Current research on mechanisms of limb bud development, and challenges for the next decade

The developmental mechanisms of limb buds have been studied in developmental biology as an excellent model of pattern formation. Chick embryos have contributed to the discovery of new principles in developmental biology, as it is easy to observe live embryos and manipulate embryonic tissues. Herein, I outline recent findings and future issues over the next decade regarding three themes, based on my research: limb positioning, proximal-distal limb elongation and digit identity determination. First, how hindlimb position is determined at the molecular level is described, with a focus on the transforming growth factor-β signaling molecule GDF11. Second, I explain how the cell population in the limb bud deforms with developmental progress, shaping the limb bud with elongation along the proximal-distal axis. Finally, I describe the developmental mechanisms that determine digit identity through the interdigits.

Direct reprogramming of non-limb fibroblasts to cells with properties of limb progenitors

The early limb bud consists of mesenchymal limb progenitors derived from the lateral plate mesoderm (LPM). The LPM also gives rise to the mesodermal components of the flank and neck. However, the cells at these other levels cannot produce the variety of cell types found in the limb. Taking advantage of a direct reprogramming approach, we find a set of factors (Prdm16, Zbtb16, and Lin28a) normally expressed in the early limb bud and capable of imparting limb progenitor-like properties to mouse non-limb fibroblasts. The reprogrammed cells show similar gene expression profiles and can differentiate into similar cell types as endogenous limb progenitors. The further addition of Lin41 potentiates the proliferation of the reprogrammed cells. These results suggest that these same four factors may play pivotal roles in the specification of endogenous limb progenitors.

Development of cartilage tissue using a stirred bioreactor and human iPSC-derived limb bud mesenchymal cells

Production of cartilaginous particles for regenerative medicine requires a large supply of chondrocytes and development of suitable production techniques. Previously, we successfully produced human induced pluripotent stem cell (hiPSC)-derived limb bud mesenchymal cells (ExpLBM cells) with a high chondrogenic differentiation potential that stably proliferate. It may be possible to use these cells in combination with a stirred bioreactor to develop a tissue-engineered cell culture technology with potential for scale-up to facilitate production of large amounts of cartilaginous particles. ExpLBM cells derived from 414C2 and Ff-I 14s04 (human leukocyte antigen homozygous) hiPSCs were seeded into a stirred bioreactor containing cartilage induction medium. To characterize the cartilaginous particles produced, we performed real-time quantitative reverse transcription-polymerase chain reaction and histological analyses. Additionally, we transplanted the cartilage tissue into osteochondral defects of immunocompromised rats to assess its functionality, and evaluated engraftment of the grafted tissue. We successfully produced large amounts of cartilaginous particles via cartilage induction culture in a stirred bioreactor. This tissue exhibited significantly increased expression levels of type II collagen (COL2), aggrecan (ACAN), and SRY-box transcription factor 9 (SOX9), as well as positive Safranin O and Toluidine blue staining, indicating that it possesses characteristics of hyaline cartilage. Furthermore, engrafted tissues in osteochondral knee defects of immunodeficient rats were positively stained for human vimentin, COL2, and ACAN as well as with Safranin O. In this study, we successfully generated large amounts of hiPSC-derived cartilaginous particles using a combination of tissue engineering techniques. This method is promising as a cartilage regeneration technology with potential for scale-up.

[Evaluation of developmental toxicity of ammonium dinitramide by micromass culture and embryonic stem cells models]

