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Background: Limb buds develop as bilateral outgrowths of the lateral plate mesoderm and are patterned along three axes. Current models of proximal to distal patterning of early amniote limb buds suggest that two signals, a distal organizing signal from the apical epithelial ridge (AER, Fgfs) and an opposing proximal (retinoic acid [RA]) act early on pattern this axis., Results: Transcriptional analysis of stage 51 Xenopus laevis hindlimb buds sectioned along the proximal-distal axis showed that the distal region is distinct from the rest of the limb. Expression of capn8.3, a novel calpain, was located in cells immediately flanking the AER. The Wnt antagonist Dkk1 was AER-specific in Xenopus limbs. Two transcription factors, sall1 and zic5, were expressed in distal mesenchyme. Zic5 has no described association with limb development. We also describe expression of two proximal genes, gata5 and tnn, not previously associated with limb development. Differentially expressed genes were associated with Fgf, Wnt, and RA signaling as well as differential cell adhesion and proliferation., Conclusions: We identify new candidate genes for early proximodistal limb patterning. Our analysis of RA-regulated genes supports a role for transient RA gradients in early limb bud in proximal-to-distal patterning in this anamniote model organism., (© 2022 The Authors. Developmental Dynamics published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Background: Major cell-to-cell signaling pathways, such as the fibroblast growth factors and their four receptors (FGF/FGFR), are conserved across a variety of animal forms. FGF/FGFRs are necessary to produce several "vertebrate-specific" structures, including the vertebrate head. Here, we examine the effects of the FGFR2 S252W mutation associated with Apert syndrome on patterns of cranial integration. Our data comprise micro-computed tomography images of newborn mouse skulls, bred to express the Fgfr2 S252W mutation exclusively in either neural crest or mesoderm-derived tissues, and mice that express the Fgfr2 S252W mutation ubiquitously., Results: Procrustes-based methods and partial least squares analysis were used to analyze craniofacial integration patterns. We found that deviations in the direction and degree of integrated shape change across the mouse models used in our study were potentially driven by the modular variation generated by differing expression of the Fgfr2 mutation in cranial tissues., Conclusions: Our overall results demonstrate that covariation patterns can be biased by the spatial distribution and magnitude of variation produced by underlying developmental-genetic mechanisms that often impact the phenotype in disproportionate ways., (© 2022 American Association for Anatomy.)

Background: PR domain containing 16 (PRDM16) is a key transcriptional regulator in the development of craniofacial, adipose, and neural tissues. Our lab identified PRDM16 expression in the epithelial cells of the Kölliker's organ (KO) that starts at ~E13.5 and is maintained until KO disappearance. A transgenic mouse model that carries a gene trap null allele of Prdm16 (Prdm16 cGT ) was used to characterize the impact of Prdm16 loss on cochlear development., Results: At P0 Prdm16 cGT null cochlea exhibited hypoplastic KO, shortened cochlear duct, increased density of hair cells (HCs) and supporting cells (SCs) in the apical turn as well as multiple isolated ectopic HCs within the KO domain. KO epithelial cells proliferation rate was reduced in the apical turn of the developing Prdm16 cGT null cochlea vs controls. Bulk RNA sequencing of cochlear duct cells at E14.5 followed by quantitative real time PCR and mRNA Fluorescence in-situ hybridization (FISH) validation identified differentially expressed genes in Prdm16 cGT null vs littermate control cochleae. Upregulated genes at E14.5 included Fgf20, as well as several Notch pathway genes (Lfng, Hes1, and Jag1)., Conclusions: This study characterizes Prdm16 expression during cochlear development and establishes its requirement for KO development., (© 2022 The Authors. Developmental Dynamics published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Background: Fibroblast growth factors (Fgfs) are required for survival and organ formation during embryogenesis. Fgfs often execute their functions redundantly. Previous analysis of Fgf3 mutants revealed effects on inner ear formation and embryonic survival with incomplete penetrance., Results: Here, we show that presence of a neomycin resistance gene (neo) replacing the Fgf3 coding region leads to reduced survival during embryogenesis and an increased penetrance of inner ear defects. Fgf3 neo/neo mutants showed reduced expression of Fgf4, which is positioned in close proximity to the Fgf3 locus in the mouse genome. Conditional inactivation of Fgf4 during inner ear development on a Fgf3 null background using Fgf3/4 cis mice revealed a redundant requirement between these Fgfs during otic placode induction. In contrast, inactivation of Fgf3 and Fgf4 in the pharyngeal region where both Fgfs are also co-expressed using a Foxg1-Cre driver did not affect development of the pharyngeal arches. However, these mutants showed reduced perinatal survival., Conclusions: These results highlight the importance of Fgf signaling during development. In particular, different members of the Fgf family act redundantly to guarantee inner ear formation and embryonic survival., (© 2021 American Association for Anatomy.)

The formation of the external genitalia is a highly complex developmental process, considering it involves a wide range of cell types and results in sexually dimorphic outcomes. Development is controlled by several secreted signalling factors produced in complex spatiotemporal patterns, including the hedgehog (HH), bone morphogenic protein (BMP), fibroblast growth factor (FGF) and WNT signalling families. Many of these factors act on or are influenced by the actions of the androgen receptor (AR) that is critical to masculinisation. This complexity of expression makes it difficult to conceptualise patterns of potential importance. Mapping expression during key stages of development is needed to develop a comprehensive model of how different cell types interact in formation of external genitalia, and the global regulatory networks at play. This is particularly true in light of the sensitivity of this process to environmental disruption during key stages of development. The goal of this review is to integrate all recent studies on gene expression in early penis development to create a comprehensive spatiotemporal map. This serves as a resource to aid in visualising potentially significant interactions involved in external genital development., (© 2021 The Authors. Developmental Dynamics published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Progress in understanding mechanisms of inner ear development has been remarkably rapid in recent years. The research community has benefited from the availability of several diverse model organisms, including zebrafish, chick, and mouse. The complexity of the inner ear has proven to be a challenge, and the complexity of the mammalian cochlea in particular has been the subject of intense scrutiny. Zebrafish lack a cochlea and exhibit a number of other differences from amniote species, hence they are sometimes seen as less relevant for inner ear studies. However, accumulating evidence shows that underlying cellular and molecular mechanisms are often highly conserved. As a case in point, consideration of the diverse functions of Fgf and its downstream effectors reveals many similarities between vertebrate species, allowing meaningful comparisons the can benefit the entire research community. In this review, I will discuss mechanisms by which Fgf controls key events in early otic development in zebrafish and provide direct comparisons with chick and mouse., (© 2021 American Association of Anatomists.)

