Dual origins of the prechordal cranium in the chicken embryo

Prechordal structures act cooperatively in early trabeculae development of gnathostome skull

The vertebrate skull is formed by mesoderm and neural crest (NC) cells. The mesoderm contributes to the skull chordal domain, with the notochord playing an essential role in this process. The NC contributes to the skull prechordal domain, prompting investigation into the embryonic structures involved in prechordal neurocranium cartilage formation. The trabeculae cartilage, a structure of the prechordal neurocranium, arises at the convergence of prechordal plate (PCP), ventral midline (VM) cells of the diencephalon, and dorsal oral ectoderm. This study examines the molecular participation of these embryonic structures in gnathostome trabeculae development. PCP-secreted SHH induces its expression in VM cells of the diencephalon, initiating a positive feedback loop involving SIX3 and GLI1. SHH secreted by the VM cells of the diencephalon acts on the dorsal oral ectoderm, stimulating condensation of NC cells to form trabeculae. SHH from the prechordal region affects the expression of SOX9 in NC cells. BMP7 and SHH secreted by PCP induce NKX2.1 expression in VM cells of the diencephalon, but this does not impact trabeculae formation. Molecular cooperation between PCP, VM cells of the diencephalon, and dorsal oral ectoderm is crucial for craniofacial development by NC cells in the prechordal domain.

Evolution of the therian face through complete loss of the premaxilla

The anatomical framework of the jawbones is highly conserved among most of the Osteichthyes, including the tetrapods. However, our recent study suggested that the premaxilla, the rostralmost upper jaw bone, was rearranged during the evolution of therian mammals, being replaced by the septomaxilla at least in the lateral part. In the present study, to understand more about the process of evolution from the ancestral upper jaw to the therian face, we re-examined the development of the therian premaxilla (incisive bone). By comparing mouse, bat, goat, and cattle fetuses, we confirmed that the therian premaxilla has dual developmental origins, the lateral body and the palatine process. This dual development is widely conserved among the therian mammals. Cell-lineage-tracing experiments using Dlx1-CreER^T2 mice revealed that the palatine process arises in the ventral part of the premandibular domain, where the nasopalatine nerve distributes, whereas the lateral body develops from the maxillary prominence in the domain of the maxillary nerve. Through comparative analysis using various tetrapods, we concluded that the palatine process should not be considered part of the ancestral premaxilla. It rather corresponds to the anterior region of the vomerine bone of nonmammalian tetrapods. Thus, the present findings indicate that the true premaxilla was completely lost during the evolution of the therian mammals, resulting in the establishment of the unique therian face as an evolutionary novelty. Reconsideration of the homological framework of the cranial skeleton based on the topographical relationships of the ossification center during embryonic development is warranted.

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

Evolution and development of the bird chondrocranium

Background

Birds exhibit an enormous diversity in adult skull shape (disparity), while their embryonic chondrocrania are considered to be conserved across species. However, there may be chondrocranial features that are diagnostic for bird clades or for Aves as a whole. We synthesized and analyzed information on the sequence of chondrification of 23 elements in ten bird species and five outgroups. Moreover, we critically considered the developmental morphology of the chondrocrania of 21 bird species and examined whether the diversity in adult skull shape is reflected in the development of the embryonic skull, and whether there are group-specific developmental patterns.

Results

We found that chondrocranial morphology is largely uniform in its major features, with some variation in the presence or absence of fenestrae and other parts. In kiwis (Apteryx), the unique morphology of the bony skull in the orbito-nasal region is reflected in its chondrocranial anatomy. Finally, differences in morphology and chondrification sequence may distinguish between different Palaeognathae and Neognathae and between the Galloanserae and Neoaves. The sequence of chondrification is largely conserved in birds, but with some variation in most regions. The peri- and prechordal areas in the base of the chondrocranium are largely conserved. In contrast to the outgroups, chondrification in birds starts in the acrochordal cartilage and the basicranial fenestra is formed secondarily. Further differences concern the orbital region, including early chondrification of the pila antotica and the late formation of the planum supraseptale.

