novelty

Parrots have developed novel head structures in their evolutionary history. The appearance of two new muscles for strong jaw adduction is especially fascinating in developmental and evolutionary contexts. However, jaw muscle development of parrots has not been described, despite its uniqueness. This report first presents the normal developmental stages of the cockatiel (Nymphicus hollandicus), comparable to that of the chick. Next, the peculiar skeletal myogenesis in the first visceral arch of parrots is described, mainly focusing on the development of two new jaw muscles. One of the parrot-specific muscles, M. ethmomandibularis, was initially detected at Nymphicus Stage 28 (N28) as the rostral budding of M. pterygoideus. After N32, the muscle significantly elongates rostrodorsally toward the interorbital septum, following a course lateral to the palatine bone. Another parrot-specific muscle, M. pseudomasseter, was first recognized at N36. The muscle branches off from the posteromedial M. adductor mandibulae externus and grows in a dorsolateral direction, almost covering the lateral surface of the jugal bar. The upper tip of the muscle is accompanied by condensed mesenchyme, which seems to be derived from cephalic neural crest cells.

Normal and aberrant craniofacial myogenesis by grafted trunk somitic and segmental plate mesoderm

Our research assesses the ability of three trunk mesodermal populations– medial and lateral halves of newly formed somites, and presomitic(segmental plate) mesenchyme – to participate in the differentiation and morphogenesis of craniofacial muscles. Grafts from quail donor embryos were placed in mesodermal pockets adjacent to the midbrain-hindbrain boundary,prior to the onset of neural crest migration, in chick host embryos. This encompasses the site where the lateral rectus and the proximal first branchial arch muscle primordia arise. The distribution and differentiation of graft-derived cells were assayed using QCPN and QH1 antibodies to identify all quail cells and quail endothelial cells, respectively. Chimeric embryos were assayed for expression of myf5, myod, paraxis and lbx1, and the synthesis of myosin heavy chain (MyHC), between 1 and 6 days later (stages 14-30). Heterotopic and control (orthotopic) transplants consistently produced invasive angioblasts, and contributed to the lateral rectus and proximal first branchial arch muscles; many also contributed to the dorsal oblique muscle. The spatiotemporal patterns of transcription factor and MyHC expression by these trunk cells mimicked those of normal head muscles. Heterotopic grafts also gave rise to many ectopic muscles. These were observed in somite-like condensations at the implant site, in dense mesenchymal aggregates adjacent to the midbrain-hindbrain boundary, and in numerous small condensations scattered deep to the dorsal margin of the eye. Cells in ectopic condensations expressed trunk transcription factors and differentiated rapidly, mimicking the trunk myogenic timetable. A novel discovery was the formation by grafted trunk mesoderm of many mononucleated myocytes and irregularly oriented myotubes deep to the eye. These results establish that the head environment is able to support the progressive differentiation of several distinct trunk myogenic progenitor populations, over-riding whatever biases were present at the time of grafting. The spatial and temporal control of head muscle differentiation and morphogenesis are very site specific, and head mesoderm outside of these sites is normally refractory to, or inhibited by, the signals that initiate ectopic myogenesis by grafted trunk mesoderm cells.

Patterns of spatial and temporal visceral arch muscle development in the Mexican axolotl (Ambystoma mexicanum)

Vertebrate head development is a classical topic that has received renewed attention during the last decade. Most reports use one of a few model organisms (chicken, mouse, zebrafish) and have focused on molecular mechanisms and the role of the neural crest, while cranial muscle development has received less attention. Here we describe cranial muscle differentiation and morphogenesis in the Mexican axolotl, Ambystoma mexicanum. To determine the onset of differentiation we use antibodies against desmin and optical sectioning using confocal laser scanning microscopy on whole-mount immunostained embryos. This technique makes it possible to document the cranial muscle in three dimensions while keeping the specimens intact. Desmin expression starts almost simultaneously in the first, second, and third visceral arch muscles (as in other amphibians studied). Muscle anlagen divide up early into the different elements which constitute the larval cranial musculature. We extend and refine earlier findings, e.g., by documenting a clear division between interhyoideus and interhyoideus posterior. The timing of cranial muscle differentiation differs among vertebrate groups, but seems to be constant within each group. This study provides a morphological foundation for further studies of muscle cell fate and early differentiation.

