Cranial neural crest cell migration in the Australian lungfish, Neoceratodus forsteri

Earliest migratory cephalic NC cells are potent to differentiate into dental ectomesenchyme of the two lungfish dentitions: tetrapodomorph ancestral condition of unconstrained capability of mesencephalic NC cells to form oral teeth

Reciprocal interactions between epithelial and neural crest-derived mesenchymal cells have been recognized in the evolutionary modulation of tetrapod odontodes, skeletal structures that include the teeth and tooth-integrated basal tissue. Using cell-tracking experiments, it has been demonstrated that mandibular neural crest cells, labelled during migration, extensively populate dental papillae of all tooth phenotypes of the lobe-finned fish, the Australian lungfish (Neoceratodus forsteri). Here, I report on an extension of this experimental study that earliest migrating NC cells are able to differentiate into odontogenic ectomesenchyme. Using vital dye cell-tracking to mark the mesencephalic neural crest prior to migration, I have found that the corresponding population of earliest migratory cells selectively relocated to dental papillae of both temporary and permanent dentitions of Neoceratodus. I noticed a gradient in distribution of the labelled cells which populated posterior teeth, pterygoid and prearticular (including associated trabecular and Meckelian cartilages; major relocation) much more densely than those in anterior marginal positions, temporary and vomeral permanent teeth (minor relocation). Contrary to mice and zebrafish, the odontogenic potency of mesencephalic neural crest cells is already programmed at the onset of the migration event in lungfish. This may imply that the morphogenic potential of mesencephalic neural crest cells to form teeth has been heterochronically shifted and constrained to later migratory populations of neural crest cells during the developmental evolution of derived tetrapods, or/and arrested in their expression in the oral development of some modern osteichthyans.

Neural crest and the patterning of vertebrate craniofacial muscles

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

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.

Spatial and temporal pattern for the dentition in the Australian lungfish revealed with sonic hedgehog expression profile

We report a temporal order of tooth addition in the Australian lungfish where timing of tooth induction is sequential in the same pattern as osteichthyans along the lower jaw. The order of tooth initiation in Neoceratodus starts from the midline tooth, together with left and right ones at jaw position 2, followed by 3 and then 1. This is the pattern order for dentary teeth of several teleosts and what we propose represents a stereotypic initiation pattern shared with all osteichthyans, including the living sister group to all tetrapods, the Australian lungfish. This is contrary to previous opinions that the lungfish dentition is otherwise derived and uniquely different. Sonic hedgehog (shh) expression is intensely focused on tooth positions at different times corresponding with their initiation order. This deployment of shh is required for lungfish tooth induction, as cyclopamine treatment results in complete loss of these teeth when applied before they develop. The temporal sequence of tooth initiation is possibly regulated by shh and is know to be required for dentition pattern in other osteichthyans, including cichlid fish and snakes. This reflects a shared developmental process with jawed vertebrates at the level of the tooth module but differs with the lack of replacement teeth.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

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

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

The fate of cranial neural crest cells in the Australian lungfish,Neoceratodus forsteri

The cranial neural crest has been shown to give rise to a diversity of cells and tissues, including cartilage, bone and connective tissue, in a variety of tetrapods and in the zebrafish. It has been claimed, however, that in the Australian lungfish these tissues are not derived from the cranial neural crest, and even that no migrating cranial neural crest cells exist in this species. We have earlier documented that cranial neural crest cells do migrate, although they emerge late, in the Australian lungfish. Here, we have used the lipophilic fluorescent dye, DiI, to label premigratory cranial neural crest cells and follow their fate until stage 43, when several cranial skeletal elements have started to differentiate. The timing and extent of their migration was investigated, and formation of mandibular, hyoid and branchial streams documented. Cranial neural crest was shown to contribute cells to several parts of the head skeleton, including the trabecula cranii and derivatives of the mandibular arch (e.g., Meckel's cartilage, quadrate), the hyoid arch (e.g., the ceratohyal) and the branchial arches (ceratobranchials I-IV), as well as to the connective tissue surrounding the myofibers in cranial muscles. We conclude that cranial neural crest migration and fate in the Australian lungfish follow the stereotyped pattern documented in other vertebrates.

Craniofacial development in marsupial mammals: Developmental origins of evolutionary change

Biologists have long studied the evolutionary consequences of the differences in reproductive and life history strategies of marsupial and eutherian mammals. Over the past few decades, the impact of these strategies on the development of the marsupial embryo and neonate has received attention. In this review, the differences in development in the craniofacial region in marsupial and eutherian mammals will be discussed. The review will highlight differences at the organogenic and cellular levels, and discuss hypotheses for shifts in the expression of important regulatory genes. The major difference in the organogenic period is a whole-scale shift in the relative timing of central nervous system structures, in particular those of the forebrain, which are delayed in marsupials, relative to the structures of the oral-facial apparatus. Correlated with the delay in development of nervous system structures, the ossification of the bones of the neurocranium are delayed, while those of the face are accelerated. This study will also review work showing that the neural crest, which provides much of the cellular material to the facial skeleton and may also carry important patterning information, is notably accelerated in its development in marsupials. Potential consequences of these observations for hypotheses on constraint, evolutionary integration, and the existence of developmental modules is discussed. Finally, the implications of these results for hypotheses on the genetic modulation of craniofacial patterning are presented.

