A Molecular Clock Operates During Chick Autopod Proximal-distal Outgrowth

Temporal control can be considered the fourth dimension in embryonic development. The identification of the somitogenesis molecular clock provided new insight into how embryonic cells measure time. We provide the first evidence of a molecular clock operating during chick fore-limb autopod outgrowth and patterning, by showing that the expression of the somitogenesis clock component hairy2 cycles in autopod chondrogenic precursor cells with a 6 h periodicity. We determined the length of time required to form an autopod skeletal limb element, and established a correlation between the latter and the periodicity of cyclic hairy2 gene expression. We suggest that temporal control exerted by cyclic gene expression can be a widespread mechanism providing cellular temporal information during vertebrate embryonic development.

Manipulations of PKA in chick limb development reveal roles in digit patterning including a positive role in Sonic Hedgehog signaling

Sonic Hedgehog (Shh) signaling by the polarizing region, at the posterior of the vertebrate limb bud, is pivotal in determining digit number and identity. Shh establishes a gradient of the bifunctional transcriptional effector, Gli3, with high levels of full-length activator (Gli3A) in the posterior bud, where digits form, and high levels of shorter repressor (Gli3R) in the anterior. Repressor formation depends on protein kinase A (PKA), but in Drosophila, PKA also plays a role in activator function. Increasing PKA levels in chick limb development using Forskolin had no effect on posterior polarizing activity but weak polarizing activity, based on ligand-independent Shh signaling, was induced in anterior limb bud cells resulting in extra digits. Manipulating PKA activity levels directly with a retrovirus expressing activated PKA induced extra digits similar to those induced by Forskolin treatment suggesting that PKA may have a previously unrecognized positive role in Shh signaling in vertebrate limbs. Expressing dominant negative PKA also induced extra, sometimes multiple digits, from anterior limb bud demonstrating the negative role in Shh signaling. PKA levels in the limb bud are high posteriorly and low anteriorly, suggesting that PKA activity may influence the outcome of Shh signaling in normal development.

Tails of the unexpected: palatal medial edge epithelium is no more specialized than other embryonic epithelium

OBJECTIVE

To determine whether palatal medial edge epithelium (MEE) is specialized in its ability to disappear compared with other embryonic, non-palatal, epithelium.

SUBJECTS

Embryonic tissues harvested from CD1 mice.

METHODS

Organs were cultured in 2 ml of DMEM/F12 supplemented with 300 microg/ml L-glutamine and 1% penicillin/streptomycin. Organs were cultured under various conditions including opposing other organs and opposing an inert material for a period of 6 days. Tissues were then processed for histological examination.

RESULTS

MEE of shelves opposing nothing persisted, whereas MEE of shelves contacting another shelf disappeared. When a tail was placed against a palatal shelf the MEE disappeared, as did the epithelium from the tail, resulting in fusion between the shelf and tail. Furthermore, when palatal shelves were placed against an inert material the MEE disappeared, suggesting pressure alone is a sufficient stimulus to initiate disappearance of the MEE, and that the interaction between the two palatal shelves is not a prerequisite for the disappearance of MEE. Moreover, when two embryonic tails were cultured in close apposition they fused, as did paired limbs. Non-palatal epithelia also disappeared after contact with inert materials. Epithelial disappearance began within 24 h of contact, but there was an age limit.

CONCLUSION

These findings suggest that embryonic epithelium from non-specific sites around the body has the ability to disappear with mechanical contact resulting in fusion of tissues. MEE may not be as specialized as once thought.

