Development of the cranium and paired fins in the zebrafish Danio rerio (Ostariophysi, Cyprinidae)

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.

Zebrafish: an emerging technology for in vivo pharmacological assessment to identify potential safety liabilities in early drug discovery

The zebrafish is a well-established model organism used in developmental biology. In the last decade, this technology has been extended to the generation of high-value knowledge on safety risks of novel drugs. Indeed, the larval zebrafish appear to combine advantages of whole organism phenotypic assays and those (rapid production of results with minimal resource engagement) of in vitro high-throughput screening techniques. Thus, if appropriately evaluated, it can offer undeniable advantages in drug discovery for identification of target and off-target effects. Here, we review some applications of zebrafish to identify potential safety liabilities, particularly before lead/candidate selection. For instance, zebrafish cardiovascular system can be used to reveal decreases in heart rate and atrial-ventricular dissociation, which may signal human ether-a-go-go-related gene (hERG) channel blockade. Another main area of interest is the CNS, where zebrafish behavioural assays have been and are further being developed into screening platforms for assessment of locomotor activity, convulsant and proconvulsant liability, cognitive impairment, drug dependence potential and impaired visual and auditory functions. Zebrafish also offer interesting possibilities for evaluating effects on bone density and gastrointestinal function. Furthermore, available knowledge of the renal system in larval zebrafish can allow identification of potential safety issues of drug candidates on this often neglected area in early development platforms. Although additional validation is certainly needed, the zebrafish is emerging as a versatile in vivo animal model to identify off-target effects that need investigation and further clarification early in the drug discovery process to reduce the current, high degree of attrition in development.

Embryonic requirements for ErbB signaling in neural crest development and adult pigment pattern formation

Vertebrate pigment cells are derived from neural crest cells and are a useful system for studying neural crest-derived traits during post-embryonic development. In zebrafish, neural crest-derived melanophores differentiate during embryogenesis to produce stripes in the early larva. Dramatic changes to the pigment pattern occur subsequently during the larva-to-adult transformation, or metamorphosis. At this time, embryonic melanophores are replaced by newly differentiating metamorphic melanophores that form the adult stripes. Mutants with normal embryonic/early larval pigment patterns but defective adult patterns identify factors required uniquely to establish, maintain or recruit the latent precursors to metamorphic melanophores. We show that one such mutant, picasso, lacks most metamorphic melanophores and results from mutations in the ErbB gene erbb3b, which encodes an EGFR-like receptor tyrosine kinase. To identify critical periods for ErbB activities, we treated fish with pharmacological ErbB inhibitors and also knocked down erbb3b by morpholino injection. These analyses reveal an embryonic critical period for ErbB signaling in promoting later pigment pattern metamorphosis, despite the normal patterning of embryonic/early larval melanophores. We further demonstrate a peak requirement during neural crest migration that correlates with early defects in neural crest pathfinding and peripheral ganglion formation. Finally, we show that erbb3b activities are both autonomous and non-autonomous to the metamorphic melanophore lineage. These data identify a very early, embryonic, requirement for erbb3b in the development of much later metamorphic melanophores, and suggest complex modes by which ErbB signals promote adult pigment pattern development.

Regulation of zebrafish skeletogenesis by ext2/dackel and papst1/pinscher

Mutations in human Exostosin genes (EXTs) confer a disease called Hereditary Multiple Exostoses (HME) that affects 1 in 50,000 among the general population. Patients with HME have a short stature and develop osteochondromas during childhood. Here we show that two zebrafish mutants, dackel (dak) and pinscher (pic), have cartilage defects that strongly resemble those seen in HME patients. We have previously determined that dak encodes zebrafish Ext2. Positional cloning of pic reveals that it encodes a sulphate transporter required for sulphation of glycans (Papst1). We show that although both dak and pic are required during cartilage morphogenesis, they are dispensable for chondrocyte and perichondral cell differentiation. They are also required for hypertrophic chondrocyte differentiation and osteoblast differentiation. Transplantation analysis indicates that dak^−/− cells are usually rescued by neighbouring wild-type chondrocytes. In contrast, pic^−/− chondrocytes always act autonomously and can disrupt the morphology of neighbouring wild-type cells. These findings lead to the development of a new model to explain the aetiology of HME.

Hereditary Multiple Exostoses is a disease that causes the formation of benign bone tumours in children. Besides causing severe skeletal deformity, the bone tumours can compress nerves or other tissue resulting in chronic pain. Although the tumours can usually be surgically removed, they sometimes recur or are in positions that prevent surgery. We have identified two strains of zebrafish whose offspring have skeletal defects that resemble those of patients with Hereditary Multiple Exostoses. We have found that each strain carries a mutated form of an essential gene. Importantly, these two genes are also found in humans, and thus by analysing their function in zebrafish, we may shed light on their role in humans. Our study has elucidated the roles of these genes during normal skeletal development and has allowed us to generate a model for how genetic changes give rise to bone tumours in humans.

Bone development in the jaw of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae)

East African cichlids have evolved feeding apparatus morphologies adapted to their diverse feeding behaviors. The evolution of the oral jaw morphologies is accomplished by the diversity of bone formation during development. To further understand this evolutionary process, we examined the skeletal elements of the jaw and their temporal and sequential emergence, categorized by developmental stages, using the Nile tilapia Oreochromis niloticus as a model cichlid. We found that chondrogenesis started in Stage 17. The deposition of osteoid for the dermal bones commenced in Stage 18. The uptake of calcium dramatically shifted from the surface of larvae to the gills in Stage 20. The bone mineralization of the skeleton began in Stage 25. These data provide important information regarding the sequential events of craniofacial development in East African cichlids and lay the groundwork for studying the molecular mechanisms underlying adaptation of jaw structure to feeding behavior.

Evolution and development of complex biomechanical systems: 300 million years of fish jaws

The jaws of teleost fishes are diverse and complex musculoskeletal systems. The focus in this review is on the major biomechanical systems in the teleost head, and the range and interplay of functional, developmental, and genetic influences that shape the modular and integrated evolution of elements. Insights possible from comparative studies are discussed in the context of traditional and new models for studies of craniofacial evolution and development.