Morphogenesis of blood vessels in the head muscles of avian embryo: spatial, temporal, and VEGF expression analyses

Adult skeletal muscle is a highly vascularized tissue, but the development of intramuscular endothelial networks has not been well studied. In quail embryos, QH1-positive angioblasts are present and moving throughout myogenic head mesoderm before the onset of primary myotube formation. On day 5 of incubation, concurrent with early myotube formation and aggregation, angioblasts establish a transient vascular plexus surrounding the myogenic condensations. Between days 5 and 9, the intramuscular vessels form an irregular network of endothelial cords and patent channels and only later are the parallel arrays of capillaries characteristic of adult muscles established. Microinjections using India ink, QH1, and Mercox resin reveal that these intramuscular capillaries are typically not connected to systemic vessels of the head until day 10, which is near the end of primary myogenesis and corresponds to the onset of muscular function. Morphometric analyses performed during primary myogenesis stages show a decrease in muscle cell density but no significant changes in intramuscular vascular density between days 5 and 9. This finding was surprising, as it is generally assumed that muscle growth requires elevated oxygen and nutrient levels. Moreover, there are no significant morphometric differences in vascular supply to embryonic fast and slow muscles. Endothelial tissue density is similar in slow muscles (oculorotatory, e.g., lateral rectus), fast muscles (mandibular depressor), and mixed muscles, in which the fiber types can be interspersed (jaw adductors) or segregated (branchiomandibular). Vascular endothelial growth factor (VEGF) protein is abundant in myotubes but not endothelial cells within both fast and slow head muscles at days 7 and 9. However, in some mixed muscles, only a minority of myotubes, which do not correspond to one specific fiber type, express VEGF. These results document a dynamic set of intramuscular and perimuscular angiogenic reorganizations during avian head myogenesis. Thus far, no vasculogenic distinctions between fast and slow muscles have been observed, although muscle heterogeneity in VEGF expression is evident.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Apoptosis of premigratory neural crest cells in rhombomeres 3 and 5: consequences for patterning of the branchial region

In the avian hindbrain, premigratory neural crest cells undergo programmed cell death (apoptosis) in rhombomeres 3 and 5 (r3, r5). Here, we have attempted to analyze the significance of the loss of neural crest cells from these odd-numbered rhombomeres. When apoptosis is prevented in r3 and r5, r3 crest migrate into the first arch and r5 into the third arch. Interestingly, these extra neural crest cells contributed to the formation of ectopic muscle attachment sites that are also found in those species in which r3 and r5 neural crest cells do not undergo apoptosis. Thus, apoptosis in the odd-numbered rhombomeres appears to be an evolutionarily derived mechanism that is required to eliminate r3 and r5 crest migration into first and third arches and thereby remove these muscle attachment sites.

Cryptic responses to tissue manipulations in avian embryos

Experimental embryology performed on avian embryos combines tissue manipulations and cell-labeling methods with increasing opportunities and demands for critical assays of the results. These approaches continue to reveal unexpected complexities in the normal patterns of cell movement and tissue origins, documentation of which is critical to unraveling the intricacies of cell and tissue interactions during embryogenesis. Viktor Hamburger's many pioneering contributions helped launch and promote the philosophical as well as technical elements of avian experimental embryology. Furthermore, his scholarship and profoundly positive presence influenced not just those of us fortunate to have trained with him, but several generations of developmental biologists. The first part of this article presents examples of the opportunities and rewards that have occurred due to his influences. Surgical manipulation of avian embryonic tissues always introduces a greater number of variables than the experimenter can control for or, often, readily identify. We present the results of dorsal and ventral lesions of hindbrain segments, which include defects in structures within, beside, and also at a considerable distance from the site of lesion. Extramedullary loops of longitudinal tract axons exit and re-enter the neural tube, and intra-medullary proliferation of blood vessels is expanded. Peripherally, the coalescence of neural crest- and placode-derived neuroblasts is disrupted. As expected, motor neurons and their projections close to the sites of lesion are compromised. However, an unexpected finding is that the normal projections of cranial nerves located distant to the lesion site were also disrupted. Following brainstem lesions in the region of rhombomeres 3, 4 or 5, trigeminal or oculomotor axons penetrated the lateral rectus muscle. Surprisingly, the ability of VIth nerve axons to reach the lateral rectus muscle was not destroyed in most cases, even though the terrain through which they needed to pass was disrupted. These axons typically followed a more ventral course than normal, and usually, the axons emerging from individual roots failed to fasciculate into a common VIth nerve, which suggests that each rootlet contains pathfinder-competent axons. The lesson from these lesions is that surgical intervention in avian embryos may have substantial effects upon tissues within, adjacent to, and distant to those that are being manipulated.