Characterization ofXenopusdigits and regenerated limbs of the froglet

Xenopus has 4 and 5 digits in a forelimb and hindlimb, respectively. It is thought that their limbs and digits develop in Xenopus by mechanisms that are almost conserved from amphibians to higher vertebrates. This is supported by some molecular evidence. The 5'hoxd genes are convenient marker genes for characterizing digits in the chick and mouse. The anteriormost digit is characterized by being hoxd13-positive and hoxd12 (hoxd11)-negative in the chick and mouse. In this study, we revealed that the anteriormost digit of the Xenopus forelimb is hoxd13-positive and hoxd11-positive, that is, a more posterior character than digit I. The order of formation of digit cartilages also suggested that Xenopus forelimb digit identity is II to V, not I to IV. We have also been interested in the relationship between digit identity and shh. The anteriormost digit develops in a shh-independent way. A limb treated with cyclopamine (a shh inhibitor) has a gene expression pattern (hoxd11-negative) similar to that in shh-deficient mice, suggesting that a hindlimb treated with cyclopamine has a digit I character. However, a Xenopus froglet regenerate (spike), which lacks shh expression during its regeneration process, does not have such an expression pattern, being hoxd11-positive. We investigated hoxd11 transcriptions in blastemas that formed in the anteriormost and posteriormost digits, and we found that the blastemas have different hoxd11 expression levels. These findings suggest that the froglet limb blastema does not have a mere digit I character in spite of shh defectiveness and that the froglet limb blastema recognizes its positional differences along the anterior-posterior axis.

Tbx5 is dispensable for forelimb outgrowth

Tbx5 is essential for initiation of the forelimb, and its deletion in mice results in the failure of forelimb formation. Misexpression of dominant-negative forms of Tbx5 results in limb truncations, suggesting Tbx5 is also required for forelimb outgrowth. Here we show that Tbx5 is expressed throughout the limb mesenchyme in progenitors of cartilage, tendon and muscle. Using a tamoxifeninducible Cre transgenic line, we map the time frame during which Tbx5 is required for limb development. We show that deletion of Tbx5 subsequent to limb initiation does not impair limb outgrowth. Furthermore, we distinguish two distinct phases of limb development: a Tbx5-dependent limb initiation phase, followed by a Tbx5-independent limb outgrowth phase. In humans, mutations in the T-box transcription factor TBX5 are associated with the dominant disorder Holt-Oram syndrome (HOS), which is characterised by malformations in the forelimb and heart. Our results demonstrate a short temporal requirement for Tbx5 during early limb development, and suggest that the defects found in HOS arise as a result of disrupted TBX5 function during this narrow time window.

TCDD-induced alterations in gene expression profiles of the developing mouse paw do not influence morphological differentiation of this potential target tissue

The aryl-hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxicity of certain halogenated aromatic hydrocarbons including 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD). These compounds are potent developmental toxicants that can alter gene expression and disrupt processes of proliferation and differentiation. It has not yet been determined which tissues during development are most sensitive to these compounds, nor which genes are directly associated with the toxicities. We developed a transgenic (TG) mouse model to delineate the temporal and spatial context of transcriptionally active AhR by utilizing a dioxin responsive element-linked LacZ reporter system. The present study focuses on the pattern of TCDD-induced transgene expression localized to the footpad and digits of the paws between gestational days (GD) 13 and 18. Paw morphology was evaluated at several developmental stages following TCDD exposure. Gene expression profiles acquired by microarray technology were evaluated in the paws of fetuses exposed at GD 14.5. The results showed that TCDD exposure in utero induced LacZ expression in the developing paws. This expression appeared to be localized to the mesenchymal cell layer. Gross morphological changes were not observed in the paws prior to or after birth following TCDD exposure in utero. However, significant alterations in gene expression profiles in the developing paws were observed at 24 h following TCDD exposure in utero. These results indicate that the developing paw is a target tissue of TCDD in terms of altered gene expression, further validating the use of this AhR responsive reporter gene TG mouse model in studying AhR ligand-mediated responsiveness. However, the linkage of these changes to detectable biological outcomes in the paw remains unclear.

Interdigital webbing retention in bat wings illustrates genetic changes underlying amniote limb diversification

Developmentally regulated programmed cell death sculpts the limbs and other embryonic organs in vertebrates. One intriguing example of species-specific differences in apoptotic extent is observed in the tissue between the digits. In chicks and mice, bone morphogenetic proteins (Bmps) trigger apoptosis of the interdigital mesenchyme, leading to freed digits, whereas in ducks, Bmp antagonists inhibit the apoptotic program, resulting in webbed feet. Here, we show that the phyllostomid bat Carollia perspicillata utilizes a distinct mechanism for maintaining interdigit tissue. We find that bat forelimb and hindlimb interdigital tissues express Bmp signaling components but that only bat hindlimbs undergo interdigital apoptosis. Strikingly, the retention of interdigital webbing in the bat forelimb is correlated with a unique pattern of Fgf8 expression in addition to the Bmp inhibitor Gremlin. By using a functional assay, we show that maintenance of interdigit tissue in the bat wing depends on the combined effects of high levels of Fgf signaling and inhibition of Bmp signaling. Our data also indicate that although there is not a conserved mechanism for maintaining interdigit tissue across amniotes, the expression in the bat forelimb interdigits of Gremlin and Fgf8 suggests that these key molecular changes contributed to the evolution of the bat wing.