Development of mandibular, hyoid and hypobranchial muscles in the zebrafish: homologies and evolution of these muscles within bony fishes and tetrapods

BACKGROUND

During vertebrate head evolution, muscle changes accompanied radical modification of the skeleton. Recent studies have suggested that muscles and their innervation evolve less rapidly than cartilage. The freshwater teleostean zebrafish (Danio rerio) is the most studied actinopterygian model organism, and is sometimes taken to represent osteichthyans as a whole, which include bony fishes and tetrapods. Most work concerning zebrafish cranial muscles has focused on larval stages. We set out to describe the later development of zebrafish head muscles and compare muscle homologies across the Osteichthyes.

RESULTS

We describe one new muscle and show that the number of mandibular, hyoid and hypobranchial muscles found in four day-old zebrafish larvae is similar to that found in the adult. However, the overall configuration and/or the number of divisions of these muscles change during development. For example, the undivided adductor mandibulae of early larvae gives rise to the adductor mandibulae sections A0, A1-OST, A2 and Aomega, and the protractor hyoideus becomes divided into dorsal and ventral portions in adults. There is not always a correspondence between the ontogeny of these muscles in the zebrafish and their evolution within the Osteichthyes. All of the 13 mandibular, hyoid and hypobranchial muscles present in the adult zebrafish are found in at least some other living teleosts, and all except the protractor hyoideus are found in at least some extant non-teleost actinopterygians. Of these muscles, about a quarter (intermandibularis anterior, adductor mandibulae, sternohyoideus) are found in at least some living tetrapods, and a further quarter (levator arcus palatini, adductor arcus palatini, adductor operculi) in at least some extant sarcopterygian fish.

CONCLUSION

Although the zebrafish occupies a rather derived phylogenetic position within actinopterygians and even within teleosts, with respect to the mandibular, hyoid and hypobranchial muscles it seems justified to consider it an appropriate representative of these two groups. Among these muscles, the three with clear homologues in tetrapods and the further three identified in sarcopterygian fish are particularly appropriate for comparisons of results between the actinopterygian zebrafish and the sarcopterygians.

Gene expression patterns underlying proximal-distal skeletal segmentation in late-stage zebrafish, Danio rerio

Timing and pattern of expression of ten candidate segmentation genes or gene pairs were reviewed or examined in developing median fins of late-stage zebrafish, Danio rerio. We found a general correspondence in timing and pattern of expression between zebrafish fin radial segmentation and tetrapod joint development, suggesting that molecular mechanisms underlying radial segmentation have been conserved over 400 million years of evolution. Gene co-expression during segmentation (5.5-6.5 mm SL) is similar between tetrapods and zebrafish: bmp2b, bmp4, chordin, and gdf5 in interradial mesenchyme and ZS; bapx1, col2a1, noggin3, and sox9a in chondrocytes. Surprisingly, wnt9a is not expressed in the developing median fins, though wnt9b is detected. In contrast to all other candidate segmentation genes we examined, bapx1 is not expressed in the caudal fin, which does not segment. Together, these data suggest a scenario of gene interactions underlying radial segmentation based on the patterns and timing of candidate gene expression.

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.

Using zebrafish to investigate cypriniform evolutionary novelties: functional development and evolutionary diversification of the kinethmoid

Although the zebrafish has become a popular model organism for biomedical studies, we propose that the wealth of morphological novelties that characterize this cypriniform fish makes it well suited for investigating the development of evolutionary innovations. Morphological novelties associated with feeding in cypriniform fishes include: a unique structure of the pharyngeal jaws in which the lower pharyngeal jaws are enlarged and opposed to a pad on the basioccipital process; a palatal organ found on the roof of the buccal chamber that is thought to help process detrital food within the buccal chamber; and, the kinethmoid, a novel ossification that effects a unique means of premaxillary protrusion. We present new morphological and developmental data and review functional data regarding the role of the kinethmoid in premaxillary protrusion in the zebrafish. Premaxillary protrusion plays an important role in effective prey acquisition in teleosts and the evolution of a unique means of premaxillary protrusion within Cypriniformes may have led to a number of trophic radiations within this clade. Ontogenetic data from zebrafish show that substantial premaxillary protrusion is not seen until these fish have undergone metamorphosis at which point the adductor mandibulae musculature becomes divided and all ligamentous attachments become established. A comparative study of families within Cypriniformes shows diverse morphologies of the kinethmoid. The morphological diversification that characterizes the kinethmoid suggests that this feeding structure has played a role in trophic radiations within Cypriniformes, since the morphology of this feature is correlated with feeding habits.

Development of the embryo, larva and early juvenile of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae). Developmental staging system

We described the developmental stages for the embryonic, larval and early juvenile periods of Nile tilapia Oreochromis niloticus to elucidate sequential events of craniofacial development. Craniofacial development of cichlids, especially differentiation and morphogenesis of the pharyngeal skeleton, progresses until about 30 days postfertilization (dpf). Because there is no comprehensive report describing the sequential processes of craniofacial development up to 30 dpf, we newly defined 32 stages using a numbered staging system. For embryonic development, we defined 18 stages (stages 1-18), which were grouped into seven periods named the zygote, cleavage, blastula, gastrula, segmentation, pharyngula and hatching periods. For larval development, we defined seven stages (stages 19-25), which were grouped into two periods, early larval and late larval. For juvenile development until 30 dpf, we defined seven stages (stages 26-32) in the early juvenile period. This developmental staging system for Nile tilapia O. niloticus will benefit researchers investigating skeletogenesis throughout tilapia ontogeny and will also facilitate comparative evolutionary developmental biology studies of haplochromine cichlids, which comprise the species flocks of Lakes Malawi and Victoria.

Fgf8 haploinsufficiency results in distinct craniofacial defects in adult zebrafish

Significant progress has been made toward understanding the role of fgf8 in directing early embryonic patterning of the pharyngeal skeleton. Considerably less is known about the role this growth factor plays in the coordinated development, growth, and remodeling of the craniofacial skeleton beyond embryonic stages. To better understand the contributions of fgf8 in the formation of adult craniofacial architecture, we analyzed the skeletal anatomy of adult ace(ti282a)/fgf8 heterozygous zebrafish. Our results revealed distinct skeletal defects including facial asymmetries, aberrant craniofacial geometry, irregular patterns of cranial suturing, and ectopic bone formation. These defects are similar in presentation to several human craniofacial disorders (e.g., craniosynostosis, hemifacial microsomia), and may be related to increased levels of bone metabolism observed in ace(ti282a)/fgf8 heterozygotes. Moreover, skeletal defects observed in ace(ti282a)/fgf8 heterozygotes are consistent with expression patterns of fgf8 in the mature craniofacial skeleton. These data reveal previously unrecognized roles for fgf8 during skeletogenesis, and provide a basis for future investigations into the mechanisms that regulate craniofacial development beyond the embryo.

