The mechanism of cartilage subdivision in the reorganization of the zebrafish pectoral fin endoskeleton

Zebrafish endochondral growth zones as they relate to human bone size, shape and disease

Research on the genetic mechanisms underlying human skeletal development and disease have largely relied on studies in mice. However, recently the zebrafish has emerged as a popular model for skeletal research. Despite anatomical differences such as a lack of long bones in their limbs and no hematopoietic bone marrow, both the cell types in cartilage and bone as well as the genetic pathways that regulate their development are remarkably conserved between teleost fish and humans. Here we review recent studies that highlight this conservation, focusing specifically on the cartilaginous growth zones (GZs) of endochondral bones. GZs can be unidirectional such as the growth plates (GPs) of long bones in tetrapod limbs or bidirectional, such as in the synchondroses of the mammalian skull base. In addition to endochondral growth, GZs play key roles in cartilage maturation and replacement by bone. Recent studies in zebrafish suggest key roles for cartilage polarity in GZ function, surprisingly early establishment of signaling systems that regulate cartilage during embryonic development, and important roles for cartilage proliferation rather than hypertrophy in bone size. Despite anatomical differences, there are now many zebrafish models for human skeletal disorders including mutations in genes that cause defects in cartilage associated with endochondral GZs. These point to conserved developmental mechanisms, some of which operate both in cranial GZs and limb GPs, as well as others that act earlier or in parallel to known GP regulators. Experimental advantages of zebrafish for genetic screens, high resolution live imaging and drug screens, set the stage for many novel insights into causes and potential therapies for human endochondral bone diseases.

Morphological evolution and diversity of pectoral fin skeletons in teleosts

The Teleostei class has the most species of the fishes. Members of this group have pectoral fins, enabling refined movements in the water. Although teleosts live in a diverse set of environments, the skeletal pattern of pectoral fins in teleosts is considered to show little morphological variability. Here, in order to elucidate variations in pectoral fin skeletons and to identify their evolutionary processes, we compared the pectoral fin skeletons from 27 species of teleosts. We identified several variations and a diversity of pectoral fin skeletal patterns within some teleost groups. Taken together with previous reports on teleost skeletons, our findings reveal that in the course of teleost evolution, there are a mixture of conserved and non-conserved components in the pectoral fin skeletons of teleosts, and that teleosts may have experienced the variation and conservation of the number and shape of the proximal radials, the loss of the mesocoracoid, and the change in the distal radial-fin ray relationship.

Direct development of the catfish pectoral fin - an alternative pectoral fin pattern of teleosts

BACKGROUND

Study of the teleosts' pectoral fin development touches on many crucial issues of evolutionary biology, from the formation of local adaptations to the tetrapod limbs' origin. Teleosts' pectoral fin is considered a rather developmentally and anatomically conservative structure. It displays larval and adult stages differing in the skeletal and soft tissues' composition. Larva-adult transition proceeds under the thyroid hormone (TH) control that defines pectoral fin ontogeny as an indirect development. However, the outstanding diversity of teleosts allows suggesting the existence of lineage specific developmental patterns.

RESULTS

We present a description of the North African catfish, Clarias gariepinus, pectoral fin development. It lacks a clear larval stage and directly develops the adult skeleton with the associated musculature and innervation. Interestingly, the development of catfish pectoral fin appears not to be under the TH dependence.

CONCLUSION

This catfish displays a direct pectoral fin developmental trajectory differing from the stereotyped teleost pattern. In the absence of the larval endoskeletal disk and TH control, the catfish's proximal radials arise in a manner somewhat similar to the metapterygial radials in basal actinopterygians and humerus in sarcopterygians. Thus, the catfish fin pattern seems homoplastic, arising by convergence with, or reversion to the ancestral developmental mechanisms. This article is protected by copyright. All rights reserved.