Microarray analysis on Runx2-deficient mouse embryos reveals novel Runx2 functions and target genes during intramembranous and endochondral bone formation

A major challenge in developmental biology is to correlate genome-wide gene expression modulations with developmental processes in vivo. In this study, we analyzed the role of Runx2 during intramembranous and endochondral bone development, by comparing gene expression profiles in 14.5 dpc wild-type and Runx2 (-/-) mice. A total of 1277, 606 and 492 transcripts were found to be significantly modulated by Runx2 in calvaria, forelimbs and hindlimbs, respectively. Bioinformatics analysis indicated that Runx2 not only controls the processes of osteoblast differentiation and chondrocyte maturation, but may also play a role in axon formation and hematopoietic cell commitment during bone development. A total of 41 genes are affected by the Runx2 deletion in both intramembranous and endochondral bone, indicating common pathways between these two developmental modes of bone formation. In addition, we identified genes that are specifically involved in endochondral ossification. In conclusion, our data show that a comparative genome-wide expression analysis of wild-type and mutant mouse models allows the examination of mutant phenotypes in complex tissues.

Invariant mantling of growth cones by Schwann cell precursors characterize growing peripheral nerve fronts

Little is known about the cytoarchitecture of growth fronts in developing mammalian nerves. We report here the first quantitative, ultrastructural analysis of growth cones (GCs) and their immediate cellular and tissue environment at tips of growing nerves that are nearing their targets in fore limbs of E14 rat embryos. Schwann cell precursor (SCP) marker, p75 neurotrophin receptor, and growth cone marker, SCG10, were used to identify nerve fronts, respectively. Using confocal 3D reconstructions and immunoelectron microscopy, we found that growth cone and Schwann cell precursor migrate together at the nerve front, where growth cone contact adjacent growth cone and Schwann cell precursor with similar frequency. Schwann cell precursor are extensively connected by adherens junctions and form elaborate scaffolds that enmantle growth cone at nerve fronts, so that 80% of the nerve front surface is covered by Schwann cell precursor. Although they interdigitate in complex ways among growth cone, the total contact area between growth cone and glial membranes is remarkably constant among the 100 growth fronts analyzed. In contrast to this consistency, other growth cone contacts varied markedly from front to front such that the frequencies of GC-GC contacts are increasing proportional to their decreasing contacts with mesenchymal tissue. Thus, at the nerve front, it is the Schwann cell precursor that are most exposed to extracellular environment while forming a surprisingly invariant substrate for advancing growth cone. This study shows for the first time that Schwann cell precursor are close and consistent cellular companions of growth cone in their approach to their final targets in the developing limb and suggests a previously unappreciated role for Schwann cell precursor in growth cone advance through the limb mesenchyme.

Prdm1 acts downstream of a sequential RA, Wnt and Fgf signaling cascade during zebrafish forelimb induction

Vertebrate limb induction is triggered in the lateral plate mesoderm (LPM)by a cascade of signaling events originating in the axial mesoderm. While it is known that Fgf, Wnt and retinoic acid (RA) signals are involved in this cascade, their precise regulatory hierarchy has not been determined in any species. tbx5 is the earliest gene expressed in the limb bud mesenchyme. Recently, another transcription factor, Prdm1, has been shown to be crucial for zebrafish forelimb development. Here, we show that Prdm1 is downstream of RA, Wnt2b and Tbx5 activity. We find that RA activity, but not Fgf signaling, is necessary for wnt2b expression. Fgf signaling is required for prdm1 expression in the fin bud, but is not necessary for the initiation of tbx5 expression. We propose a model in which RA signaling from the somitic mesoderm leads to activation of wnt2bexpression in the intermediate mesoderm, which then signals to the LPM to trigger tbx5 expression. tbx5 is required for Fgf signaling in the limb bud leading to activation of prdm1 expression, which in turn is required for downstream activation of fgf10 expression.