Median accessory digastric muscle: radiological and surgical correlation

A median accessory digastric muscle was revealed during a dissection of the submental region. The muscle was located between the anterior bellies of the digastrics, external to the mylohyoid and deep to the platysma. It appeared as a flat quadrilateral sheet. Its base arose from the front of the body of the hyoid bone near its upper border. Lower portion of the fibers, at the right base of the muscle, initially traveled perpendicular to the lower fibers of the right mylohyoid. The rest of the muscle had a median course, with the cranial end inserting into the mandible between the digastric fossae. The muscle elevated the hyoid bone and depressed the mandible when appropriate stress was applied. No other morphologic abnormalities were found in this region. Aberrant anterior bellies of the digastric muscles are uncommon and occur bilaterally or unilaterally. This observation of a median accessory digastric muscle has not previously been reported. Knowledge of this variant will help to avoid confusion with pathological conditions of the floor of the mouth and the submental region. It is relevant both for the interpretation of radiological images and during surgical procedures such as dissection of the anterior belly of the digastric for a malignant disease and graft positioning.

Homeotic transformation of branchial arch identity after Hoxa2 overexpression

Overexpression of Hoxa2 in the chick first branchial arch leads to a transformation of first arch cartilages, such as Meckel's and the quadrate, into second arch elements, such as the tongue skeleton. These duplicated elements are fused to the original in a similar manner to that seen in the Hoxa2 knockout, where the reverse transformation of second to first arch morphology is observed. This confirms the role of Hoxa2 as a selector gene specifying second arch fate. When first arch neural crest alone is targeted, first arch elements are lost, but the Hoxa2-expressing crest is unable to develop into second arch elements. This is not due to Hoxa2 preventing differentiation of cartilages. Upregulation of a second arch marker in the first arch, and homeotic transformation of cartilage elements is only produced after global Hoxa2 overexpression in the crest and the surrounding tissue. Thus, although the neural crest appears to contain some patterning information, it needs to read cues from the environment to form a coordinated pattern. Hoxa2 appears to exert its effect during differentiation of the cartilage elements in the branchial arches, rather than during crest migration, implying that pattern is determined quite late in development.

Compensatory defects associated with mutations in Hoxa1 restore normal palatogenesis to Hoxa2 mutants

The rhombencephalic neural crest play several roles in craniofacial development. They give rise to the cranial sensory ganglia and much of the craniofacial skeleton, and are vital for patterning of the craniofacial muscles. The loss of Hoxa1 or Hoxa2 function affects the development of multiple neural crest-derived structures. To understand how these two genes function together in craniofacial development, an allele was generated that disrupts both of these linked genes. Some of the craniofacial defects observed in the double mutants were additive combinations of those that exist in each of the single mutants, indicating that each gene functions independently in the formation of these structures. Other defects were found only in the double mutants demonstrating overlapping or synergistic functions. We also uncovered multiple defects in the attachments and trajectories of the extrinsic tongue and hyoid muscles in Hoxa2 mutants. Interestingly, the abnormal trajectory of two of these muscles, the styloglossus and the stylohyoideus, blocked the attachment of the hyoglossus to the greater horn of the hyoid, which in turn correlated exactly with the presence of cleft palate in Hoxa2 mutants. We suggest that the hyoglossus, whose function is to depress the lateral edges of the tongue, when unable to make its proper attachment to the greater horn of the hyoid, forces the tongue to adopt an abnormal posture which blocks closure of the palatal shelves. Unexpectedly, in Hoxa1/Hoxa2 double mutants, the penetrance of cleft palate is dramatically reduced. We show that two compensatory defects, associated with the loss of Hoxa1 function, restore normal attachment of the hyoglossus to the greater horn thereby allowing the palatal shelves to lift and fuse above the flattened tongue.

