Retinoid signaling is required for the establishment of a ZPA and for the expression of Hoxb-8, a mediator of ZPA formation

LHX2 Mediates the FGF-to-SHH Regulatory Loop during Limb Development

During limb development, fibroblast growth factors (Fgfs) govern proximal⁻distal outgrowth and patterning. FGFs also synchronize developmental patterning between the proximal⁻distal and anterior⁻posterior axes by maintaining Sonic hedgehog (Shh) expression in cells of the zone of polarizing activity (ZPA) in the distal posterior mesoderm. Shh, in turn, maintains Fgfs in the apical ectodermal ridge (AER) that caps the distal tip of the limb bud. Crosstalk between Fgf and Shh signaling is critical for patterned limb development, but the mechanisms underlying this feedback loop are not well-characterized. Implantation of Fgf beads in the proximal posterior limb bud can maintain SHH expression in the former ZPA domain (evident 3 h after application), while prolonged exposure (24 h) can induce SHH outside of this domain. Although temporally and spatially disparate, comparative analysis of transcriptome data from these different populations accentuated genes involved in SHH regulation. Comparative analysis identified 25 candidates common to both treatments, with eight linked to SHH expression or function. Furthermore, we demonstrated that LHX2, a LIM Homeodomain transcription factor, is an intermediate in the FGF-mediated regulation of SHH. Our data suggest that LHX2 acts as a competency factor maintaining distal posterior SHH expression subjacent to the AER.

A newly established culture method highlights regulatory roles of retinoic acid on morphogenesis and calcification of mammalian limb cartilage

During mammalian embryogenesis, sclerotome-derived chondrocytes in the limb bud are arranged into a complicated bone shape with specific areas undergoing hypertrophy and calcification, creating a region-specific mineralized pattern in the cartilage. To follow chondrogenesis progression in vitro, we isolated limb cartilage from mice on embryonic day 13 (E13) and cultured it at the air-liquid interface after microsurgical removal of the ectoderm/epidermis. Explants underwent proper morphogenesis, giving rise to complete templates for limb bones in vitro. We found that region-specific calcification patterns resembling limbs of prepartum mature embryos could be induced in explants using culture medium containing high concentrations of CaCl2 (Ca), ascorbic acid (AA), and β-glycerophosphoric acid (BGP). In this culture system, excess amounts of all-trans retinoic acid (RA) severely disrupted morphogenesis and calcification patterns in limb cartilage. These effects were more pronounced in forearms than in phalanges. Although dissociated, the nascent chondrocytes in culture did not give rise to cartilage units even though augmented calcification was induced in these cell aggregates in the presence of RA. Taken together, our newly established culture system revealed that RA independently regulates three-dimensional morphogenesis and calcification.

Application of synthetic photostable retinoids induces novel limb and facial phenotypes during chick embryogenesis in vivo

We have recently developed a range of synthetic retinoid analogues which include the compounds EC23 and EC19. They are stable on exposure to light and are predicted to be resistant to the normal metabolic processes involved in the inactivation of retinoids in vivo. Based on the position of the terminal carboxylic acid groups in the compounds we suggest that EC23 is a structural analogue of all-trans retinoic acid (ATRA), and EC19 is an analogue of 13-cis retinoic acid. Their effects on the differentiation of pluripotent stem cells has been previously described in vitro and are consistent with this hypothesis. We present herein the first description of the effects of these molecules in vivo. Retinoids were applied to the anterior limb buds of chicken embryos in ovo via ion-exchange beads. We found that retinoid EC23 produces effects on the wing digits similar to ATRA, but does so at two orders of magnitude lower concentration. When larger quantities of EC23 are applied, a novel phenotype is obtained involving production of multiple digit 1s on the anterior limb. This corresponds to differential effects of ATRA and EC23 on sonic hedgehog (shh) expression in the developing limb bud. With EC23 application we also find digit 1 phenotypes similar to thumb duplications described in the clinical literature. EC23 and ATRA are shown to have effects on the entire proximal–distal axis of the limb, including hitherto undescribed effects on the scapula. This includes suppression of expression of the scapula marker Pax1. EC23 also produces effects similar to those of ATRA on the developing face, producing reductions of the upper beak at concentrations two orders of magnitude lower than ATRA. In contrast, EC19, which is structurally very similar to EC23, has novel, less severe effects on the face and rarely alters limb development. EC19 and ATRA are effective at similar concentrations. These results further demonstrate the ability of retinoids to influence embryonic development. Moreover, EC23 represents a useful new tool to investigate developmental processes and probe the mechanisms underlying congenital abnormalities in vertebrates including man.

A misplaced lncRNA causes brachydactyly in humans

Translocations are chromosomal rearrangements that are frequently associated with a variety of disease states and developmental disorders. We identified 2 families with brachydactyly type E (BDE) resulting from different translocations affecting chromosome 12p. Both translocations caused downregulation of the parathyroid hormone-like hormone (PTHLH) gene by disrupting the cis-regulatory landscape. Using chromosome conformation capturing, we identified a regulator on chromosome 12q that interacts in cis with PTHLH over a 24.4-megabase distance and in trans with the sex-determining region Y-box 9 (SOX9) gene on chromosome 17q. The element also harbored a long noncoding RNA (lncRNA). Silencing of the lncRNA, PTHLH, or SOX9 revealed a feedback mechanism involving an expression-dependent network in humans. In the BDE patients, the human lncRNA was upregulated by the disrupted chromosomal association. Moreover, the lncRNA occupancy at the PTHLH locus was reduced. Our results document what we believe to be a novel in cis- and in trans-acting DNA and lncRNA regulatory feedback element that is reciprocally regulated by coding genes. Furthermore, our findings provide a systematic and combinatorial view of how enhancers encoding lncRNAs may affect gene expression in normal development.