Background: The regeneration of larvae zebrafish fin emerged as a new model of regeneration in the last decade. In contrast to genetic tools to study fin regeneration, chemical probes to modulate and interrogate regeneration processes are not well developed., Results: We set up a zebrafish larvae fin regeneration assay system and tested activities of natural product compounds and extracts, prepared from various microbes. Colomitide C, a recently isolated product from a fungus obtained from Antarctica, inhibited larvae fin regeneration. Using fluorescent reporter transgenic lines, we show that colomitide C inhibited fibroblast growth factor (FGF) signaling and WNT/β-catenin signaling, which were activated after larvae fin amputation. By using the endothelial cell reporter line and immunofluorescence, we showed that colomitide C did not affect migration of the blood vessel and nerve into the injured larvae fin. Colomitide C did not show any cytotoxic activities when tested against FGF receptor-amplified human cancer cell lines., Conclusion: Colomitide C, a natural product, modulated larvae fin regeneration likely acting upstream of FGF and WNT signaling. Colomitide C may serve as a template for developing new chemical probes to study regeneration and other biological processes., (© 2020 Wiley Periodicals LLC.)

Background: Fibroblast Growth Factor 20 (FGF20)-FGF receptor 1 (FGFR1) signaling is essential for cochlear hair cell (HC) and supporting cell (SC) differentiation. In other organ systems, FGFR1 signals through several intracellular pathways including MAPK (ERK), PI3K, phospholipase C ɣ (PLCɣ), and p38. Previous studies implicated MAPK and PI3K pathways in HC and SC development. We hypothesized that one or both would be important downstream mediators of FGF20-FGFR1 signaling for HC differentiation., Results: By inhibiting pathways downstream of FGFR1 in cochlea explant cultures, we established that both MAPK and PI3K pathways are required for HC differentiation while PLCɣ and p38 pathways are not. Examining the canonical PI3K pathway, we found that while AKT is necessary for HC differentiation, it is not sufficient to rescue the Fgf20 -/- phenotype. To determine whether PI3K functions downstream of FGF20, we inhibited Phosphatase and Tensin Homolog (PTEN) in Fgf20 -/- explants. Overactivation of PI3K resulted in a partial rescue of the Fgf20 -/- phenotype, demonstrating a requirement for PI3K downstream of FGF20. Consistent with a requirement for the MAPK pathway for FGF20-regulated HC differentiation, we show that treating Fgf20 -/- explants with FGF9 increased levels of dpERK., Conclusions: Together, these data provide evidence that both MAPK and PI3K are important downstream mediators of FGF20-FGFR1 signaling during HC and SC differentiation., (© 2020 Wiley Periodicals LLC.)

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles., (© 2020 American Association of Anatomists.)

Background: Neural crest cells (NCCs) delaminate from the neural tube (NT) and migrate ventrally to generate the trunk peripheral nervous system (PNS). Although several signaling pathways have been identified that steer NCCs once they are on their ventral trajectory, no molecules have been identified that are required for the initial migration between the NT and the dorsal root ganglion. Given the critical role of fibroblast growth factor (FGF) signaling in embryogenesis, we investigated its function in this initial migration., Results: FGFR1 signaling is required for the migration of newly delaminated NCCs onto the ventral pathway. Live imaging of migrating NCCs revealed that inhibition of FGFR1 signaling caused the dorsally stalled NCCs to lose their dorsal/ventral oriented polarity and instead adopt a rounded morphology while dynamically extending filopodia. FGF8, an FGFR1 ligand, increased motility of NCCs away from the NT by acting chemokinetically. Finally, we provide evidence that inhibition of FGFR1-mediated chemokinesis is partially rescued by increasing Akt signaling, inhibiting RhoA, and activation of N-cadherin signaling., Conclusion: These data support a model in which NCCs are stimulated chemokinetically by FGF:FGFR1 signaling, and that this activation positions and orients NCCs on their ventral migratory route-a process that is essential for patterning the trunk PNS., (© 2020 Wiley Periodicals, Inc.)

Background: The basic ground plan of vertebrate hindbrain is established through a process of segmentation, which generates eight transient lineage-restricted cellular compartments called rhombomeres (r). The segments adopt distinct individual identities in response to axial patterning signals. It is unclear whether signaling between rhombomeres plays a conserved role in regulating segmental patterning during hindbrain development., Results: Using tissue manipulations of rhombomeres in chicken embryos, we have uncovered roles for r2 and r4 in regulating the expression of EphA4 in r3 and r5. Perturbations of signaling pathways reveal that these regulatory inputs from r2 and r4 into EphA4 expression are mediated independent of inputs from Krox20 through cues involving fibroblast growth factor (FGF) signaling. These interactions are stage dependent and are set up in embryos with <10 somites., Conclusions: We show that r2 and r4 function as temporally dynamic signaling centers in the early patterning of adjacent hindbrain segments and this activity is dependent upon the FGF pathway. These results reveal that inter-rhombomeric signaling is a conserved feature of the regulatory networks that control the specification of individual rhombomere identities in vertebrate hindbrain segmentation. However, the timing of when restricted domains of FGF signaling are coupled to formation of r4 may vary between the species., (© 2019 Wiley Periodicals, Inc.)

Background: Fibroblast growth factor 18 (FGF18) functions in the development of several tissues, including the lung, limb bud, palate, skeleton, central nervous system, and hair follicle. Mice containing a germline knockout of Fgf18 (Fgf18 -/- ) die shortly after birth. Postnatally, FGF18 is being evaluated for pathogenic roles in fibrosis and several types of cancer. The specific cell types that express FGF18 have been difficult to identify, and the function of FGF18 in postnatal development and tissue homeostasis has been hampered by the perinatal lethality of Fgf18 null mice., Results: We engineered a floxed allele of Fgf18 (Fgf18 flox ) that allows conditional gene inactivation and a CreER T2 knockin allele (Fgf18 CreERT2 ) that allows the precise identification of cells that express Fgf18 and their lineage. We validated the Fgf18 flox allele by targeting it in mesenchymal tissue and primary mesoderm during embryonic development, resulting in similar phenotypes to those observed in Fgf18 null mice. We also use the Fgf18 CreERT2 allele, in combination with a conditional fluorescent reporter to confirm known and identify new sites of Fgf18 expression., Conclusion: These alleles will be useful to investigate FGF18 function during organogenesis and tissue homeostasis, and to target specific cell lineages at embryonic and postnatal time points., (© 2019 Wiley Periodicals, Inc.)

