Developmental Dynamics

Extracellular matrix (ECM) plays fundamental roles in animal development, regeneration, and disease. The difficulty of tagging endogenous matrix proteins in vertebrates has limited the understanding of ECM composition and dynamics in complex tissues. To visualize vertebrate ECM components, we tagged zebrafish Laminin, gamma 1 (Lamc1), Collagen, type I, alpha 2 (Col1a2), and Transforming growth factor, beta-induced (Tgfbi) using C-terminus in-fusion genome editing. Analysis of these knock-in lines revealed distinct expression of each protein in various tissues during development and regeneration. Fluorescent recover after photobleaching (FRAP) analysis further indicated that Lamc1 is stable in fin fold matrix but more dynamic in myoseptal matrix of developing zebrafish, while Col1a2 and Tgfbi are stable matrix components in myosepta. Strikingly, we found that Col1a2-mScarlet protein accumulates at the amputation plane during tailfin regeneration, where it remains concentrated for several days and distant from the regeneration blastema. This "foundation" region also displayed a distinct transcriptome suggesting active and dedicated events at the base of the regenerating appendage. Our resource enables live capture of ECM dynamics that can identify new events in developing and regenerating zebrafish. Summary statementExtracellular matrix resources for zebrafish development and tissue regeneration.

AO_SCPLOWBSTRACTC_SCPLOWSkin appendages, such as hair follicles and scales, represent evolutionary adaptations that vary among different species. Mouse and rat tails exhibit distinct appendage types, with mice developing hair follicles and rats developing scales. The study investigated whether the differential expression of Edar and Xedar, critical regulators of ectodermal development, could explain these distinct developmental outcomes by measuring Edar and Xedar expression in mouse and rat tail tissues during embryonic development using quantitative real-time PCR. Expression levels were normalized to actin and analyzed using mathematical modeling and statistical approaches. It was found that distinct temporal patterns of gene expression between species. Mouse tissues showed a dramatic peak in Xedar expression during days 3-4, coinciding with hair follicle initiation, while rat tissues maintained relatively stable Xedar expression. Edar expression showed opposing trends between species, with a gradual increase in mice and a decrease in rats. These findings provide the first quantitative evidence for species-specific regulation of Edar and Xedar during appendage development, suggesting a molecular basis for determining hair versus scale fate.

Ectodermal appendages in mammals, such as teeth, mammary glands, sweat glands and hair follicles, are generated during embryogenesis through a series of mesenchymal-epithelial interactions. Canonical Wnt signaling and its inhibitors are implicated in the early steps of ectodermal appendage development and patterning. To study the activation dynamics of the Wnt target and inhibitor Dickkopf4 (Dkk4) in ectodermal appendages, we used CRSIPR/Cas9 to generate a Dkk4-Cre knock-in mouse (Mus musculus) line, where the Cre recombinase cDNA replaces the expression of endogenous Dkk4. Using Cre reporters, the Dkk4-Cre activity was evident at the prospective sites of ectodermal appendages, overlapping with the Dkk4 mRNA expression. Unexpectedly, a predominantly mesenchymal cell population in the embryo posterior also showed Dkk4-Cre activity. Lineage-tracing suggested that these cells are likely derived from a few Dkk4-Cre-expressing cells in the epiblast at early gastrulation. Finally, our analyses of Dkk4-Cre-expressing cells in developing hair follicle epithelial placodes revealed intra- and inter-placodal cellular heterogeneity, supporting emerging data on the positional and transcriptional variability in placodes. Collectively, we propose the new Dkk4-Cre knock-in mouse line as a suitable model to study Wnt and DKK4 inhibitor dynamics in early mouse development and ectodermal appendage morphogenesis.