The "Phantom Effect" of the Rexinoid LG100754: structural and functional insights

Retinoic acid receptors (RARs) and Retinoid X nuclear receptors (RXRs) are ligand-dependent transcriptional modulators that execute their biological action through the generation of functional heterodimers. RXR acts as an obligate dimer partner in many signalling pathways, gene regulation by rexinoids depending on the liganded state of the specific heterodimeric partner. To address the question of the effect of rexinoid antagonists on RAR/RXR function, we solved the crystal structure of the heterodimer formed by the ligand binding domain (LBD) of the RARα bound to its natural agonist ligand (all-trans retinoic acid, atRA) and RXRα bound to a rexinoid antagonist (LG100754). We observed that RARα exhibits the canonical agonist conformation and RXRα an antagonist one with the C-terminal H12 flipping out to the solvent. Examination of the protein-LG100754 interactions reveals that its propoxy group sterically prevents the H12 associating with the LBD, without affecting the dimerization or the active conformation of RAR. Although LG100754 has been reported to act as a ‘phantom ligand’ activating RAR in a cellular context, our structural data and biochemical assays demonstrate that LG100754 mediates its effect as a full RXR antagonist. Finally we show that the ‘phantom ligand effect’ of the LG100754 is due to a direct binding of the ligand to RAR that stabilizes coactivator interactions thus accounting for the observed transcriptional activation of RAR/RXR.

Retinoic acid promotes limb induction through effects on body axis extension but is unnecessary for limb patterning

Retinoic acid (RA) is thought to be a key signaling molecule involved in limb bud patterning along the proximodistal or anteroposterior axes functioning through induction of Meis2 and Shh, respectively. Here, we utilize Raldh2-/- and Raldh3-/- mouse embryos lacking RA synthesis to demonstrate that RA signaling is not required for limb expression of Shh and Meis2. We demonstrate that RA action is required outside of the limb field in the body axis during forelimb induction but that RA is unnecessary at later stages when hindlimb budding and patterning occur. We provide evidence for a model of trunk mesodermal RA action in which forelimb induction requires RA repression of Fgf8 in the developing trunk similar to how RA controls somitogenesis and heart development. We demonstrate that pectoral fin development in RA-deficient zebrafish embryos can be rescued by an FGF receptor antagonist SU5402. In addition, embryo ChIP assays demonstrate that RA receptors bind the Fgf8 promoter in vivo. Our findings suggest that RA signaling is not required for limb proximodistal or anteroposterior patterning but that RA inhibition of FGF8 signaling during the early stages of body axis extension provides an environment permissive for induction of forelimb buds.

Shared developmental mechanisms pattern the vertebrate gill arch and paired fin skeletons

Here, we describe the molecular patterning of chondrichthyan branchial rays (gill rays) and reveal profound developmental similarities between gill rays and vertebrate appendages. Sonic hedgehog (Shh) and fibroblast growth factor 8 (Fgf8) regulate the outgrowth and patterning of the chondrichthyan gill arch skeleton, in an interdependent manner similar to their roles in gnathostome paired appendages. Additionally, we demonstrate that paired appendages and branchial rays share other conserved developmental features, including Shh-mediated mirror-image duplications of the endoskeleton after exposure to retinoic acid, and Fgf8 expression by a pseudostratified distal epithelial ridge directing endoskeletal outgrowth. These data suggest that the skeletal patterning role of the retinoic acid/Shh/Fgf8 regulatory circuit has a deep evolutionary origin predating vertebrate paired appendages and may have functioned initially in patterning pharyngeal structures in a deuterostome ancestor of vertebrates.

Mutant Hoxd13 induces extra digits in a mouse model of synpolydactyly directly and by decreasing retinoic acid synthesis

Individuals with the birth defect synpolydactyly (SPD) have 1 or more digit duplicated and 2 or more digits fused together. One form of SPD is caused by polyalanine expansions in homeobox d13 (Hoxd13). Here we have used the naturally occurring mouse mutant that has the same mutation, the SPD homolog (Spdh) allele, and a similar phenotype, to investigate the molecular pathogenesis of SPD. A transgenic approach and crossing experiments showed that the Spdh allele is a combination of loss and gain of function. Here we identify retinaldehyde dehydrogenase 2 (Raldh2), the rate-limiting enzyme for retinoic acid (RA) synthesis in the limb, as a direct Hoxd13 target and show decreased RA production in limbs from Spdh/Spdh mice. Intrauterine treatment with RA restored pentadactyly in Spdh/Spdh mice. We further show that RA and WT Hoxd13 suppress chondrogenesis in mesenchymal progenitor cells, whereas Hoxd13 encoded by Spdh promotes cartilage formation in primary cells isolated from Spdh/Spdh limbs, and that this was associated with increased expression of Sox6/9. Increased Sox9 expression and ectopic cartilage formation in the interdigital mesenchyme of limbs from Spdh/Spdh mice suggest uncontrolled differentiation of these cells into the chondrocytic lineage. Thus, we propose that mutated Hoxd13 causes polydactyly in SPD by inducing extraneous interdigital chondrogenesis, both directly and indirectly, via a reduction in RA levels.

Raldh expression in embryos of the direct developing frog Eleutherodactylus coqui and the conserved retinoic acid requirement for forelimb initiation

Embryos of the direct developing frog, Eleutherodactylus coqui, provide opportunities to examine frog early limb development that are not available in species with tadpoles. We cloned two retinaldehyde dehydrogenase genes, EcRaldh1 and EcRaldh2, to see which enzyme likely supplies retinoic acid for limb development. EcRaldh1 is expressed in the dorsal retina, otic vesicle, pronephros, and pronephric duct, but not in the limb. EcRaldh2 is expressed early at the blastoporal lip and then in the mesoderm in the neurula, so this expression could function in forelimb initiation. Later EcRaldh2 is expressed in the mesoderm at the base of the limbs and in the ventral spinal cord where motor neurons innervating the limbs emerge. These observations on a frog support the functional conservation of EcRaldh2 in forelimb initiation in Osteichthyans and in limb patterning and motor neuron specification in tetrapods.