Objective: To evaluated the potential developmental toxicity and teratogenicity of ammonium dinitroamide (ADN) by micromass test (MM Test) and embryonic stem cell test models. Methods: In September 2018, rat embryos were isolated and limb bud cells were collected. The limb bud cells were treated with different concentrations of ADN (0, 312.50, 625.00, 1250.00, 2500.00, 5000.00, 10000.00 μg/ml) . Half proliferation inhibitory concentration and half differentiation inhibitory concentration were calculated and the teratogenic effects were evaluated according to the criteria. For the embryonic stem cell test, the effects of different concentrations of ADN (0, 39.06, 78.13, 156.25, 312.50, 625.00, 1250.00, 2500.00 μg/ml) on the differentiation of mouse embryonic stem cells (mESCs) into myocardial cells and the cytotoxicity of mESCs and 3T3 cells were detected. The embryonic toxicity was evaluated according to the criteria. In this study, both 5-fluorouracil (5-FU) , a known strong embryonic toxic drug, and penicillin-G (P-G) , a non-embryonic toxic drug, were used to verify the effectiveness of the model, and the validated test model was applied to evaluate the embryonic toxicity of ADN. Results: In the MM Test, the inhibition rates of proliferation and differentiation of limb bud cells in ADN groups were higher than that in control group (P<0.05) . And the half proliferation inhibitory concentration and half differentiation inhibitory concentration of ADN on limb bud cells were 7480.32 and 4526.09 μg/ml, respectively. ADN was determined to be non-teratogenic by standard. In the embryonic stem cell test, the inhibition rates of mESCs proliferation in ADN groups were higher than that in control group, and the inhibition rates of 3T3 cells in 156.25, 312.50, 625.00, 1250.00, 2500.00 μg/ml ADN groups were higher than that in control group (P<0.05) . The half proliferation inhibitory concentration and half differentiation inhibitory concentration of ADN on mESCs were 1851.73 and 1796.39 μg/ml, respectively, and the half proliferation inhibitory concentration on 3T3 cells was 3334.35 μg/ml. ADN was determined to be non-embryotoxic by standard. Conclusion: After evaluation by MM Test and embryonic stem cell models, ADN has no embryo toxicity and is a non-teratogenic substance.

Effects of an Environmentally Relevant Mixture of Organophosphate Esters Derived From House Dust on Endochondral Ossification in Murine Limb Bud Cultures

Organophosphate esters (OPEs) are used widely as flame retardants and plasticizers but much remains unknown about their potential toxicity. Previously, we reported that 4 individual OPEs suppress endochondral ossification in murine limb bud cultures. However, real-life exposure is to complex OPE mixtures. In the present study, we tested the hypothesis that a Canadian household dust-based OPE mixture will affect endochondral ossification in gestation day 13 CD1 mouse embryo limb buds expressing fluorescent markers for the major cell populations involved in the process: collagen type II alpha 1-enhanced cyan fluorescent protein (proliferative chondrocytes), collagen type X alpha 1-mCherry (hypertrophic chondrocytes), and collagen type I alpha 1-yellow fluorescent protein (osteoblasts). Limbs were cultured for 6 days in the presence of vehicle or dilutions of the OPE mixture (1/1 000 000, 1/600 000, and 1/300 000). All 3 OPE mixture dilutions affected cartilage template development and the progression of endochondral ossification, as indicated by the fluorescent markers. The expression of Sox9, the master regulator of chondrogenesis, was unchanged, but the expression of Runx2 and Sp7, which drive chondrocyte hypertrophy and osteoblastogenesis, was dilution-dependently suppressed. RNA-seq revealed that exposure to the 1/300 000 dilution of the OPE mixture for 24 h downregulated 153 transcripts and upregulated 48 others by at least 1.5-fold. Downregulated transcripts were enriched for those related to the immune system and bone formation. In contrast, upregulated transcripts were enriched for those with stress response functions known to be regulated by ATF4 activation. Thus, exposure to the mixture of OPEs commonly found in house dust may have adverse effects on bone formation.

