Cartilage is a metazoan tissue; integrating data from nonvertebrate sources

Development of the vertebra and fin skeleton in the lamprey and its implications for the homology of vertebrate vertebrae

BACKGROUND

Although vertebrae are the defining character of vertebrates, they are found only in rudimentary form in extant agnathans. In addition, the vertebrae of agnathans possess several unique features, such as elastin-like molecules as the main matrix component and late (post-metamorphosis) differentiation of lamprey vertebrae. In this study, by tracing the developmental process of vertebrae in lamprey, we examined the homology of vertebrae between lampreys and gnathostomes.

RESULTS

We found that the lamprey somite is first subdivided mediolaterally, with myotome cells differentiating medially and non-myotome cells emerging laterally. Subsequently, collagen-positive non-myotome cells surround the myotome. This pattern of somitogenesis is rather similar to that in amphioxi and sheds doubt on the presence of a sclerotome, in terms of mesenchyme cells induced by a signal from the notochord, in lamprey. Further tracing of non-myotome cell development revealed that fin cartilage develops in ammocoete larvae approximately 35 mm in body length. The development of the fin cartilage occurs much earlier than that of the vertebra whose development proceeds during metamorphosis.

CONCLUSION

We propose that the homology of vertebrae between agnathans and gnathostomes should be discussed carefully, because the developmental process of the lamprey vertebra is different from that of gnathostomes.

Trophic level and proteobacteria abundance drive antibiotic resistance levels in fish from coastal New England

BACKGROUND

The natural marine environment represents a vast reservoir of antimicrobial resistant bacteria. The wildlife that inhabits this environment plays an important role as the host to these bacteria and in the dissemination of resistance. The relationship between host diet, phylogeny, and trophic level and the microbiome/resistome in marine fish is not fully understood. To further explore this relationship, we utilize shotgun metagenomic sequencing to define the gastrointestinal tract microbiomes of seven different marine vertebrates collected in coastal New England waters.

RESULTS

We identify inter and intraspecies differences in the gut microbiota of these wild marine fish populations. Furthermore, we find an association between antibiotic resistance genes and host dietary guild, which suggests that higher trophic level organisms have a greater abundance of resistance genes. Additionally, we demonstrate that antibiotic resistance gene burden is positively correlated with Proteobacteria abundance in the microbiome. Lastly, we identify dietary signatures within the gut of these fish and find evidence of possible dietary selection for bacteria with specific carbohydrate utilization potential.

CONCLUSIONS

This work establishes a link between host lifestyle/dietary guild, and microbiome composition and the abundance of antibiotic resistance genes within the gastrointestinal tract of marine organisms. We expand the current understanding of marine organism-associated microbial communities and their role as reservoirs of antimicrobial resistance genes.

Mineralized belemnoid cephalic cartilage from the late Triassic Polzberg Konservat-Lagerstätte (Austria)

Although hyaline cartilage is widely distributed in various invertebrate groups such as sabellid polychaetes, molluscs (cephalopods, gastropods) and a chelicerate arthropod group (horseshoe crabs), the enigmatic relationship and distribution of cartilage in taxonomic groups remains to be explained. It can be interpreted as a convergent trait in animal evolution and thus does not seem to be a vertebrate invention. Due to the poor fossil record of cartilaginous structures, occurrences of mineralized fossil cartilages are important for evolutionary biology and paleontology. Although the biochemical composition of recent cephalopod cartilage differs from vertebrate cartilage, histologically the cartilages of these animal groups resemble one another remarkably. In this study we present fossil material from the late Triassic Polzberg Konservat-Lagerstätte near Lunz am See (Lower Austria, Northern Calcareous Alps). A rich Carnian fauna is preserved here, whereby a morphogroup (often associated with belemnoid remains) of black, amorphous appearing fossils still remained undetermined. These multi-elemental, symmetrical fossils show remarkable similarities to recent cartilage. We examined the conspicuous micro- and ultrastructure of these enigmatic fossils by thin-sectioning and Scanning Electron Microscopy (SEM). The geochemical composition analyzed by Microprobe and Energy Dispersive X-ray Spectroscopy (SEM-EDX) revealed carbonization as the taphonomic pathway for this fossil group. Mineralization of soft tissues permits the 3D preservation of otherwise degraded soft tissues such as cartilage. We examined eighty-one specimens from the Polzberg locality and seven specimens from Cave del Predil (formerly Raibl, Julian Alps, Italy). The study included morphological examinations of these multi-elemental fossils and a focus on noticeable structures like grooves and ridges. The detected grooves are interpreted to be muscular attachment areas, and the preserved branched system of canaliculi is comparable to a channel system that is also present in recent coleoid cartilage. The new findings on these long-known enigmatic structures strongly point to the preservation of cephalic cartilage belonging to the belemnoid Phragmoteuthis bisinuata and its homologization to the cephalic cartilage of modern coleoids.