Retinoid signaling is involved in governing the waiting period for axons in chick hindlimb

During embryonic development in chick, axons pause in a plexus region for approximately 1 day prior to invading the limb. We have previously shown that this "waiting period" is governed by maturational changes in the limb. Here we provide a detailed description of the spatiotemporal pattern of Raldh2 expression in lumbosacral motoneurons and in the limb, and show that retinoid signaling in the limb contributes significantly to terminating the waiting period. Raldh2, indicative of retinoid signaling, first appears in hindlimb mesenchyme near the end of the waiting period. Transcripts are more abundant in connective tissue associated with predominantly fast muscles than predominantly slow muscles, but are not expressed in muscle cells themselves. The tips of ingrowing axons are always found in association with domains of Raldh2, but development of Raldh2 expression is not regulated by the axons. Instead, retinoid signaling appears to regulate axon entry into the limb. Supplying exogenous retinoic acid to proximal limb during the waiting period caused both motor and sensory axons to invade the limb prematurely and altered the normal stereotyped pattern of axon ingrowth without obvious effects on limb morphogenesis or motoneuron specification. Conversely, locally decreasing retinoid synthesis reduced axon growth into the limb. Retinoic acid significantly enhanced motor axon growth in vitro, suggesting that retinoic acid may directly promote axon growth into the limb in vivo. In addition, retinoid signaling may indirectly affect the waiting period by regulating the maturation of other gate keeping or guidance molecules in the limb. Together these findings reveal a novel function of retinoid signaling in governing the timing and patterning of axon growth into the limb.

Cadmium exposure induces expression of the HOXB8 gene in COS-7 cells

Cadmium (Cd) is a serious toxic metal, which is classified as a possible human carcinogen. We assessed the effects of Cd on the expression levels of homeobox genes, which are associated with carcinogenesis. Among 6 homeobox genes examined in this study, only HOXB8 exhibited increased mRNA expression in COS-7 cells treated with 10 microM CdCl(2). Semiquantitative reverse transcription-polymerase chain reaction analysis revealed that the HOXB8 mRNA level was increased by a maximum of 5.4-fold after 6h of Cd exposure. The levels of HOXA7, A9, C4, C9 and C10 mRNAs decreased from 0.1 to 0.3-fold. Silencing of HOXB8 mRNA expression using a siRNA increased HOXC9 and C10 mRNA expression levels by 6.6- and 1.9-fold, respectively. These results suggest that HOXB8 upregulation is associated with suppression of HOXC9 and C10, and that decreased expression of HOXC9 and C10 after Cd exposure is partly due to HOXB8 induction. In conclusion, Cd disrupts the HOX network. Comprehensive analyses of all the HOX gene expression levels in the presence of Cd may afford clues toward understanding Cd-induced carcinogenesis and teratogenesis.

Rescue of morphogenetic defects and of retinoic acid signaling in retinaldehyde dehydrogenase 2 (Raldh2) mouse mutants by chimerism with wild-type cells

Retinoic acid (RA), the active vitamin A derivative, is an important developmental signaling molecule in vertebrates. In this study, we have assessed whether minimal numbers and/or specific distributions of RA-producing cells can support normal mouse embryonic development. Retinaldehyde dehydrogenase 2 (RALDH2) is the main RA-synthesizing enzyme acting during development. We have generated an embryonic stem (ES) cell line homozygous for an Raldh2 gene disruption, and have analyzed chimeric embryos with various contributions of wild-type cells. Whereas embryos almost completely derived from Raldh2(-/-) cells phenocopy the corresponding germline null mutants, the presence of even small numbers (<10%) of wild-type cells can rescue most of the morphogenetic defects, including embryonic turning and axial elongation, and left-right looping of the heart tube. No consistent bias in the distribution of wild-type cells was observed in the phenotypically rescued Raldh2(-/-) chimeras. Analysis of an RA-sensitive transgene indicates that RA can diffuse from wild-type cells and elicit a widespread transcriptional response in Raldh2-deficient cells. Our results show that few wild-type RA-producing cells, even when present in apparent random distributions, can support early morphogenesis of the mouse embryo. However, the Raldh2(-/-) chimeric fetuses display lung abnormalities, persistent truncus arteriosus, and abnormal myocardial differentiation, showing that subsequent RA-dependent events cannot be fully rescued by the mosaic presence of wild-type cells.

Anteroposterior patterning in the limb and digit specification: contribution of mouse genetics

The limb has been a privileged object of investigation and reflection for scientists over the past two centuries and continues to provide a heuristic framework to analyze vertebrate development. Recently, accumulation of new data has significantly changed our view on the mechanisms of limb patterning, in particular along the anterior-posterior axis. These data have led us to revisit the mode of action of the zone of polarizing activity. They shed light on the molecular and cellular mechanisms of patterning linked to the Shh-Gli3 signaling pathway and give insights into the mechanism of activation of these cardinal factors, as well as the consequences of their activity. These new data are in good part the result of systematic Application of tools used in contemporary mouse molecular genetics. These have extended the power of mouse genetics by introducing mutational strategies that allow fine-tuned modulation of gene expression, interchromosomal deletions and duplication. They have even made the mouse embryo amenable to cell lineage analysis that used to be the realm of chick embryos. In this review, we focus on the data acquired over the last five years from the analysis of mouse limb development and discuss new perspectives opened by these results.

Signaling during lens regeneration

The newt is one of the few organisms that is able to undergo lens regeneration as an adult. This review will examine the signaling pathways that are involved in this amazing phenomenon. In addition to outlining the current research involved in elucidating the key signaling molecules in lens regeneration, we will also highlight some of the similarities and differences between lens regeneration and development.

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

Hoxb8 has been suggestively implicated in the formation of the zone of polarizing activity (ZPA) in the limb bud. However, as hoxb8-/- mice did not show any defects in their limb development, the role of Hoxb8 during limb development has not been fully elucidated. Here, we report the identification of the medaka hoxb8a mutant, unextended-fin (ufi), in which all the fin tissues were malformed. Since the abnormal phenotype was observed in the caudal fin, the ufi phenotype suggests that the medaka Hoxb8a has a fundamental role in the formation of appendages protruding from the trunk. Our analyses revealed that the expression of wnt5a, a regulator of cell migration that signals through the non-canonical Wnt/Ca2+ pathway, was down-regulated in the ufi fin-folds. In fact, we found that the proximal-distal cell migration was impaired in ufi mutants and that the defect could be reversed by the injection of a Wnt5a protein. Moreover, we show herein that the numbers of proliferating cells and osteoblastic cells were increased in the ufi mutants. According to these results, we propose that the medaka Hoxb8a protein functions in the outgrowth of appendages through the regulation of cell migration and osteoblast differentiation.