Conservation and divergence of Bmp2a, Bmp2b, and Bmp4 expression patterns within and between dentitions of teleost fishes

The diversity of tooth location in teleost fishes provides an excellent system for comparing genetic divergence between teeth in different species (phylogenetic homologs) with divergence between teeth within one species (iterative homologs). We have chosen to examine the expression of three members of the bone morphogenetic protein (Bmp) family because they are known to play multiple roles in tooth development and evolution in tetrapod vertebrates. We characterized expression of Bmp2a, Bmp2b, and Bmp4 during the development of oral and pharyngeal dentitions in three species of teleost fishes, the zebrafish (Danio rerio), Mexican tetra (Astyanax mexicanus), and Japanese medaka (Oryzias latipes). We found that expression in teleosts is generally highly conserved, with minor differences found among both iteratively homologous and phylogenetically homologous teeth. Expression of orthologous genes differs in several ways between the teeth of teleost fishes and those of the mouse, but between these vertebrate groups the summed expression pattern of Bmp genes is highly conserved. Significantly, the toothless oral region of the zebrafish lacks Bmp expression domains found in teleosts with oral teeth, implicating these genes in evolutionary tooth loss. We conclude that Bmp expression has been largely conserved in vertebrate tooth development over evolutionary time, and that loss of Bmp expression is correlated with region-specific loss of the dentition in a major group of fishes.

A hierarchy of Runx transcription factors modulate the onset of chondrogenesis in craniofacial endochondral bones in zebrafish

The Runx (runt-related) family of transcription factors are important regulators of cell fate decisions in early embryonic development, and in differentiation of tissues including blood, neurons, and bone. During skeletal development in mammals, while only Runx2 is essential for osteoblast differentiation, all family members seem to be involved in chondrogenesis. Runx2 and Runx3 control chondrocyte maturation. Both Runx1 and Runx2 are expressed early in mesenchymal condensations, but how they contribute to the initial stages of chondrocyte differentiation is unclear. Here we show that a hierarchy of Runx transcriptional regulation promotes the early program of chondrocyte differentiation from pre-cartilage mesenchyme in the zebrafish head skeleton. We have previously characterized the zebrafish orthologs for all Runx genes. Zebrafish runx2 is duplicated, but not runx1 or runx3. In the work presented here, we determined the early expression pattern of the runx genes in the craniofacial region. The earliest expression detected was that of runx3 in the pharyngeal endoderm, then runx2a and b in mesenchymal condensations, and later runx1 in the epithelium. Using antisense morpholino knockdown analysis, we examined their respective activities in early chondrogenesis. Depletion of runx2b (but not runx2a) and runx3 severely compromised craniofacial cartilage formation. Because runx2b expression was abolished in Runx3 morphants, we propose that endodermal Runx3 has a role in influencing signaling activities from the endoderm to promote chondrocyte differentiation. We also show that, in contrast to data from mouse studies, zebrafish Runx1 is not required in the initial steps of chondrogenesis leading to endochondral bone formation.

Developmental and morphological variation in the teleost craniofacial skeleton reveals an unusual mode of ossification

We investigated the morphology and development of the scleral ossicles within the eyes of three species from three basal teleost orders, namely, the alewife (Alosa pseudoharengus; Clupeiformes), the surface morph of the Mexican tetra (Astyanax mexicanus; Characiformes) and zebrafish (Danio rerio; Cypriniformes). Two morphologies, circular and elongated, and one variation, fused elements, were identified. Zebrafish have small circular ossicles, whereas the alewife and the Mexican tetra have elongated ossicles. Surprisingly in the Mexican tetra these elements fuse at one end forming a continuous element with an antero-ventral opening; this may be typical for the Order Characiformes. Regardless of morphology, the ossicles develop via unilateral perichondral ossification of the scleral cartilage from two centers opposite one another in the eye. This unilateral type of ossification, in which only the perichondrium furthest from the retina contributes to the ossicles, has not previously been reported in any vertebrate. Because either the perichondrium and/or an extension of the perichondrium can transform into the scleral ossicle, we refer to the transitional tissue as periskeletal. Although the functional significance of the different shaped ossicles is unclear, skeletal muscle attaches directly to these bones, implying voluntary control. The morphological and developmental variation of teleost scleral ossicles makes them an ideal system for determining the genetic basis underlying phenotypic variation as well as for studying mechanisms underlying osteogenic and chondrogenic processes in teleosts. These data support our previous finding that scleral ossicles in teleosts may not be homologous to those in other vertebrates, such as reptiles.

Morphing the hyomandibular skeleton in development and evolution

How might changes in developmental regulatory pathways underlie evolutionary changes in morphology? Here we focus on a particular pathway regulated by a secreted, signaling peptide, Endothelin1 (Edn1). Developmental genetic analyses show the Edn1-pathway to be crucial for hyomandibular patterning, and we discuss our work with zebrafish suggesting how the signal may function in regulating numbers of skeletal elements, their sizes and their shapes. We then review a broader collection of comparative studies that examine morphological evolution of a subset of the same skeletal elements-the opercular-branchiostegal series of bones of the hyoid arch. We find that phenotypic changes in zebrafish mutants copy evolutionary changes that recur along many actinopterygian lineages. Hence the developmental genetic studies are informative for providing candidate pathways for macroevolution of facial morphology, as well as for our understanding of how these pathways work.

Heterochrony and the development of the escape response: prehatching movements in the rainbow troutOncorhynchus mykiss

Teleost fishes produce coordinated escape responses (C-starts) at hatching. This implies that essential swimming morphologies and motor behaviors develop during the incubation interval while the embryo is in the chorion. We examined prehatching motor behaviors in rainbow trout Oncorhycus mykiss (considered morphologically mature at hatching) and compared this species with zebrafish Danio rerio (considered morphologically immature) and assessed two hypotheses concerning the development of escape behavior. (1) Escape behavior is associated with the formation of key elements of the musculoskeletal and nervous systems; thus, the escape response appears early in ontogeny, when these elements form. (2) Escape behavior is not directly associated with the formation of underlying morphological elements; instead, it appears at hatching (i.e. when needed). We find that rainbow trout, like zebrafish, respond to touch early in the incubation interval, but do not demonstrate a complete C-start (including the second, propulsive stage) until shortly before hatching. At hatching, rainbow trout and zebrafish are similar in the degree of development of the chondocranium, paired fins and visceral arches (which comprise the larval jaw and gill support); however, rainbow trout have incipient rays in their unpaired fins (dorsal, anal and caudal), whereas zebrafish retain the embryonic fin fold. Although rainbow trout are more mature in axial swimming morphology at hatching, the essential neural and musculoskeletal systems that produce a coordinated escape response are functional at hatching in both species. This finding supports the evolutionary hypothesis that an effective escape response is critical for the survival of newly hatched teleost fishes.