Comparative ultrastructure of the radiolar crown in Sabellida (Annelida)

Three major clades of tube-dwelling annelids are grouped within Sabellida: Fabriciidae, Serpulidae and Sabellidae. The most characteristic feature of these animals is the often spectacularly colorful and flower-like radiolar crown. Holding up such delicate, feathery appendages in water currents requires some sort of internal stabilization. Each of the above-mentioned family-ranked groups has overcome this problem in a different way. Herein we describe the arrangement, composition and ultrastructure of radiolar tissues for fabriciids, sabellids and serpulids using transmission electron microscopy, histology and immunohistochemistry. Our sampling of 12 species spans most of the phylogenetic lineages across Sabellida and, from within Sabellidae, includes representatives of Myxicolinae, Sabellinae and the enigmatic sabellin Caobangia. We further characterize the ultrastructure of the chordoid cells that make up the supporting cellular axis in Sabellidae and discuss the evolution of radiolar tissues within Sabellida in light of the recently published phylogeny of the group.

Embryonic origin and serial homology of gill arches and paired fins in the skate, Leucoraja erinacea

Paired fins are a defining feature of the jawed vertebrate body plan, but their evolutionary origin remains unresolved. Gegenbaur proposed that paired fins evolved as gill arch serial homologues, but this hypothesis is now widely discounted, owing largely to the presumed distinct embryonic origins of these structures from mesoderm and neural crest, respectively. Here, we use cell lineage tracing to test the embryonic origin of the pharyngeal and paired fin skeleton in the skate (Leucoraja erinacea). We find that while the jaw and hyoid arch skeleton derive from neural crest, and the pectoral fin skeleton from mesoderm, the gill arches are of dual origin, receiving contributions from both germ layers. We propose that gill arches and paired fins are serially homologous as derivatives of a continuous, dual-origin mesenchyme with common skeletogenic competence, and that this serial homology accounts for their parallel anatomical organization and shared responses to axial patterning signals.

Identification of a unique lamprey gene with tandemly repeated sequences and pharyngeal chondrocyte-specific expression

Recent studies have revealed a common cartilage genetic regulatory network among vertebrates, cephalochordates, and arthropods. It has been proposed that this network was originally established for the dense connective tissues of ancestral invertebrates and subsequently recruited for chondrocyte differentiation in various lineages. This reasoning prompted questions about whether the evolution of cartilage from dense connective tissues occurred in the common ancestors of vertebrates. Alternatively, the evolution of cartilage may have occurred independently in agnathans and in gnathostomes, because extant agnathans (cyclostomes) are known to possess a matrix composition different from that of gnathostomes. Here, we identified the gene which is likely to encode one of the matrix proteins unique to lamprey cartilage, which we designated pharymprin. Pharymprin shows specific expression in larval pharyngeal chondrocytes. Like lamprins, which are the known matrix proteins of lamprey trabecular cartilage, pharymprin is also composed of repeated sequences. However, the repeated sequence is distinct from that of lamprins. The presence of two distinct matrix proteins in lamprey cartilage supports the hypothesis that true cartilage evolved independently in cyclostomes and gnathostomes.