Differentiation of avian craniofacial muscles: I. Patterns of early regulatory gene expression and myosin heavy chain synthesis

Myogenic populations of the avian head arise within both epithelial (somitic) and mesenchymal (unsegmented) mesodermal populations. The former, which gives rise to neck, tongue, laryngeal, and diaphragmatic muscles, show many similarities to trunk axial, body wall, and appendicular muscles. However, muscle progenitors originating within unsegmented head mesoderm exhibit several distinct features, including multiple ancestries, the absence of several somite lineage-determining regulatory gene products, diverse locations relative to neuraxial and pharyngeal tissues, and a prolonged and necessary interaction with neural crest cells. The object of this study has been to characterize the spatial and temporal patterns of early muscle regulatory gene expression and subsequent myosin heavy chain isoform appearance in avian mesenchyme-derived extraocular and branchial muscles, and compare these with expression patterns in myotome-derived neck and tongue muscles. Myf5 and myoD transcripts are detected in the dorsomedial (epaxial) region of the occipital somites before stage 12, but are not evident in the ventrolateral domain until stage 14. Within unsegmented head mesoderm, myf5 expression begins at stage 13.5 in the second branchial arch, followed within a few hours in the lateral rectus and first branchial arch myoblasts, then other eye and branchial arch muscles. Expression of myoD is detected initially in the first branchial arch beginning at stage 14.5, followed quickly by its appearance in other arches and eye muscles. Multiple foci of myoblasts expressing these transcripts are evident during the early stages of myogenesis in the first and third branchial arches and the lateral rectus-pyramidalis/quadratus complex, suggesting an early patterned segregation of muscle precursors within head mesoderm. Myf5-positive myoblasts forming the hypoglossal cord emerge from the lateral borders of somites 4 and 5 by stage 15 and move ventrally as a cohort. Myosin heavy chain (MyHC) is first immunologically detectable in several eye and branchial arch myofibers between stages 21 and 22, although many tongue and laryngeal muscles do not initiate myosin production until stage 24 or later. Detectable synthesis of the MyHC-S3 isoform, which characterizes myofibers as having "slow" contraction properties, occurs within 1-2 stages of the onset of MyHC synthesis in most head muscles, with tongue and laryngeal muscles being substantially delayed. Such a prolonged, 2- to 3-day period of regulatory gene expression preceding the onset of myosin production contrasts with the interval seen in muscles developing in axial (approximately 18 hr) and wing (approximately 1-1.5 days) locations, and is unique to head muscles. This finding suggests that ongoing interactions between head myoblasts and their surroundings, most likely neural crest cells, delay myoblast withdrawal from the mitotic pool. These descriptions define a spatiotemporal pattern of muscle regulatory gene and myosin heavy chain expression unique to head muscles. This pattern is independent of origin (somitic vs. unsegmented paraxial vs. prechordal mesoderm), position (extraocular vs. branchial vs. subpharyngeal), and fiber type (fast vs. slow) and is shared among all muscles whose precursors interact with cephalic neural crest populations. Dev Dyn 1999;216:96-112.

Myogenic determination and differentiation of the mouse palatal muscle in relation to the developing mandibular nerve

The vertebrate palatal muscles are derived from the cranial paraxial mesoderm and start myogenesis by the expression of myogenic regulatory factors (MRFs). Predetermined myogenic cells migrate from the cranial paraxial mesoderm into the branchial arches, followed by myogenic differentiation. The objective of this study was to elucidate whether the determination, migration, and differentiation of myogenic cells during the myogenesis of the palatal muscles, particularly the tensor veli palatini (TVP), are related to the extending mandibular nerve in mouse embryos. By immunohistochemical staining at embryonic day (E) 9.5, MyoD1 and myogenin have been expressed in the mandibular arch, into which the mandibular nerve had not yet extended. At E11.5, these myogenic cells encircled the extending mandibular nerve and were distributed from the distal and lateral to the trigeminal ganglion and into the mandibular arch to form the muscle plate, a girdle-like structure. By E12.5, these myogenic cells lost their girdle-like pattern, vacated the trunk area of the mandibular nerve, and were separated into several incompletely divided masses encircling the collateral branches of the mandibular nerve. The TVP started differentiation at E13.5 with the appearance of myofilaments and acetylcholinesterase (AchE), whereas the other palatal muscles began differentiation at E14.5. We defined the differentiation process of mouse palatal muscles into five stages based on the present findings. These results suggest that the determination and initial migration of the palatal myogenic cells into the mandibular arch occur before the mandibular nerve extends out of the trigeminal ganglion, whereas the myogenic cells migrating into the final sites of differentiation intimately relate to the extending nerve.

