Effects of fin fold mesenchyme ablation on fin development in zebrafish

wnt10a is required for zebrafish median fin fold maintenance and adult unpaired fin metamorphosis

BACKGROUND

Mutations of human WNT10A are associated with odonto-ectodermal dysplasia syndromes. Here, we present analyses of wnt10a loss-of-function mutants in the zebrafish.

RESULTS

wnt10a mutant zebrafish embryos display impaired tooth development and a collapsing median fin fold (MFF). Rescue experiments show that wnt10a is essential for MFF maintenance both during embryogenesis and later metamorphosis. The MFF collapse could not be attributed to increased cell death or altered proliferation rates of MFF cell types. Rather, wnt10a mutants show reduced expression levels of dlx2a in distal-most MFF cells, followed by compromised expression of col1a1a and other extracellular matrix proteins encoding genes. Transmission electron microscopy analysis shows that although dermal MFF compartments of wnt10a mutants initially are of normal morphology, with regular collagenous actinotrichia, positioning of actinotrichia within the cleft of distal MFF cells becomes compromised, coinciding with actinotrichia shrinkage and MFF collapse.

CONCLUSIONS

MFF collapse of wnt10a mutant zebrafish is likely caused by the loss of distal properties in the developing MFF, strikingly similar to the proposed molecular pathomechanisms underlying the teeth defects caused by the loss of Wnt10 in fish and mammals. In addition, it points to thus fur unknown mechanisms controlling the linear growth and stability of actinotrichia and their collagen fibrils.

Origin and diversification of fibroblasts from the sclerotome in zebrafish

Fibroblasts play an important role in maintaining tissue integrity by secreting components of the extracellular matrix and initiating response to injury. Although the function of fibroblasts has been extensively studied in adults, the embryonic origin and diversification of different fibroblast subtypes during development remain largely unexplored. Using zebrafish as a model, we show that the sclerotome, a sub-compartment of the somite, is the embryonic source of multiple fibroblast subtypes including tenocytes (tendon fibroblasts), blood vessel associated fibroblasts, fin mesenchymal cells, and interstitial fibroblasts. High-resolution imaging shows that different fibroblast subtypes occupy unique anatomical locations with distinct morphologies. Long-term Cre-mediated lineage tracing reveals that the sclerotome also contributes to cells closely associated with the axial skeleton. Ablation of sclerotome progenitors results in extensive skeletal defects. Using photoconversion-based cell lineage analysis, we find that sclerotome progenitors at different dorsal-ventral and anterior-posterior positions display distinct differentiation potentials. Single-cell clonal analysis combined with in vivo imaging suggests that the sclerotome mostly contains unipotent and bipotent progenitors prior to cell migration, and the fate of their daughter cells is biased by their migration paths and relative positions. Together, our work demonstrates that the sclerotome is the embryonic source of trunk fibroblasts as well as the axial skeleton, and local signals likely contribute to the diversification of distinct fibroblast subtypes.

Whole-body variational modularity in the zebrafish: an inside-out story of a model species

Actinopterygians are the most diversified clade of extant vertebrates. Their impressive morphological disparity bears witness to tremendous ecological diversity. Modularity, the organization of biological systems into quasi-independent anatomical/morphological units, is thought to increase evolvability of organisms and facilitate morphological diversification. Our study aims to quantify patterns of variational modularity in a model actinopterygian, the zebrafish (Danio rerio), using three-dimensional geometric morphometrics on osteological structures isolated from micro-CT scans. A total of 72 landmarks were digitized along cranial and postcranial ossified regions of 30 adult zebrafishes. Two methods were used to test modularity hypotheses, the covariance ratio and the distance matrix approach. We find strong support for two modules, one comprised paired fins and the other comprised median fins, that are best explained by functional properties of subcarangiform swimming. While the skull is tightly integrated with the rest of the body, its intrinsic integration is relatively weak supporting previous findings that the fish skull is a modular structure. Our results provide additional support for the recognition of similar hypotheses of modularity identified based on external morphology in various teleosts, and at least two variational modules are proposed. Thus, our results hint at the possibility that internal and external modularity patterns may be congruent.

