Medaka unextended-fin mutants suggest a role for Hoxb8a in cell migration and osteoblast differentiation during appendage formation

Adult caudal fin shape is imprinted in the embryonic fin fold

Appendage shape is formed during development (and re-formed during regeneration) according to spatial and temporal cues that orchestrate local cellular morphogenesis. The caudal fin is the primary appendage used for propulsion in most fish species, and exhibits a range of distinct morphologies adapted for different swimming strategies, however the molecular mechanisms responsible for generating these diverse shapes remain mostly unknown. In zebrafish, caudal fins display a forked shape, with longer supportive bony rays at the periphery and shortest rays at the center. Here, we show that a premature, transient pulse of sonic hedgehog a (shha) overexpression during late embryonic development results in excess proliferation and growth of the central rays, causing the adult caudal fin to grow into a triangular, truncate shape. Both global and regional ectopic shha overexpression are sufficient to alter fin shape, and forked shape may be rescued by subsequent treatment with an antagonist of the canonical Shh pathway. The induced truncate fins show a decreased fin ray number and fail to form the hypural diastema that normally separates the dorsal and ventral fin lobes. While forked fins regenerate their original forked morphology, truncate fins regenerate truncate, suggesting that positional memory of the fin rays can be permanently altered by a transient treatment during embryogenesis. Ray finned fish have evolved a wide spectrum of caudal fin morphologies, ranging from truncate to forked, and the current work offers insights into the developmental mechanisms that may underlie this shape diversity.

Mechanism of pectoral fin outgrowth in zebrafish development

Fins and limbs, which are considered to be homologous paired vertebrate appendages, have obvious morphological differences that arise during development. One major difference in their development is that the AER (apical ectodermal ridge), which organizes fin/limb development, transitions into a different, elongated organizing structure in the fin bud, the AF (apical fold). Although the role of AER in limb development has been clarified in many studies, little is known about the role of AF in fin development. Here, we investigated AF-driven morphogenesis in the pectoral fin of zebrafish. After the AER-AF transition at ∼36 hours post-fertilization, the AF was identifiable distal to the circumferential blood vessel of the fin bud. Moreover, the AF was divisible into two regions: the proximal AF (pAF) and the distal AF (dAF). Removing the AF caused the AER and a new AF to re-form. Interestingly, repeatedly removing the AF led to excessive elongation of the fin mesenchyme, suggesting that prolonged exposure to AER signals results in elongation of mesenchyme region for endoskeleton. Removal of the dAF affected outgrowth of the pAF region, suggesting that dAF signals act on the pAF. We also found that the elongation of the AF was caused by morphological changes in ectodermal cells. Our results suggest that the timing of the AER-AF transition mediates the differences between fins and limbs, and that the acquisition of a mechanism to maintain the AER was a crucial evolutionary step in the development of tetrapod limbs.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

miR-196 regulates axial patterning and pectoral appendage initiation

Vertebrate Hox clusters contain protein-coding genes that regulate body axis development and microRNA (miRNA) genes whose functions are not yet well understood. We overexpressed the Hox cluster microRNA miR-196 in zebrafish embryos and found four specific, viable phenotypes: failure of pectoral fin bud initiation, deletion of the 6th pharyngeal arch, homeotic aberration and loss of rostral vertebrae, and reduced number of ribs and somites. Reciprocally, miR-196 knockdown evoked an extra pharyngeal arch, extra ribs, and extra somites, confirming endogenous roles of miR-196. miR-196 injection altered expression of hox genes and the signaling of retinoic acid through the retinoic acid receptor gene rarab. Knocking down rarab mimicked the pectoral fin phenotype of miR-196 overexpression, and reporter constructs tested in tissue culture and in embryos showed that the rarab 3'UTR is a miR-196 target for pectoral fin bud initiation. These results show that a Hox cluster microRNA modulates development of axial patterning similar to nearby protein-coding Hox genes, and acts on appendicular patterning at least in part by modulating retinoic acid signaling.

The art of medaka genetics and genomics: what makes them so unique?

The medaka fish, Oryzias latipes, is an emerging vertebrate model and now has a high quality draft genome and a number of unique mutants. The long history of medaka research in Japan has provided medaka with unique features, which are complementary to other vertebrate models. A large collection of spontaneous mutants collected over a century, the presence of highly polymorphic inbred lines established over decades, and the recently completed genome sequence all give the medaka a big boost. This review focuses on the state of the art in medaka genetics and genomics, such as the first isolation of active transposons in vertebrates, the influence of chromatin structure on sequence variation, fine quantitative trait locus (QTL) analysis, and versatile mutants as human disease models.

Pelvic fins in teleosts: structure, function and evolution

The pelvic fins of teleosts are paired appendages that are considered to be homologous to the hind limbs of tetrapods. Because they are less important for swimming, their morphology and function can be flexibly modified, and such modifications have probably facilitated the adaptations of teleosts to various environments. Recently, among these modifications, pelvic-fin loss has gained attention in evolutionary developmental biology. Pelvic-fin loss, however, has only been investigated in a few model species, and various biological aspects of pelvic fins in teleosts in general remain poorly understood. This review summarizes the current state of knowledge regarding pelvic fins, such as their structure, function and evolution, to elucidate their contribution to the considerable diversity of teleosts. This information could be invaluable for future investigations into various aspects of pelvic fins, which will provide clues to understanding the evolution, diversity and adaptations of teleosts.