Mandibular disorders are among the most common birth defects in humans, yet the etiological factors are largely unknown. Most of the neonates affected by mandibular abnormalities have a sequence of secondary anomalies, including airway obstruction and feeding problems, that reduce the quality of life. In the event of lacking corrective surgeries, patients with mandibular congenital disorders suffer from additional lifelong problems such as sleep apnea and temporomandibular disorders, among others. The goal of this systematic review is to gather evidence on hormonal and genetic factors that are involved in signaling pathways and interactions that are potentially associated with the nonsyndromic mandibular disorders. We found that members of FGF and BMP pathways, including FGF8/10, FGFR2/3, BMP2/4/7, BMPR1A, ACVR1, and ACVR2A/B, have a prominent number of gene-gene interactions among all identified genes in this review. Gene ontology of the 154 genes showed that the functional gene sets are involved in all aspects of cellular processes and organogenesis. Some of the genes identified by the genome-wide association studies of common mandibular disorders are involved in skeletal formation and growth retardation based on animal models, suggesting a potential direct role as genetic risk factors in the common complex jaw disorders. Developmental Dynamics 248:162-172, 2019. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

Background: Spatiotemporal perturbation of signaling pathways in vivo remains challenging and requires precise transgenic control of signaling effectors. Fibroblast growth factor (FGF) signaling guides multiple developmental processes, including body axis formation and cell fate patterning. In zebrafish, mutants and chemical perturbations affecting FGF signaling have uncovered key developmental processes; however, these approaches cause embryo-wide perturbations, rendering assessment of cell-autonomous vs. non-autonomous requirements for FGF signaling in individual processes difficult., Results: Here, we created the novel transgenic line fgfr1-dn-cargo, encoding dominant-negative Fgfr1a with fluorescent tag under combined Cre/lox and heatshock control to perturb FGF signaling spatiotemporally. Validating efficient perturbation of FGF signaling by fgfr1-dn-cargo primed with ubiquitous CreERT2, we established that primed, heatshock-induced fgfr1-dn-cargo behaves similarly to pulsed treatment with the FGFR inhibitor SU5402. Priming fgfr1-dn-cargo with CreERT2 in the lateral plate mesoderm triggered selective cardiac and pectoral fin phenotypes without drastic impact on overall embryo patterning. Harnessing lateral plate mesoderm-specific FGF inhibition, we recapitulated the cell-autonomous and temporal requirement for FGF signaling in pectoral fin outgrowth, as previously inferred from pan-embryonic FGF inhibition., Conclusions: As a paradigm for rapid Cre/lox-mediated signaling perturbations, our results establish fgfr1-dn-cargo as a genetic tool to define the spatiotemporal requirements for FGF signaling in zebrafish. Developmental Dynamics 247:1146-1159, 2018. © 2018 Wiley Periodicals, Inc., (© 2018 Wiley Periodicals, Inc.)

Background: Deficiency of Suppressor of Fused (SuFu), an intracellular mediator of Hedgehog signaling, in the murine mid-hindbrain disrupts cerebellar morphogenesis and cell differentiation in a manner that is rescued by constitutive expression of GLI3 transcriptional repressor (GLI3R). Here, we determined SuFu functions in cerebellar radial precursors following the stage of mid-hindbrain specification using a Blbp-Cre transgene., Results: SuFu-deficient cerebella were severely dysplastic, and characterized by laminar disorganization, and delayed differentiation of ventricular zone-derived precursors. In vitro analysis of cerebellar precursors isolated from control and mutant mice demonstrated an increased proportion of radial glial precursors vs. Tuj1-positive neurons in mutant cultures. Abnormal cell differentiation in SuFu-deficient precursors was rescued by a constitutively expressed GLI3R knock-in allele, albeit with variable penetrance. Using RNA expression analysis in control and SuFu-deficient cerebellar anlage, we identified up-regulation of Fgf15 in mutant tissue. Strikingly, exogenous hFGF19, a mFGF15 ortholog, inhibited neuronal differentiation in cultures of wild-type cerebellar precursors. Moreover, siRNA-mediated knockdown of Fgf15 in SuFu-deficient cerebellar precursors rescued their delayed differentiation to neurons., Conclusions: Together, our results show that SuFu promotes cerebellar radial precursor differentiation to neurons. SuFu function is mediated in part by GLI3R and down-regulation of Fgf15 expression. Developmental Dynamics 247:156-169, 2018. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

Fibroblast growth factors (FGFs) are essential for self-renewal of skeletal muscle stem cells (satellite cells) and required for maintenance and repair of skeletal muscle. Satellite cells express high levels of FGF receptors 1 and 4, low levels of FGF receptor 3, and little or no detectable FGF receptor 2. Of the multiple FGFs that influence satellite cell function in culture, FGF2 and FGF6 are the only members that regulate satellite cell function in vivo by activating ERK MAPK, p38α/β MAPKs, PI3 kinase, PLCγ and STATs. Regulation of FGF signaling is complex in satellite cells, requiring Syndecan-4, a heparan sulfate proteoglycan, as well as ß1-integrin and fibronectin. During aging, reduced responsiveness to FGF diminishes satellite cell self-renewal, leading to impaired skeletal muscle regeneration and depletion of satellite cells. Mislocalization of ß1-integrin, reductions in fibronectin, and alterations in heparan sulfate content all contribute to reduced FGF responsiveness in satellite cells. How these cell surface proteins regulate satellite cell self-renewal is incompletely understood. Here we summarize the current knowledge, highlighting the role(s) for FGF signaling in skeletal muscle regeneration, satellite cell behavior, and age-induced muscle wasting. Developmental Dynamics 246:359-367, 2017. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

In this commentary we focus on the function of FGFs during limb development and morphogenesis. Our goal is to understand, interpret and, when possible, reconcile the interesting findings and conflicting results that remain unexplained. For example, the cell death pattern observed after surgical removal of the AER versus genetic removal of the AER-Fgfs is strikingly different and the field is at an impasse with regard to an explanation. We also discuss the idea that AER function may involve signaling components in addition to the AER-FGFs and that signaling from the non-AER ectoderm may also have a significant contribution. We hope that a re-evaluation of current studies and a discussion of outstanding questions will motivate new experiments, especially considering the availability of new technologies, that will fuel further progress toward understanding the intricate ectoderm-to-mesoderm crosstalk during limb development. Developmental Dynamics 246:208-216, 2017. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