Myotube heterogeneity in developing chick craniofacial skeletal muscles

Avian skeletal muscles consist of myotubes that can be categorized according to contraction and fatigue properties, which are based largely on the types of myosins and metabolic enzymes present in the cells. Most mature muscles in the head are mixed, but they display a variety of ratios and distributions of fast and slow muscle cells. We examine the development of all head muscles in chick and quail embryos, using immunohistochemical assays that distinguish between fast and slow myosin heavy chain (MyHC) isoforms. Some muscles exhibit the mature spatial organization from the onset of primary myotube differentiation (e.g., jaw adductor complex). Many other muscles undergo substantial transformation during the transition from primary to secondary myogenesis, becoming mixed after having started as exclusively slow (e.g., oculorotatory, neck muscles) or fast (e.g., mandibular depressor) myotube populations. A few muscles are comprised exclusively of fast myotubes throughout their development and in the adult (e.g., the quail quadratus and pyramidalis muscles, chick stylohyoideus muscles). Most developing quail and chick head muscles exhibit identical fiber type composition; exceptions include the genioglossal (chick: initially slow, quail: mixed), quadratus and pyramidalis (chick: mixed, quail: fast), and stylohyoid (chick: fast, quail: mixed). The great diversity of spatial and temporal scenarios during myogenesis of head muscles exceeds that observed in the limbs and trunk, and these observations, coupled with the results of precursor mapping studies, make it unlikely that a lineage based model, in which individual myoblasts are restricted to fast or slow fates, is in operation. More likely, spatiotemporal patterning of muscle fiber types is coupled with the interactions that direct the movements of muscle precursors and subsequent segregation of individual muscles from common myogenic condensations. In the head, most of these events are facilitated by connective tissue precursors derived from the neural crest. Whether these influences act upon uncommitted, or biased but not restricted, myogenic mesenchymal cells remains to be tested.

Response of "naive" cutaneous and muscle afferents to potential targets in vitro

It is now well documented that motoneurons are specified to innervate particular target muscles prior to axon outgrowth. Here we investigate whether sensory neurons are similarly specified to innervate target skin or muscle, taking advantage of the avian trigeminal system where cutaneous and muscle afferents are anatomically separate. Using this system, we have previously shown that by embryonic day 10 (E10) (approximately 4-5 days after target innervation), regenerating cutaneous and muscle afferents differ in their response to various potential targets in vitro, in manners consistent with their normal innervation patterns in vivo. Thus, by E10 these two populations of sensory neurons have distinct identities as skin and muscle afferents. In contrast, we report here that the responses of younger, naive cutaneous and muscle afferents that have not yet, or only recently, innervated peripheral targets are indistinguishable, regardless of the target tissue tested. These findings suggest that at stages when innervation is being established, cutaneous and muscle afferents, unlike motoneurons, may not yet have acquired rigidly specified identities and/or the ability to recognize and respond selectively to their appropriate peripheral targets.