Analyses of Sox-B and Sox-E Family Genes in the Cephalopod Sepia officinalis: Revealing the Conserved and the Unusual

Cephalopods provide an unprecedented opportunity for comparative studies of the developmental genetics of organ systems that are convergent with analogous vertebrate structures. The Sox-family of transcription factors is an important class of DNA-binding proteins that are known to be involved in many aspects of differentiation, but have been largely unstudied in lophotrochozoan systems. Using a degenerate primer strategy we have isolated coding sequence for three members of the Sox family of transcription factors from a cephalopod mollusk, the European cuttlefish Sepia officinalis: Sof-SoxE, Sof-SoxB1, and Sof-SoxB2. Analyses of their expression patterns during organogenesis reveals distinct spatial and temporal expression domains. Sof-SoxB1 shows early ectodermal expression throughout the developing epithelium, which is gradually restricted to presumptive sensory epithelia. Expression within the nervous system appears by mid-embryogenesis. Sof-SoxB2 expression is similar to Sof-SoxB1 within the developing epithelia in early embryogenesis, however appears in largely non-overlapping expression domains within the central nervous system and is not expressed in the maturing sensory epithelium. In contrast, Sof-SoxE is expressed throughout the presumptive mesodermal territories at the onset of organogenesis. As development proceeds, Sof-SoxE expression is elevated throughout the developing peripheral circulatory system. This expression disappears as the circulatory system matures, but expression is maintained within undifferentiated connective tissues throughout the animal, and appears within the nervous system near the end of embryogenesis. SoxB proteins are widely known for their role in neural specification in numerous phylogenetic lineages. Our data suggests that Sof-SoxB genes play similar roles in cephalopods. In contrast, Sof-SoxE appears to be involved in the early stages of vasculogenesis of the cephalopod closed circulatory system, a novel role for a member of this gene family.

The genetic program for cartilage development has deep homology within Bilateria

The evolution of novel cell types led to the emergence of new tissues and organs during the diversification of animals. The origin of the chondrocyte, the cell type that synthesizes cartilage matrix, was central to the evolution of the vertebrate endoskeleton. Cartilage-like tissues also exist outside the vertebrates, although their relationship to vertebrate cartilage is enigmatic. Here we show that protostome and deuterostome cartilage share structural and chemical properties, and that the mechanisms of cartilage development are extensively conserved--from induction of chondroprogenitor cells by Hedgehog and β-catenin signalling, to chondrocyte differentiation and matrix synthesis by SoxE and SoxD regulation of clade A fibrillar collagen (ColA) genes--suggesting that the chondrogenic gene regulatory network evolved in the common ancestor of Bilateria. These results reveal deep homology of the genetic program for cartilage development in Bilateria and suggest that activation of this ancient core chondrogenic network underlies the parallel evolution of cartilage tissues in Ecdysozoa, Lophotrochozoa and Deuterostomia.

Marine Structural Biomaterials in Medical Biomimicry

Marine biomaterials display properties, behaviors, and functions that have not been artificially matched in relation to their hierarchical construction, crack-stopping properties, growth adaptation, and energy efficiency. The discovery and understanding of such features that are characteristic of natural biomaterials can be used to manufacture more energy-efficient and lightweight materials. However, a more detailed understanding of the design of natural biomaterials with good performance and the mechanism of their design is required. Far-reaching biomolecular characterization of biomaterials and biostructures from the ocean world is possible with sophisticated analytical methods, such as whole-genome RNA-seq, and de novo transcriptome sequencing and mass spectrophotometry-based sequencing. In combination with detailed material characterization, the elements in newly discovered biomaterials and their properties can be reconstituted into biomimetic or bio-inspired materials. A major aim of harnessing marine biomaterials is their translation into biomimetic counterparts. To achieve full translation, the genome, proteome, and hierarchical material characteristics, and their profiles in space and time, have to be associated to allow for smooth biomimetic translation. In this article, we highlight the novel science of marine biomimicry from a materials perspective. We focus on areas of material design and fabrication that have excelled in marine biological models, such as embedded interfaces, chiral organization, and the use of specialized composite material-on-material designs. Our emphasis is primarily on key materials with high value in healthcare in which we evaluate their future prospects. Marine biomaterials are among the most exquisite and powerful aspects in materials science today.