Background: Syngnathia is a congenital craniofacial disorder characterized by bony or soft tissue fusion of upper and lower jaws. Previous studies suggested some causative signals, such as Foxc1 or Bmp4, cause the disruption of maxillomandibular identity, but their location and the interactive signals involved remain unexplored. We wanted to examine the embryonic origin of syngnathia based on the assumption that it may be located at the separation between the maxillary and mandibular processes. This region, known as the maxillomandibular junction (MMJ), is involved in segregation of cranial neural crest-derived mesenchyme into the presumptive upper and lower jaws., Results: Here we investigated the role of Fgf, Bmp, and retinoid signaling during development of MMJ in chicken embryos. By changing the levels of these signals with bead implants, we induced syngnathia with microstomia on the treated side, which showed increased Barx1 and neural cell adhesion molecule (NCAM) expression. Redistribution of proliferating cells was also observed at the proximal region to maxillary and mandibular arch around MMJ., Conclusions: We propose that interactive molecular signaling by Fgfs, Bmps, and retinoids around MMJ is required for normal separation of the maxilla and mandible, as well as the proper positioning of beak commissure during early facial morphogenesis. Developmental Dynamics 246:28-40, 2017. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

Background: Mutations of WNT3, WNT5A, WNT9B, and WNT11 genes are associated with orofacial birth defects, including nonsyndromic cleft lip with cleft palate in humans. However, the source of Wnt ligands and their signaling effects on the orofacial morphogenetic process remain elusive., Results: Using Foxg1-Cre to impair Wnt secretion through the inactivation of Gpr177/mWls, we investigate the relevant regulation of Wnt production and signaling in nasal-facial development. Ectodermal ablation of Gpr177 leads to severe facial deformities resulting from dramatically reduced cell proliferation and increased cell death due to a combined loss of WNT, FGF and BMP signaling in the developing facial prominence. In the invaginating nasal pit, the Gpr177 disruption also causes a detrimental effect on migration of the olfactory epithelial cells into the mesenchymal region. The blockage of Wnt secretion apparently impairs the olfactory epithelial cells through modulation of JNK signaling., Conclusions: Our study thus suggests the head ectoderm, including the facial ectoderm and the neuroectoderm, as the source of canonical as well as noncanonical Wnt ligands during early development of the nasal-facial prominence. Both β-catenin-dependent and -independent signaling pathways are required for proper development of these morphogenetic processes., (© 2015 Wiley Periodicals, Inc.)

Background: The adaptation to terrestrial life required the development of an organ capable of efficient air-blood gas exchange. To meet the metabolic load of cellular respiration, the mammalian respiratory system has evolved from a relatively simple structure, similar to the two-tube amphibian lung, to a highly complex tree-like system of branched epithelial airways connected to a vast network of gas exchanging units called alveoli. The development of such an elaborate organ in a relatively short time window is therefore an extraordinary feat and involves an intimate crosstalk between mesodermal and endodermal cell lineages., Results: This review describes the molecular processes governing lung development with an emphasis on the current knowledge on the role of Wnt and FGF signaling in lung epithelial differentiation., Conclusions: The Wnt and FGF signaling pathways are crucial for the dynamic and reciprocal communication between epithelium and mesenchyme during lung development. In addition, some of this developmental crosstalk is reemployed in the adult lung after injury to drive regeneration, and may, when aberrantly or chronically activated, result in chronic lung diseases. Novel insights into how the Wnt and FGF pathways interact and are integrated into a complex gene regulatory network will not only provide us with essential information about how the lung regenerates itself, but also enhance our understanding of the pathogenesis of chronic lung diseases, as well as improve the controlled differentiation of lung epithelium from pluripotent stem cells., (© 2014 Wiley Periodicals, Inc.)

Background: The definitive endoderm arises as a naive epithelial sheet that produces the entire gut tube and associated organs including the liver, pancreas and lungs. Murine explant studies demonstrate that fibroblast growth factor (FGF) signaling from adjacent tissues is required to induce hepatic gene expression from isolated foregut endoderm. The requirement of FGF signaling during liver development is examined by means of small molecule inhibition during whole embryo culture., Results: Loss of FGF signaling before hepatic induction results in morphological defects and gene expression changes that are confined to the anterior liver bud. In contrast the posterior portion of the liver bud remains relatively unaffected. Because FGF is thought to act as a morphogen during endoderm organogenesis, the ventral pancreas was also examined after FGF inhibition. Although the size of the ventral pancreas is not affected, loss of FGF signaling results in a significantly higher density of ventral pancreas cells., Conclusions: The requirement for FGF-mediated induction of hepatic gene expression differs across the anterior/posterior axis of the developing liver bud. These results underscore the importance of studying tissue differentiation in the context of the whole embryo., (© 2014 Wiley Periodicals, Inc.)

Background: During the course of development, the vertebrate nephric duct (ND) extends and migrates from the place of its initial formation, adjacent to the anterior somites, until it inserts into the bladder or cloaca in the posterior region of the embryo. The molecular mechanisms that guide ND migration are poorly understood., Results: A novel Gata3-enhancer-Gfp-based chick embryo live imaging system was developed that permits documentation of ND migration at the individual cell level for the first time. FGF Receptors and FGF response genes are expressed in the ND, and FGF ligands are expressed in surrounding tissues. FGF receptor inhibition blocked nephric duct migration. Individual inhibitors of the Erk, p38, or Jnk pathways did not affect duct migration, but inhibition of all three pathways together did inhibit migration of the duct. A localized source of FGF8 placed adjacent to the nephric duct did not affect the duct migration path., Conclusions: FGF signaling acts as a "motor" that is required for duct migration, but other signals are needed to determine the directionality of the duct migration pathway. Developmental Dynamics 244:157-167, 2015. © 2014 Wiley Periodicals, Inc., (© 2014 Wiley Periodicals, Inc.)

Background: At birth, marsupial neonates have precociously developed forelimbs. The development of the tammar wallaby (Macropus eugenii) hindlimbs lags significantly behind that of the forelimbs. This differs from the grey short-tailed opossum, Monodelphis domestica, which has relatively similar fore- and hindlimbs at birth. This study examines the expression of the key patterning genes TBX4, TBX5, PITX1, FGF8, and SHH in developing limb buds in the tammar wallaby., Results: All genes examined were highly conserved with orthologues from opossum and mouse. TBX4 expression appeared earlier in development than in the mouse, but later than in the opossum. SHH expression is restricted to the zone of polarising activity, while TBX5 (forelimb) and PITX1 (hindlimb) showed diffuse mRNA expression. FGF8 is specifically localised to the apical ectodermal ridge, which is more prominent than in the opossum., Conclusions: The most marked divergence in limb size in marsupials occurs in the kangaroos and wallabies. The faster development of the fore limb compared to that of the hind limb correlates with the early timing of the expression of the key patterning genes in these limbs., (Copyright © 2013 Wiley Periodicals, Inc.)