Wnt9a signaling is required for joint integrity and regulation of Ihh during chondrogenesis

Joints, which separate skeleton elements, serve as important signaling centers that regulate the growth of adjacent cartilage elements by controlling proliferation and maturation of chondrocytes. Accurate chondrocyte maturation is crucial for endochondral ossification and for the ultimate size of skeletal elements, as premature or delayed maturation results predominantly in shortened elements. Wnt9a has previously been implicated as being a player in joint induction, based on gain-of function experiments in chicken and mouse. We show that loss of Wnt9a does not affect joint induction, but results to synovial chondroid metaplasia in some joints. This phenotype can be enhanced by removal of an additional Wnt gene, Wnt4, suggesting that Wnts are playing a crucial role in directing bi-potential chondro-synovioprogenitors to become synovial connective tissue, by actively suppressing their chondrogenic potential. Furthermore, we show that Wnt9a is a temporal and spatial regulator of Indian hedgehog (Ihh), a central player of skeletogenesis. Loss of Wnt9a activity results in transient downregulation of Ihh and reduced Ihh-signaling activity at E12.5-E13.5. The canonical Wnt/beta-catenin pathway probably mediates regulation of Ihh expression in prehypertrophic chondrocytes by Wnt9a, because embryos double-heterozygous for Wnt9a and beta-catenin show reduced Ihh expression, and in vivo chromatin immunoprecipitation demonstrates a direct interaction between the beta-catenin/Lef1 complex and the Ihh promoter.

Mammalian motoneuron axon targeting requires receptor protein tyrosine phosphatases sigma and delta

The leukocyte common antigen-related (LAR) subfamily of receptor protein tyrosine phosphatases (RPTPs), LAR, RPTP-sigma, and RPTP-delta, regulate neuroendocrine development, axonal regeneration, and hippocampal long-term potentiation in mammals. In Drosophila, RPTPs are required for appropriate axon targeting during embryonic development. In contrast, deletion of any one of the three LAR-RPTP family members in mammals does not result in gross axon targeting defects. Both RPTP-sigma and RPTP-delta are highly expressed in the developing mammalian nervous system, suggesting they might be functionally redundant. To test this hypothesis, we generated RPTP-sigma and RPTP-delta (RPTP-sigma/delta) double-mutant mice. Although embryonic day 18.5 RPTP-sigma and RPTP-delta single-mutant embryos were viable, RPTP-sigma/delta double mutants were paralyzed, were never observed to draw a breath, and died shortly after cesarean section. RPTP-sigma/delta double mutants exhibit severe muscle dysgenesis and severe loss of motoneurons in the spinal cord. Detailed analysis of the projections of phrenic nerves in RPTP-sigma/delta double mutants indicated that these motoneuron axons emerge normally from the cervical spinal cord, but stall on reaching the diaphragm. Our results demonstrate that RPTP-sigma and RPTP-delta complement each other functionally during mammalian development, and reveal an essential contribution of RPTP-sigma and RPTP-delta to appropriate motoneuron axon targeting during mammalian axonogenesis.

Novel retinoid targets in the mouse limb during organogenesis

Bioactive retinoids are potent limb teratogens, upregulating apoptosis, decreasing chondrogenesis, and producing limb-reduction defects. To target the origins of these effects, we examined gene expression changes in the developing murine limb after 3 h of culture with teratogenic concentrations of vitamin A. Embryonic day 12 CD-1 limbs were cultured in the absence or presence of vitamin A (retinol acetate) at 1.25 and 62.5muM (n = 5). Total RNA was used to probe Atlas 1.2 cDNA arrays. Eighty-one genes were significantly upregulated by retinol exposure; among these were key limb development signaling molecules, extracellular matrix and adhesion proteins, oncogenes, and a large number of transcriptional regulators, including Eya2, Id3, Snail, and Hes1. To relate these expression changes to teratogenic outcome, the response of these four genes was assessed after culture with vitamin A and retinoid receptor antagonists that are able to rescue retinoid-induced malformations; expression levels were correlated with limb malformations. Lastly, pathways analysis revealed that a large number of the genes significantly affected by retinoid treatment are functionally linked through direct interactions. Several regulatory gene cascades emerged relevant to morphogenesis, cell-fate, and chondrogenesis; moreover, members of these cascades crosstalk with one other. These results indicate that retinoids act in a coordinated fashion to disrupt development at multiple levels. In sum, this work proposes several unifying mechanisms for retinoid-induced limb malformations, identifies novel retinoid targets, and highlights Eya2, Id3, Snail, and Hes1 as potential key teratogenic effectors.