The role of Gdf5 in the development of the zebrafish fin endoskeleton

BACKGROUND

The development of the vertebrate limb skeleton requires a complex interaction of multiple factors to facilitate the correct shaping and positioning of bones and joints. Growth and differentiation factor 5 (Gdf5) is involved in patterning appendicular skeletal elements including joints. Expression of gdf5 in zebrafish has been detected in fin mesenchyme condensations and segmentation zones as well as the jaw joint, however, little is known about the functional role of Gdf5 outside of Amniota.

RESULTS

We generated CRISPR/Cas9 knockout of gdf5 in zebrafish and analyzed the resulting phenotype at different developmental stages. Homozygous gdf5 mutant zebrafish displayed changes in segmentation of the endoskeletal disc and, as a consequence, loss of posterior radials in the pectoral fins. Mutant fish also displayed disorganization and reduced length of endoskeletal elements in the median fins, while joints and mineralization seemed unaffected.

CONCLUSIONS

Our study demonstrates the importance of Gdf5 in the development of the zebrafish pectoral and median fin endoskeleton and reveals that the severity of the effect increases from anterior to posterior elements. Our findings are consistent with phenotypes observed in the human and mouse appendicular skeleton in response to Gdf5 knockout, suggesting a broadly conserved role for Gdf5 in Osteichthyes.

Latent developmental potential to form limb-like skeletal structures in zebrafish

Changes in appendage structure underlie key transitions in vertebrate evolution. Addition of skeletal elements along the proximal-distal axis facilitated critical transformations, including the fin-to-limb transition that permitted generation of diverse modes of locomotion. Here, we identify zebrafish mutants that form supernumerary long bones in their pectoral fins. These new bones integrate into musculature, form joints, and articulate with neighboring elements. This phenotype is caused by activating mutations in previously unrecognized regulators of appendage patterning, vav2 and waslb, that function in a common pathway. This pathway is required for appendage development across vertebrates, and loss of Wasl in mice causes defects similar to those seen in murine Hox mutants. Concordantly, formation of supernumerary bones requires Hox11 function, and mutations in the vav2/wasl pathway drive enhanced expression of hoxa11b, indicating developmental homology with the forearm. Our findings reveal a latent, limb-like pattern ability in fins that is activated by simple genetic perturbation.

Terrestrial acclimation and exercise lead to bone functional response in Polypterus senegalus pectoral fins

The ability of bones to sense and respond to mechanical loading is a central feature of vertebrate skeletons. However, the functional demands imposed on terrestrial and aquatic animals differ vastly. The pectoral girdle of the basal actinopterygian fish Polypterus senegalus was previously shown to exhibit plasticity following terrestrial acclimation, but the pectoral fin itself has yet to be examined. We investigated skeletal plasticity in the pectoral fins of P. senegalus after exposure to terrestrial loading. Juvenile fish were divided into three groups: a control group was kept under aquatic conditions without intervention, an exercised group was also kept in water but received daily exercise on land, and a terrestrial group was kept in a chronic semi-terrestrial condition. After 5 weeks, the pectoral fins were cleared and stained with Alcian Blue and Alizarin Red to visualize cartilage and bone, allowing measurements of bone length, bone width, ossification and curvature to be taken for the endochondral radial bones. Polypterus senegalus fin bones responded most strongly to chronic loading in the terrestrial condition. Fish that were reared in a terrestrial environment had significantly longer bones compared with those of aquatic controls, wider propterygia and metapterygia, and more ossified metapterygia and medial radials, and they showed changes in propterygial curvature. Exercised fish also had longer and more ossified medial radials compared with those of controls. Polypterus senegalus fin bones exhibit plasticity in response to novel terrestrial loading. Such plasticity could be relevant for transitions between water and land on evolutionary scales, but key differences between fish and tetrapod bone make direct comparisons challenging.