Background: Regulation of developmental signaling pathways is essential for embryogenesis. The small putative zinc finger protein, Churchill (ChCh) has been implicated in modulation of both TGF-β and FGF signaling., Results: We used zinc finger nuclease (ZFN) mediated gene targeting to disrupt the zebrafish chch locus and generate the first chch mutations. Three induced lesions produce frameshift mutations that truncate the protein in the third of five β-strands that comprise the protein. Surprisingly, zygotic and maternal zygotic chch mutants are viable. Mutants have elevated expression of mesodermal markers, but progress normally through early development. chch mutants are sensitive to exogenous Nodal. However, neither misregulation of FGF targets nor sensitivity to exogenous FGF was detected. Finally, chch mutant cells were found to undergo inappropriate migration in cell transplant assays., Conclusions: Together, these results suggest that chch is not essential for survival, but functions to modulate early mesendodermal gene expression and limit cell migration., (Copyright © 2013 Wiley Periodicals, Inc.)

Background: Supernumerary teeth are often observed in patients suffering from cleidocranial dysplasia due to a mutation in Runx2 that results in haploinsufficiency. However, the underlying molecular mechanisms are poorly defined. In this study, we assessed the roles of Runx2 and its functional antagonist Twist1 in regulating fibroblast growth factor (FGF) signaling using in vitro biochemical approaches., Results: We showed that Twist1 stimulated Fgfr2 and Fgf10 expression in a mesenchymal cell line and that it formed heterodimers with ubiquitously expressed E12 (together with E47 encoded by E2A gene) and upregulated Fgfr2 and Fgf10 promoter activities in a dental mesenchyme-derived cell line. We further demonstrated that the bHLH domain of Twist1 was essential for its synergistic activation of Fgfr2 promoter with E12 and that the binding of E12 stabilized Twist1 by preventing it from undergoing lysosomal degradation. Although Runx2 had no apparent effects on Fgfr2 and Fgf10 promoter activities, it inhibited the stimulatory activity of Twist1 on Fgfr2 promoter., Conclusions: These findings suggest that Runx2 haploinsufficiency might result in excessive unbound Twist1 that can freely bind to E12 and enhance FGF signaling, thereby promoting the formation of extra teeth., (Copyright © 2012 Wiley Periodicals, Inc.)

Background: The vertebrate nuclear receptor subfamily 2, group f (nr2f) genes encode orphan receptors that have the capacity to act as negative regulators of retinoic acid (RA) signaling., Results: We describe embryonic and larval expression of four of the six zebrafish nr2f genes, nr2f1a, nr2f1b, nr2f2, and nr2f5. These genes show highly regulated patterns of expression within the central nervous system, including in the developing hindbrain, as well as in the mesoderm and endoderm. We also investigated the role of RA and fibroblast growth factor (Fgf) signaling in regulating early nr2f gene expression. RA is not required for nr2f expression in the hindbrain; however, exogenous RA can repress this expression. Conversely, we find that RA positively regulates nr2f1a expression in trunk endoderm and mesoderm. Fgf signaling is not required for nr2f expression onset in the hindbrain; however, it may play a role in maintaining rhombomere-specific expression., Conclusions: We report detailed expression analysis of four nr2f genes in all three germ layers. The onset of nr2f expression in the hindbrain does not require RA or Fgf signals. Our finding that RA positively regulates nr2f1a expression in the trunk supports the possibility that Nr2fs function in a negative feedback loop to modulate RA signaling in this region., (Copyright © 2012 Wiley Periodicals, Inc.)

The pharyngeal endoderm is hypothesized as the source of local signals that specify the identity of neural crest-derived mesenchyme in the arches. Sox9 is induced and maintained in prechondrogenic cells during condensation formation and endochondral ossification. Using explant culture, we determined that pharyngeal endoderm was sufficient, but not necessary for specifying prechondrogenic identity, as surrounding tissues including the otic vesicle can compensate for signals from the pharyngeal endoderm. Multiple Fgf genes are expressed specifically in the pharyngeal endoderm subjacent to the neural crest-derived mesenchyme. Fibroblast growth factor (FGF) signaling is both sufficient and required for specification of Sox9 expression and specification of prechondrogenic identity, as demonstrated by the addition of recombinant FGF protein or the FGF receptor inhibitor (SU5402) to explanted tissue, respectively. However, FGF signaling cannot maintain Sox9 expression or initiate the chondrogenic program as indicated by the absence of Col2a1 transcripts. Bone morphogenetic protein (BMP) 4 signaling can induce and maintain Sox9 expression in isolated mesenchyme, but only in combination with FGF signaling induce Col2a1 expression, and thus, chondrogenesis. Given the spatiotemporal expression patterns of FGFs and BMPs in the pharyngeal arches, we suggest that this may represent a general mechanism of local signals specifying prechondrogenic identity and initiation of the chondrogenic program., (Copyright © 2012 Wiley Periodicals, Inc.)

Background: The early telencephalon shares molecular features with the early mid-hindbrain region. In particular, these two developing brain areas each have a signaling center that secretes FGFs and an adjacent one that secretes WNTs. WNTs and FGFs each play essential roles in regulating cell fates in both the telencephalon and mid-hindbrain. Despite this similarity, telencephalic and mid-hindbrain precursors express distinct genes and ultimately generate different cell types, tissue morphologies, and neural functions., Results: Here we show that genetically increasing the level of β-catenin, a mediator of canonical WNT signaling, in the anterior neural plate causes a loss of telencephalic characteristics and a gain of mid-hindbrain characteristics., Conclusion: These results, together with previous ones demonstrating that increased WNT signaling in the anterior neural plate increases FGF expression, suggest that the levels of WNT and FGF signaling regulate telencephalic versus mid-hindbrain fates., (Copyright © 2011 Wiley-Liss, Inc.)

While the paired forelimb and hindlimb buds of vertebrates are initially morphologically homogeneous, as the limb progenitors differentiate, each individual tissue element attains a characteristic limb-type morphology that ultimately defines the constitution of the forelimb or hindlimb. This review focuses on contemporary understanding of the regulation of limb bud initiation and formation of limb-type specific morphologies and how these regulatory mechanisms evolved in vertebrates. We also attempt to clarify the definition of the terms limb-type identity and limb-type morphology that have frequently been used interchangeably. Over the last decade, three genes, Tbx4, Tbx5, and Pitx1, have been extensively studied for their roles in limb initiation and determining limb-type morphologies. The role of Tbx4 and Tbx5 in limb initiation is clearly established. However, their putative role in the generation of limb-type morphologies remains controversial. In contrast, all evidence supports a function for Pitx1 in determination of hindlimb morphologies., (Copyright © 2011 Wiley-Liss, Inc.)