MicroRNAs in mammalian development and tumorigenesis

Mammals have evolved a variety of gene regulatory mechanisms to ensure the proper development of tissues during embryonic organogenesis. Recently, microRNAs (miRNAs) have been shown to regulate protein dosage during mammalian development. miRNAs are tiny RNA molecules that function to regulate diverse cellular processes by inhibiting gene expression posttranscriptionally. Since their discovery in mammals in 2000, much has been learned about the biogenesis, mechanisms of action, and expression of miRNAs. This knowledge combined with the identification of new mRNA targets has provided valuable insights into the functions of these RNA regulatory molecules. It is now clear that miRNAs are involved in modulating a variety of developmental and physiological processes. This review is designed to highlight recent advances in the study of miRNAs with a particular emphasis on their roles in mammalian development and cancer progression.

The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development

MicroRNAs (miRNAs) are an abundant class of gene regulatory molecules (reviewed in refs 1, 2). Although computational work indicates that miRNAs repress more than a third of human genes, their roles in vertebrate development are only now beginning to be determined. Here we show that miR-196 acts upstream of Hoxb8 and Sonic hedgehog (Shh) in vivo in the context of limb development, thereby identifying a previously observed but uncharacterized inhibitory activity that operates specifically in the hindlimb. Our data indicate that miR-196 functions in a fail-safe mechanism to assure the fidelity of expression domains that are primarily regulated at the transcriptional level, supporting the idea that many vertebrate miRNAs may function as a secondary level of gene regulation.

Deer antlers as a model of Mammalian regeneration

Deer antlers are cranial appendages that develop after birth as extensions of a permanent protuberance (pedicle) on the frontal bone. Pedicles and antlers originate from a specialized region of the frontal bone; the 'antlerogeneic periosteum' and the systemic cue which triggers their development in the fawn is an increase in circulating androgen. These primary antlers are then shed and regenerated the following year in a larger, more complex form. Antler growth is extremely rapid-an adult red deer can produce a pair of antlers weighing approximately 30kg in three months, and involves both endochondral and intramembranous ossification. Since antlers are sexual secondary characteristics, their annual cycles of growth have evolved to be closely coordinated to the reproductive cycle which, in temperate species, is linked to the photoperiod. Cessation of antler growth and death of the overlying skin (velvet) coincides with a rise in circulating testosterone as the autumn breeding season approaches. The 'dead' antlers remain attached to the pedicle until they are shed (cast) the following spring when circulating testosterone levels fall. In red deer, the species that we study, casting of the old set of antlers is followed immediately by growth of the new set. Although the anatomy of antler growth and the endocrine changes associated with it have been well documented, the molecular mechanisms involved remain poorly understood. The case for continuing to decipher them remains compelling, despite the obvious limitations of using deer as an experimental model, because this research will help provide insight into why humans and other mammals have lost the ability to regenerate organs. From the information so far available, it would appear that the signaling pathways that control the development of skeletal elements are recapitulated in regenerating antlers. This apparent lack of any specific 'antlerogenic molecular machinery' suggests that the secret of deers' ability to regenerate antlers lies in the particular cues to which multipotential progenitor/stem cells in an antler's 'regeneration territory' are exposed. This in turn suggests that with appropriate manipulation of the environment, pluripotential cells in other adult mammalian tissues could be stimulated to increase the healing capacity of organs, even if not to regenerate them completely. The need for replacement organs in humans is substantial. The benefits of increasing individuals' own capacity for regeneration and repair are self evident.

Retinoic acid in alveolar development, maintenance and regeneration

Recent data suggest that exogenous retinoic acid (RA), the biologically active derivative of vitamin A, can induce alveolar regeneration in a rat model of experimental emphysema. Here, we describe a mouse model of disrupted alveolar development using dexamethasone administered postnatally. We show that the effects of dexamethasone are concentration dependent, dose dependent, long lasting and result in a severe loss of alveolar surface area. When RA is administered to these animals as adults, lung architecture and the surface area per unit of body weight are completely restored to normal. This remarkable effect may be because RA is required during normal alveolar development and administering RA re-awakens gene cascades used during development. We provide evidence that RA is required during alveologenesis in the mouse by showing that the levels of the retinoid binding proteins, the RA receptors and two RA synthesizing enzymes peak postnatally. Furthermore, an inhibitor of RA synthesis, disulphiram, disrupts alveologenesis. We also show that RA is required throughout life for the maintenance of lung alveoli because when rats are deprived of dietary retinol they lose alveoli and show the features of emphysema. Alveolar regeneration with RA may therefore be an important novel therapeutic approach to the treatment of respiratory diseases characterized by a reduced gas-exchanging surface area such as bronchopulmonary dysplasia and emphysema for which there are currently no treatments.

Ventral tail bud mesenchyme is a signaling center for tail paraxial mesoderm induction

A large body of evidence from several systems indicates that formation of the vertebrate tail is morphogenetically continuous with gastrulation, including neural inducing activity in descendants of the gastrula organizer. However, the signaling centers and molecular events regulating tail mesoderm induction and its organized elongation remain poorly defined. In mammals, the ventral ectoderm ridge (VER) is essential to maintain ongoing formation of paraxial mesoderm and somitogenesis in cultures of intact tail. Avian tail buds contain a similar VER structure. Here, we report that the chick ventral tail bud operates as a signaling center for paraxial mesoderm induction. By using "organizer" style grafting assays to early host embryos, we found that ventral tail bud was able to induce elongated paraxial mesodermal extensions and that the ventral tail bud mesenchyme underlying the VER is both necessary and sufficient for the induction in this assay system. Our observations combined with those of others suggest that interplay between several different signaling centers in the amniote tail bud regulates the coordinate induction and elongation of axial and paraxial structures in the developing tail.

Cellular and molecular mechanisms of development of the external genitalia

The limb and external genitalia are appendages of the body wall. Development of these structures differs fundamentally in that masculine development of the external genitalia is androgen dependent, whereas development of the limb is not. Despite this fundamental difference in developmental regulation, epithelial-mesenchymal interactions play key roles in the development of both structures, and similar regulatory molecules are utilized as mediators of morphogenetic cell-cell interactions during development of both the limb and external genitalia. Given the relatively high incidence of hypospadias, a malformation of penile development, it is appropriate and timely to review the morphological, endocrine, and molecular mechanisms of development of the genital tubercle (GT), the precursor of the penis in males and the clitoris in females. Morphological observations comparing development of the GT in humans and mouse emphasize the validity of the mouse as an animal model of GT development and validate the results of experimental studies. Accordingly, the use of mutant mice provides important insights into the roles of specific regulatory molecules in development of the external genitalia. While our current understanding of the morphological and molecular mechanisms of mammalian external genitalia development is still rudimentary, this review summarizes the current state of our knowledge and whenever possible draws from the rich experimental embryology literature on other relevant organs such as the developing limb. Future research on the hormonal and molecular mechanisms of GT development may yield strategies to prevent or reduce the incidence of hypospadias and to elucidate the molecular genetic mechanisms of GT morphogenesis, especially in relation to common organogenetic pathways utilized in other organ systems.