Effects of Phosphodiesterase 5 Inhibitor on Pulmonary Vascular Reactivity in the Fetal Lamb

BACKGROUND

Nitric oxide released by pulmonary vascular endothelium is a potent vasodilator related to increased cyclic guanosine monophosphate (cGMP) content. Hydrolysis of cGMP is achieved predominately by cGMP-specific phosphodiesterases. Sildenafil is a selective phosphodiesterase-5 (PDE5) inhibitor. The purpose of the study is to assess the effects of sildenafil on pulmonary vascular circulation during the perinatal period.

METHODS

Thirty-two pregnant ewes were operated on at the end of gestation, and fetal lambs were prepared with catheters placed into the aorta, vena cava, pulmonary artery, and left atrium. An ultrasonic flow transducer and an inflatable vascular occluder were placed respectively around the left pulmonary artery and the ductus arteriosus. Fetal lambs were randomly divided into two groups: (1) sildenafil group, infused continuously with sildenafil for 24 hours at a rate of 1 mg/h; or (2) control group, infused with saline for 24 hours. After 24 hours of infusion, we compared basal pulmonary vascular resistance and the pulmonary vascular responses to increase in fetal PaO2 and to acute ductus arteriosus compression causing "shear stress."

RESULTS

Sildenafil infusion did not change mean aortic and pulmonary artery pressures, increased mean left pulmonary blood flow by 160%, and decreased pulmonary vascular resistance by 60% (p < 0.05). However, both mean flow (Q) and pulmonary vascular resistance returned to baseline values after 2 hours of sildenafil infusion. Despite similar baseline values, pulmonary vascular resistance during maternal O2 inhalation was lower in the sildenafil group than in the control group (0.21 +/- 0.03 versus 0.33 +/- 0.03 mm Hg.mL(-1).min(-1), respectively; p < 0.01). Furthermore, drop in pulmonary vascular resistance during acute ductus arteriosus compression was greater in the sildenafil group (from 0.56 +/- 0.06 to 0.26 +/- 0.04 mm Hg.mL(-1).min(-1)) than in the control group (from 0.55 +/- 0.05 to 0.39 +/- 0.03 mm Hg.mL(-1).min(-1); p < 0.01).

CONCLUSIONS

Although sildenafil induces a transient pulmonary vasodilation, it mediates a sustained change in vascular reactivity, especially to birth-related stimuli in the ovine fetal lung. These data suggest that PDE5 is involved in the regulation of pulmonary vascular reactivity during the perinatal period and may potentiate birth-related pulmonary vasodilator stimuli.

Distinct roles for secreted semaphorin signaling in spinal motor axon guidance

Neuropilins, secreted semaphorin coreceptors, are expressed in discrete populations of spinal motor neurons, suggesting they provide critical guidance information for the establishment of functional motor circuitry. We show here that motor axon growth and guidance are impaired in the absence of Sema3A-Npn-1 signaling. Motor axons enter the limb precociously, showing that Sema3A controls the timing of motor axon in-growth to the limb. Lateral motor column (LMC) motor axons within spinal nerves are defasciculated as they grow toward the limb and converge in the plexus region. Medial and lateral LMC motor axons show dorso-ventral guidance defects in the forelimb. In contrast, Sema3F-Npn-2 signaling guides the axons of a medial subset of LMC neurons to the ventral limb, but plays no major role in regulating their fasciculation. Thus, Sema3A-Npn-1 and Sema3F-Npn-2 signaling control distinct steps of motor axon growth and guidance during the formation of spinal motor connections.