Incremental evolution of the neural crest, neural crest cells and neural crest-derived skeletal tissues

Urochordates (ascidians) have recently supplanted cephalochordates (amphioxus) as the extant sister taxon of vertebrates. Given that urochordates possess migratory cells that have been classified as 'neural crest-like'- and that cephalochordates lack such cells--this phylogenetic hypothesis may have significant implications with respect to the origin of the neural crest and neural crest-derived skeletal tissues in vertebrates. We present an overview of the genes and gene regulatory network associated with specification of the neural crest in vertebrates. We then use these molecular data--alongside cell behaviour, cell fate and embryonic context--to assess putative antecedents (latent homologues) of the neural crest or neural crest cells in ascidians and cephalochordates. Ascidian migratory mesenchymal cells--non-pigment-forming trunk lateral line cells and pigment-forming 'neural crest-like cells' (NCLC)--are unlikely latent neural crest cell homologues. Rather, Snail-expressing cells at the neural plate of border of urochordates and cephalochordates likely represent the extent of neural crest elaboration in non-vertebrate chordates. We also review evidence for the evolutionary origin of two neural crest-derived skeletal tissues--cartilage and dentine. Dentine is a bona fide vertebrate novelty, and dentine-secreting odontoblasts represent a cell type that is exclusively derived from the neural crest. Cartilage, on the other hand, likely has a much deeper origin within the Metazoa. The mesodermally derived cellular cartilages of some protostome invertebrates are much more similar to vertebrate cartilage than is the acellular 'cartilage-like' tissue in cephalochordate pharyngeal arches. Cartilage, therefore, is not a vertebrate novelty, and a well-developed chondrogenic program was most likely co-opted from mesoderm to the neural crest along the vertebrate stem. We conclude that the neural crest is a vertebrate novelty, but that neural crest cells and their derivatives evolved and diversified in a step-wise fashion--first by elaboration of neural plate border cells, then by the innovation or co-option of new or ancient metazoan cell fates.

Ultrastructure of book gill development in embryos and first instars of the horseshoe crab Limulus polyphemus L. (Chelicerata, Xiphosura)

BACKGROUND

The transmission electron microscope (TEM) is used for the first time to study the development of book gills in the horseshoe crab. Near the end of the nineteenth century the hypothesis was presented for homology and a common ancestry for horseshoe crab book gills and arachnid book lungs. The present developmental study and the author's recent ones of book gills (SEM) and scorpion book lungs (TEM) are intended to clarify early histological work and provide new ultrastructural details for further research and for hypotheses about evolutionary history and relationships.

RESULTS

The observations herein are in agreement with earlier reports that the book gill lamellae are formed by proliferation and evagination of epithelial cells posterior to opisthosomal branchial appendages. A cartilage-like endoskeleton is produced in the base of the opisthosomal appendages. The lamellar precursor cells in the appendage base proliferate, migrate outward and secrete the lamellar cuticle from their apical surface. A series of external, posteriorly-directed lamellae is formed, with each lamella having a central channel for hemolymph and pillar-type space holders formed from cells of the opposed walls. This repeated, page-like pattern results also in water channels (without space holders) between the sac-like hemolymph lamellae.