Skeletal elements in the vertebrate eye and adnexa: morphological and developmental perspectives

Although poorly appreciated, the vertebrate eye and adnexa are relatively common sites for skeletogenesis. In many taxa, the skeleton contributes to internal reinforcement in addition to the external housing of the eye (e.g., the circumorbital bones and eyelids). Eyeball elements such as scleral cartilage and scleral ossicles are present within a broad diversity of vertebrates, albeit not therian mammals, and have been used as important models for the study of condensations and epithelial-mesenchymal interactions. In contrast, other elements invested within the eye or its close surroundings remain largely unexplored. The onset and mode of development of these skeletal elements are often variable (early versus late; involving chondrogenesis, osteogenesis, or both), and most (if not all) of these elements appear to share a common neural crest origin. This review discusses the development and distribution of the skeletal elements within and associated with the developing eye and comments on homology of the elements where these are questionable.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

A rapid, high content, in vivo model of glucocorticoid-induced osteoporosis

Glucocorticoid-induced osteoporosis (GIOP) is a major clinical problem given the widespread use of steroids and limited efficacy of biphosphonates. Existing animal models of GIOP are both slow and expensive. Hence, there is a need both for adjunctive modelling systems, as well as more efficacious therapies for the treatment of GIOP. We have addressed this issue through the creation of a zebrafish model of GIOP, which can be used for 96-well plate in vivo screening with an assay time of 5 days. The model demonstrates key similarities to human GIOP including a partial response to bisphosphonates. The ability to extract detailed pharmacological data, including concentration-response analyses, enables the screening and ranking of candidate therapeutic compounds. In addition, the zebrafish model is highly relevant for pathway dissection through genetic knockdown and overexpression studies.

Differential expression of two tropoelastin genes in zebrafish

Elastin is the extracellular matrix protein responsible for properties of extensibility and elastic recoil in large blood vessels, lung and skin of most vertebrates. Elastin is synthesized as a monomer, tropoelastin, but is rapidly transformed into its final polymeric form in the extracellular matrix. Until recently information on sequence and developmental expression of tropoelastins was limited to mammalian and avian species. We have recently identified and characterized two expressed tropoelastin genes in zebrafish. This was the first example of a species with multiple tropoelastin genes, raising the possibility of differential expression and function of these tropoelastins in elastic tissues of the zebrafish. Here we have investigated the temporal expression and tissue distribution of the two tropoelastin genes in developing and adult zebrafish. Expression was detected early in skeletal cartilage structures of the head, in the developing outflow tract of the heart, including the bulbus arteriosus and the ventral aorta, and in the wall of the swim bladder. While the temporal pattern of expression was similar for both genes, the upregulation of eln2 was much stronger than that of eln1. In general, both genes were expressed and their gene products deposited in most of the elastic tissues examined, with the notable exception of the bulbus arteriosus in which eln2 expression and its gene product was predominant. This finding may represent a sub-specialization of eln2 to provide the unique architecture of elastin and the specific mechanical properties required by this organ.

Development of zebrafish (Danio rerio) pectoral fin musculature

During posthatching development the fins of fishes undergo striking changes in both structure and function. In this article we examine the development of the pectoral fins from larval through adult life history stages in the zebrafish (Danio rerio), describing in detail their pectoral muscle morphology. We explore the development of muscle structure as a way to interpret the fins' role in locomotion. Genetic approaches in the zebrafish model are providing new tools for examining fin development and we take advantage of transgenic lines in which fluorescent protein is expressed in specific tissues to perform detailed three-dimensional, in vivo fin imaging. The fin musculature of larval zebrafish is organized into two thin sheets of fibers, an abductor and adductor, one on each side of an endoskeletal disk. Through the juvenile stage the number of muscle fibers increases and muscle sheets cleave into distinct muscle subdivisions as fibers orient to the developing fin skeleton. By the end of the juvenile period the pectoral girdle and fin muscles have reoriented to take on the adult organization. We find that this change in morphology is associated with a switch of fin function from activity during axial locomotion in larvae to use in swim initiation and maneuvering in adults. The examination of pectoral fins of the zebrafish highlights the yet to be explored diversity of fin structure and function in subadult developmental stages. J. Morphol. (c) 2005 Wiley-Liss, Inc.

Osteocalcin and matrix Gla protein in zebrafish (Danio rerio) and Senegal sole (Solea senegalensis): comparative gene and protein expression during larval development through adulthood

Bone Gla protein (Bgp or osteocalcin) and matrix Gla protein (Mgp) are important in calcium metabolism and skeletal development, but their precise roles at the molecular level remain poorly understood. Here, we compare the tissue distribution and accumulation of Bgp and Mgp during larval development and in adult tissues of zebrafish (Danio rerio) and throughout metamorphosis in Senegal sole (Solea senegalensis), two fish species with contrasting environmental calcium levels and degrees of skeletal reorganization at metamorphosis. Mineral deposition was investigated in parallel using a modified Alizarin red/Alcian blue protocol allowing sensitive simultaneous detection of bone and cartilage. In zebrafish, bgp and mgp mRNAs were localized in all mineralized tissues during and after calcification including bone and calcified cartilage of branchial arches. Through immunohistochemistry we demonstrated that these proteins accumulate mainly in the matrix of skeletal structures already calcified or under calcification, confirming in situ hybridization results. Interestingly, some accumulation of Bgp was also observed in kidney, possibly due to the presence of a related protein, nephrocalcin. Chromosomal localization of bgp and mgp using a zebrafish radiation hybrid panel indicated that both genes are located on the same chromosome, in contrast to mammals where they map to different chromosomes, albeit in regions showing synteny with the zebrafish location. Results in Senegal sole further indicate that, during metamorphosis, there is an increase in expression of both bgp and mgp, paralleling calcification of axial skeleton structures. In contrast with results obtained for previously studied marine fishes, in zebrafish and Senegal sole Mgp accumulates in both calcified tissues and non-mieralized vessel walls of the vascular system. These results suggest different patterns of Mgp accumulation between fish and mammals.