Conclusion

Synthesizing information on chondrocranial development confronts terminological issues and a lack of comparable methods used (e.g., different staining; whole-mounts versus histology). These issues were taken into consideration when assessing differences across species. The summary of works on avian chondrocranial development, covered more than a century, and a comparison of the chondrification sequence among birds could be conducted. Future studies could test the hypothesis that chondrocranial disparity in Aves, in terms of the shape and proportion of individual elements, could be as large as adult skull disparity, despite conserved developmental patterns and the richness of forms in other (dermal) portions of the skull.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00406-z.

Evo-devo studies of cyclostomes and the origin and evolution of jawed vertebrates

Modern vertebrates consist of two sister groups: cyclostomes and gnathostomes. Cyclostomes are a monophyletic jawless group that can be further divided into hagfishes and lampreys, which show conspicuously different developmental and morphological patterns. However, during early pharyngula development, there appears to be a stage when the embryos of hagfishes and lampreys resemble each other by showing an "ancestral" craniofacial pattern; this pattern enables morphological comparison of hagfish and lamprey craniofacial development at late stages. This cyclostome developmental pattern, or more accurately, this developmental pattern of the jawless grade of vertebrates in early pharyngula was very likely shared by the gnathostome stem before the division of the nasohypophyseal placode led to the jaw and paired nostrils. The craniofacial pattern of the modern jawed vertebrates seems to have begun in fossil ostracoderms (including galeaspids), and was completed by the early placoderm lineages. The transition from jawless to jawed vertebrates was thus driven by heterotopy of development, mainly caused by separation and shift of ectodermal placodes and resultant ectomesenchymal distribution, and shifts of the epithelial-mesenchymal interactions that underlie craniofacial differentiation. Thus, the evolution of the jaw was not a simple modification of the mandibular arch, but a heterotopic shift of the developmental interactions involving not only the mandibular arch, but also the premandibular region rostral to the mandibular arch.

The neural crest and origin of the neurocranium in vertebrates

Cranium of jawed vertebrates is composed of dorsal moiety that encapsulates the brain, or the neurocranium, and the is called the neurocranium, and the ventral moiety, the viscerocranium, that supports the pharynx. In modern jawed vertebrates (crown gnathostomes), the viscerocranium is predominantly of neural crest origin, and for the neurocranium, the rostral part is derived from neural crest cells, whereas the posterior part from the mesoderm. In the cyclostome cranium, the mesoderm/neural crest boundary of the neurocranium used to be enigmatic, let alone the morphological comparison of neurocranial between two cyclostome groups, lampreys and hagfishes. By examining the hagfish development it has become clear that cyclostomes share a common craniofacial embryonic pattern that is not shared by modern gnathostomes, and cyclostome cranium can be compared among the group as developmental modular units with comparable mesoderm/neural crest boundary within the neuroranium. Also, the dual origin of the jawed vertebrate neurocranium has now turned out to represent a derived condition, and ancestrally, the neurocranium would likely have been predominantly of mesodermal origin. Enlargement of the forebrain and reorganization of the oral apparatus seem to have led to the involvement of the neural crest in the rostral neurocranium.

Palaeospondylus as a primitive hagfish

BACKGROUND

The taxonomic position of the Middle Devonian fish-like animal Palaeospondylus has remained enigmatic, due mainly to the inability to identify homologous cranial elements. This animal has been classified into nearly all of the major vertebrate taxa over a century of heuristic taxonomic research, despite the lack of conclusive morphological evidence.

RESULTS

Here we report the first comparative morphological analysis of hagfish embryos and Palaeospondylus, and a hitherto overlooked resemblance in the chondrocranial elements of these animals; i.e., congruence in the arrangement of the nasal capsule, neurocranium and mandibular arch-derived velar bar. The large ventral skeletal complex of Palaeospondylus is identified as a cyclostome-specific lingual apparatus. Importantly, the overall morphological pattern of the Palaeospondylus cranium coincides well with the cyclostome pattern of craniofacial development, which is not shared with that of crown gnathostomes. Previously, the presence of the vertebral column in Palaeospondylus made its assignment problematic, but the recent identification of this vertebral element in hagfish is consistent with an affinity between this group and Palaeospondylus.

CONCLUSION

These lines of evidence support the hagfish affinity of Palaeospondylus. Moreover, based on the less specialized features in its cranial morphology, we conclude that Palaeospondylus is likely a stem hagfish.