Differentiation of chondrocytes and scleroblasts during dorsal fin skeletogenesis in flounder larvae

In teleosts, the embryonic fin fold consists of a peridermis, an underlying epidermis and a small number of mesenchymal cells. Beginning from such a simple structure, the fin skeletons, including the proximal and distal radials and lepidotrichia (finrays), develop in the dorsal fin fold at the larval stage. Their process of skeletogenesis and embryonic origin are unclear. Using flounder larvae, we report the differentiation process for chondrocytes and scleroblasts prior to fin skeletogenesis and the effects of retinoic acid (RA) on it. In early larvae, the mesenchymal cells grow between the epidermis and spinal cord to form a line of periodical condensations, which are proximal radial primordia, to produce chondrocytes. The prescleroblasts, which ossify the proximal radial cartilages, differentiate in the mesenchymal cells remaining between the cartilages. Then, mesenchymal condensations occur between the distal ends of the proximal radials, forming distal radial primordia, to produce chondrocytes. Simultaneously, condensations occur between the distal radial primordia and peridermis, which are lepidotrichia primordia, to produce prescleroblasts. Exogenous RA specifically inhibits the mesenchymal condensation prior to the proximal radial formation together with the down-regulation of sonic hedgehog (shh) and patched (pta) expression, resulting in the loss of proximal radials. Thus, it was indicated that differentiation of the precursor cells of radials and lepidotrichia begins in the proximal part of the fin fold and that the initial mesenchymal condensation prior to the proximal radial formation is highly susceptible to the effects of RA. Lepidotrichia formation does not occur where proximal radials are absent, indicating that lepidotrichia differentiation requires interaction with the radial cartilages. To examine the suggestion that neural crest cells contribute to the medial fin skeletons, we localized the HNK-1 positive cells in flounder embryos and slug and msxb-positive cells in pufferfish, Fugu rubripes, embryos. That the positive cells commonly arrive at the proximal part of the fin fold does not contradict the suggestion, but their final destiny as radial chondrocytes or lepidotrichia scleroblasts, should be further investigated.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Retinoic acid signalling centres in the avian embryo identified by sites of expression of synthesising and catabolising enzymes

Retinoic acid is an important signalling molecule in the developing embryo, but its precise distribution throughout development is very difficult to determine by available techniques. Examining the distribution of the enzymes by which it is synthesised by using in situ hybridisation is an alternative strategy. Here, we describe the distribution of three retinoic acid synthesising enzymes and one retinoic acid catabolic enzyme during the early stages of chick embryogenesis with the intention of identifying localized retinoic acid signalling regions. The enzymes involved are Raldh1, Raldh2, Raldh3, and Cyp26A1. Although some of these distributions have been described before, here we assemble them all in one species and several novel sites of enzyme expression are identified, including Hensen's node, the cardiac endoderm, the presumptive pancreatic endoderm, and the dorsal lens. This study emphasizes the dynamic pattern of expression of the enzymes that control the availability of retinoic acid as well as the role that retinoic acid plays in the development of many regions of the embryo throughout embryogenesis. This strategy provides a basis for understanding the phenotypes of retinoic acid teratology and retinoic acid-deficiency syndromes.

Molecular genetics of renal development

The understanding of the complex branching morphogenesis of the early kidney is at an early stage; however, a framework is emerging that suggests numerous active genes sequentially switching on and reciprocally influencing each other. Much of our understanding of this process comes from studies of rodents specifically engineered to lack a particular gene responsible for an inductive agent or receptor. This review attempts to place newly discovered genetic programs within an organized framework of sequential renal development.

Expression of rigf, a member of avian VEGF family, correlates with vascular patterning in the developing chick limb bud

In a differential display screening for genes regulated by retinoic acid in the developing chick limb bud, we have isolated a novel gene, termed rigf, retinoic-acid induced growth factor, that encodes a protein belonging to the vascular endothelial growth factor (VEGF) family. Rigf transcripts were found in the posterior region of the limb bud in a region-specific manner as well as in other embryonic tissues and regions, including the notochord, head and trunk mesenchyme, retinal pigment epithelium, and branchial arches. Several manipulations revealed that retinoic acid and sonic hedgehog signaling pathways regulate rigf expression in the limb bud. VEGF family members, which promote the migration, differentiation and proliferation of endothelial cells in both blood and lymphatic vessels, are important factors for the formation of blood and lymphatic vasculatures during development. We demonstrated that the anterior border of the rigf expression domain in the limb bud corresponds with the position of the primary central artery (the subclavian artery in the forelimb), which is a main artery for supplying blood to the limb. These observations taken together with results from some experimental manipulations suggest that the limb tissue attracts blood vessels into the limb bud and that rigf is involved in the pattern formation of blood vessels in the limb.