Cranial neural crest cell migration in cockatielNymphicus hollandicus (Aves: Psittaciformes)

Parrots have developed unique jaw muscles in their evolutionary history. The M. pseudomasseter, which completely covers the lateral side of the jugal bar, is regarded as a jaw muscle unique to parrots. In a previous study, I presented a hypothesis on the relevance of modifications in the regulation of cranial neural crest cell (NCC) development to the generation of this novel jaw muscle based on histological analyses (Tokita [2004] J Morphol 259:69-81). In the present study, I investigated distribution and migration patterns of cranial neural crest cells (NCCs) through parrot embryogenesis with immunohistochemical techniques to further understand the role of cranial NCCs in the evolution of the M. pseudomasseter, and to provide new information on the relative plasticity in cranial NCC migration at early stages of avian development. The basic nature of cranial NCC development was mostly conserved between chick and parrot. In both, cranial NCCs migrated from the dorsal tip of the neural tube in a ventral direction. Three major populations were identified in their cranial NCCs. Migration pathways of these cells were almost identical between chick and parrot. The principal difference was seen in the relative timing of cranial NCC migration. In the parrot, cranial NCC migration into the first pharyngeal arch was more advanced than in the chick at early stages of development. Such a temporal shift in cranial NCC migration might influence architectural patterning of parrot jaw muscles that generates new muscle like M. pseudomasseter.

Evolution of cranial development and the role of neural crest: insights from amphibians

Contemporary studies of vertebrate cranial development document the essential role played by the embryonic neural crest as both a source of adult tissues and a locus of cranial form and patterning. Yet corresponding and basic features of cranial evolution, such as the extent of conservation vs. variation among species in the contribution of the neural crest to specific structures, remain to be adequately resolved. Investigation of these features requires comparable data from species that are both phylogenetically appropriate and taxonomically diverse. One key group are amphibians, which are uniquely able to inform our understanding of the ancestral patterns of ontogeny in fishes and tetrapods as well as the evolution of presumably derived patterns reported for amniotes. Recent data support the hypothesis that a prominent contribution of the neural crest to cranial skeletal and muscular connective tissues is a fundamental property that evolved early in vertebrate history and is retained in living forms. The contribution of the neural crest to skull bones appears to be more evolutionarily labile than that of cartilages, although significance of the limited comparative data is difficult to establish at present. Results underline the importance of accurate and reliable homology assessments for evaluating the contrasting patterns of derivation reported for the three principal tetrapod models: mouse, chicken and frog.

Cephalic neural crest cells and the evolution of craniofacial structures in vertebrates: morphological and embryological significance of the premandibular-mandibular boundary

The vertebrate head characteristically has two types of mesenchyme: the neural crest-derived ectomesenchyme and the mesoderm derived mesenchyme. Conserved patterns of development in various animal taxa imply the presence of shared inductive events for cephalic mesenchyme. These developmental programs can serve as developmental constraints that emerge as morphological homology of embryonic patterns. To understand the evolutionary changes in the developmental programs that shape the skull, we need to separate ancestral and derived patterns of vertebrate craniogenesis. This review deals with the terminology for neural crest cell subpopulations at each developmental stage, based on the topographical relationships and possible mechanisms for specification. The aim is to identify the changes that could have occurred in the evolutionary history of vertebrates. From comparisons of a lamprey species, Lethenteron japonicum, with gnathostomes it is clear that the initial distribution of cephalic crest cells is identical in the two animal lineages. In all vertebrate embryos, the trigeminal crest (TC) cells of an early pharyngula are subdivided into three subpopulations. At this stage, only the posterior subpopulation of the TC cells is specified as the mandibular arch, as compared to the more rostral components, the 'premandibular crest cells'. Later in development, the local specification patterns of the lamprey and the gnathostomes differ, so that homology cannot be established in the craniofacial primordia, including the oral apparatus. Therefore, embryological terminology should reflect these hierarchical patterns in developmental stages and phylogeny.

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.