Collagen9a1c localizes to collagen fibers called actinotrichia in zebrafish fins

Teleost fish fins are supported by spear-shaped collagen crystals called actinotrichia. Actinotrichia are distributed radially at the distal end of the fins and thought to be necessary for proper formation of the fin and fin-bones. We previously reported that collagen9a1c ( col9a1c ) gene product is essential for the regular arrangement of actinotrichia using col9a1c -knockout zebrafish. Here, we examined the localization pattern of the EGFP-tagged Col9a1c protein in the fins to understand its role in the arrangement of actinotrichia. We found that EGFP-Col9a1c specifically localizes to actinotrichia.

Effect of monosultap on notochord development in zebrafish (Danio rerio) embryos

Monosultap (Mon) is a broad-spectrum insecticide used in agricultural production to control stem borers in rice fields. Currently, little evidence shows how Mon affects notochord development in zebrafish (Danio rerio). In our study, zebrafish embryos were exposed to 0.25, 0.5, and 0.75 mg/L Mon to determine the effects of different concentrations of Mon on notochord development. Mon exposure reduced the body length, decreased the heart rate and hatchability, and induced notochord deformity in zebrafish. The effects of Mon exposure on the internal organization of the notochord and the structural abnormalities were determined based on histological staining of paraffinized tissue sections. Quantitative polymerase chain reaction (qPCR) and in situ hybridization findings revealed that the expression levels of genes related to notochord development (shha, col2a, and ptch2) showed an increasing trend in a concentration-dependent manner. An abnormal increase of apoptosis and cell proliferation in some parts of the notochord suggested that Mon exposure could cause developmental abnormality of the notochord. This study revealed the toxicity of Mon in notochord development. Our findings provide information in assessing the risk of Mon to the ecological environment and human health.

Mechanical role of actinotrichia in shaping the caudal fin of zebrafish

Spear-like collagen complexes, known as actinotrichia, underlie the epidermal cell layer in the tip of teleost fins and are known to contribute toward fin formation; however, their specific role remains largely unclear. In this study, we investigated of actinotrichia in the role of caudal fin formation by generating collagen9a1c (col9a1c)-knockout zebrafish. Although actinotrichia were initially produced normally and aligned correctly in the knockout fish, the number of actinotrichia decreased as the fish grew and their alignment became disordered. Simultaneously, the fin tip gradually shortened in the dorsal-ventral direction and the entire fin became oval-shaped, while the fin-rays rarely bifurcated and instead underwent fusion, suggesting that actinotrichia are essential for spreading fins dorsoventrally. Furthermore, the epithelial cells that are usually thinly spread in normal fish became spherical in the knockout fish, reducing the area covered by each cell and thus the area of the fin tip. Together, these findings suggest that the tight alignment of actinotrichia provides physical support in the dorsal-ventral direction that allows caudal fins to expand in a triangular-shape.

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.

The Physical Role of Mesenchymal Cells Driven by the Actin Cytoskeleton Is Essential for the Orientation of Collagen Fibrils in Zebrafish Fins

Fibrous collagen imparts physical strength and flexibility to tissues by forming huge complexes. The density and orientation of collagen fibers must be correctly specified for the optimal physical property of the collagen complex. However, little is known about its underlying cellular mechanisms. Actinotrichia are collagen fibers aligned at the fin-tip of bony fish and are easily visible under the microscope due to their thick, linear structure. We used the actinotrichia as a model system to investigate how cells manipulate collagen fibers. The 3D image obtained by focused ion beam scanning electron microscopy (FIB-SEM) showed that the pseudopodia of mesenchymal cells encircle the multiple actinotrichia. We then co-incubated the mesenchymal cells and actinotrichia in vitro, and time-lapse analysis revealed how cells use pseudopods to align collagen fiber orientation. This in vitro behavior is dependent on actin polymerization in mesenchymal cells. Inhibition of actin polymerization in mesenchymal cells results in mis-orientation of actinotrichia in the fin. These results reveal how mesenchymal cells are involved in fin formation and have important implications for the physical interaction between cells and collagen fibers.

Developmental constraints on fin diversity

The fish fin is a breathtaking repository full of evolutionary diversity, novelty, and convergence. Over 500 million years, the adaptation to novel habitats has provided landscapes of fin diversity. Although comparative anatomy of evolutionarily divergent patterns over centuries has highlighted the fundamental architectures and evolutionary trends of fins, including convergent evolution, the developmental constraints on fin evolution, which bias the evolutionary trajectories of fin morphology, largely remain elusive. Here, we review the evolutionary history, developmental mechanisms, and evolutionary underpinnings of paired fins, illuminating possible developmental constraints on fin evolution. Our compilation of anatomical and genetic knowledge of fin development sheds light on the canalized and the unpredictable aspects of fin shape in evolution. Leveraged by an arsenal of genomic and genetic tools within the working arena of spectacular fin diversity, evolutionary developmental biology embarks on the establishment of conceptual framework for developmental constraints, previously enigmatic properties of evolution.