Muscle and tendon morphogenesis in the avian hind limb

The proper development of the musculoskeletal system in the tetrapod limb requires the coordinated development of muscle, tendon and cartilage. This paper examines the morphogenesis of muscle and tendon in the developing avian hind limb. Based on a developmental series of embryos labeled with myosin and tenascin antibodies in whole mount, an integrative description of the temporal sequence and spatial pattern of muscle and tendon morphogenesis and their relationship to cartilage throughout the chick hind limb is presented for the first time. Anatomically distinct muscles arise by the progressive segregation of muscle: differentiated myotubes first appear as a pair of dorsal and ventral muscle masses; these masses subdivide into dorsal and ventral thigh, shank and foot muscle masses; and finally these six masses segregate into individual muscles. From their initial appearance, most myotubes are precisely oriented and their pattern presages the pattern of future, individual muscles. Anatomically distinct tendons emerge from three tendon primordia associated with the major joints of the limb. Contrary to previous reports, comparison of muscle and tendon reveals that much of their morphogenesis is temporally and spatially closely associated. To test whether reciprocal muscle-tendon interactions are necessary for correct muscle-tendon patterning or whether morphogenesis of each of these tissues is autonomous, two sets of experiments were conducted: (1) tendon development was examined in muscleless limbs produced by coelomic grafting of early limb buds and (2) muscle development was analyzed in limbs where tendon had been surgically altered. These experiments demonstrate that in the avian hind limb the initial morphogenetic events, formation of tendon primordia and initial differentiation of myogenic precursors, occur autonomously with respect to one another. However, later morphogenetic events, such as subdivision of muscle masses and segregation of tendon primordia into individual tendons, do require to various degrees reciprocal interactions between muscle and tendon. The dependence of these later morphogenetic events on tissue interactions differs between different proximodistal regions of the limb.

Rhombencephalic neural crest segmentation is preserved throughout craniofacial ontogeny

To investigate the influence of hindbrain segmentation on craniofacial patterning we have studied the long term fate of neural crest (NC) subpopulations of individual rhombomeres (r), using quail-chick chimeras. Mapping of all skeletal and muscle connective tissues developing from these small regions revealed several novel features of the cranial neural crest. First, the mandibular arch skeleton has a composite origin in which the proximal elements are r1+r2 derived, whereas more distal ones are exclusively midbrain derived. The most proximal region of the lower jaw is derived from second arch (r4) NC. Second, both the lower jaw and tongue skeleton display an organisation which precisely reflects the rostrocaudal order of segmental crest deployment from the embryonic hindbrain. Third, cryptic intraskeletal boundaries, which do not correspond to anatomical landmarks, form sharply defined interfaces between r1+r2, r4 and r6+r7 crest. Cells that survive the early apoptotic elimination of premigratory NC in r3 and r5 are restricted to tiny contributions within the 2nd arch (r4) skeleton. Fourth, a highly constrained pattern of cranial skeletomuscular connectivity was found that precisely respects the positional origin of its constitutive crest: each rhombomeric population remains coherent throughout ontogeny, forming both the connective tissues of specific muscles and their respective attachment sites onto the neuro- and viscerocranium. Finally, focal clusters of crest cells, confined to the attachment sites of branchial muscles, intrude into the otherwise mesodermal cranial base. In the viscerocranium, an equally strict, rhombomere-specific matching of muscle connective tissues and their attachment sites is found for all branchial and tongue (hypoglossal) muscles. This coherence of segmental crest populations explains how cranial skeletomuscular pattern can be implemented and conserved despite evolutionary changes in the shapes of skeletal elements.

Gene targeting the myf-5 locus with nlacZ reveals expression of this myogenic factor in mature skeletal muscle fibres as well as early embryonic muscle

We have introduced the nlacZ reporter gene into the locus of the myogenic factor gene myf-5 by homologous recombination in embryonic stem (ES) cells. Targeted ES clones were injected into precompaction morula, and the beta-galactosidase expression pattern was monitored. These mice permit the sensitive visualization of myf-5 expression throughout the embryo, and provide a standard for comparing it with that seen with different myf-5/nlacZ transgenes. Thus, in a comparison using ES cells in chimaeric embryos containing the targeted or randomly integrated myf-5/nlacZ construct, we demonstrate that 5.5 kbp of myf-5 upstream flanking sequence including exon1 and most of intron1 directs some skeletal muscle expression, but this is neither qualitatively nor quantitatively equivalent to that of the endogenous gene. Myf-5 is expressed early, before terminal myogenesis takes place in the medial half of the somite, and subsequently it is a major myogenic factor as skeletal muscle forms. All skeletal muscle shows beta-galactosidase activity, even after birth, indicating that myf-5 expression is not confined to primary myotubes, which are derived from embryonic myoblasts, but is also present in muscles containing different adult fibre types. The presence of myf-5 transcripts from the endogenous gene in older muscle was confirmed by in situ hybridization. These results suggest that the myf-5 gene is not activated in only a subset of muscle cells and are consistent with the results on the MyoD knockout mice.