CONCLUSIONS

The developmental observations herein and in an earlier study (TEM) of scorpion book lungs show that the lamellae in book gills and book lungs result from some similar activities and features of the precursor epithelial cells: proliferation, migration, alignment and apical/basal polarity with secretion of cuticle from the apical surface and the basal surface in contact with hemolymph. These cellular similarities and the resulting book-like structure suggest a common ancestry, but there are also substantial developmental differences in producing these organs for gas exchange in the different environments, aqueous and terrestrial. For scorpion book lungs, the invaginated precursor cells align in rows and secrete rows of cell fragments that are the basis for the internal, anterior-directed air sacs. The hemolymph sacs of book gills are formed by epithelial evagination or outfolding from the posterior surface of the branchial appendages.

Cilia-like structures anchor the amphioxus notochord to its sheath

Body stiffness is important during undulatory locomotion in fish. In amphioxus, the myosepta play an important role in transmission of muscular forces to the notochord. In order to define the specific supporting role of the notochord in amphioxus during locomotion, the ultrastructure of 10 adult amphioxus specimens was analyzed using transmission electron microscopy. Numerous cilia-like structures were found on the surface of each notochordal cell at the sites of their attachment to the notochordal sheath. Ultrastructurally, these structures consisted of the characteristic arrangement of peripheral and central microtubular doublets and were anchored to the inner layer of the notochordal sheath. Immunohistochemically, a positive reaction to applied dynein and β-tubulin antibodies characterized the area of the cilia-like structures. We propose that reduced back-and-forth movements of the cilia-like structures might contribute to the flow of the fluid content inside the notochord, thus modulating the stiffness of the amphioxus body during its undulatory locomotion.

Chapter 2. Evolution of vertebrate cartilage development

Major advances in the molecular genetics, paleobiology, and the evolutionary developmental biology of vertebrate skeletogenesis have improved our understanding of the early evolution and development of the vertebrate skeleton. These studies have involved genetic analysis of model organisms, human genetics, comparative developmental studies of basal vertebrates and nonvertebrate chordates, and both cladistic and histological analyses of fossil vertebrates. Integration of these studies has led to renaissance in the area of skeletal development and evolution. Among the major findings that have emerged is the discovery of an unexpectedly deep origin of the gene network that regulates chondrogenesis. In this chapter, we discuss recent progress in each these areas and identify a number of questions that need to be addressed in order to fill key gaps in our knowledge of early skeletal evolution.

Expression of Sox and fibrillar collagen genes in lamprey larval chondrogenesis with implications for the evolution of vertebrate cartilage

Lampreys possess unique types of cartilage in which elastin-like proteins are the dominant matrix component, whereas gnathostome cartilage is mainly composed of fibrillar collagen. Despite the differences in protein composition, the Sox-col2a1 genetic cascade was suggested to be conserved between lamprey pharyngeal cartilage and gnathostome cartilage. We examined whether the cascade is conserved in another type of lamprey cartilage, the trabecular cartilage. We found that SoxD and SoxE are expressed in both trabecular and pharyngeal cartilages. However, trabecular cartilage shows no clade A fibrillar collagen gene expression, including genes expressed in pharyngeal cartilage of this animal. On the basis of these observations, we propose that lampreys possess an ancestral type of cartilage that is similar to amphioxus gill cartilage, and in this respect, gnathostome cartilage can be regarded as derived for the loss of elastin-like protein as a cartilage component and recruitment of fibrillar collagen, which is included as a minor component in the ancestral cartilage, as the main component.

Genome duplication and the origin of the vertebrate skeleton

During vertebrate embryonic development, tissue patterning and differentiation are regulated by members of multigene families. Evolutionary expansion of these families is thought to have played a role in the evolution of anatomical complexity, including the origins of new cell and tissue types. A defining feature of vertebrates is an endoskeleton, the primary components of which are cartilage and bone. The molecular control of skeletal patterning has been the subject of intensive investigation for over two decades. More recently, comparative studies of organisms at key phylogenetic positions have highlighted the importance of gene duplication in the evolutionary diversification of connective tissues. Understanding the natural histories of gene families involved in skeletogenesis is therefore central to the issue of vertebrate skeletal evolution.