A role for retinoic acid in regulating the regeneration of deer antlers

Deer antlers are the only mammalian organs that can be repeatedly regenerated; each year, these complex structures are shed and then regrow to be used for display and fighting. To date, the molecular mechanisms controlling antler regeneration are not well understood. Vitamin A and its derivatives, retinoic acids, play important roles in embryonic skeletal development. Here, we provide several lines of evidence consistent with retinoids playing a functional role in controlling cellular differentiation during bone formation in the regenerating antler. Three receptors (alpha, beta, gamma) for both the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families show distinct patterns of expression in the growing antler tip, the site of endochondral ossification. RAR alpha and RXR beta are expressed in skin ("velvet") and the underlying perichondrium. In cartilage, which is vascularised, RXR beta is specifically expressed in chondrocytes, which express type II collagen, and RAR alpha in perivascular cells, which also express type I collagen, a marker of the osteoblast phenotype. High-performance liquid chromatography analysis shows significant amounts of Vitamin A (retinol) in antler tissues at all stages of differentiation. The metabolites all-trans-RA and 4-oxo-RA are found in skin, perichondrium, cartilage, bone, and periosteum. The RXR ligand, 9-cis-RA, is found in perichondrium, mineralised cartilage, and bone. To further define sites of RA synthesis in antler, we immunolocalised retinaldehyde dehydrogenase type 2 (RALDH-2), a major retinoic acid-generating enzyme. RALDH-2 is expressed in the skin and perichondrium and in perivascular cells in cartilage, although chondroprogenitors and chondrocytes express very low levels. At sites of bone formation, differentiated osteoblasts which express the bone-specific protein osteocalcin express high levels of RALDH2. The effect of RA on antler cell differentiation was studied in vitro; all-trans-RA inhibits expression of the chondrocyte phenotype, an effect that is blocked by addition of the RAR antagonist Ro41-5253. In monolayer cultures of mesenchymal progenitor cells, all-trans-RA increases the expression of alkaline phosphatase, a marker of the osteoblastic phenotype. In summary, this study has shown that antler tissues contain endogenous retinoids, including 9-cis RA, and the enzyme RALDH2 that generates RA. Sites of RA synthesis in antler correspond closely with the localisation of cells which express receptors for these ligands and which respond to the effects of RA.

Efficient ectopic gene expression targeting chick mesoderm

The chick model has been instrumental in illuminating genes that regulate early vertebrate development and pattern formation. Targeted ectopic gene expression is critical to dissect further the complicated gene interactions that are involved. In an effort to develop a consistent method to ectopically introduce and focally express genes in chick mesoderm, we evaluated and optimized several gene delivery methods, including implantation of 293 cells laden with viral vectors, direct adenoviral injection, and electroporation (EP). We targeted the mesoderm of chick wing buds between stages 19 and 21 (Hamburger and Hamilton stages) and used beta-galactosidase and green fluorescent protein (GFP) to document gene transfer. Expression constructs using the cytomegalovirus (CMV) promoter, the beta-actin promoter, and vectors with an internal ribosomal entry sequence linked to GFP (IRES-GFP) were also compared. After gene transfer, we monitored expression for up to 3 days. The functionality of ectopic expression was demonstrated with constructs containing the coding sequences for Shh, a secreted signaling protein, or Hoxb-8, a transcription factor, both of which can induce digit duplication when ectopically expressed in anterior limb mesoderm. We identified several factors that enhance mesodermal gene transfer. First, the use of a vector with the beta-actin promoter coupled to the 69% fragment of the bovine papilloma virus yielded superior mesodermal expression both by markers and functional results when compared with several CMV-driven vectors. Second, we found the use of mineral oil to be an important adjuvant for EP and direct viral injection to localize and contain vector within the mesoderm at the injection site. Lastly, although ectopic expression could be achieved with all three methods, we favored EP confined to the mesoderm with insulated microelectrodes (confined microelectroporation- CMEP), because vector construction is rapid, the method is efficient, and results were consistent and reproducible.

Mutual genetic antagonism involving GLI3 and dHAND prepatterns the vertebrate limb bud mesenchyme prior to SHH signaling

The bHLH transcription factor dHAND is required for establishment of SHH signaling by the limb bud organizer in posterior mesenchyme, a step crucial to development of vertebrate paired appendages. We show that the transcriptional repressor GLI3 restricts dHAND expression to posterior mesenchyme prior to activation of SHH signaling in mouse limb buds. dHAND, in turn, excludes anterior genes such as Gli3 and Alx4 from posterior mesenchyme. Furthermore, genetic interaction of GLI3 and dHAND directs establishment of the SHH/FGF signaling feedback loop by restricting the BMP antagonist GREMLIN posteriorly. These interactions polarize the nascent limb bud mesenchyme prior to SHH signaling.

Patterning mechanisms controlling vertebrate limb development

Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anterior-posterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-beta/BMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.

Differential response of Shh expression between chick forelimb and hindlimb buds by FGF-4

The interactions of Sonic hedgehog (Shh) and fibroblast growth factor (FGF) play important roles in vertebrate limb pattern formation. In the posterior region of the chick limb bud, Shh and FGF-4 each maintain expression in a positive feedback loop. In the anterior region, Shh can also induce Fgf-4 expression in the anterior apical ectodermal ridge. However, the possibility of Shh induction by FGF protein is unclear. Because many experiments to analyze gene expression have been carried out by using the forelimb bud of the chick embryo, we investigated gene expression of the cells in the anterior region of the chick hindlimb bud after FGF-4 application and compared the results with those for the forelimb bud. When an FGF-4-containing bead was implanted into the anterior region of a stage 20 hindlimb bud, ectopic expression of Shh was induced in the mesenchyme beneath the anterior end of the apical ectodermal ridge at 36 hr after implantation. Subsequent to Shh activation, Hoxd13 was also observed in the anterior-distal region of the limb bud. Furthermore, FGF-4 implantation to the hindlimb bud caused additional digit formation accompanying respecification of positional value in the anterior tissue. Ectopic Shh was induced in cells located distal to the FGF-4 bead, and the cells of the flank region did not contribute to ectopic Shh induction. On the other hand, no ectopic Shh and Hoxd13 expression was detected by grafting an FGF-4 bead into the forelimb bud. Although FGF-4 implantation to the forelimb bud occasionally induced extra digit 2 formation, no embryos had an extra digit 3 or digit 4, and many specimens exhibited normal skeletal pattern. These results demonstrate the difference between the fore- and hindlimb buds in the cell competence of Shh induction in response to FGF-4, suggesting the possibility that the responsiveness of mesenchymal cells in signaling molecules is not the same in the fore- and hindlimb buds.