Muscle cell death during the development of head and neck muscles in the chick embryo

Degenerating myofibers have been reported in the embryos and neonates of a number of birds and mammals, but neither the pervasiveness of the phenomenon nor the spatio-temporal patterns of degeneration has been examined in detail. Using transmission electron microscopy, we determined the patterns of muscle cell death in the chick biventer cervicis, a head extensor muscle. Cell death is most pronounced at incubation days 10 through 15, and occurs throughout the muscle. This is the period during which many myofiber clusters segregate into individual fibers, each with a separate basal lamina, and secondary myofibers become demarcated. Cells of largest diameter, presumably the primary myofibers, are preferentially affected. Degenerating cells exhibit a cohort of cytological features consistent with apoptosis, including the presence of dense, darkly-staining, hypercontracted myofibrils, misshapen nuclei with irregular chromatin condensations along the nuclear envelope, and scores of cytoplasmic vesicles and vacuoles. In cross section some large diameter muscle cells are characterized by sparse, flocculent cytoplasm that is devoid of myofibrils and organelles. Some show disintegrating cell membranes. In longitudinal section 200-300 microns long regions of hypercontracted myofibrils alternate with areas devoid of fibrils; this arrangement suggests that the myofibrils break into segments that are in register along one part of a muscle fiber and entirely absent from the adjacent length of fiber. We have observed similar patterns of muscle cell degeneration in the complexus, splenius cervicis, depressor mandibulae, and branchiomandibularis muscles. By day 18 of incubation most signs of degeneration are absent and by hatching (day 21) the muscle fibers all appear healthy. Many of these cytological changes in embryonic head muscle cells are characteristic of programmed cell death. We hypothesize that large-scale death of myocytes is a normal part of avian myogenesis and an important mechanism for affecting the transformation from embryonic to hatching muscle patterning.

Development of craniofacial musculature in Monodelphis domestica (Marsupialia, Didelphidae)

Development of craniofacial muscles of Monodelphis domestica (Marsupialia, Didelphidae) is described. In a period of 4-6 days all craniofacial muscles in M. domestica progress from myoblast condensation, to striated myofibers that are aligned in the direction of adult muscles and possess multiple, lateral nuclei. This process begins 1 to 2 days before birth and continues during the first few days after birth. Compared to other aspects of cranial development, muscle development in M. domestica is rapid. This rapid and more or less simultaneous emergence of craniofacial muscles differs from the previously described pattern of development of the cranial skeleton in marsupials, which displays a mosaic of acceleration and deceleration of regions and individual elements. Unlike the skeletal system, craniofacial muscles show no evidence of regional specialization during development. M. domestica resembles eutherian mammals in the relatively rapid and more or less simultaneous differentiation of all craniofacial muscles. It differs from eutherian taxa in that most stages of myogenesis occur postnatally, following the onset of function. The timing of the development of muscular and skeletal structures is compared and it is concluded that the relatively early development of muscle is not reflected by any particular acceleration of the differentiation or growth of skeletal structures. Finally, the difficulties in accounting for complex internal arrangements of muscles such as the tongue, given current models of myogenesis are summarized.

Developmental relations between sixth nerve motor neurons and their targets in the chick embryo