Cartilage differentiation in cephalopod molluscs

Amongst the various metazoan lineages that possess cartilage, tissues most closely resembling vertebrate hyaline cartilage in histological section are those of cephalopod molluscs. Although elements of the adult skeleton have been described, the development of these cartilages has not. Using serial histology of sequential developmental stages of the European cuttlefish, Sepia officinalis, we investigate these skeletal elements and offer the first description of the formation of any cellular invertebrate cartilage. Our data reveal that cuttlefish cartilage most often differentiates from uncondensed mesenchymal cells near the end of embryonic development, but that the earliest-forming cartilages differentiate from a cellular condensation which goes through a protocartilage stage in a manner typical of vertebrate primary cartilage formation. We further investigate the distribution and degree of differentiation of cartilages at the time of hatching in an additional four cephalopod species. We find that the timing of cartilage development varies between elements within a single species, as well as between species. We identify a tendency towards cartilage differentiation from uncondensed connective tissue in elements that form at the end of embryogenesis or after hatching. These data suggest a form of metaplasia from connective tissue is the ancestral mode of cartilage formation in this lineage.

Development and evolution of chordate cartilage

Deuterostomes are a monophyletic group of animals containing vertebrates, lancelets, tunicates, hemichordates, echinoderms, and xenoturbellids. Four out of these six extant groups-vertebrates, lancelets, tunicates, and hemichordates-have pharyngeal gill slits. All groups of deuterostome animals that have pharyngeal gill slits also have a pharyngeal skeleton supporting the pharyngeal openings, except tunicates. We previously found that pharyngeal cartilage in hemichordates and cephalochordates contains a fibrillar collagen protein similar to vertebrate type II collagen, but unlike vertebrate cartilage, the invertebrate deuterostome cartilages are acellular. We found SoxE and fibrillar collagen expression in the pharyngeal endodermal cells adjacent to where the cartilages form. These same endodermal epithelial cells also express Pax1/9, a marker of pharyngeal endoderm in vertebrates, lancelets, tunicates, and hemichordates. In situ experiments with a cephalochordate fibrillar collagen also showed expression in pharyngeal endoderm, as well as the ectoderm and the mesodermal coelomic pouches lining the gill bars. These results indicate that the pharyngeal endodermal cells are responsible for secretion of the cartilage in hemichordates, whereas in lancelets, all the pharyngeal cells surrounding the gill bars, ectodermal, endodermal, and mesodermal may be responsible for cartilage formation. We propose that endoderm secretion was primarily the ancestral mode of making pharyngeal cartilages in deuterostomes. Later the evolutionary origin of neural crest allowed co-option of the gene network for the secretion of pharyngeal cartilage matrix in the new migratory neural crest cell populations found in vertebrates.

Morphology of the prosomal endoskeleton of Scorpiones (Arachnida) and a new hypothesis for the evolution of cuticular cephalic endoskeletons in arthropods

Skeletomuscular anatomy of the scorpion prosoma is examined in an attempt to explain the evolution of two endoskeletal features, a muscular diaphragm dividing the prosoma and opisthosoma and cuticular epistomal entapophyses with a uniquely complex arrangement of muscles, tendons and ligaments. Both structures appear to be derived from modifications of the mesodermal intersegmental endoskeleton that is primitive for all major arthropod groups. The scorpion diaphragm is a compound structure comprising axial muscles and pericardial ligaments of segments VI to VIII and extrinsic muscles of leg 4 brought into contact by longitudinal reduction of segment VII and integrated into a continuous subvertical sheet. This finding reconciles a long-standing conflict between one interpretation of opisthosomal segmentation based on scorpion embryology and another derived from comparative skeletomuscular anatomy. A new evolutionary-developmental mechanism is proposed to account for the complex morphology of the epistomal entapophyses. Each entapophysis receives 14 muscles and tendons that in other taxa would attach to the anterior connective endoskeleton in the same relative positions. This observation suggests that the embryological precursor to the connective endoskeleton can initiate and guide ectodermal invagination and thereby serve as a spatial template for the development of cuticular apodemes. This mesoderm-template model of ectodermal invagination is potentially applicable to all arthropods and may explain structural diversity and convergence in cephalic apodemes throughout the group. The model is used to interpret the cephalic endoskeletons of two non-chelicerate arthropods, Archaeognatha (Hexapoda) and Symphyla (Myriapoda), to demonstrate the generality of the model.