To understand developmental mechanisms of evolutionary change, we must first know how different morphologies form. The vast majority of our knowledge on the developmental genetics of tooth formation derives from studies in mice, which have relatively derived mammalian dentitions. The marsupial Monodelphis domestica has a more plesiomorphic heterodont dentition with incisors, canines, premolars, and molars on both the upper and the lower jaws, and a deciduous premolar. The complexity of the M. domestica dentition ranges from simple, unicusped incisors to conical, sharp canines to multicusped molars. We examine the development of the teeth in M. domestica, with a specific focus on the enamel knot, a signaling center in the embryonic tooth that controls shape. We show that the tooth germs of M. domestica express fibroblast growth factor (FGF) genes and Sprouty genes in a manner similar to wild-type mouse molar germs, but with a few key differences., (© 2010 Wiley-Liss, Inc.)

In embryonic hearts explanted on collagen gels, epicardial cells delaminate and form vascular tubes, thus providing a model for coronary tubulogenesis. Using this model, we show that fibroblast growth factors (FGFs) 1, 2, 4, 8, 9, and 18 contribute to tubulogenesis and that the availability of multiple FGFs provides the optimal tubulogenic response. Moreover, the FGF effects are vascular endothelial growth factor (VEGF) -dependent, while VEGF-induced tubulogenesis requires FGF signaling. The number of endothelial cells (ECs) is increased by all of the FGFs, while EC migration is significantly enhanced only by FGF-2 and FGF-18. Finally, addition of embryonic mesenchymal stem cells (EMSC) to the explants markedly enhances EC numbers and a 23-fold increase in stromal derived factor-1α (SDF-1α), which is FGF dependent. Both explants and EMSCs produce SDF-1α. In conclusion, coronary tubulogenesis of embryonic epicardium: (1) is responsive to many FGF family members, (2) requires both FGF and VEGFA signaling, and (3) is responsive to EMSCs., (© 2010 Wiley-Liss, Inc.)

The proepicardium forms at the venous pole of the embryonic heart and gives rise to several cell types of the mature heart. We investigated the role of fibroblast growth factors (FGFs) during proepicardium formation in the chick embryo. Several FGF ligands (Fgf2, Fgf10, and Fgf12) and receptors (Fgfr1, Fgfr2, and Fgfr4) are expressed in the proepicardium. Experimental modulation of FGF signaling in explant cultures affected cell proliferation and survival. In contrast, expression of Tbx18, Wt1, or Tbx5 were unaffected by FGF inhibition. In agreement with the explant data, villous outgrowth of the proepicardium was strongly impaired by FGF inhibition in vivo, however Tbx18 expression was maintained. These data suggest that during proepicardium formation, FGF ligands act as autocrine or paracrine growth factors to prevent apoptosis, maintain proliferation, and to promote villous outgrowth of the proepicardium. However, FGF is not involved in the induction or maintenance of proepicardium-specific marker gene expression., (Developmental Dynamics 239:2393-2403, 2010. © 2010 Wiley-Liss, Inc.)

Fgf signaling plays pivotal roles in mouse gastrulation and left-right axis formation. However, although genetic analyses have revealed important aspects of Fgf signaling in these processes, the temporal resolution of genetic studies is low. Here, we combined whole-embryo culture with application of chemical compounds to inhibit Fgf signaling at specific time points. We found that sodium chlorate and PD173074 are potent inhibitors of Fgf signaling in early mouse embryos. Fgf signaling is required for the epithelial-to-mesenchymal transition of the primitive streak before the onset of gastrulation. Once gastrulation begins, Fgf signaling specifies mesodermal fates via the Ras/MAPK downstream cascade. Finally, Fgf signaling on the posterior side of the embryo during gastrulation induces Nodal expression in the node via Tbx6-Dll1, the initial event required for Nodal expression in the left lateral plate mesoderm.

Many studies have shown the importance of the fibroblast growth factor (FGF) family of factors in the development of the mammalian cochlea. There are four fibroblast growth factor receptors (FGFR1-4) and all four are expressed in the cochlea during development. While there are examples in the literature of expression patterns of some of the receptors at specific stages of cochlear development there has been no systematic study. We have assembled a full analysis of the patterns of receptor expression during cochlear development for all four Fgfrs using in situ hybridization. We have analyzed the expression patterns from embryonic day 13.5 through postnatal ages. We find that Fgfr1, 2, and 3 are expressed in the epithelium of the cochlear duct and Fgfr4 is limited in its expression to the mesenchyme surrounding the duct. We compare the receptor expression pattern to markers of the sensory domain (p27kip1) and the early hair cells (math1)., (Copyright (c) 2010 Wiley-Liss, Inc.)

The frontonasal mass gives rise to the facial midline and fuses with the maxillary prominence to form the upper lip. Here we focus on the regulation and function of TBX22, a repressor dynamically expressed in the frontonasal mass. Both FGF and Noggin (a BMP antagonist) strongly induce gTBX22, however, each has opposite effects on morphogenesis - Noggin inhibits whereas FGF stimulates growth. To determine whether TBX22 mediates these effects, we used retroviruses to locally increase expression levels. RCAS::hTBX22 decreased proliferation, reduced expression of MSX2 and DLX5 and caused cleft lip. Decreased levels of endogenous gTBX22 were also observed but were not the primary cause of the phenotype as determined in rescue experiments. Our data suggest that genetic or environmental insults such as those affecting the BMP pathway could lead to a gain-of-function of TBX22 and predispose an individual to cleft lip.

Cell-type specific regulation of a small number of growth factor signal transduction pathways generates diverse developmental outcomes. The zinc finger protein Churchill (ChCh) is a key effector of fibroblast growth factor (FGF) signaling during gastrulation. ChCh is largely thought to act by inducing expression of the multifunctional Sip1 (Smad Interacting Protein 1). We investigated the function of ChCh and Sip1a during zebrafish somitogenesis. Knockdown of ChCh or Sip1a results in misshapen somites that are short and narrow. As in wild-type embryos, cycling gene expression occurs in the developing somites in ChCh and Sip1a compromised embryos, but expression of her1 and her7 is maintained in formed somites. In addition, tail bud fgf8 expression is expanded anteriorly in these embryos. Finally, we found that blocking FGF8 restores somite morphology in ChCh and Sip1a compromised embryos. These results demonstrate a novel role for ChCh and Sip1a in repression of FGF activity.