During embryogenesis the nascent Caenorhabditis elegans epidermis secretes an apical extracellular matrix (aECM) that serves as an external stabilizer, preventing deformation of the epidermis by mechanical forces exerted during morphogenesis. We showed that two conserved proteins linked to this process, SYM-3/FAM102A and SYM-4/WDR44, colocalize to intracellular and membrane-associated puncta and likely function together in a complex. Proteomics data also suggested potential roles for FAM102A and WDR44 family proteins in intracellular trafficking, consistent with their localization patterns. Nonetheless, we found no evidence to support a clear function for SYM-3 or SYM-4 in the apical deposition of two aECM components, FBN-1 and NOAH. Surprisingly, loss of MEC-8/RBPMS2, a conserved splicing factor and regulator of fbn-1, had little effect on the abundance or deposition of FBN-1 to the aECM. Using a focused screening approach, we identified 32 additional proteins that likely contribute to the structure and function of the embryonic aECM. Lastly, we examined morphogenesis defects in embryos lacking mir-51 microRNA family members, which display a related embryonic phenotype to mec-8; sym double mutants. Collectively, our findings add to our knowledge of pathways controlling embryonic morphogenesis. SUMMARY STATEMENTWe identify new proteins in apical ECM biology in C. elegans and provide evidence that SYM-3/FAM102A and SYM-4/WDR44 function together in trafficking but do not regulate apical ECM protein deposition.

BackgroundCollective cell migration underlies many essential processes, including sculpting organs during embryogenesis, wound healing in the adult, and metastasis of cancer cells. At mid-oogenesis, Drosophila border cells undergo collective migration. Border cells round up into a small group, detach from the epithelium, and migrate - at first rapidly through the surrounding tissue, then slower, with the cluster rotating several times before stopping at the oocyte. While specific genes that promote cell signaling, polarization of the cluster, formation of protrusions, and cell-cell adhesion are known to regulate border cell migration, there may be additional genes that promote these distinct dynamic phases of border cell migration. Therefore, we sought to identify genes whose expression patterns changed during border cell migration. ResultsWe performed RNA-sequencing on border cells isolated at pre-, mid-, and late-migration stages. We report that 1,729 transcripts, in nine co-expression gene clusters, are temporally and differentially expressed across the three migration stages. Gene ontology analyses and constructed protein-protein interaction networks identified genes expected to function in collective migration, such as regulators of the cytoskeleton, adhesion, and tissue morphogenesis, but also a notable enrichment of genes involved in immune signaling, ribosome biogenesis, and stress responses. Finally, we validated the in vivo expression and function of a subset of identified genes in border cells. ConclusionsOverall, our results identified differentially and temporally expressed genetic networks that may facilitate the efficient development and migration of border cells. The genes identified here represent a wealth of new candidates to investigate the molecular nature of dynamic collective cell migrations in developing tissues.

Cell fate determination is a necessary and tightly regulated process for producing different cell types and structures during development. Cranial neural crest cells (CNCCs) are unique to vertebrate embryos and emerge from the neural fold borders into multiple cell lineages that differentiate into bone, cartilage, neurons, and glial cells. We previously reported that Irf6 genetically interacts with Twist1 during CNCC-derived tissue formation. Here, we investigated the mechanistic role of Twist1 and Irf6 at early stages of craniofacial development. Our data indicates that TWIST1 interacts with /{beta}/{gamma}-CATENINS during neural tube closure, and Irf6 is involved in the structural integrity of the neural tube. Twist1 suppresses Irf6 and other epithelial genes in CNCCs during epithelial-to-mesenchymal transition (EMT) process and cell migration. Conversely, a loss of Twist1 leads to a sustained expression of epithelial and cell adhesion markers in migratory CNCCs. Disruption of TWIST1 phosphorylation in vivo leads to epidermal blebbing, edema, neural tube defects, and CNCC-derived structural abnormalities. Altogether, this study describes an uncharacterized function of Twist1 and Irf6 in the neural tube and CNCCs and provides new target genes of Twist1 involved in cytoskeletal remodeling. Furthermore, the association between DNA variations within TWIST1 putative enhancers and human facial morphology is also investigated. SUMMARY STATEMENTThis study uncovers a new function of Twist1 in neural tube development and epithelial-to-mesenchymal transition in cranial neural crest cells. Data further shows that Twist1-interacting Irf6 is involved in regulating neural tube integrity.