Characterization of two new zebrafish members of the hedgehog family: atypical expression of a zebrafish indian hedgehog gene in skeletal elements of both endochondral and dermal origins

We have characterized two new members of the Hedgehog (Hh) family in zebrafish, ihha and dhh, encoding for orthologues of the tetrapod Indian Hedgehog (Ihh) and Desert Hedgehog (Dhh) genes, respectively. Comparison of ihha and Type X collagen (col10a1) expression during skeletal development show that ihha transcripts are located in hypertrophic chondrocytes of cartilaginous elements of the craniofacial and fin endoskeleton. Surprisingly, col10a1 expression was also detected in cells forming intramembranous bones of the head and in flat cells surrounding cartilaginous structures. The expression of col10a1 in both endochondral and intramembranous bones reflects an atypical composition of the extracellular matrix of the zebrafish craniofacial skeleton. In addition, during fin ray regeneration, both ihha and col10a1 are detected in scleroblasts, osteoblast-like cells secreting the matrix of the dermal bone fin ray. The presence of cartilage markers suggests that the dermal fin ray possesses an intermediate phenotype between cartilage and bone.

Early Hedgehog signaling from neural to oral epithelium organizes anterior craniofacial development

Hedgehog (Hh) signaling plays multiple roles in the development of the anterior craniofacial skeleton. We show that the earliest function of Hh is indirect, regulating development of the stomodeum, or oral ectoderm. A subset of post-migratory neural crest cells, that gives rise to the cartilages of the anterior neurocranium and the pterygoid process of the palatoquadrate in the upper jaw, condenses upon the upper or roof layer of the stomodeal ectoderm in the first pharyngeal arch. We observe that in mutants for the Hh co-receptor smoothened (smo) the condensation of this specific subset of crest cells fails, and expression of several genes is lost in the stomodeal ectoderm. Genetic mosaic analyses with smo mutants show that for the crest cells to condense the crucial target tissue receiving the Hh signal is the stomodeum, not the crest. Blocking signaling with cyclopamine reveals that the crucial stage, for both crest condensation and stomodeal marker expression, is at the end of gastrulation--some eight to ten hours before crest cells migrate to associate with the stomodeum. Two Hh genes, shh and twhh, are expressed in midline tissue at this stage, and we show using mosaics that for condensation and skeletogenesis only the ventral brain primordium, and not the prechordal plate, is an important Hh source. Thus, we propose that Hh signaling from the brain primordium is required for proper specification of the stomodeum and the stomodeum, in turn, promotes condensation of a subset of neural crest cells that will form the anterior neurocranial and upper jaw cartilage.

Reassessing the Dlx code: the genetic regulation of branchial arch skeletal pattern and development

The branchial arches are meristic vertebrate structures, being metameric both between each other within the rostrocaudal series along the ventrocephalic surface of the embryonic head and within each individual arch: thus, just as each branchial arch must acquire a unique identity along the rostrocaudal axis, each structure within the proximodistal axis of an arch must also acquire a unique identity. It is believed that regional specification of metameric structures is controlled by the nested expression of related genes resulting in a regional code, a principal that is though to be demonstrated by the regulation of rostrocaudal axis development in animals exerted by the nested HOM-C/Hox homeobox genes. The nested expression pattern of the Dlx genes within the murine branchial arch ectomesenchyme has more recently led to the proposal of a Dlx code for the regional specification along the proximodistal axis of the branchial arches (i.e. it establishes intra-arch identity). This review re-examines this hypothesis, and presents new work on an allelic series of Dlx loss-of-function mouse mutants that includes various combinations of Dlx1, Dlx2, Dlx3, Dlx5 and Dlx6. Although we confirm fundamental aspects of the hypothesis, we further report a number of novel findings. First, contrary to initial reports, Dlx1, Dlx2 and Dlx1/2 heterozygotes exhibit alterations of branchial arch structures and Dlx2-/- and Dlx1/2-/- mutants have slight alterations of structures derived from the distal portions of their branchial arches. Second, we present evidence for a role for murine Dlx3 in the development of the branchial arches. Third, analysis of compound Dlx mutants reveals four grades of mandibular arch transformations and that the genetic interactions of cis first-order (e.g. Dlx5 and Dlx6), trans second-order (e.g. Dlx5 and Dlx2) and trans third-order paralogues (e.g. Dlx5 and Dlx1) result in significant and distinct morphological differences in mandibular arch development. We conclude by integrating functions of the Dlx genes within the context of a hypothesized general mechanism for the establishment of pattern and polarity in the first branchial arch of gnathostomes that includes regionally secreted growth factors such as Fgf8 and Bmp and other transcription factors such as Msx1, and is consistent both with the structure of the conserved gnathostome jaw bauplan and the elaboration of this bauplan to meet organismal end-point designs.

Eya1 is required for lineage-specific differentiation, but not for cell survival in the zebrafish adenohypophysis

The homeodomain transcription factor Six1 and its modulator, the protein phosphatase Eya1, cooperate to promote cell differentiation and survival during mouse organ development. Here, we studied the effects caused by loss of eya1 and six1 function on pituitary development in zebrafish. eya1 and six1 are co-expressed in all adenohypophyseal cells. Nevertheless, eya1 (aal, dog) mutants show lineage-specific defects, defining corticotropes, melanotropes, and gonadotropes as an Eya1-dependent lineage, which is complementary to the Pit1 lineage. Furthermore, eya1 is required for maintenance of pit1 expression, leading to subsequent loss of cognate hormone gene expression in thyrotropes and somatotropes of mutant embryos, whereas prolactin expression in lactotropes persists. In contrast to other organs, adenohypophyseal cells of eya1 mutants do not become apoptotic, and the adenohypophysis remains at rather normal size. Also, cells do not trans-differentiate, as in the case of pit1 mutants, but display morphological features characteristic for nonsecretory cells. Some of the adenohypophyseal defects of eya1 mutants are moderately enhanced in combination with antisense-mediated loss of Six1 function, which per se does not affect pituitary cell differentiation. In conclusion, this is the first report of an essential role of Eya1 during pituitary development in vertebrates. Eya1 is required for lineage-specific differentiation of adenohypophyseal cells, but not for their survival, thereby uncoupling the differentiation-promoting and anti-apoptotic effects of Eya proteins seen in other tissues.