An experimental analysis of the developmental capacities of distal parts of avian leg buds

The development, differentiation, and pattern formation of isolated distal parts of avian leg buds that had grown ectopically on the chorioallantoic membrane (CAM) or in the coelomic cavity were studied. The grafts grown on the CAM invariably gave rise to cartilage and soft connective tissue. In some cases muscle tissue was also found. The CAM grafts did not undergo overt morphogenesis and pattern formation. A high percentage of grafts grown in the coelomic cavity showed a close approximation to normal limbs. The presence of proximal structures depended on the stage of development of the donor at the time of the operation, on the size of the grafts, and on the site to which the graft was attached within the coelom. The presence of anteroposterior structures depended on the shape of the graft. The pattern formation of this axis was found to be independent of the presence of the zone of polarizing activity at the proximal posterior border of the bud. The distance from the tip of the bud to the line of most distal colonization by myogenic cells was determined. The speed of migration of the myogenic cells can be considered to be constant. In muscleless legs, tendons developed at the levels of the phalanges and the tarsometatarse. They degenerated, however, in the absence of muscle from day 9 on, from proximal to distal areas. CAM grafts as well as coelomic grafts were well vascularized. The endothelial cells of the blood vessels were of host origin. In coelomic grafts, nerves were present with Schwann cells of host origin. The nerves and blood vessels showed a distribution that resembled that in normal legs.

BMP-2-Modulated Chondrogenic Differentiation In Vitro Involves Down-Regulation of Membrane-Bound Beta-Catenin

Bone morphogenetic proteins (BMPs) are important regulators of cellular differentiation and embryonic development. Beta catenin mediated nuclear signaling has been implicated in BMP-2-modulated chondrogenic differentiation in the pluripotential stem cell line C3H10T1/2. However, there is little information on the functional role of beta catenin in BMP-2-modulated differentiation of primary nontransformed mesenchymal cells. Here, we present evidence to show that BMP-2-induced chondrogenic differentiation in high-density primary mesenchymal culture is associated with a significant decrease in membrane-bound beta catenin by 72 hours compared to controls. Nuclear localization of beta catenin is not detectable by immunofluorescence and the TCF-responsive reporter vector TOPFLASH shows only background activity during chondrogenic differentiation. BMP-2-treated cultures show reduced cell-cell adhesion by 72 hours, which correlates with the changes in levels of membrane-bound beta catenin. Up-regulation of membrane-bound beta catenin blocks the effect of BMP-2 on both chondrogenic differentiation and cell-cell adhesiveness. These findings suggest that BMP-2 can modulate the adhesivity of adherens junctions through regulation of membrane bound beta catenin.

Growth and development of the placenta in the capybara (Hydrochaeris hydrochaeris)

BACKGROUND

The guinea pig is an attractive model for human pregnancy and placentation, mainly because of its haemomonochorial placental type, but is rather small in size. Therefore, to better understand the impact of body mass, we studied placental development in the capybara which has a body mass around 50 kg and a gestation period of around 150 days. We paid attention to the development of the lobulated arrangement of the placenta, the growth of the labyrinth in the course of gestation, the differentiation of the subplacenta, and the pattern of invasion by extraplacental trophoblast.

METHODS

Material was collected from six animals at pregnancy stages ranging from the late limb bud stage to mid gestation. Methods included latex casts, standard histology, immunohistochemistry for cytokeratin, vimentin, alpha-smooth muscle actin, and proliferating cell nuclear antigen as well as transmission electron microscopy.

RESULTS

At the limb bud stage, the placenta was a pad of trophoblast covered by a layer of mesoderm from which fetal vessels were beginning to penetrate at folds in the surface. By 70 days, the placenta comprised areas of labyrinth (lobes) separated by interlobular areas. Placental growth resulted predominantly from proliferation of cellular trophoblast situated in nests at the fetal side of the placenta and along internally directed projections on fetal mesenchyme. Additional proliferation was demonstrated for cellular trophoblast within the labyrinth.Already at the limb bud stage, there was a prominent subplacenta comprising cellular and syncytial trophoblast with mesenchyme and associated blood vessels. At 90 days, differentiation was complete and similar to that seen in other hystricognath rodents. Overlap of fetal vessels and maternal blood lacunae was confirmed by latex injection of the vessels. At all stages extraplacental trophoblast was associated with the maternal arterial supply and consisted of cellular trophoblast and syncytial streamers derived from the subplacenta.

CONCLUSION

All important characteristics of placental development and organization in the capybara resembled those found in smaller hystricognath rodents including the guinea pig. These features apparently do not dependent on body size. Clearly, placentation in hystricognaths adheres to an extraordinarily stable pattern suggesting they can be used interchangeably as models of human placenta.