The choice of fixation method significantly impacts tissue morphology and protein visualization after immunohistochemistry (IHC). In this study, we compared the effects of paraformaldehyde (PFA) and trichloroacetic acid (TCA) fixation prior to IHC on chicken embryos. Our findings underscore the importance of validating fixation methods for accurate interpretation of IHC results, with implications for antibody validation and tissue-specific protein localization studies. We found that TCA fixation resulted in larger and more circular nuclei compared to PFA fixation. Additionally, TCA fixation altered the appearance of subcellular localization and fluorescence intensity of various proteins, including transcription factors and cytoskeletal proteins. Notably, TCA fixation revealed protein localization domains that may be inaccessible with PFA fixation. These results highlight the need for optimization of fixation protocols depending on the target epitope and model system, emphasizing the importance of methodological considerations in biological analyses.

The rete ovarii (RO) is an epithelial structure that arises during fetal development in close proximity to the ovary and persists throughout adulthood in mice. However, the functional significance of the RO remains elusive, and it has been absent from recent discussions of female reproductive anatomy. The RO comprises three distinct regions: the intraovarian rete (IOR) within the ovary, the extraovarian rete (EOR) in the periovarian tissue, and the connecting rete (CR) linking the EOR and IOR. We hypothesize that the RO plays a pivotal role in maintaining ovarian homeostasis and responding to physiological changes. To uncover the nature and function of RO cells, we conducted transcriptome analysis, encompassing bulk, single-cell, and nucleus-level sequencing of both fetal and adult RO tissues using the Pax8-rtTA; Tre- H2B-GFP mouse line, where all RO regions express nuclear GFP. This study presents three datasets, which highlight RO-specific gene expression signatures and reveal differences in gene expression across the three RO regions during development and in adulthood. The integration and rigorous validation of these datasets will advance our understanding of the ROs roles in ovarian development, female maturation, and adult female fertility. Short narrativeThis study employs comprehensive bulk, single cell and single nucleus transcriptome analysis to uncover gene expression signatures of the fetal and adult rete ovarii (RO).

Zebrafish hatching, a critical developmental milestone, occurs around 48-72 hours post-fertilization (hpf). It is regulated by the specialized secretory organ called the hatching gland. Voltage-gated potassium channels (Kv) are known for their roles in maintaining plasma membrane potential and regulating intracellular protein traffic and secretion. Previous studies on zebrafish mutants of Kv2.1 channel subunits - the electrically active subunit Kcnb1 and the modulatory subunit Kcng4b - revealed antagonistic functions in the development of brain ventricles, ear, and Reissner fiber. In this study, we investigated their functional role in the hatching gland. The loss of either subunit resulted in a significant delay in normal hatching. Using in situ hybridization and immunohistochemistry, we show that both mutants exhibited severe defects in the hatching gland patterning, including a reduced number of hatching gland cells. The mutants displayed changes in the transcript levels of several hatching gland markers and reduced cell proliferation in this organ. These developmental defects were intensified by a late-stage functional failure characterized by decreased cathepsin synthesis, reduced proteolytic activity, and delay in the period of secretion in both mutants. Together, our findings establish that Kv2.1 subunits, Kcnb1, and Kcng4b are essential during the development of the zebrafish hatching gland and its secretion.

Regulation of cell cycle progression is essential for cell proliferation during regeneration following injury. After appendage amputation, the axolotl (Ambystoma mexicanum) regenerates missing structures through an accumulation of proliferating cells known as the blastema. To study cell division during blastema growth, we generated a transgenic line of axolotls that ubiquitously expresses a bicistronic version of the Fluorescent Ubiquitination-based Cell Cycle Indicator (FUCCI). We demonstrate near-ubiquitous expression of FUCCI expression in developing and adult tissues and validate these expression patterns with DNA synthesis and mitosis phase markers. We demonstrate the utility of FUCCI for live and whole-mount imaging, showing the predominantly local contribution of cells during limb and tail regeneration. We also show that spinal cord amputation results in increased proliferation at least 5 mm from the injury. Finally, we use multimodal staining to provide cell type information for cycling cells by combining fluorescence in-situ hybridization, EdU click-chemistry, and immunohistochemistry on a single FUCCI tissue section. This new line of animals will be useful for studying cell cycle dynamics using in-situ endpoint assays and in-vivo imaging in developing and regenerating animals. Summary statementWe generated a ubiquitous transgenic fluorescence ubiquitin cell cycle indicator (FUCCI) axolotl line for examination of cell cycle dynamics during tissue regeneration.