Skeletal elements within teleost eyes and a discussion of their homology

Scleral ossicles and scleral cartilages form part of the craniofacial skeleton of many vertebrates. Some vertebrates, including all birds and most reptiles, but excluding most mammals, have scleral cartilages as well as scleral ossicles supporting their eyes. The teleost equivalent of these elements has received little attention in the literature. From radiographic and whole-mount analyses of over 400 individuals from 376 teleost species, we conclude that the teleost scleral skeletal elements (ossicles and cartilage) differ significantly from those of reptiles (including birds). Scleral ossicles in teleosts have different developmental origins, different positions within the eyeball, and different relationships with the scleral cartilaginous element than those in reptiles. From whole-mount staining of a growth series of four species of teleost (Danio rerio, Salmo salar, Esox lucius, and Alosa pseudoharengus), we interpret the development of these elements and show that they arise from within an Alcian blue-staining cartilaginous ring that develops around the eye earlier in development. We present possible scenarios on the evolution of these scleral skeletal elements from a common gnathostome ancestor, and consider that teleost scleral skeletal elements may not be homologous to those in reptiles. Our study indicates that homology cannot be assumed for these elements, despite the fact that they share the same name, scleral ossicles.

Expression of thedlx gene family during formation of the cranial bones in the zebrafish (Danio rerio): Differential involvement in the visceral skeleton and braincase

We have used dlx genes to test the hypothesis of a separate developmental program for dermal and cartilage bones within the neuro- and splanchnocranium by comparing expression patterns of all eight dlx genes during cranial bone formation in zebrafish from 1 day postfertilization (dPF) to 15 dPF. dlx genes are expressed in the visceral skeleton but not during the formation of dermal or cartilage bones of the braincase. The spatiotemporal expression pattern of all the members of the dlx gene family, support the view that dlx genes impart cellular identity to the different arches, required to make arch-specific dermal bones. Expression patterns seemingly associated with cartilage (perichondral) bones of the arches, in contrast, are probably related to ongoing differentiation of the underlying cartilage rather than with differentiation of perichondral bones themselves. Whether dlx genes originally functioned in the visceral skeleton only, and whether their involvement in the formation of neurocranial bones (as in mammals) is secondary, awaits clarification.

High-throughput in vivo screening for bone anabolic compounds with zebrafish

Osteoporosis and diseases of bone loss are a major public health problem for the present and the future since longevity and prevalence of the disease are increasing in all parts of the world. The bisphosphonates, widely used in the treatment of osteoporosis, act by inhibiting bone resorption. However, there are few agents that promote or increase bone formation in patients who have suffered substantial bone loss. To facilitate the identification of novel anabolic therapies, the authors have developed a rapid, high-throughput in vivo screen using larval zebrafish (Danio rerio) in which they are able to identify agents with anabolic effects in the skeleton within a 6-day time period. Vitamin D3 analogs and intermittent parathyroid hormone (PTH) result in dose-dependent increases in the formation of mineralized bone, whereas continuous exposure to PTH results in net bone loss. Because this model is fast, economical, and genetically tractable, it provides a powerful adjunct to mammalian models for the identification of new anabolic bone agents and offers the potential for genetic elucidation of pathways important in osteoblastic activity.

Hedgehog signaling is required for cranial neural crest morphogenesis and chondrogenesis at the midline in the zebrafish skull

Neural crest cells that form the vertebrate head skeleton migrate and interact with surrounding tissues to shape the skull, and defects in these processes underlie many human craniofacial syndromes. Signals at the midline play a crucial role in the development of the anterior neurocranium, which forms the ventral braincase and palate, and here we explore the role of Hedgehog (Hh) signaling in this process. Using sox10:egfp transgenics to follow neural crest cell movements in the living embryo, and vital dye labeling to generate a fate map, we show that distinct populations of neural crest form the two main cartilage elements of the larval anterior neurocranium: the paired trabeculae and the midline ethmoid. By analyzing zebrafish mutants that disrupt sonic hedgehog (shh) expression, we demonstrate that shh is required to specify the movements of progenitors of these elements at the midline, and to induce them to form cartilage. Treatments with cyclopamine, to block Hh signaling at different stages, suggest that although requirements in morphogenesis occur during neural crest migration beneath the brain, requirements in chondrogenesis occur later, as cells form separate trabecular and ethmoid condensations. Cell transplantations indicate that these also reflect different sources of Shh, one from the ventral neural tube that controls trabecular morphogenesis and one from the oral ectoderm that promotes chondrogenesis. Our results suggest a novel role for Shh in the movements of neural crest cells at the midline, as well as in their differentiation into cartilage, and help to explain why both skeletal fusions and palatal clefting are associated with the loss of Hh signaling in holoprosencephalic humans.

Defective skeletogenesis with kidney stone formation in dwarf zebrafish mutant for trpm7

Development of the adult form requires coordinated growth and patterning of multiple traits in response to local gene activity as well as to global endocrine and physiological effectors. An excellent example of such coordination is the skeleton. Skeletal development depends on the differentiation and morphogenesis of multiple cell types to generate elements with distinct forms and functions throughout the body. We show that zebrafish touchtone/nutria mutants exhibit severe growth retardation and gross alterations in skeletal development in addition to embryonic melanophore and touch-response defects. These alterations include accelerated endochondral ossification but delayed intramembranous ossification, as well as skeletal deformities. We show that the touchtone/nutria phenotype results from mutations in trpm7, which encodes a transient receptor potential (TRP) family member that functions as both a cation channel and kinase. We find trpm7 expression in the mesonephric kidney and show that mutants develop kidney stones, indicating renal dysfunction. These results identify a requirement for trpm7 in growth and skeletogenesis and highlight the potential of forward genetic approaches to uncover physiological mechanisms contributing to the development of adult form.