Differential expression of chicken CYP26 in anterior versus posterior limb bud in response to retinoic acid

Multiple studies indicate that quantitative control of the levels of all-trans-retinoic acid (RA) in the vertebrate embryo is necessary for correct development. The function of RA in cells is regulated by a number of coordinated mechanisms. One of those mechanisms involves controls on the rate of RA catabolism. Recently, enzymes capable of catabolizing RA were found to constitute a new family, called CYP26, within the cytochrome P450 superfamily. CYP26 homologues have been isolated from human, mouse, zebra fish, and recently from the chick. In this study, we examined the regulation of chicken CYP26 (cCYP26) expression by RA during the early phase of chick limb outgrowth. In the anterior limb mesenchyme and apical ectodermal ridge (AER), cCYP26 expression was induced in a concentration dependent manner by implanting beads soaked in 0.1, 1, and 5 mg/ml RA. The RA-induced expression of cCYP26 in anterior limb mesenchyme and the AER was detected as early as 1 hr after treatment and was not affected by the presence of cycloheximide. In contrast to the anterior limb, the induction of cCYP26 was dramatically reduced (or absent) when RA beads were implanted in the posterior limb mesenchyme. Furthermore, induction of cCYP26 expression in the anterior mesenchyme was inhibited by transplantations of the zone of polarizing activity (ZPA) and by Shh-soaked beads. Our data suggest that different mechanisms regulate retinoid homeostasis in the AER and mesenchyme during limb bud outgrowth. J. Exp. Zool. 290:136-147, 2001.

Cell biology of limb patterning

Of vertebrate organ systems, the developing limb has been especially well characterized. Morphological studies have yielded a wealth of information describing limb outgrowth and have allowed for the identification of a multitude of important factors. In terms of the latter, key signaling pathways are known to control numerous aspects of limb development, including establishment of the early limb field, determination of limb identity, elongation of the limb bud, specification of digit pattern, and sculpting of the digits. Modification of underlying signaling pathways can thus result in dramatic alterations of the limb phenotype, accounting for many of the diverse limb patterns observed in nature. Given this, it is clear that signaling pathways regulate the highly orchestrated and tightly controlled sequence of cellular events necessary for limb outgrowth; however, exactly how molecular signals interface with the cell biology of limb development remains largely a mystery. In this review we first provide an overview of a number of the morphogenetic signaling pathways that have been identified in the developing limb and then review how a subset of these signals are known to modify cell behaviors important for limb outgrowth.

Perspectives on the evolutionary origin of tetrapod limbs

The study of the origin and evolution of the tetrapod limb has benefited enormously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponentially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan.

Evidence that members of the Cut/Cux/CDP family may be involved in AER positioning and polarizing activity during chick limb development

In vertebrates, the apical ectodermal ridge (AER) is a specialized epithelium localized at the dorsoventral boundary of the limb bud that regulates limb outgrowth. In Drosophila, the wing margin is also a specialized region located at the dorsoventral frontier of the wing imaginal disc. The wingless and Notch pathways have been implicated in positioning both the wing margin and the AER. One of the nuclear effectors of the Notch signal in the wing margin is the transcription factor cut. Here we report the identification of two chick homologues of the Cut/Cux/CDP family that are expressed in the developing limb bud. Chick cux1 is expressed in the ectoderm outside the AER, as well as around ridge-like structures induced by (β)-catenin, a downstream target of the Wnt pathway. cux1 overexpression in the chick limb results in scalloping of the AER and limb truncations, suggesting that Cux1 may have a role in limiting the position of the AER by preventing the ectodermal cells around it from differentiating into AER cells. The second molecule of the Cut family identified in this study, cux2, is expressed in the pre-limb lateral plate mesoderm, posterior limb bud and flank mesenchyme, a pattern reminiscent of the distribution of polarizing activity. The polarizing activity is determined by the ability of a certain region to induce digit duplications when grafted into the anterior margin of a host limb bud. Several manipulations of the chick limb bud show that cux2 expression is regulated by retinoic acid, Sonic hedgehog and the posterior AER. These results suggest that Cux2 may have a role in generating or mediating polarizing activity. Taking into account the probable involvement of Cut/Cux/CDP molecules in cell cycle regulation and differentiation, our results raise the hypothesis that chick Cux1 and Cux2 may act by modulating proliferation versus differentiation in the limb ectoderm and polarizing activity regions, respectively.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

The bHLH transcription factor dHAND controls Sonic hedgehog expression and establishment of the zone of polarizing activity during limb development

Limb outgrowth and patterning of skeletal elements are dependent on complex tissue interactions involving the zone of polarizing activity (ZPA) in the posterior region of the limb bud and the apical ectodermal ridge. The peptide morphogen Sonic hedgehog (SHH) is expressed specifically in the ZPA and, when expressed ectopically, is sufficient to mimic its functions, inducing tissue growth and formation of posterior skeletal elements. We show that the basic helix-loop-helix transcription factor dHAND is expressed posteriorly in the developing limb prior to Shh and subsequently occupies a broad domain that encompasses the Shh expression domain. In mouse embryos homozygous for a dHAND null allele, limb buds are severely underdeveloped and Shh is not expressed. Conversely, misexpression of dHAND in the anterior region of the limb bud of transgenic mice results in formation of an additional ZPA, revealed by ectopic expression of Shh and its target genes, and resulting limb abnormalities that include preaxial polydactyly with duplication of posterior skeletal elements. Analysis of mouse mutants in which Hedgehog expression is altered also revealed a feedback mechanism in which Hedgehog signaling is required to maintain the full dHAND expression domain in the developing limb. Together, these findings identify dHAND as an upstream activator of Shh expression and important transcriptional regulator of limb development.

Role of dHAND in the anterior-posterior polarization of the limb bud: implications for the Sonic hedgehog pathway

dHAND is a basic helix-loop-helix (bHLH) transcription factor essential for cardiovascular development. Here we analyze its pattern of expression and functional role during chick limb development. dHAND expression was observed in the lateral plate mesoderm prior to emergence of the limb buds. Coincident with limb initiation, expression of dHAND became restricted to the posterior half of the limb bud. Experimental procedures that caused mirror-image duplications of the limb resulted in mirror-image duplications of the pattern of dHAND expression along the anterior-posterior axis. Retroviral overexpression of dHAND in the limb bud produced preaxial polydactyly, corresponding to mild polarizing activity at the anterior border. At the molecular level, misexpression of dHAND caused ectopic activation of members of the Sonic hedgehog (Shh) pathway, including Gli and Patched, in the anterior limb bud. A subset of infected embryos displayed ectopic anterior activation of Shh. Other factors implicated in anterior-posterior polarization of the bud such as the most 5′ Hoxd genes and Bmp2 were also ectopically activated at the anterior border. Our results indicate a role for dHAND in the establishment of anterior-posterior polarization of the limb bud.