The Wnt1-Cre2 driver, designed to address the effect of Wnt1 overactivation in the ventral neural tube in the original Wnt1-Cre line, was recently shown to have ectopic expression in the male germline. When crossed with a reporter mouse, we observed fluorescent protein expression in non-neural-crest cell types in the gut. Here, we characterize the pattern of Cre-mediated recombination in the Wnt1-Cre2 driver using three transgenic reporter lines. We find aberrant reporter activation in the gut endoderm in embryonic and postnatal timepoints, starting as early as E8.5. This pattern of recombination was independent of the age, sex, and type of reporter line used, with the Wnt1-Cre2 allele inherited from either sires or dams resulting in ectopic fluorescence in the intestinal epithelium. We also detect reporter activity in the ventral neural tube. However, expression in the neural crest and its derivatives remained consistent with previous studies. We further quantify differences in the non-specific recombination observed across reporter lines using flow cytometry. Interestingly, the penetrance of reporter activation between reporter lines was different, with R26RmTmG showing less ectopic activation than the R26RtdTom and R26ReYFP lines. Finally, we propose a potential mechanism whereby genes surrounding the Wnt1-Cre2 insertion site on mouse chromosome 2 contribute to its Wnt1-independent activation in the endoderm. Taken together, our results suggest that users should exercise caution when using the Wnt1-Cre2 driver line for neural crest studies in the mouse.

IRF6 is a key genetic determinant of syndromic and non-syndromic cleft lip and palate. The ability to interrogate post-embryonic requirements of Irf6 has been hindered, as global Irf6 ablation in the mouse causes neonatal lethality. Prior work analyzing Irf6 in mouse models defined its role in the embryonic surface epithelium and periderm where it is required to regulate cell proliferation and differentiation. Several reports have also described Irf6 gene expression in other cell types, such as muscle, and neuroectoderm. However, analysis of a functional role in non-epithelial cell lineages has been incomplete due to the severity and lethality of the Irf6 knockout model and the paucity of work with a conditional Irf6 allele. Here we describe the generation and characterization of a new Irf6 floxed mouse model and analysis of Irf6 ablation in periderm and neural crest lineages. This work found that loss of Irf6 in periderm recapitulates a mild Irf6 null phenotype, suggesting that Irf6-mediated signaling in periderm plays a crucial role in regulating embryonic development. Further, conditional ablation of Irf6 in neural crest cells resulted in an anterior neural tube defect of variable penetrance. The generation of this conditional Irf6 allele allows for new insights into craniofacial development and new exploration into the post-natal role of Irf6.

The development of cranial nerves, including the trigeminal nerve, and the formation of neuromuscular junctions (NMJs) are crucial processes for craniofacial motor function. Mllt11/Af1q/Tcf7c (henceforth Mllt11), a novel type of cytoskeletal-interacting protein, has been implicated in neuronal migration and neuritogenesis during central nervous system development. However, its role in peripheral nerve development and NMJ formation remains poorly understood. This study investigates the function of Mllt11 during trigeminal ganglion development and its impact on motor innervation of the masseter muscle. We report Mllt11 expression in the developing trigeminal ganglia, suggesting a potential role in cranial nerve development. Using a conditional knockout mouse model to delete Mllt11 in Wnt1-expressing neural crest cells, we assessed trigeminal ganglion development and innervation of the masseter muscle in the jaw. Surprisingly, we find that Mllt11 loss does not affect the initial formation of the trigeminal ganglion but disrupts its cellular composition, with a reduction in the ratio of neural crest-derived Sox10+ cells relative to placode-derived Isl1/2+ cells. Furthermore, our study demonstrates that conditional Mllt11 knockout leads to reduction of neurofilament density and NMJs within the masseter muscle, indicating altered trigeminal motor innervation. Our findings show that Mllt11 regulates the cellular composition of the trigeminal ganglion and is essential for proper trigeminal motor innervation in the masseter muscle.