Stepwise enforcement of the notochord and its intersection with the myoseptum: an evolutionary path leading to development of the vertebra?

The notochord constitutes the main axial support during the embryonic and larval stages, and the arrangement of collagen fibrils within the notochord sheath is assumed to play a decisive role in determining its functional properties as a fibre-wound hydrostatic skeleton. We have found that during early ontogeny in Atlantic salmon stepwise changes occur in the configuration of the collagen fibre-winding of the notochord sheath. The sheath consists of a basal lamina, a layer of type II collagen, and an elastica externa that delimits the notochord; and these constituents are secreted in a specific order. Initially, the collagen fibrils are circumferentially arranged perpendicular to the longitudinal axis, and this specific spatial fibril configuration is maintained until hatching when the collagen becomes reorganized into distinct layers or lamellae. Within each lamella, fibrils are parallel to each other, forming helices around the longitudinal axis of the notochord, with a tangent angle of 75-80 degrees to the cranio-caudal axis. The helical geometry shifts between adjacent lamellae, forming enantiomorphous left- and right-handed coils, respectively, thus enforcing the sheath. The observed changes in the fibre-winding configuration may reflect adaptation of the notochord to functional demands related to stage in ontogeny. When the vertebral bodies initially form as chordacentra, the collagen lamellae of the sheath in the vertebral region are fixed by the deposition of minerals; in the intervertebral region, however, they represent a pre-adaptation providing torsional stability to the intervertebral joint. Hence, these modifications of the sheath transform the notochord per se into a functional vertebral column. The elastica externa, encasing the notochord, has serrated surfaces, connected inward to the type II collagen of the sheath, and outward to type I collagen of the mesenchymal connective tissue surrounding the notochord. In a similar manner, the collagen matrix of the neural and haemal arch cartilages is tightly anchored to the outward surface of the elastic membrane. Hence, the elastic membrane may serve as an interface between the notochord and the adjacent structures, with an essential function related to transmission of tensile forces from the musculature. The interconnection between the notochord and the myosepta is discussed in relation to function and to evolution of the arches and the vertebra. Contrary to current understanding, this study also shows that notochord vacuolization does not result in an increased elongation of the embryo, which agrees with the circular arrangement of type II collagen that probably only enables a restricted increase in girth upon vacuolization, not aiding elongation. As the vacuolization occurs during the egg stage, this type of collagen disposition, in combination with an elastica externa, also probably facilitates flexibility and curling of the embryo.

Hagfish and lancelet fibrillar collagens reveal that type II collagen-based cartilage evolved in stem vertebrates

The origin of vertebrates was defined by evolution of a skeleton; however, little is known about the developmental mechanisms responsible for this landmark evolutionary innovation. In jawed vertebrates, cartilage matrix consists predominantly of type II collagen (Col2alpha1), whereas that of jawless fishes has long been thought to be noncollagenous. We recently showed that Col2alpha1 is present in lamprey cartilage, indicating that type II collagen-based cartilage evolved earlier than previously recognized. Here, we investigate the origin of vertebrate cartilage, and we report that hagfishes, the sister group to lampreys, also have Col2alpha1-based cartilage, suggesting its presence in the common ancestor of crown-group vertebrates. We go on to show that lancelets, a sister group to vertebrates, possess an ancestral clade A fibrillar collagen (ColA) gene that is expressed in the notochord. Together, these results suggest that duplication and diversification of ColA genes at the chordate-vertebrate transition may underlie the evolutionary origin of vertebrate skeletal tissues.