Interactive effects of cadmium and all-trans-retinoic acid on the induction of forelimb ectrodactyly in C57BL/6 mice

BACKGROUND

Most toxicological studies have tested single chemical agents at relatively high doses, and fewer studies have addressed the toxic effects of chemical interactions. It is important to understand the toxicity of chemical mixtures in order to assess the more realistic risks of environmental and occupational exposures. A number of chemicals are known to induce a predominantly postaxial forelimb ectrodactyly in C57BL/6 mice, including acetazolamide, ethanol, cadmium, valproic acid, carbon dioxide, dimethadione, phenytoin, and 13-cis-retinoic acid and all-trans-retinoic acid (RA). In the present study, the interactive effects of coadministration of cadmium and RA on developing limbs were investigated.

METHODS

Pregnant C57BL/6 mice were treated with different intraperitoneal (IP) doses of cadmium chloride (CdCl2) and/or RA on gestational day (GD) 9.5, and fetuses were collected on GD 18 and double stained for examination of skeletal defects.

RESULTS

When RA was given simultaneously with cadmium, a significant increase in the incidence and severity of forelimb ectrodactyly (predominantly postaxial) was observed compared to the results with corresponding doses of cadmium or RA alone. When mice were exposed to subthreshold doses of both cadmium (0.5 mg/kg) and RA (1 mg/kg), the combined treatment exceeded the threshold, resulting in forelimb ectrodactyly in 19% of the fetuses. Moreover, coadministration of cadmium and RA at doses exceeding the respective thresholds showed a synergistic effect, that is, 92% of fetuses were found with the forelimb defect as opposed to 10% if the response were additive.

CONCLUSIONS

The findings demonstrate that concurrent exposure to these teratogens can have a synergistic effect and that subteratogenic doses may combine to exceed a threshold.

Birth and death of cells in limb development: A mapping study

Cell death and cell proliferation are basic cellular processes that need to be precisely controlled during embryonic development. The developing vertebrate limb illustrates particularly well how correct morphogenesis depends on the appropriate spatial and temporal balance between cell death and cell proliferation. Precise knowledge of the patterns of cell proliferation and cell death during limb development is required to understand how their modifications may contribute to the generation of the great diversity of limb phenotypes that result from spontaneous mutations or induced genetic manipulations. We have performed a comprehensive analysis of the patterns of cell death, assayed by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL), and cell proliferation, assayed by anti-phosphorylated histone H3 immunohistochemistry, in consecutive sections of forelimbs and hindlimbs covering an extensive period of chick and mouse limb development. Our results confirm and expand previous reports and show common and specific areas of cell death for each species. Mitotic cells were found scattered in a uniform distribution across the early limb bud, with the exception of the areas of cell death in which mitotic cells were scarce. At later stages, mitotic cells were seen more abundantly in the digital tips. The aim of the present study was to satisfy the need for organized data sets describing these processes, which will allow the side-by-side comparison between the two major model organisms of limb development, i.e., the mouse and the chick.

A framework for three-dimensional simulation of morphogenesis

We present COMPUCELL3D, a software framework for three-dimensional simulation of morphogenesis in different organisms. COMPUCELL3D employs biologically relevant models for cell clustering, growth, and interaction with chemical fields. COMPUCELL3D uses design patterns for speed, efficient memory management, extensibility, and flexibility to allow an almost unlimited variety of simulations. We have verified COMPUCELL3D by building a model of growth and skeletal pattern formation in the avian (chicken) limb bud. Binaries and source code are available, along with documentation and input files for sample simulations, at http:// compucell.sourceforge.net.

Embryos of an Early Jurassic Prosauropod Dinosaur and Their Evolutionary Significance

Articulated embryos from the Lower Jurassic Elliot Formation of South Africa are referable to the prosauropod Massospondylus carinatus and, together with other material, provide substantial insights into the ontogenetic development in this early dinosaur. The large forelimbs and head and the horizontally held neck indicate that the hatchlings were obligate quadrupeds. In contrast, adult Massospondylus were at least facultatively bipedal. This suggests that the quadrupedal gait of giant sauropods may have evolved by retardation of postnatal negative allometry of the forelimbs. Embryonic body proportions and an absence of well-developed teeth suggest that hatchlings of this dinosaur may have required parental care.