Previous studies on retinoic acid receptor (RAR) mutants suggested that retinoic acid (RA) is required for loss of interdigital mesenchyme during digit formation. Here, we report that the RA-generating enzyme retinaldehyde dehydrogenase-2 (Raldh2) is expressed in the interdigital mesenchyme whereas Cyp26b1, controlling RA degradation, is expressed in digits, limiting autopodal RA action to the interdigital zones. Embryonic day 13.5 Raldh2-/- mouse embryos lose expression of the RARE-lacZ RA-reporter transgene and matrix metalloproteinase-11 (Mmp11) throughout the interdigital mesenchyme, while expression of RARb, Fgf18, and high mobility group N1 (Hmgn1) is lost at the digit-interdigit junction. Raldh2-/- autopods exhibit reduced interdigital apoptosis associated with loss of Bmp7 expression, but Bmp2, Bmp4, Msx2, and Fgf8 were unaffected. Although interdigital expression of Hmgn1 was greatly down-regulated in Raldh2-/- autopods, complementary expression of Sox9 in digit cartilage was unaffected. Regulation of Hmgn1 and Fgf18 at the digit-interdigit junction suggests RA controls tissue remodeling as well as apoptosis.

The process of segmentation in vertebrates is described by a clock and wavefront model consisting of a Notch signal and an fibroblast growth factor-8 (FGF8) gradient, respectively. To further investigate the segmentation process, we screened gene expression profiles for downstream targets of the segmentation clock. The Rnd1 and Rnd3 GTP-binding proteins comprise a subgroup of the Rho GTPase family that show a specific expression pattern similar to the Notch signal component ESR5, suggesting an association between Rnd1/3 and the segmentation clock. Rnd1/3 expression patterns are disrupted by overexpression of dominant-negative or active forms of Notch signaling genes, and responds to the FGF inhibitor SU5402 by a posterior shift analogous to other segmentation-related genes, suggesting that Rnd1/3 expressions are regulated by the segmentation clock machinery. We also show that antisense morpholino oligonucleotides to Rnd1/3 inhibit somite segmentation and differentiation in Xenopus embryos. These results suggest that Rnd1/3 are required for Xenopus somitogenesis., (Copyright 2009 Wiley-Liss, Inc.)

Multiple tissue interactions and signaling within the pharyngeal arches are required for development of the craniofacial skeleton. Here, we focus on the role of the transcription factor prdm1a in the differentiation of the posterior skeleton. prdm1a is expressed in the presumptive pharyngeal arch region and later in an endodermal pouch, the otic vesicle, and pharyngeal teeth. prdm1a mutants display a reduction in pharyngeal arch markers, a loss of posterior ceratobranchial cartilages, and a reduction in most neural crest-derived dermal bones. This is likely caused by a decrease in the number of proliferating cells but not an increase in cell death. Finally, a reduction in two key developmental signaling pathways, Fgf and retinoic acid, alters prdm1a expression, suggesting that prdm1a expression is mediated by these signaling pathways to pattern the posterior craniofacial skeleton. Together, these results indicate an essential role for prdm1a in the development of the zebrafish craniofacial skeleton.

Induction of otic placodes requires Fgf from surrounding tissues. We tested the hypothesis that mesendodermally derived Bmp-antagonists Chordin, Follistatin-a, and Crossveinless-2 cooperate in this process. Injecting morpholinos for all three genes, or treatment with the Nodal inhibitor SB431542 to block mesoderm-formation, reduces otic induction and strongly enhances the effects of disrupting fgf3 or fgf8. In contrast, using a lower dose of SB431542, combined with partial loss of Fgf, causes a dramatic medial expansion of otic tissue and formation of a single, large otic vesicle spanning the width of the hindbrain. Under these conditions, paraxial cephalic mesoderm forms ectopically at the midline, migrates into the head, and later transfates to form otic tissue beneath the hindbrain. Blocking expression of Bmp-antagonists blocks formation of medial otic tissue. These data show the importance of mesendodermal Bmp-antagonists for otic induction and that paraxial cephalic mesendoderm can facilitate its own otic differentiation under certain circumstances. Developmental Dynamics 238:1582-1594, 2009. (c) 2009 Wiley-Liss, Inc.

The pancreas is both an exocrine and endocrine endodermal organ involved in digestion and glucose homeostasis. During embryogenesis, the anlagen of the pancreas arise from dorsal and ventral evaginations of the foregut that later fuse to form a single organ. To better understand the molecular genetics of early pancreas development, we sought to isolate markers that are uniquely expressed in this tissue. Microarray analysis was performed comparing dissected pancreatic buds, liver buds, and the stomach region of tadpole stage Xenopus embryos. A total of 912 genes were found to be differentially expressed between these organs during early stages of organogenesis. K-means clustering analysis predicted 120 of these genes to be specifically enriched in the pancreas. Of these, we report on the novel expression patterns of 24 genes. Our analyses implicate the involvement of previously unsuspected signaling pathways during early pancreas development. Developmental Dynamics 238:1455-1466, 2009. (c) 2009 Wiley-Liss, Inc.

Fibroblast growth factor (FGF) signalling plays a major role during early vertebrate development. It is involved in the specification of the mesoderm, control of morphogenetic movements, patterning of the anterior-posterior axis, and neural induction. In mammals, 22 FGF ligands have been identified, which can be grouped into seven subfamilies according to their sequence homology and function. We have cloned 17 fgf genes from Xenopus tropicalis and have analysed their temporal expression by RT-PCR and spatial expression by whole mount in situ hybridisation at key developmental stages. It reveals the diverse expression pattern of fgf genes during early embryonic development. Furthermore, our analysis shows the transient nature of expression of several fgfs in a number of embryonic tissues. This study constitutes the most comprehensive description of the temporal and spatial expression pattern of fgf ligands and receptors during vertebrate development to date. Developmental Dynamics 238:1467-1479, 2009. (c) 2009 Wiley-Liss, Inc.