Identification of genes associated with regenerative success of Xenopus laevis hindlimbs

BACKGROUND

Epimorphic regeneration is the process by which complete regeneration of a complex structure such as a limb occurs through production of a proliferating blastema. This type of regeneration is rare among vertebrates but does occur in the African clawed frog Xenopus laevis, traditionally a model organism for the study of early development. Xenopus tadpoles can regenerate their tails, limb buds and the lens of the eye, although the ability of the latter two organs to regenerate diminishes with advancing developmental stage. Using a heat shock inducible transgene that remains silent unless activated, we have established a stable line of transgenic Xenopus (strain N1) in which the BMP inhibitor Noggin can be over-expressed at any time during development. Activation of this transgene blocks regeneration of the tail and limb of Xenopus tadpoles.

RESULTS

In the current study, we have taken advantage of the N1 transgenic line to directly compare morphology and gene expression in same stage regenerating vs. BMP signalling deficient non-regenerating hindlimb buds. The wound epithelium of N1 transgenic hindlimb buds, which forms over the cut surface of the limb bud after amputation, does not transition normally into the distal thickened apical epithelial cap. Instead, a basement membrane and dermis form, indicative of mature skin. Furthermore, the underlying mesenchyme remains rounded and does not expand to form a cone shaped blastema, a normal feature of successful regeneration. Using Affymetrix Gene Chip analysis, we have identified genes linked to regenerative success downstream of BMP signalling, including the BMP inhibitor Gremlin and the stress protein Hsp60 (no blastema in zebrafish). Gene Ontology analysis showed that genes involved in embryonic development and growth are significantly over-represented in regenerating early hindlimb buds and that successful regeneration in the Xenopus hindlimb correlates with the induction of stress response pathways.

CONCLUSION

N1 transgenic hindlimbs, which do not regenerate, do not form an apical epithelial cap or cone shaped blastema following amputation. Comparison of gene expression in stage matched N1 vs. wild type hindlimb buds has revealed several new targets for regeneration research.

The Xenopus tadpole: a new model for regeneration research

The Xenopus tadpole is a favourable organism for regeneration research because it is suitable for a wide range of micromanipulative procedures and for a wide range of transgenic methods. Combination of these techniques enables genes to be activated or inhibited at specific times and in specific tissue types to a much higher degree than in any other organism capable of regeneration. Regenerating systems include the tail, the limb buds and the lens. The study of tail regeneration has shown that each tissue type supplies the cells for its own replacement: there is no detectable de-differentiation or metaplasia. Signalling systems needed for regeneration include the BMP and Notch signalling pathways, and perhaps also the Wnt and FGF pathways. The limb buds will regenerate completely at early stages, but not once they are fully differentiated. This provides a good opportunity to study the loss of regenerative ability using transgenic methods.

Growth and differentiation of the developing limb bud from the perspective of chondrogenesis

Limb skeletal elements develop from a cartilage template, which is formed by the process termed chondrogenesis. This process is crucial in determining the shape and size of definitive bones in vertebrates. During chondrogenesis, aggregated mesenchymal cells undergo a highly organized process of proliferation and maturation along with secretion of extracellular matrix followed by programmed cell death and replacement by bone. The molecular mechanisms underlying this sophisticated process have been extensively studied. It has been demonstrated that several transcription factors such as Sox genes and Runx genes are indispensable for the major steps in chondrogenesis. Additionally, a number of signaling molecules including Bmps, Fgfs and Ihh/PTHrP are known to regulate chondrogenesis through highly coordinated interactions. This review is meant to provide an overview of the current knowledge of chondrogenesis with particular emphasis on the cellular and molecular aspects.