Adipose fin development and its relation to the evolutionary origins of median fins

The dorsal, anal and caudal fins of vertebrates are proposed to have originated by the partitioning and transformation of the continuous median fin fold that is plesiomorphic to chordates. Evaluating this hypothesis has been challenging, because it is unclear how the median fin fold relates to the adult median fins of vertebrates. To understand how new median fins originate, here we study the development and diversity of adipose fins. Phylogenetic mapping shows that in all lineages except Characoidei (Characiformes) adipose fins develop from a domain of the larval median fin fold. To inform how the larva's median fin fold contributes to the adipose fin, we study Corydoras aeneus (Siluriformes). As the fin fold reduces around the prospective site of the adipose fin, a fin spine develops in the fold, growing both proximally and distally, and sensory innervation, which appears to originate from the recurrent ramus of the facial nerve and from dorsal rami of the spinal cord, develops in the adipose fin membrane. Collectively, these data show how a plesiomorphic median fin fold can serve as scaffolding for the evolution and development of novel, individuated median fins, consistent with the median fin fold hypothesis.

Roles of basal keratinocytes in actinotrichia formation

The embryonic fins and the tip of adult fins of teleost fish are supported by rows of straight, unmineralized fibrils called actinotrichia. The proximal ends of the actinotrichia are bundled and the mineralized bones called lepidotrichia are made along them. Since malformation in actinotrichia causes wavy fin bones, the correct configuration of actinotrichia is essential for the correct construction of the fin shape. Past studies suggested that two types of cells, basal keratinocytes, and mesenchymal cells involve in the formation of actinotrichia. However, the mechanism how these cells contribute is unknown. In this study, we elucidated the role of basal keratinocytes in actinotrichia formation. First, we developed the imaging tool that specifically visualizes the basal keratinocytes and actinotrichia. Then, we established the in vitro culture method of the basal keratinocytes and found that the keratinocytes developed fine needle-like structures in it. The TEM image of them showed the specific shadow pattern of actinotrichia, indicating that the fine needle-like structures are the newly made actinotrichia. Finally, we cultured the basal keratinocytes with mature actinotrichia and observed that the basal keratinocytes actively holded actinotrichia with their membrane, and often generated a linear array of cells holding a single actinotrichium. This behavior suggested a mechanism with which long actinotrichia are made by relatively small cells. Our results clarified the role of basal keratinocyte and provided a novel insight into understanding the mechanism of actinotrichia formation.

Problems in Fish-to-Tetrapod Transition: Genetic Expeditions Into Old Specimens

The fish-to-tetrapod transition is one of the fundamental problems in evolutionary biology. A significant amount of paleontological data has revealed the morphological trajectories of skeletons, such as those of the skull, vertebrae, and appendages in vertebrate history. Shifts in bone differentiation, from dermal to endochondral bones, are key to explaining skeletal transformations during the transition from water to land. However, the genetic underpinnings underlying the evolution of dermal and endochondral bones are largely missing. Recent genetic approaches utilizing model organisms—zebrafish, frogs, chickens, and mice—reveal the molecular mechanisms underlying vertebrate skeletal development and provide new insights for how the skeletal system has evolved. Currently, our experimental horizons to test evolutionary hypotheses are being expanded to non-model organisms with state-of-the-art techniques in molecular biology and imaging. An integration of functional genomics, developmental genetics, and high-resolution CT scanning into evolutionary inquiries allows us to reevaluate our understanding of old specimens. Here, we summarize the current perspectives in genetic programs underlying the development and evolution of the dermal skull roof, shoulder girdle, and appendages. The ratio shifts of dermal and endochondral bones, and its underlying mechanisms, during the fish-to-tetrapod transition are particularly emphasized. Recent studies have suggested the novel cell origins of dermal bones, and the interchangeability between dermal and endochondral bones, obscuring the ontogenetic distinction of these two types of bones. Assimilation of ontogenetic knowledge of dermal and endochondral bones from different structures demands revisions of the prevalent consensus in the evolutionary mechanisms of vertebrate skeletal shifts.