Evolution and development of facial bone morphology in threespine sticklebacks

How do developmental mechanisms evolve to control changing skeletal morphology, the shapes and sizes of individual bones? We address this question with studies of the opercle (OP), a large facial bone that has undergone marked morphological evolution in the ray-finned fish. Attributes for developmental analysis motivated us to examine how OP shape and size evolve and develop in threespine sticklebacks, a model system for understanding vertebrate evolution. We find that when Alaskan anadromous fish take up permanent residence in lakes, they evolve smaller and reshaped OPs. The change is a reduction in the amount of bone laid down along one body axis, and it arises at or shortly after the onset of OP development. A quantitative trait locus is present on linkage group 19 that contributes in a major way to this phenotype.

Development of the paired fins in the paddlefish, Polyodon spathula

In Polyodon spathula, the pectoral fin radials, with the exception of the metapterygium, are derived from the decomposition of a single continuous cartilage fin plate that is continuous with the scapulocoracoid. This cartilage sheet develops two interior splits to form three precursor pieces, and these decompose in a predictable way to generate the propterygium and radials. The metapterygium is an extension of the scapulocoracoid that segments off of it during early development. To our knowledge, this has not been reported for acipenserids or other basal actinopterygians. In teleosts, the proximal radials also develop from the "break up" of an initially continuous paddle-like sheet of cartilage along the posterior edge of the scapulocoracoid, and in Polypterus and sharks a similar pattern holds. Thus, the pattern observed in Polyodon may represent the basal developmental condition for the gnathostome pectoral fin. The process underlying development of the superficially similar cartilages of the pelvic and pectoral fins is different. In the pectoral fin, the metapterygium is segmented off of the scapulocoracoid and other radials form from the decomposition of the cartilage plate. In contrast, individual rod-like basipterygial elements form in a close one-to-one correspondence with the middle radials of the pelvic fin, but later fuse to form an anterior element that is branched in appearance. To evaluate further claims of similarity among the pectoral and pelvic fin elements of various fishes, the course of the development of these structures must be observed. The pectoral fin and girdle in Polyodon ossifies in a different sequence than that proposed as ancestral (and highly conserved) for actinopterygians: the supracleithrum ossifies significantly before the cleithrum. The later ossification of the cleithrum in Polyodon may be related to the primary use of the caudal fin vs. the pectoral fins in their locomotion.

A new origin for the maxillary jaw

One conserved feature of craniofacial development is that the first pharyngeal arch has two components, the maxillary and mandibular, which then form the upper and lower jaws, respectively. However, until now, there have been no tests of whether the maxillary cells originate entirely within the first pharyngeal arch or whether they originate in a separate condensation, cranial to the first arch. We therefore constructed a fate map of the pharyngeal arches and environs with a series of dye injections into stage 13-17 chicken embryos. We found that from the earliest stage examined, the major contribution to the maxillary bud is from post-optic mesenchyme with a relatively minor contribution from the maxillo-mandibular cleft. Cells labeled within the first pharyngeal arch contributed exclusively to the mandibular prominence. Gene expression data showed that there were different molecular codes for the cranial and caudal maxillary prominence. Two of the genes examined, Rarbeta (retinoic acid receptor beta) and Bmp4 (bone morphogenetic protein) were expressed in the post-optic mesenchyme and epithelium prior to formation of the maxillary prominence and then were restricted to the cranial half of the maxillary prominence. In order to determine the derivatives of the maxillary prominence, we performed focal injections of CM-DiI into the stage 24 maxillary prominence. Labeled cells contributed to the maxillary, palatine, and jugal bones, but not the other elements of the upper beak, the premaxilla and prenasal cartilage. We also determined that the cranial cells give rise to more distal parts of the upper beak, whereas caudal cells form proximal structures. Grafts of stage 24 maxillary prominences were also analyzed to determine skeletal derivatives and these results concurred with the DiI maps. These early and later fate maps indicate that the maxillary prominence and its skeletal derivatives are not derived from the first pharyngeal arch but rather from a separate maxillary condensation that occurs between the eye and the maxillo-mandibular cleft. These data also suggest that during evolution, recession of the first pharyngeal arch-derived palatoquadrate cartilage to a more proximal position gave way to the bony upper jaw of amniotes.

Zebrafishacvr2a andacvr2b exhibit distinct roles in craniofacial development

To examine the roles of activin type II receptor signaling in craniofacial development, full-length zebrafish acvr2a and acvr2b clones were isolated. Although ubiquitously expressed as maternal mRNAs and in early embryogenesis, by 24 hr postfertilization (hpf), acvr2a and acvr2b exhibit restricted expression in neural, hindbrain, and neural crest cells (NCCs). A morpholino-based targeted protein depletion approach was used to reveal discrete functions for each acvr2 gene product. The acvr2a morphants exhibited defects in the development of most cranial NCC-derived cartilage, bone, and pharyngeal tooth structures, whereas acvr2b morphant defects were largely restricted to posterior arch structures and included the absence and/or aberrant migration of posterior NCC streams, defects in NCC-derived posterior arch cartilages, and dysmorphic pharyngeal tooth development. These studies revealed previously uncharacterized roles for acvr2a and acvr2b in hindbrain and NCC patterning, in NCC derived pharyngeal arch cartilage and joint formation, and in tooth development.

The Zebrafish (Danio rerio): A Model System for Cranial Suture Patterning

The zebrafish (Danio rerio) is an alluring model system currently used to study early embryonic development, organogenesis and gene functional analysis. However, few studies have been devoted to post-embryonic development. We have explored the possibility of using this organism to analyze how cranial suture patterning occurs. This study reports on the establishment of the zebrafish skull vault anatomy, calvarial osteogenesis, and cranial suture morphology. Our results demonstrate that the anatomy of the zebrafish cranial vault and cranial sutures is very similar to that of mammalian organisms. Indeed, the zebrafish represents a versatile and valuable model system for the study of the biogenesis of cranial sutures.

Characterization of dermacan, a novel zebrafish lectican gene, expressed in dermal bones

We report here the isolation and characterization of a cDNA encoding zebrafish dermacan, a novel member of hyaluronan (HA)-binding proteoglycans, which was termed after its characteristic expression in the zebrafish dermal bones. The deduced protein sequence shares the typical modular elements of lecticans. Sequence comparison covering the C-terminal globular domain demonstrated that dermacan shows high homology with zebrafish versican but is distinct from any other identified lecticans. Genomic DNA analysis demonstrated that dermacan and versican were encoded by distinct genes in the zebrafish genome. The expression of dermacan is initiated in the sclerotome and cephalic paraxial mesoderm at 16 h postfertilization. During the pharyngular period, dermacan transcripts were detected in the sclerotome, tail fin bud, pharyngular arch primordial region, and otic vesicle. In the development of craniofacial bones, dermacan expression was detected typically in the opercle and dentary. These regions belong to the craniofacial dermal bones. aggrecan expression, in contrast, was observed in the elements of craniofacial cartilage bones. In the dermacan-morpholino-injected embryos, dermal bones, e.g. opercle, dentary, and branchiostegal rays, as well as axial skeleton in the trunk, showed decreased ossification. We conclude that dermacan is a novel lectican gene, and that zebrafish lectican genes have genetically diverged. In addition, our data suggest the involvement of dermacan in zebrafish dermal bone development.