Midline dural filum of the sellar floor: Its relationship to the septum attachment to the sellar floor and the ossification in the sphenoid sinus

OBJECTIVES

It is important to identify and maintain a midline orientation during endoscopic transsphenoidal surgery (ETSS) for sellar lesions to prevent critical injury to the internal carotid artery. Therefore, the preoperative neuroradiological assessment of the bony structures in the sphenoid sinus, including the septum attachment to the sellar floor and its surrounding structures, is essential. It has been reported that the midline filum of the sellar dura can function as a useful intraoperative orientation guide during ETSS. However, the relationship between the midline dural filum and the intrasinus bony structures, such as the sellar floor, the intrasinus septation and the ossification, remains unclear and the mechanisms underlying development of the midline dural filum have also not yet been explored.

METHODS

This retrospective study included 160 patients undergoing ETSS to assess both the midline dural filum and the intrasinus bony structures, using video recording reviews. The intrasinus septum and the ossification in the sphenoid sinus were evaluated on the computed tomography images of the bone window.

RESULTS

A midline dural filum was identified in 66 (41.3%) of 160 patients. Attachment of the septum to the sellar midline was found in 61 (39.4%) of 155 patients, after excluding 5 patients with the conchal type of sphenoid sinus, 55 (90.2%) of 61 patients with a septum on the midline and only 6 (6.4%) of the remaining 94 patients without a septum on the midline had a midline dural filum. The relationship between a midline dural filum and a septum on midline was statistically significant (p<0.001), regardless of the number of intrasphenoidal septa. In terms of the types of sphenoid sinus, the midline dural filum was predominantly detected in patients where ossification extended over the midline filum. In patients with the sellar type of sphenoid sinus, 49 (36.0%) of 136 had a midline dural filum, meanwhile, 16 (84.2%) of 19 patients with the pre-sellar type (p=0.039) and all 5 patients (100%) with the choncal type harbored a midline dural filum (p<0.001).

CONCLUSION

This study clearly verified the importance of the midline dural filum in a large series and evaluated the obvious relationship between the midline dural filum and the bony structures on the sellar floor. Our results strongly suggest that, during developing of the midline dural filum, the sellar dura becomes tethered to the bony elements attached to the sellar surface, such as the septum on the midline and the ossification in both the pre-sellar and the conchal type of sphenoid sinus.

An Fgf-Shh signaling hierarchy regulates early specification of the zebrafish skull

The neurocranium generates most of the craniofacial skeleton and consists of prechordal and postchordal regions. Although development of the prechordal is well studied, little is known of the postchordal region. Here we characterize a signaling hierarchy necessary for postchordal neurocranial development involving Fibroblast growth factor (Fgf) signaling for early specification of mesodermally-derived progenitor cells. The expression of hyaluron synthetase 2 (has2) in the cephalic mesoderm requires Fgf signaling and Has2 function, in turn, is required for postchordal neurocranial development. While Hedgehog (Hh)-deficient embryos also lack a postchordal neurocranium, this appears primarily due to a later defect in chondrocyte differentiation. Inhibitor studies demonstrate that postchordal neurocranial development requires early Fgf and later Hh signaling. Collectively, our results provide a mechanistic understanding of early postchordal neurocranial development and demonstrate a hierarchy of signaling between Fgf and Hh in the development of this structure.

A molecular mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history

The avian beak is a key evolutionary innovation whose flexibility has permitted birds to diversify into a range of disparate ecological niches. We approached the problem of the mechanism behind this innovation using an approach bridging paleontology, comparative anatomy, and experimental developmental biology. First, we used fossil and extant data to show the beak is distinctive in consisting of fused premaxillae that are geometrically distinct from those of ancestral archosaurs. To elucidate underlying developmental mechanisms, we examined candidate gene expression domains in the embryonic face: the earlier frontonasal ectodermal zone (FEZ) and the later midfacial WNT-responsive region, in birds and several reptiles. This permitted the identification of an autapomorphic median gene expression region in Aves. To test the mechanism, we used inhibitors of both pathways to replicate in chicken the ancestral amniote expression. Altering the FEZ altered later WNT responsiveness to the ancestral pattern. Skeletal phenotypes from both types of experiments had premaxillae that clustered geometrically with ancestral fossil forms instead of beaked birds. The palatal region was also altered to a more ancestral phenotype. This is consistent with the fossil record and with the tight functional association of avian premaxillae and palate in forming a kinetic beak.