Embryonic staging system for the short-tailed fruit bat,Carollia perspicillata, a model organism for the mammalian orderChiroptera, based upon timed pregnancies in captive-bred animals

There are approximately 4,800 extant species of mammals that exhibit tremendous morphological, physiological, and developmental diversity. Yet embryonic development has been studied in only a few mammalian species. Among mammals, bats are second only to rodents with regard to species number and habitat range and are the most abundant mammals in undisturbed tropical regions. Bat development, though, remains relatively unstudied. Here, we describe and illustrate a staging series of embryonic development for the short-tailed fruit bat, Carollia perspicillata, based on embryos collected at timed intervals after captive matings. As Carollia can be readily maintained and propagated in captivity and is extremely abundant in the wild, it offers an attractive choice as a chiropteran model organism. This staging system provides a framework for studying Carollia embryogenesis and should prove useful as a guide for embryological studies of other bat species and for comparisons with other orders of mammals.

Murine teratology of fluconazole: evaluation of developmental phase specificity and dose dependence

The potential of in utero exposure to fluconazole to initiate teratogenesis was analyzed in ICR (CD-1) mice. Developmental phase specificity was determined by treating mice with single oral doses of 700 mg/kg on gestational day 8, 9, 10, 11, or 12. Control animals received vehicle on gestational days 8-12. Gestational day 10 was identified as the phase of maximal sensitivity for induction of cleft palate, the predominant teratogenic effect induced by fluconazole, with 50% of fetuses exposed on this developmental phase being affected. After treatments on gestational day 8, 9, 11, or 12, cleft palate occurred with lower frequencies: 12, 21, 28.7, and 2.7%, respectively. Examination of skeletal morphology revealed anomalies of the middle ear apparatus in 15% of the fetuses that were exposed on gestational day 8. Dysmorphic tympanic ring and absence of the incus were the more common ear anomalies recorded. Reduced humeral length was noted in 22% of fetuses that were exposed on gestational day 10. Dose-response relationship was investigated by treating animals with 0 (vehicle), 87.5, 175, or 350 mg/kg on gestational day 10, coincident with the phase of peak teratogenic sensitivity. Besides showing that fluconazole operates under a strict dose-response mechanism, the study identified 175 mg/kg as the lowest observed adverse effect level for cleft palate induction, with 7.6% of the exposed fetuses being affected.

Tbx3 can alter limb position along the rostrocaudal axis of the developing embryo

The limbs of the vertebrate embryo form at precise locations along the body and these positions are fixed across different species. The mechanisms that control this process are not understood. Ectopic expression of Tbx3, a transcriptional repressor that belongs to the Tbx2/3/4/5 subfamily of T-box transcriptional regulators, in the forelimb results in a rostral shift in the position of the limb along the main body axis. By contrast, a transcriptional activator form of Tbx3 shifts the limb to more caudal locations. We also show that dHand and Gli3, genes previously implicated in anteroposterior pre-patterning of the limb-forming region, are also involved in refining the position of the limbs. Our data suggest a new role for Tbx3 in positioning the limb along the main body axis through a genetic interplay between dHand and Gli3.

Expression of Fgf and Tgfbeta signaling related genes during embryonic endochondral ossification

Disturbed fibroblast growth factor (Fgf) and transforming growth factor beta (Tgfbeta) signaling lead to a variety of human skeletal disorders. To reveal the possible function and interaction of these signaling systems we have started to analyze the expression patterns of signaling factors, antagonists, receptors and transducers of these pathways in forelimbs of mouse embryos and compared them to the expression of established markers including Ihh. In addition to defining their expression domains in the developing bone, our study identified new subpopulations of chondrocytes characterized by the expression of distinct combinations of markers.

Early developmental arrest of mammalian limbs lacking HoxA/HoxD gene function

Vertebrate HoxA and HoxD cluster genes are required for proper limb development. However, early lethality, compensation and redundancy have made a full assessment of their function difficult. Here we describe mice that are lacking all Hoxa and Hoxd functions in their forelimbs. We show that such limbs are arrested early in their developmental patterning and display severe truncations of distal elements, partly owing to the absence of Sonic hedgehog expression. These results indicate that the evolutionary recruitment of Hox gene function into growing appendages might have been crucial in implementing hedgehog signalling, subsequently leading to the distal extension of tetrapod appendages. Accordingly, these mutant limbs may be reminiscent of an ancestral trunk extension, related to that proposed for arthropods.