All-trans retinoic acid enhances the long-term repopulating activity of cultured hematopoietic stem cells

The retinoic acid receptor (RAR) agonist, all-trans retinoic acid (ATRA), is a potent inducer of terminal differentiation of malignant promyelocytes, but its effects on more primitive hematopoietic progenitors and stem cells are less clear. We previously reported that pharmacologic levels (1 μmol) of ATRA enhanced the generation of colony-forming cell (CFC) and colony-forming unit-spleen (CFU-S) in liquid suspension cultures of lin− c-kit+ Sca-1+ murine hematopoietic precursors. In this study, we further investigated the effects of ATRA as well as an RAR antagonist, AGN 193109, on the generation of transplantable cells, including pre–CFU-S, short-term repopulating stem cells (STRCs), and long-term repopulating stem cells (LTRCs). ATRA enhanced the ex vivo maintenance and production of competitive repopulating STRCs and LTRCs from lin− c-kit+ Sca-1+ cells cultured in liquid suspension for 14 days. In addition, ATRA prevented the differentiation of these primitive stem cells into more mature pre–CFU-S during the 14 days of culture. In marked contrast, lin− c-kit+ Sca-1+ cells cultured with AGN 193109 for 7 days had virtually no short- or long-term repopulating ability, but displayed an approximately 6-fold increase in the pre–CFU-S population. The data suggest that the RAR agonist ATRA enhances the maintenance and self-renewal of short- and long-term repopulating stem cells. In contrast, the RAR antagonist AGN 193109 abrogates reconstituting ability, most likely by promoting the differentiation of the primitive stem cells. These results imply an important and unexpected role of retinoids in regulating hematopoietic stem cell differentiation.

Retinoic acid is essential for Shh/Hoxd signaling during rat limb outgrowth but not for limb initiation

Retinoids long have been implicated in limb development and their endogenous contributions to this process are finally being elucidated. Here we use an established model of retinoid depletion during specific gestational windows to investigate the role of endogenous retinoic acid (RA) in supporting limb outgrowth. Rat embryos were deprived of RA starting at days-postcoitum (dpc) 3.0, 5.5, or 7.0 and harvested at the 35-somite stage (dpc 12-12.5). Although embryos from all these windows possessed many characteristics of gestational retinoid deficiency (frontonasal hypoplasia, straight tail, reduced CRBPI and RAR beta), their limb buds emerged with only modest size reductions. Molecular analysis of RA-deficient limb buds revealed enhanced gli-3 and reduced hoxd-12, hoxd-13, shh, and fgf-4, while fgf-8, en-1, and wnt-7a expression remained unaltered. Occasional posterior truncations were observed at low incidence in the longest deficiency window; otherwise, the deficiency window length had no discernable impact on the severity of these changes. At the 45-somite stage, RA-deficient limbs had additional losses of hoxd-13 and fgf-8, accompanied by a flattened AER, suggestive of an ultimate failure in limb bud outgrowth. Results could not confirm a function for endogenous retinoids in limb initiation, but show they are required to maintain the signaling loops between the developing mesenchyme and AER that govern limb outgrowth after the initial emergence of limb bud.

Complementary domains of retinoic acid production and degradation in the early chick embryo

Excess retinoids as well as retinoid deprivation cause abnormal development, suggesting that retinoid homeostasis is critical for proper morphogenesis. RALDH-2 and CYP26, two key enzymes that carry out retinoic acid (RA) synthesis and degradation, respectively, were cloned from the chick and show significant homology with their orthologs in other vertebrates. Expression patterns of RALDH-2 and CYP26 genes were determined in the early chick embryo by in situ hybridization. During gastrulation and neurulation RALDH-2 and CYP26 were expressed in nonoverlapping regions, with RALDH-2 transcripts localized to the presumptive presomitic and lateral plate mesoderm and CYP26 mRNA to the presumptive mid- and forebrain. The two domains of expression were separated by an approximately 300-micrometer-wide gap, encompassing the presumptive hindbrain. In the limb region, a similar spatial segregation of RALDH-2 and CYP26 expression was found at stages 14 and 15. Limb region mesoderm expressed RALDH-2, whereas the overlying limb ectoderm expressed CYP26. RA-synthesizing and -degrading enzymatic activities were measured biochemically in regions expressing RALDH-2 or CYP26. Regions expressing RALDH-2 generated RA efficiently from precursor retinal but degraded RA only inefficiently. Conversely, tissue expressing CYP26 efficiently degraded but did not synthesize RA. Localized regions of RA synthesis and degradation mediated by these two enzymes may therefore provide a mechanism to regulate RA homeostasis spatially in vertebrate embryos.

Differential distribution of retinoic acid synthesis in the chicken embryo as determined by immunolocalization of the retinoic acid synthetic enzyme, RALDH-2

Retinaldehyde dehydrogenase type 2 (RALDH-2) is a major retinoic acid generating enzyme in the early embryo. Here we report the immunolocalization of this enzyme (RALDH-2-IR) in stage 6-29 chicken embryos; we also show that tissues that exhibit strong RALDH-2-IR in the embryo contain RALDH-2 and synthesize retinoic acid. RALDH-2-IR indicates dynamic and discrete patterns of retinoic acid synthesis in the embryo, particularly within the somitic mesoderm, lateral mesoderm, kidney, heart, and spinal motor neurons. Prior to somitogenesis, RALDH-2-IR is present in the paraxial mesoderm with a rostral boundary at the level of the presumptive first somite; as the somites form, they exhibit strong RALDH-2-IR. Cervical presomitic mesoderm exhibits RALDH-2-IR but thoracic presomitic mesoderm does not. Neural crest cells do not express detectable levels of RALDH-2, but migrating crest cells are associated with RALDH-2 expressing mesoderm. The developing limb mesoderm expresses little RALDH-2-IR; however, RALDH-2-IR is strongly expressed in tissues adjacent to the limb. The most lateral, earliest-projecting motor neurons at all levels of the spinal cord exhibit RALDH-2-IR. Subsequently, many additional motor neurons in the brachial and lumbar cord regions express RALDH-2-IR. Motor neuronal expression of RALDH-2-IR is present in the growing axons as they extend to the periphery, indicating a potential role of retinoic acid in nerve influences on peripheral differentiation. With the exception of a transient expression in the facial/vestibulocochlear nucleus, cranial motor neurons do not express detectable levels of RALDH-2-IR.