Anomalies in the developing neural and visceral head skeleton of the Australian lungfish, Neoceratodus forsteri

Several anomalies occur in the developing neural and visceral head skeleton of young specimens of Neoceratodus forsteri that have been reared under laboratory conditions. These include anomalies of the basicranium and its derivatives, aberrations of the anterior mandible and hyoid apparatus, and abnormalities in the articulation of the jaws and the elements that produce them. Apart from the occasional absence of the basihyal, and failure of the quadrate processes to form, the anomalies are not deficiencies. Most involve malformations of parts of the neurocranium and visceral skeleton, inappropriate articulations or fusions between elements, disunity in structures that are normally fused and the appearance of supernumerary elements. The incidence of chondral anomalies, generally higher than aberrations that occur in the dermal skeleton in juvenile lungfish, ranges from 1-10% in laboratory reared individuals that have not been subjected to experimental interference. The anomalies differ from those found in many amphibian populations, in the field and in the laboratory, because they involve the cranium, and not the limbs, and the lungfish have not been exposed to the factors that cause anomalies in the amphibians. It is unlikely that the existence of those anomalies, if it is reflected in the wild population, places a selective pressure on the lungfish, because, in a normal season, less than 1% of the total number of eggs produced survive to be recruited into the adult population.

Early differentiation and migration of cranial neural crest in the opossum, Monodelphis domestica

Marsupial mammals are born at a highly altricial state. Nonetheless, the neonate must be capable of considerable functional independence. Comparative studies have shown that in marsupials the morphogenesis of many structures critical to independent function are advanced relative to overall development. Many skeletal and muscular elements in the facial region show particular heterochrony. Because neural crest cells are crucial to forming and patterning much of the face, this study investigates whether the timing of cranial neural crest differentiation is also advanced. Histology and scanning electron microscopy of Monodelphis domestica embryos show that many aspects of cranial neural crest differentiation and migration are conserved in marsupials. For example, as in other vertebrates, cranial neural crest differentiates at the neural ectoderm/epidermal boundary and migrates as three major streams. However, when compared with other vertebrates, a number of timing differences exist. The onset of cranial neural crest migration is early relative to both neural tube development and somite formation in Monodelphis. First arch neural crest cell migration is particularly advanced and begins before any somites appear or regional differentiation exists in the neural tube. Our study provides the first published description of cranial neural crest differentiation and migration in marsupials and offers insight into how shifts in early developmental processes can lead to morphological change.

Cranial neural crest emergence and migration in the Mexican axolotl (Ambystoma mexicanum)

The timing and pattern of cranial neural crest cell emergence and migration in the Mexican axolotl, Ambystoma mexicanum, are assessed using scanning electron microscopy (SEM). Cranial neural crest cells emerge and begin to migrate at the time of neural fold closure and soon form three distinct streams. The most anterior (mandibular) stream emerges first, at the level of the mesencephalon. Cells in this stream migrate rostroventrally around the optic vesicle. The second (hyoid) and third (branchial) streams emerge in close succession at the level of the rhombencephalon and extend ventrolaterally. Cells forming the hyoid stream migrate rostral to the otic vesicle, whereas the branchial stream divides into two parallel streams, which migrate caudal to the otic vesicle. At later stages (stage 26 onwards) the cranial neural crest cells disperse into the adjacent mesoderm and can no longer be followed by dissection and SEM. The pattern of cranial neural crest emergence and migration, and division into migratory streams is similar to that in other amphibians and in the Australian lungfish (Neoceratodus forsteri). Emergence of crest cells from the neural tube, relative to the time of neural tube closure, occurs relatively late in comparison to anurans, but much earlier than in the Australian lungfish. These results establish a morphological foundation for studies in progress on the further development and fate of cranial neural crest cells in the Mexican axolotl, as well as for studies of the role of cranial neural crest in cranial patterning.

Using heterochrony plots to detect the dissociated coevolution of characters

The comparison of developmental sequences among species is notoriously difficult. Here, heterochrony plots are introduced as a new graphic method to detect temporal shifts in the development of characters in pair-wise species comparisons. Plotting the timing of character development in one species against the timing of character development in another species allows us to compare a principally unlimited number of characters simultaneously and can detect whether suites of characters are dissociated from one another or not. Such heterochrony plots can be embedded into a comparative phylogenetic analysis in order to establish whether observed patterns of character codissociation are indeed due to their dissociated coevolution. Comparative phylogenetic analysis may also reveal multiple independent events of dissociated coevolution of the same suite of characters in a certain lineage, suggesting that the characters of this suite reciprocally constrain their evolutionary modifiability, thereby forming a unit of evolution. This ability to identify units of evolution is a prerequisite for assessing the validity of recently proposed scenarios, suggesting that modules of development and/or function tend to act as units of evolution. Starting from a detailed heterochrony plot comparing development in the direct developing frog Eleutherodactylus coqui and in the biphasically developing frog Discoglossus pictus, this comparative approach is illustrated focusing on the evolution of development of limbs, the nervous system and the pharyngeal arches in amphibians.