Evolution of the vertebrate skeleton: morphology, embryology, and development

Two major skeletal systems-the endoskeleton and exoskeleton-are recognized in vertebrate evolution. Here, we propose that these two systems are distinguished primarily by their relative positions, not by differences in embryonic histogenesis or cell lineage of origin. Comparative embryologic analyses have shown that both types of skeleton have changed their mode of histogenesis during evolution. Although exoskeletons were thought to arise exclusively from the neural crest, recent experiments in teleosts have shown that exoskeletons in the trunk are mesodermal in origin. The enameloid and dentine-coated postcranial exoskeleton seen in many vertebrates does not appear to represent an ancestral condition, as previously hypothesized, but rather a derived condition, in which the enameloid and dentine tissues became accreted to bones. Recent data from placoderm fossils are compatible with this scenario. In contrast, the skull contains neural crest-derived bones in its rostral part. Recent developmental studies suggest that the boundary between neural crest- and mesoderm-derived bones may not be consistent throughout evolution. Rather, the relative positions of bony elements may be conserved, and homologies of bony elements have been retained, with opportunistic changes in the mechanisms and cell lineages of development.

Late-emigrating trunk neural crest cells in turtle embryos generate an osteogenic ectomesenchyme in the plastron

BACKGROUND

The turtle plastron is composed of a keratinized epidermis overlying nine dermal bones. Its developmental origin has been controversial; recent evidence suggests that the plastral bones derive from trunk neural crest cells (NCCs).

RESULTS

This study extends the observations that there is a turtle-specific, second wave of trunk NCC delamination and migration, after the original NCCs have reached their destination and differentiated. This second wave was confirmed by immunohistochemistry in whole-mounts and serial sections, by injecting DiI (1,1', di-octadecyl-3,3,3',3',-tetramethylindo-carbocyanine perchlorate) into the lumen of the neural tube and tracing labeled cells into the plastron, and by isolating neural tubes from older turtle embryos and observing delaminating NCCs. This later migration gives rise to a plastral ectomesenchyme that expresses NCC markers and can be induced to initiate bone formation.

CONCLUSIONS

The NCCs of this second migration have properties similar to those of the earlier NCCs, but also express markers characteristic of cranial NCCs. The majority of the cells of the plastron mesenchyme express neural crest markers, and have osteogenic differentiation capabilities that are similar or identical to craniofacial ectomesenchyme. Our evidence supports the contention that turtle plastron bones are derived from a late emigrating population of cells derived from the trunk neural crest.

The buccohypophyseal canal is an ancestral vertebrate trait maintained by modulation in sonic hedgehog signaling

Background

The pituitary gland is formed by the juxtaposition of two tissues: neuroectoderm arising from the basal diencephalon, and oral epithelium, which invaginates towards the central nervous system from the roof of the mouth. The oral invagination that reaches the brain from the mouth is referred to as Rathke’s pouch, with the tip forming the adenohypophysis and the stalk disappearing after the earliest stages of development. In tetrapods, formation of the cranial base establishes a definitive barrier between the pituitary and oral cavity; however, numerous extinct and extant vertebrate species retain an open buccohypophyseal canal in adulthood, a vestige of the stalk of Rathke’s pouch. Little is currently known about the formation and function of this structure. Here we have investigated molecular mechanisms driving the formation of the buccohypophyseal canal and their evolutionary significance.

Results

We show that Rathke’s pouch is located at a boundary region delineated by endoderm, neural crest-derived oral mesenchyme and the anterior limit of the notochord, using CD1, R26R-Sox17-Cre and R26R-Wnt1-Cre mouse lines. As revealed by synchrotron X-ray microtomography after iodine staining in mouse embryos, the pouch has a lobulated three-dimensional structure that embraces the descending diencephalon during pituitary formation. Polaris^fl/fl; Wnt1-Cre, Ofd1^-/- and Kif3a^-/- primary cilia mouse mutants have abnormal sonic hedgehog (Shh) signaling and all present with malformations of the anterior pituitary gland and midline structures of the anterior cranial base. Changes in the expressions of Shh downstream genes are confirmed in Gas1^-/- mice. From an evolutionary perspective, persistence of the buccohypophyseal canal is a basal character for all vertebrates and its maintenance in several groups is related to a specific morphology of the midline that can be related to modulation in Shh signaling.