The developmental relations between abducens (VI) nerves and their targets, the lateral rectus, quadratus, and pyramidalis muscles, have been examined in the chick embryo from early neural tube stages through 10 days of incubation. Sites of myoblast origins were determined by microinjection of replication-incompetent retroviruses containing the LacZ reporter into paraxial mesoderm corresponding to somitomeres 3-5. Motor neurons and axons were identified by Bodian staining, immunocytochemistry, and application of DiI and DiO to dissected peripheral nerves. Anlage of the dorsal oblique originate in somitomere 3, close to the ventrolateral margin of the mid-to-caudal mesencephalon. Precursors of the lateral rectus arise deep within somitomere 4, beside the future metencephalon (rhombomere "A"). Quadratus and pyramidalis precursors are located between and partially segregated from these other two anlage. VIth nerve axons exit rhombomeres 5 and 6 via multiple median roots, fasciculate, and by stage 17 have elongated rostrally beneath the hindbrain. Immediately caudal to a mesenchymal pre-muscle condensation located deep to rhombomere 2, the VIth nerve separates into two branches. One branch enters the rostral portion of the condensation, from which quadratus and pyramidalis muscles will segregate. This branch projects exclusively from rhombomere 5 and is the accessory abducens nerve. The other branch enters the caudal, presumptive lateral rectus, region of the condensation. This is the abducens nerve, and it projects from cells located in both rhombomeres 5 and 6. These findings indicate that specific matching of motor nerves with their presumptive targets begins prior to the differentiation and segregation of myogenic populations, and that spatial organization of developing eye muscles is initiated well before they interact with connective tissue precursors derived from the neural crest.

Development of the mesencephalic nucleus of the trigeminal nerve in chick embryos: target innervation, neurotrophin receptors, and cell death

The goal of this study was to determine whether processes of neurons in the mesencephalic nucleus of the trigeminal nerve (Mes V) of chick embryos arrive in their peripheral target prior to the period of developmental cell death, and to determine whether neurons with early target contact survive to a greater extent than neurons with processes that reach their peripheral target later. The arrival of Mes V nerve fibers in the masticatory muscles was determined by injecting the fluorescent tracer DiI, and the position of labeled and unlabeled neurons was mapped in subdivisions of the Mes V nucleus. Developmental changes in the numerical configuration of Mes V subdivisions were studied in DiI-labeled as well as Nissl-stained material. The expression of low-affinity (p75) neurotrophin receptors was investigated throughout development of the Mes V nucleus with in situ hybridization to assess whether and how levels of expression of this trophic receptor may relate to target innervation and cell death. The extent of cell death was evaluated by counting pyknotic nuclei. Processes of Mes V neurons invade their peripheral target between 5 and 7 days of incubation (E5-7). At E7-12, between 800 and 1,400 labeled Mes V neurons were distributed throughout the two main subdivisions of the Mes V nucleus, the tectal commissure and the optic tectum. Only few Mes V neurons were labeled in the posterior commissure or outside the brain. Cell counts in Nissl-stained material from E7-13 revealed that the numbers of Mes V neurons in the optic tectum decreased to about 40-60%, and in the tectal commissure to 20-25%, whereas Mes V neurons in the posterior commissure disappeared almost entirely. Few Mes V neurons remained in the leptomeninges at E8-10, but a considerable number was found outside the midbrain at E11, indicating ongoing migration of some Mes V neurons. Neurotrophin receptors were differentially expressed in the Mes V nucleus: Before and after the period of cell death, 90-100% of Mes V neurons expressed neurotrophin receptors, whereas during, and immediately preceding the period of developmental cell death (E9-E13), merely 70% of Mes V neurons expressed this receptor. These findings are consistent with the hypothesis that early target contact may provide an advantage for the survival of Mes V neurons and that competition for trophic factors may occur in the peripheral target of this nucleus prior to the period of cell death.

Interconnections of muscles in the adductor mandibulae complex of birds

In addition to a variety of connections at or near the attachment of muscles, some of the adjacent bellies in the adductor mandibulae complex of birds merge with one another across the whole or most of their contiguous sides. Such continuous intermuscular transitions are found between the adductor posterior and the deep part of the adductor externus, the posterior adductor and quadratomandibularis muscle, the adductor posterior and pterygoideus muscle and between the quadratomandibularis and pseudotemporalis muscles. Since a complete breakup of the adductor premuscle mass occurs early in development and there is no indication of adult intermuscular transitions in any of the studied avian embryos, the muscles must coalesce at some stage of growth. Therefore, the intermuscular transitions of adult birds cannot be derived with any certainty from the primary connections in the adductor mandibulae of early gnathostomes. The coalescence of developmentally discrete muscles is invariably seamless and appears to be contingent not only upon the contiguity of muscles but also upon the congruent arrangement of their fibers.