Six proteins regulate the activation of Myf5 expression in embryonic mouse limbs

Myf5, a member of the myogenic regulatory factor family, plays a major role in determining myogenic cell fate at the onset of skeletal muscle formation in the embryo. Spatiotemporal control of its expression during development requires multiple enhancer elements spread over >100 kb at the Myf5 locus. Transcription in embryonic limbs is regulated by a 145-bp element located at -57.5 kb from the Myf5 gene. In the present study we show that Myf5 expression is severely impaired in the limb buds of Six1(-/-) and Six1(-/-)Six4(-/+) mouse mutants despite the presence of myogenic progenitor cells. The 145-bp regulatory element contains a sequence that binds Six1 and Six4 in electromobility shift assays in vitro and in chromatin immunoprecipitation assays with embryonic extracts. We further show that Six1 is able to transactivate a reporter gene under the control of this sequence. In vivo functionality of the Six binding site is demonstrated by transgenic analysis. Mutation of this site impairs reporter gene expression in the limbs and in mature somites where the 145-bp regulatory element is also active. Six1/4 therefore regulate Myf5 transcription, together with Pax3, which was previously shown to be required for the activity of the 145-bp element. Six homeoproteins, which also directly regulate the myogenic differentiation gene Myogenin and lie genetically upstream of Pax3, thus control hypaxial myogenesis at multiple levels.

Tinkering with Constraints in the Evolution of the Vertebrate Limb Anterior-Posterior Polarity

Genes belonging to both HoxA and HoxD clusters are required for proper vertebrate limb development. Mice lacking all, or parts of, Hoxa and Hoxd functions in forelimbs, as well as mice with a gain of function of these genes in the early limb bud, have helped us to understand functional and regulatory issues associated with these genes, such that, for example, the tight mechanistic interdependency that exists between the production of the limb and its anterior to posterior (AP) polarity. Our studies suggest that the evolutionary recruitment of Hox gene function into growing appendages was crucial to implement hedgehog signalling, subsequently leading to the distal extension of tetrapod appendages, with an already built-in AP polarity. We propose that this process results from the evolutionary co-option, in the developing limbs, of a particular regulatory mechanism (collinearity), which is necessary to pattern the developing trunk. This major regulatory constraint imposed a polarity to our limbs as the most parsimonious solution to grow appendages.

Wnt/beta-catenin signaling has an essential role in the initiation of limb regeneration

Anuran (frog) tadpoles and urodeles (newts and salamanders) are the only vertebrates capable of fully regenerating amputated limbs. During the early stages of regeneration these amphibians form a "blastema", a group of mesenchymal progenitor cells that specifically directs the regrowth of the limb. We report that wnt-3a is expressed in the apical epithelium of regenerating Xenopus laevis limb buds, at the appropriate time and place to play a role during blastema formation. To test whether Wnt/beta-catenin signaling is required for limb regeneration, we created transgenic X. laevis tadpoles that express Dickkopf-1 (Dkk1), a specific inhibitor of Wnt/beta-catenin signaling, under the control of a heat-shock promoter. Heat-shock immediately before limb amputation or during early blastema formation blocked limb regeneration but did not affect the development of contralateral, un-amputated limb buds. When the transgenic tadpoles were heat-shocked following the formation of a blastema, however, they retained the ability to regenerate partial hindlimb structures. Furthermore, heat-shock induced Dkk1 blocked fgf-8 but not fgf-10 expression in the blastema. We conclude that Wnt/beta-catenin signaling has an essential role during the early stages of limb regeneration, but is not absolutely required after blastema formation.

Oxidative stress-induced poly(ADP-ribosyl)ation in chick limb bud-derived chondrocytes

Oxidative stress has been implicated in the pathogenesis of various diseases affecting chondrogenesis or the function of articular cartilage. DNA damage caused by oxidative stress may trigger the activation of the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP-1) which may contribute to tissue injury. We aimed at investigating the effects of peroxynitrite (100-600 microM) and hydrogen peroxide (0.1-4 mM) on PARP activation and extracellular matrix production of high density micromass cultures (HDC) prepared from chick limb bud mesenchymal cells. We found that both oxidative species strongly inhibited matrix formation of HDCs treated on day 2 but not on day 5. The PARP inhibitor 3-aminobenzamide (3-AB) stimulated matrix production in non-stressed cells and prevented suppressed matrix production in oxidatively stressed cells. Both hydrogen peroxide and peroxynitrite induced PARP activation and poly(ADP-ribose) accumulation. Decreased proliferation, viability and NAD+ content were not or only slightly improved by 3-AB, indicating that 3-AB directly affects matrix formation. In conclusion, oxidative stress stimulates poly(ADP-ribose) metabolism and inhibits extracellular matrix production of HDCs in a PARP-dependent manner. Our findings may have implications for potential therapeutic approaches aimed at restoring the matrix production capacity of chondrogenic cells.