Sex specific differences in size and distribution of cell types have been observed in mammalian brains. How sex-specific differences in the brain are established and to what extent sexual dimorphism contributes to sex-biased neurodevelopment and neurological disorders is not well understood. Microglia are the resident immune cells of the nervous system and have been implicated in masculinizing the mammalian brain and refining neural connections to promote remodeling of neural circuitry, yet their contributions to developmental brain patterning and plasticity in zebrafish remains unclear. Here, we report anatomical and cellular differences between juvenile brains and adult female and male brains. Leveraging the plasticity of the zebrafish female brain and genetic models lacking microglia and tumor suppressor factors, we provide insight into the mechanisms that establish sex-specific brain dimorphism in zebrafish. Specifically, we identified sexually dimorphic features in the adult zebrafish brain that depend on microglia and Chek2, which may have broader implications and represent therapeutic targets for sex-biased neurological disorders. Plain language summaryMales and females of species can have significant differences in appearance, including differences in size, color, or sex specific anatomical structures. In addition to overt morphological differences, sex specific differences in size and distribution of cell types have been observed in mammalian brains. How these sex-specific differences in the brain are established and to what extent these differences contribute to sex-specific neurodevelopment and neurological disorders that differentially impact males and females is not well understood. Despite an incomplete picture of the mechanisms regulating sex-specific development, some of the cell types involved include microglia. Microglia are the resident immune cells of the nervous system and have been implicated in promoting features that are typical in the male mammalian brain. Specifically, microglia may refine neural connections and promote remodeling of neural circuitry and influence sex-specific behaviors. The contributions of microglia to developmental brain patterning and plasticity in zebrafish remain unclear. Here, we report anatomical and cellular differences between juvenile brains and adult female and male brains. Leveraging zebrafish genetic models lacking microglia and tumor suppressor factors, and the unique plasticity of the zebrafish female brain, we investigated and provide insight into the mechanisms that establish sex-specific brain differences in zebrafish. Specifically, we identified sexually distinct features in the adult zebrafish brain that depend on microglia and the tumor suppressor Chek2. If these or similar mechanisms operate in other species, our findings may have broader implications for sex-specific brain development and represent therapeutic targets for sex-biased neurological disorders. HighlightsO_LITissue clearing and immunostaining of juvenile and adult whole-mount zebrafish brains allows analysis of sex differences. C_LIO_LIAnatomical and cellular sexual dimorphism in the adult vertebrate brain appears after gonadal sex differentiation. C_LIO_LISexual dimorphism in the adult brain is driven by differences in cell death regulation. C_LIO_LIMicroglia colonization of brain areas involved in courtship is sexually dimorphic. C_LIO_LIMicroglia involvement in establishing sex-specific differences in the adult brain. C_LI

BackgroundEndocytosis constitutes a fundamental cellular process governing development through coordinated regulation of plasma membrane remodeling and ciliogenesis, processes essential for cell shape changes and embryonic development. Although Twist1 null embryos display complete cranial neural tube closure defects and conditional knockout in neuroectoderm disrupts cranial neural crest cell fate determination and delamination, the function of TWIST1 in neural tube morphogenesis remains unknown. We investigated the basis underlying neuroectodermal morphological abnormalities in TWIST1 mutant embryos, specifically the formation of ectopic lateral bending points and cellular disorganization, by examining TWIST1 function in cilia formation, adherens junction integrity, and endocytic vesicle dynamics. ResultsImmunofluorescence analysis revealed that cytosolic TWIST1 colocalizes with {beta}-catenin and endocytic regulators LRP2 and RAB11B along the apical surface of cranial neuroectoderm. Twist1 knockout resulted in reduced ciliary length and number. Quantitative PCR and Western blot analyses demonstrated upregulation of RAB11B and {beta}-catenin at mRNA and protein levels in Twist1 mutants. This molecular dysregulation coincided with increased accumulation of apical endocytic vesicles and altered expression profiles of endocytic component genes, ultimately modifying the apical neuroectodermal cell-cell junctions. ConclusionOur findings establish TWIST1 as a regulator of neuroectodermal morphology, demonstrating its ability to modulate ciliogenesis, endocytic vesicle dynamics, and cell-cell integrity.