Conclusion

These results provide insight into a poorly understood ancestral vertebrate structure. It appears that the opening of the buccohypophyseal canal depends upon Shh signaling and that modulation in this pathway most probably accounts for its persistence in phylogeny.

Development of head and trunk mesoderm in the dogfish, Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures

Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance for understanding the developmental body plan of vertebrates and its evolutionary origin. In particular, the segmentation of the mesoderm is central to the problem. The shark embryo has provided a canonical morphological scheme of the head, with its epithelialized coelomic cavities (head cavities), which have often been regarded as head somites. To understand the evolutionary significance of the head cavities, the embryonic development of the mesoderm was investigated at the morphological and histological levels in the shark, Scyliorhinus torazame. Unlike somites and some enterocoelic mesodermal components in other vertebrates, the head cavities in S. torazame appeared as irregular cyst(s) in the originally unsegmented mesenchymal head mesoderm, and not via segmentation of an undivided coelom. The mandibular cavity appeared first in the paraxial part of the mandibular mesoderm, followed by the hyoid cavity, and the premandibular cavity was the last to form. The prechordal plate was recognized as a rhomboid roof of the preoral gut, continuous with the rostral notochord, and was divided anteroposteriorly into two parts by the growth of the hypothalamic primordium. Of those, the posterior part was likely to differentiate into the premandibular cavity, and the anterior part disappeared later. The head cavities and somites in the trunk exhibited significant differences, in terms of histological appearance and timing of differentiation. The mandibular cavity developed a rostral process secondarily; its homology to the anterior cavity reported in some elasmobranch embryos is discussed.

Distinct spatiotemporal roles of hedgehog signalling during chick and mouse cranial base and axial skeleton development

The cranial base exerts a supportive role for the brain and includes the occipital, sphenoid and ethmoid bones that arise from cartilaginous precursors in the early embryo. As the occipital bone and the posterior part of the sphenoid are mesoderm derivatives that arise in close proximity to the notochord and floor plate, it has been assumed that their development, like the axial skeleton, is dependent on Sonic hedgehog (Shh) and modulation of bone morphogenetic protein (Bmp) signalling. Here we examined the development of the cranial base in chick and mouse embryos to compare the molecular signals that are required for chondrogenic induction in the trunk and head. We found that Shh signalling is required but the molecular network controlling cranial base development is distinct from that in the trunk. In the absence of Shh, the presumptive cranial base did not undergo chondrogenic commitment as determined by the loss of Sox9 expression and there was a decrease in cell survival. In contrast, induction of the otic capsule occurred normally demonstrating that induction of the cranial base is uncoupled from formation of the sensory capsules. Lastly, we found that the early cranial mesoderm is refractory to Shh signalling, likely accounting for why development of the cranial base occurs after the axial skeleton. Our data reveal that cranial and axial skeletal induction is controlled by conserved, yet spatiotemporally distinct mechanisms that co-ordinate development of the cranial base with that of the cranial musculature and the pharyngeal arches.

Developmental and evolutionary significance of the mandibular arch and prechordal/premandibular cranium in vertebrates: revising the heterotopy scenario of gnathostome jaw evolution

The cephalic neural crest produces streams of migrating cells that populate pharyngeal arches and a more rostral, premandibular domain, to give rise to an extensive ectomesenchyme in the embryonic vertebrate head. The crest cells forming the trigeminal stream are the major source of the craniofacial skeleton; however, there is no clear distinction between the mandibular arch and the premandibular domain in this ectomesenchyme. The question regarding the evolution of the gnathostome jaw is, in part, a question about the differentiation of the mandibular arch, the rostralmost component of the pharynx, and in part a question about the developmental fate of the premandibular domain. We address the developmental definition of the mandibular arch in connection with the developmental origin of the trabeculae, paired cartilaginous elements generally believed to develop in the premandibular domain, and also of enigmatic cartilaginous elements called polar cartilages. Based on comparative embryology, we propose that the mandibular arch ectomesenchyme in gnathostomes can be defined as a Dlx1-positive domain, and that the polar cartilages, which develop from the Dlx1-negative premandibular ectomesenchyme, would represent merely posterior parts of the trabeculae. We also show, in the lamprey embryo, early migration of mandibular arch mesenchyme into the premandibular domain, and propose an updated version of the heterotopy theory on the origin of the jaw.