Postembryonic development of the cranial lateral line canals and neuromasts in zebrafish

The development of the cranial lateral line canals and neuromast organs are described in postembryonic zebrafish (0-80 days postfertilization). Cranial canal development commences several weeks after hatch, is initiated in the vicinity of individual neuromasts, and occurs in four discrete stages that are described histologically. Neuromasts remain in open canal grooves for several weeks during which they dramatically change shape and increase in size by adding hair cells at a rate one-tenth that in the zebrafish inner ear. Scanning electron microscopy demonstrates that neuromasts elongate perpendicular to the canal axis and the axis of hair cell polarization and that they lack a prominent nonsensory cell population surrounding the hair cells-features that make zebrafish neuromasts unusual among fishes. These results demand a reassessment of neuromast and lateral line canal diversity among fishes and highlight the utility of the lateral line system of postembryonic zebrafish for experimental and genetic studies of the development and growth of hair cell epithelia.

Developmental morphology of the axial skeleton of the zebrafish, Danio rerio (Ostariophysi: Cyprinidae)

Before our rapidly increasing knowledge of gene interactions can be connected with the morphologic defects in mutant zebrafish, the normal course of skeletal development must be understood. Here, we describe the developmental morphology of the axial skeleton of zebrafish and review it in relation to the morphology of related species. The relative sequence of ossification in the skeleton is described. Two separate centers of development were found in the axial skeleton (Weberian apparatus and caudal fin) in contrast to tetrapods, which have a single anterior center. Slight variation was found in the overall relative timing of development. The extensive ichthyological literature on teleost anatomy and recent genetic data form the basis for our review and interpretation of homologies of various elements of the axial skeleton. Because homology forms the basis for all evolutionary comparisons, these data are critical for integration in evo-devo studies.

The scapulocoracoid of the Queensland lungfish Neoceratodus forsteri (Dipnoi: Sarcopterygii): morphology, development and evolutionary implications for bony fishes (Osteichthyes)

Among bony fishes, the ontogenetic sequence by which the actinopterygian scapulocoracoid develops has been well described, but that of the sarcopterygian scapulocoracoid is poorly known, as the majority of taxa are only known from fossils. To rectify this, the cartilaginous scapulocoracoid of the extant lungfish Neoceratodus forsteri is examined. In initial stages of its development, the scapulocoracoid of Neoceratodus has a simple rounded shape, and supports the glenoid fossa. It appears nearly contemporaneously with the proximal endochondral element (humerus) of the pectoral fin. Pectoral fin elements develop by segmentation from a continuous field of cartilaginous precursor cells extending distally from the glenoid region of the scapulocoracoid. Subsequent scapulocoracoid development produces a ventromedial process, which is not associated with this field of precursor cells. A dorsal process also develops outside this field. Thus, the scapulocoracoid of Neoceratodus may consist of at least two developmentally distinct regions; (1) the ventromedial being homologous with the coracoid of actinopterygians, tetrapods and other jawed vertebrates and (2) a smaller dorsal process, homologous to the scapular region. The two, together with the glenoid region, give an overall triangular shape. The scapulocoracoids of fossil lungfish and other sarcopterygian fishes are also triangular and are composed of scapular and coracoid regions, rather than the 'buttresses' associated with scapulocoracoids of the Actinopterygii and Tetrapoda.

Radiographic analysis of zebrafish skeletal defects

Systematic identification of skeletal dysplasias in model vertebrates provides insight into the pathogenesis of human skeletal disorders and can aid in the identification of orthologous human genes. We are undertaking a mutagenesis screen for skeletal dysplasias in adult zebrafish, using radiography to detect abnormalities in skeletal anatomy and bone morphology. We have isolated chihuahua, a dominant mutation causing a general defect in bone growth. Heterozygous chihuahua fish have phenotypic similarities to human osteogenesis imperfecta, a skeletal dysplasia caused by mutations in the type I collagen genes. Mapping and molecular characterization of the chihuahua mutation indicates that the defect resides in the gene encoding the collagen I(alpha1) chain. Thus, chihuahua accurately models osteogenesis imperfecta at the biologic and molecular levels, and will prove an important resource for studies on the disease pathophysiology. Radiography is a practical screening tool to detect subtle skeletal abnormalities in the adult zebrafish. The identification of chihuahua demonstrates that mutant phenotypes analogous to human skeletal dysplasias will be discovered.

Conserved molecular program regulating cranial and appendicular skeletogenesis

The majority of in vivo studies on bone and cartilage differentiation are carried out using the appendicular skeleton as a model system, with the implicit assumption that skeletal formation is equivalent throughout the body. This assumption persists, despite differences in the cellular origins of the skeletogenic precursors. To test the hypothesis that a fundamental set of genes directs skeletal cell differentiation throughout the body, we analyzed cartilage and bone of the chick limb and head during mesenchymal condensation, and when the skeletal tissues had matured. First, we analyzed the expression patterns of transcription factors in early skeletogenic condensations, which revealed similarities among skeletal cell specification in the limb and head. For example, skeletogenic condensations that undergo endochondral ossification had equivalent expression patterns of skeletogenic transcription factors in both limb and head. In the head, many elements also differentiate through intramembranous ossification, or through persistent cartilage formation. Our analyses of these skeletogenic condensations revealed that a unique expression pattern of transcription factors distinguishes among three skeletal tissue fates. The vasculature was excluded from all three skeletogenic condensations, demonstrating that this is not a characteristic unique to endochondral ossification. Second, we compared three different types of more mature cartilage and bone tissue in both the limb and the head, by analyzing a variety of skeletal collagens and signaling molecules. Histological and molecular markers of cartilage and bone generally were conserved between the limb and head skeletons, although we uncovered subtle differences in signaling pathways that might influence cranial and appendicular skeletogenesis.