BackgroundMorphogenesis depends on spatial and temporal coordination of signaling pathways, yet the colocalization of proteins across pathways remains poorly understood. Here we examine cellular and histological localization of regulatory proteins forming core craniofacial developmental pathways during beak morphogenesis of the zebra finch (Taeniopygia guttata). ResultsWe present an atlas of spatiotemporal coexpression of {beta}-catenin, Bmp4, CaM, Dkk3, Fgf8, Ihh, Tgf{beta}2, and Wnt4 across embryonic stages HH29-42 revealing both established and novel patterns of expression. Overall, in the earliest stages (HH29-32), most proteins show broad and overlapping expression across epithelial and mesenchymal tissues. By stage HH36, expression becomes increasingly compartmentalized, with pronounced differentiation among tissue types. Notably, at later stages, proteins showed tissue-specific distributions in boundary versus core regions of chondrogenic and osteogenic domains indicating coordinated cross-pathway patterning during cartilage and bone formation. ConclusionsOsteogenesis in the zebra finch beak is organized by coordinated signaling between boundary-associated cells and differentiating cores, with cross-pathway feedback establishing bone and cartilage differentiation while maintaining boundaries. Our results corroborated core elements of craniofacial signaling dynamics, while revealing unexpected subcellular localization for several proteins that showed regulatory complexity not captured by prior transcript-level maps. This atlas provides a protein-level baseline for comparative and mechanistic studies of beak morphogenesis.

Epiboly is a crucial morphogenetic process during early animal embryogenesis that expands surface area of embryonic tissues while thinning them. During zebrafish development, epiboly spreads the superficial enveloping layer (EVL), germ layers, and yolk syncytial layer to cover the yolk cell. Here we investigated functions of the three zebrafish dchs genes, dchs1a, dchs1b and dchs2 that encode large atypical cadherins and report that they have partially overlapping functions in epiboly progression. We have inserted GFP at the C-terminal Dchs1b intracellular domain of the endogenous dchs1b locus using homologous recombination. We observed the resulting Dchs1b-GFP fusion protein localized in both the cell membrane and the cytoplasm of EVL and embryonic cells during gastrulation. The dynamic microtubule and actin cytoskeleton of the yolk cell are essential for epiboly. Our studies of the yolk microtubule network demonstrate that these microtubules are more bundled and show faster polymerization during epiboly in dchs triple loss-of-function mutant embryos than in wild-type embryos, indicating that dchs genes are required for limiting microtubule polymerization and promoting dynamics during epiboly. The epiboly progression defects of dchs1b deficient mutants were suppressed by mutations in the tetratricopeptide repeat protein 28 (ttc28) gene encoding a cytoplasmic protein previously shown to bind to Dchs1b intracellular domain and alter microtubule dynamics during early cleavages. We further demonstrate that MZdchs1b mutants exhibit abnormal organization and dynamics of yolk cell actin cytoskeleton during epiboly. Together, these lines of evidence as well as our transcriptomic analyses support the notion that like during early embryonic cleavages, Dchs1b plays a major role, while Dchs1a and Dchs2 proteins have supporting roles in regulating microtubule dynamics and organization of both microtubule and actin cytoskeleton to ensure normal epiboly.