Evolution and development of the chordates: collagen and pharyngeal cartilage

Chordates evolved a unique body plan within deuterostomes and are considered to share five morphological characters, a muscular postanal tail, a notochord, a dorsal neural tube, an endostyle, and pharyngeal gill slits. The phylum Chordata typically includes three subphyla, Cephalochordata, Vertebrata, and Tunicata, the last showing a chordate body plan only as a larva. Hemichordates, in contrast, have pharyngeal gill slits, an endostyle, and a postanal tail but appear to lack a notochord and dorsal neural tube. Because hemichordates are the sister group of echinoderms, the morphological features shared with the chordates must have been present in the deuterostome ancestor. No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. We review the development of chordate characters in hemichordates and present new data characterizing the pharyngeal gill slits and their cartilaginous gill bars. We show that hemichordate gill bars contain collagen and proteoglycans but are acellular. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.

Consideration of the neural crest and its skeletal derivatives in the context of novelty/innovation

I examine the neural crest and skeletal tissues derived from neural crest cells in the context of novelty/innovation by asking whether the neural crest is a novel tissue and whether the evolutionary origin of the neural crest required innovative developmental processes. As a vertebrate autapomorphy, the neural crest is a novel structure. I equate novelty with innovation and take a hierarchical approach. Some other workers separate the two, using novelty for new structures not found in an ancestor and not homologous with a feature in an ancestor, and innovation for the new processes required to generate the novel structure. While development clearly evolves, I do not separate those processes that result in the production of novel features from those that lead to change in existing structures, whether that change is a transition or transformation from one homologous feature to another (fins-->tetrapod limbs or locomotory appendages-->crustacean maxilliped feeding appendages). The existence of novelties causes us to consider the concept of latent homology. Neural crest cells form cartilage, dentine and bone. Cartilage is found in invertebrates and so is not a vertebrate innovation. No invertebrate cartilage mineralizes in vivo, although some can be induced to mineralize in vitro. Mineralization of cartilage in vivo is a vertebrate innovation. Dentine is a novel tissue that only forms from neural crest cells. Bone is a vertebrate innovation but not one exclusive to the neural crest. The developmental processes responsible for the neural crest and for these skeletal tissues did not arise de novo with the vertebrates. Novelty/innovation results from tinkering with existing processes, from the flexibility that arises from modifications of existing gene networks, and from the selective advantage provided by gene duplications or modifications.

The nature and significance of invertebrate cartilages revisited: distribution and histology of cartilage and cartilage-like tissues within the Metazoa

Tissues similar to vertebrate cartilage have been described throughout the Metazoa. Often the designation of tissues as cartilage within non-vertebrate lineages is based upon sparse supporting data. To be considered cartilage, a tissue should meet a number of histological criteria that include composition and organization of the extracellular matrix. To re-evaluate the distribution and structural properties of these tissues, we have re-investigated the histological properties of many of these tissues from fresh material, and review the existing literature on invertebrate cartilages. Chondroid connective tissue is common amongst invertebrates, and differs from invertebrate cartilage in the structure and organization of the cells that comprise it. Groups having extensive chondroid connective tissue include brachiopods, polychaetes, and urochordates. Cartilage is found within cephalopod mollusks, chelicerate arthropods and sabellid polychaetes. Skeletal tissues found within enteropneust hemichordates are unique in that the extracellular matrix shares many properties with vertebrate cartilage, yet these tissues are completely acellular. The possibility that this tissue may represent a new category of cartilage, acellular cartilage, is discussed. Immunoreactivity of some invertebrate cartilages with antibodies that recognize molecules specific to vertebrate bone suggests an intermediate phenotype between vertebrate cartilage and bone. Although cartilage is found within a number of invertebrate lineages, we find that not all tissues previously reported to be cartilage have the appropriate properties to merit their distinction as cartilage.