The ophthalmic trigeminal (opV) placode gives rise exclusively to sensory neurons of the peripheral nervous system, providing an advantageous model for understanding neurogenesis. The signaling pathways governing opV placode development have only recently begun to be elucidated. Here, we investigate the fibroblast growth factor receptor-4 (FGFR4), an opV expressed gene, to examine if and how FGF signaling regulates opV placode development. After inhibiting FGFR4, Pax3+ opV placode cells failed to delaminate from the ectoderm and did not contribute to the opV ganglion. Blocking FGF signaling also led to a loss of the early and late neuronal differentiation markers Ngn2, Islet-1, NeuN, and Neurofilament. In addition, without FGF signaling, cells that stalled in the ectoderm lost their opV placode-specific identity by down-regulating Pax3. We conclude that FGF signaling, through FGFR4, is necessary for delamination and differentiation of opV placode cells.

The vertebrate mid-hindbrain boundary (MHB) is a crucial morphological structure required for patterning and neural differentiation of the midbrain and anterior hindbrain. We isolated a novel zebrafish mutant, MHB gone (mgo), that exhibited a defective MHB. Expression of engrailed3 in the prospective MHB was absent at the 1-somite stage, suggesting that initiation of the isthmic organizer was disrupted in mgo mutants. Complementation test with mgo and noi, in which the pax2a gene is mutated, infer that the mgo mutant may represent a novel noi allele. However, pronephric, otic vesicle, and commissural axonal defects described in noi mutants were not associated with mgo mutants. Genetic mapping revealed that the mgo mutation is linked to the Pax2a locus, but no mutation was detected in pax2a exons or within intron-exon boundaries. Based on these findings, we propose that the mgo mutation genetically interacts with pax2a required for the initiation of MHB formation., (Copyright 2009 Wiley-Liss, Inc.)

Fibroblast growth factors (FGFs) are important signaling molecules that regulate many stages of endochondral bone development. During the healing of a skeletal fracture, several features of endochondral bone development are reactivated. To better understand the role of FGFs in skeletal fracture healing, we quantitatively evaluated the temporal expression patterns of Fgfs, Fgf receptors (Fgfrs), and molecular markers of bone development over a 14-day period following long bone fracture in a mouse model. These studies identify distinct groups of FGFs that are differentially expressed and suggest active stage-specific roles for FGF signaling during the fracture repair process., ((c) 2009 Wiley-Liss, Inc.)

LIM-homeodomain (LIM-hd) proteins form a multifunctional family of transcription factors that plays critical roles in the development of progenitor and post-mitotic cells. Considerable work has focused on what regulates their expression post-mitotically in the spinal cord. However, little is known about what regulates LIM-hd genes at earlier developmental stages. To address this question, we explored the role of fibroblast growth factor (FGF) signalling in regulating the expression of a Xenopus laevis Lhx9 orthologue (XLhx9). XLhx9 is first expressed in the eye field and hindbrain, and when FGF receptor (FGFR) activation was inhibited prior to its onset, both brain and eye field expression was diminished. However, when FGFRs were inhibited after XLhx9 onset, retinal expression remained strong and brain expression was again diminished. These data suggest that while FGF signalling initiates and maintains brain XLhx9 expression, in the eye primordium the requirement of FGFs for expression is rapidly lost.

Fibroblast growth factor (Fgf) signaling plays important roles in development and metabolism. Mouse Fgf16 was predominantly expressed in cardiomyocytes. To elucidate the physiological roles of Fgf16, we generated Fgf16 knockout mice. Although the mice were apparently normal and fertile, heart weight and cardiomyocyte cell numbers were slightly decreased at 6 months of age. However, blood pressure, heart rate, and cardiac performance were essentially unchanged. In addition, the expression of most cardiac marker genes examined was also essentially unchanged. However, the expression of Bnp was significantly decreased, indicating potential roles of Fgf16 in the heart under pathological conditions. In contrast, the proliferation of embryonic cardiomyocytes was significantly decreased, indicating that Fgf16 is a growth factor for these cells. The embryonic heart phenotype is similar to that of the Fgf9 knockout heart, indicating Fgf9 and Fgf16 to synergistically act as growth factors for embryonic cardiomyocytes., (Copyright (c) 2008 Wiley-Liss, Inc.)

FGF signaling is important for the formation of mesoderm in vertebrates, and when it is perturbed in Xenopus, most trunk and tail mesoderm fails to form. Here we have further dissected the activities of FGF in patterning the embryo by addressing its inductive and maintenance roles. We show that FGF signaling is necessary for the establishment of xbra expression in addition to its well-characterized role in maintaining xbra expression. The role of FGF signaling in organizer formation is not clear in Xenopus. We find that FGF signaling is essential for the initial specification of paraxial mesoderm but not for activation of several pan-mesodermal and most organizer genes; however, early FGF signaling is necessary for the maintenance of organizer gene expression into the neurula stage. Inhibition of FGF signaling prevents VegT activation of specific mesodermal transcripts. These findings illuminate how FGF signaling contributes to the establishment of distinct types of mesoderm.

We examined the role of WNT signaling in pituitary development by characterizing the pituitary phenotype of three WNT knockout mice and assessing the expression of WNT pathway components. Wnt5a mutants have expanded domains of Fgf10 and bone morphogenetic protein expression in the ventral diencephalon and a reduced domain of LHX3 expression in Rathke's pouch. Wnt4 mutants have mildly reduced cell differentiation, reduced POU1F1 expression, and mild anterior lobe hypoplasia. Wnt4, Wnt5a double mutants exhibit an additive pituitary phenotype of dysmorphology and mild hypoplasia. Wnt6 mutants have no obvious pituitary phenotype. We surveyed WNT expression and identified transcripts for numerous Wnts, Frizzleds, and downstream pathway members in the pituitary and ventral diencephalon. These findings support the emerging model that WNT signaling affects the pituitary gland via effects on ventral diencephalon signaling, and suggest additional Wnt genes that are worthy of functional studies., ((c) 2008 Wiley-Liss, Inc.)

Cell proliferation, differentiation, and vascular growth are coordinated processes that are essential for embryonic development, tissue repair, and disease pathogenesis. Of interest, whether these critical processes are dependent upon each other has not been thoroughly explored. We have generated mice that conditionally express the cell cycle inhibitor p27(Kip1), following Cre-mediated recombination, as a tool to separate tissue proliferation from other cellular processes. Using the embryonic heart as a model, we show that myocardial proliferation and coronary development are genetically separable processes. Forced expression of p27, in both a wild-type and in a genetically sensitized background, resulted in ventricular hypoplasia without having any substantial effects on coronary development. We further demonstrate that Hedgehog signaling, which is essential for coronary vascular growth, does not control myocardial proliferation. Together, these studies strongly suggest that myocardial cell proliferation and coronary development are genetically separable programs exemplifying one of the many potential uses of this genetic tool., ((c) 2008 Wiley-Liss, Inc.)