Wnt signaling is a crucial developmental pathway involved in early development as well as stem cell maintenance in adults and its misregulation leads to numerous diseases. Thus, understanding the regulation of this pathway becomes vitally important. Axin2 and Nkd1 are widely utilized negative feedback regulators in Wnt signaling where Axin2 functions to destabilize cytoplasmic {beta}-catenin, and Nkd1 functions to inhibit the nuclear localization of {beta}-catenin. Here, we set out to further understand how Axin2 and Nkd1 regulate Wnt signaling by creating axin2-/-, nkd1-/- single mutants and axin2-/-;nkd1-/- double mutant zebrafish using sgRNA/Cas9. All three Wnt regulator mutants were viable and had impaired heart looping, neuromast migration defects, and behavior abnormalities in common, but there were no signs of synergy in the axin2-/-;nkd1-/- double mutants. Further, Wnt target gene expression by qRT-PCR, and RNA-seq analysis and protein expression by mass spectrometry demonstrated that the double axin2-/-;nkd1-/- mutant resembled the nkd1-/- phenotype demonstrating that Axin2 functions upstream of Nkd1 and that loss of Nkd1 is epistatic to the loss of Axin2. In support of this, the data further demonstrates that Axin2 uniquely alters the properties of {beta}-catenin-dependent transcription having novel readouts of Wnt activity compared to nkd1-/- or the axin2-/-;nkd1-/- double mutant. We also tested the sensitivity of the Wnt regulator mutants to exacerbated Wnt signaling, where the single mutants displayed characteristic heightened Wnt sensitivity, resulting in an eyeless phenotype. Surprisingly, this phenotype was rescued in the double mutant, where we speculate that cross-talk between Wnt/{beta}-catenin and Wnt/Planar Cell Polarity pathways could lead to altered Wnt signaling in some scenarios. Collectively, the data emphasizes both the commonality and the complexity in the feedback regulation of Wnt signaling.

Craniofacial development is a complex and tightly regulated process and disruptions can lead to structural birth defects, the most common being nonsyndromic cleft lip and palate (NSCLP). Previously, we identified FOS as a candidate regulator of NSCLP through family-based association studies, yet its specific contributions to oral and palatal formation are poorly understood. This study investigated the role of fos during zebrafish craniofacial development through genetic disruption and knockdown approaches. Fos was expressed in the periderm, olfactory epithelium and other cell populations in the head. Genetic perturbation of fos produced an abnormal craniofacial phenotype with a hypoplastic oral cavity that showed significant changes in midface dimensions by quantitative facial morphometric analysis. Loss and knockdown of fos caused increased cell apoptosis in the head, followed by a significant reduction in cranial neural crest cells (CNCCs) populating the upper and lower jaws. These changes resulted in abnormalities of cartilage, bone and pharyngeal teeth formation. Periderm cells surrounding the oral cavity showed altered morphology and a subset of cells in the upper and lower lip showed disrupted Wnt/{beta}-catenin activation, consistent with modified inductive interactions between mesenchymal and epithelial cells. Taken together, these findings demonstrate that perturbation of fos has detrimental effects on oral epithelial and CNCC-derived tissues suggesting that it plays a critical role in zebrafish craniofacial development and a potential role in NSCLP. Summary statementPerturbation of fos, a candidate gene associated with nonsyndromic cleft lip and palate in humans, causes a distinctive orofacial phenotype in zebrafish as a result of abnormal development of craniofacial tissues. Disruption of fos in the oral epithelial, cranial neural crest and Wnt responsive cell populations around the oral cavity causes anomalies that suggest a potential role in the etiology of NSCLP.

Teleost species possess complex caudal musculoskeletal systems. While mid-trunk muscles exhibit simple segmental patterns, several caudal skeletal muscles display intricate orientations in their muscle fibers. Due to this distinctive morphology, both early and recent researchers have studied the structure and development of the caudal musculoskeletal system. However, the early developmental origin of the cell populations within the caudal muscle system remains largely unknown. In this study, we performed lineage tracing of caudal muscle primordia in zebrafish using a transgenic line expressing EGFP in somite derivatives following tamoxifen induction. This approach allowed us to observe the specific cell populations that contribute to caudal muscle tissue formation at the early larval stage. By monitoring the growth of these labeled cells from the early larval stage, we identified the origins of muscle fibers in caudal fin muscles unique to teleosts, such as the adductor caudalis and flexor caudalis. Our findings provide descriptions that aid in understanding how fish-specialized caudal muscle structures were formed through the modification of developmental processes during evolution.