Pattern of fin rays along the antero-posterior axis based on their connection to distal radials

BACKGROUND

Teleost paired fins are composed of two endoskeletal domains, proximal and distal radials, and an exoskeletal domain, the fin ray. The zebrafish pectoral fin displays elaborately patterned radials along the anteroposterior (AP) axis. Radials are considered homologous to tetrapod limb skeletons, and their patterning mechanisms in embryonic development are similar to those of limb development. Nevertheless, the pattern along the AP axis in fin rays has not been well described in the zebrafish pectoral fin, although several recent reports have revealed that fin ray development shares some cellular and genetic properties with fin/limb endoskeleton development. Thus, fin ray morphogenesis may involve developmental mechanisms for AP patterning in the fin/limb endoskeleton, and may have a specific pattern along the AP axis.

RESULTS

We conducted detailed morphological observations on fin rays and their connection to distal radials by comparing intra- and inter-strain zebrafish specimens. Although the number of fin rays varied, pectoral fin rays could be categorized into three domains along the AP axis, according to the connection between the fin rays and distal radials; additionally, the number of fin rays varied in the posterior part of the three domains. This result was confirmed by observation of the morphogenesis process of fin rays and distal radials, which showed altered localization of distal radials in the middle domain. We also evaluated the expression pattern of lhx genes, which have AP patterning activity in limb development, in fin rays and during distal radial development and found these genes to be expressed during morphogenesis in both fin rays and distal radials.

CONCLUSION

The fin ray and its connection to the endoskeleton are patterned along the AP axis, and the pattern along the AP axis in the fin ray and the radial connection is constructed by the developmental mechanism related to AP patterning in the limb/fin bud. Our results indicate the possibility that the developmental mechanisms of fin rays and their connection are comparable to those of the distal element of the limb skeleton.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.

TALEN-mediated mutagenesis in zebrafish reveals a role for r-spondin 2 in fin ray and vertebral development

R-spondin (Rspo) encodes a multi-domain protein that modulates the Wnt-signaling pathway. Two distinct rspo2 zebrafish mutants were generated by TALEN-mediated mutagenesis: a null mutant, rspo2(null), lacking all functional domains, and a hypomorphic mutant, rspo2(tsp), lacking the two N-terminal domains. Mutants were analyzed mainly for abnormalities in the skeletal system. Fin ray skeletons were formed normally in the rspo2(tsp) mutants, but were absent from the rspo2(null) mutants. Hypoplasia of the neural/hemal arches and ribs was observed in both mutants. Thus, the two rspo2 mutants help to identify the functions of Rspo2 in skeletogenesis, as well as functional differences among multiple Rspo2 domains.

Zebrabow: multispectral cell labeling for cell tracing and lineage analysis in zebrafish

Advances in imaging and cell-labeling techniques have greatly enhanced our understanding of developmental and neurobiological processes. Among vertebrates, zebrafish is uniquely suited for in vivo imaging owing to its small size and optical translucency. However, distinguishing and following cells over extended time periods remains difficult. Previous studies have demonstrated that Cre recombinase-mediated recombination can lead to combinatorial expression of spectrally distinct fluorescent proteins (RFP, YFP and CFP) in neighboring cells, creating a 'Brainbow' of colors. The random combination of fluorescent proteins provides a way to distinguish adjacent cells, visualize cellular interactions and perform lineage analyses. Here, we describe Zebrabow (Zebrafish Brainbow) tools for in vivo multicolor imaging in zebrafish. First, we show that the broadly expressed ubi:Zebrabow line provides diverse color profiles that can be optimized by modulating Cre activity. Second, we find that colors are inherited equally among daughter cells and remain stable throughout embryonic and larval stages. Third, we show that UAS:Zebrabow lines can be used in combination with Gal4 to generate broad or tissue-specific expression patterns and facilitate tracing of axonal processes. Fourth, we demonstrate that Zebrabow can be used for long-term lineage analysis. Using the cornea as a model system, we provide evidence that embryonic corneal epithelial clones are replaced by large, wedge-shaped clones formed by centripetal expansion of cells from the peripheral cornea. The Zebrabow tool set presented here provides a resource for next-generation color-based anatomical and lineage analyses in zebrafish.