Brpf1, a subunit of the MOZ histone acetyl transferase complex, maintains expression of anterior and posterior Hox genes for proper patterning of craniofacial and caudal skeletons

The epigenetic mechanism involving chromatin modification plays a critical role in the maintenance of the expression of Hox genes. Here, we characterize a mutant of the medaka fish, named biaxial symmetries (bis), in which brpf1, a subunit of the MOZ histone acetyl transferase (HAT) complex, is mutated. The bis mutant displayed patterning defects both in the anterior-posterior axis of the craniofacial skeleton and the dorsal-ventral axis of the caudal one. In the anterior region, the bis mutant exhibited craniofacial cartilage homeosis. The expression of Hox genes was decreased in the pharyngeal arches, suggesting that the pharyngeal segmental identities were altered in the bis mutant. In the posterior region, the bis mutant exhibited abnormal patterning of the caudal skeleton, which ectopically formed at the dorsal side of the caudal fin. The expression of Zic genes was decreased at the posterior region, suggesting that the dorsal-ventral axis formation of the posterior trunk was disrupted in the bis mutant. We also found that the MOZ-deficient mice exhibited an abnormal patterning of their craniofacial and cervical skeletons and a decrease of Hox transcripts. We propose a common role of the MOZ HAT complex in vertebrates, a complex which is required for the proper patterning for skeletal development.

Wnt5a regulates directional cell migration and cell proliferation via Ror2-mediated noncanonical pathway in mammalian palate development

Tissue and molecular heterogeneities are present in the developing secondary palate along the anteroposterior (AP) axis in mice. Here, we show that Wnt5a and its receptor Ror2 are expressed in a graded manner along the AP axis of the palate. Wnt5a deficiency leads to a complete cleft of the secondary palate, which exhibits distinct phenotypic alterations at histological, cellular and molecular levels in the anterior and posterior regions of the palate. We demonstrate that there is directional cell migration within the developing palate. In the absence of Wnt5a, this directional cell migration does not occur. Genetic studies and in vitro organ culture assays further demonstrate a role for Ror2 in mediating Wnt5a signaling in the regulation of cell proliferation and migration during palate development. Our results reveal distinct regulatory roles for Wnt5a in gene expression and cell proliferation along the AP axis of the developing palate, and an essential role for Wnt5a in the regulation of directional cell migration.

Hedgehog signaling patterns the outgrowth of unpaired skeletal appendages in zebrafish

BACKGROUND

Little is known about the control of the development of vertebrate unpaired appendages such as the caudal fin, one of the key morphological specializations of fishes. Recent analysis of lamprey and dogshark median fins suggests the co-option of some molecular mechanisms between paired and median in Chondrichthyes. However, the extent to which the molecular mechanisms patterning paired and median fins are shared remains unknown.

RESULTS

Here we provide molecular description of the initial ontogeny of the median fins in zebrafish and present several independent lines of evidence that Sonic hedgehog signaling emanating from the embryonic midline is essential for establishment and outgrowth of the caudal fin primordium. However, gene expression analysis shows that the primordium of the adult caudal fin does not harbor a Sonic hedgehog-expressing domain equivalent to the Shh secreting zone of polarizing activity (ZPA) of paired appendages.

CONCLUSION

Our results suggest that Hedgehog proteins can regulate skeletal appendage outgrowth independent of a ZPA and demonstrates an unexpected mechanism for mediating Shh signals in a median fin primordium. The median fins evolved before paired fins in early craniates, thus the patterning of the median fins may be an ancestral mechanism that controls the outgrowth of skeletogenic appendages in vertebrates.

Multinucleate osteoclasts in medaka as evidence of active bone remodeling

Putative sites of bone resorption in the acellular bony skeleton of the medaka fish (Oryzias latipes) were investigated primarily by RNA in situ hybridization and histological analysis. Numerous cells that displayed intense enzymatic activity of tartrate-resistant acid phosphatase (TRAP), the main marker of osteoclasts, were distributed in the pharyngeal region of this fish. Moreover, these cells expressed cathepsin K, an osteoclast-specific gene, as well as the genes for TRAP and vacuolar-type proton ATPase (V-ATPase). Some of the TRAP-positive cells displayed all of the morphological characteristics equivalent to those of mammalian- and bird-type osteoclasts. These cells were associated primarily with the shedding teeth and their supporting bones (pedicles), where alkaline phosphatase (ALPase)-positive osteoblasts were also located, implying progressive bone remodeling associated with tooth replacement in these regions. In contrast, the inner aspects of the neural and hemal arches of the vertebral column, which were the only sites of bone resorption other than the tooth-bearing bones, showed sporadically aligned flat mononuclear TRAP-positive cells without a ruffled border, indicating a different mode of bone remodeling in these regions. These results suggest the feasibility of medaka as a model animal for the investigation of bone-related abnormalities and their genetic backgrounds.