Regeneration potency of mouse limbs

Mammalians have a low potency for limb regeneration compared to that of amphibians. One explanation for the low potency is the deficiency of cells for regenerating amputated limbs in mammals. Amphibians can form a blastema with dedifferentiated cells, but mammals have few such cells. In this paper, we report limb formation, especially bone/cartilage formation in amputated limbs, because bone/cartilage formation is a basic step in limb pattern regeneration. After the amputation of limbs of a neonatal mouse, hypertrophy of the stump bone was observed at the amputation site, which was preceded by cell proliferation and cartilage formation. However, no new elements of bone/cartilage were formed. Thus, we grafted limb buds of mouse embryo into amputated limbs of neonatal mice. When the intact limb bud of a transgenic green fluorescent protein (GFP) mouse was grafted to the limb stump after amputation at the digit joint level, the grafted limb bud grew and differentiated into bone, cartilage and soft tissues, and it formed a segmented pattern that was constituted by bone and cartilage. The skeletal pattern was more complicated when limb buds at advanced stages were used. To examine if the grafted limb bud autonomously develops a limb or interacts with stump tissue to form a limb, the limb bud was dissociated into single cells and reaggregated before grafting. The reaggregated limb bud cells formed similar digit-like bone/cartilage structures. The reaggregated grafts also formed segmented cartilage. When the reaggregates of bone marrow mesenchymal cells were grafted into the stump, these cells formed cartilage, as do limb bud cells. Finally, to examine the potency of new bone formation in the stump tissue without exogenously supplied cells, we grafted gelatin gel containing BMP-7. BMP induced formation of several new bone elements, which was preceded by cartilage formation. The results suggest that the environmental tissues of the stump allow the formation of cartilage and bone at least partially, and that limb formation will be possible by supplying competent cells endogenously or exogenously in the future.

Hyaluronan in limb morphogenesis

Hyaluronan (HA) is a large glycosaminoglycan that is not only a structural component of extracellular matrices, but also interacts with cell surface receptors to promote cell proliferation, migration, and intracellular signaling. HA is a major component of the extracellular matrix of the distal subapical mesenchymal cells of the developing limb bud that are undergoing proliferation, directed migration, and patterning in response to the apical ectodermal ridge (AER), and has the functional potential to be involved in these processes. Here we show that the HA synthase Has2 is abundantly expressed by the distal subridge mesodermal cells of the chick limb bud and also by the AER itself. Has2 expression and HA production are downregulated in the proximal central core of the limb bud during the formation of the precartilage condensations of the skeletal elements, suggesting that downregulation of HA may be necessary for the close juxtaposition of cells and the resulting cell-cell interactions that trigger cartilage differentiation during condensation. Overexpression of Has2 in the mesoderm of the chick limb bud in vivo results in the formation of shortened and severely malformed limbs that lack one or more skeletal elements. Skeletal elements that do form in limbs overexpressing Has2 are reduced in length, exhibit abnormal morphology, and are positioned inappropriately. We also demonstrate that sustained HA production in micromass cultures of limb mesenchymal cells inhibits formation of precartilage condensations and subsequent chondrogenesis, indicating that downregulation of HA is indeed necessary for formation of the precartilage condensations that trigger cartilage differentiation. Taken together these results suggest involvement of HA in various aspects of limb morphogenesis.

Regulatory constraints in the evolution of the tetrapod limb anterior-posterior polarity

The anterior to posterior (A-P) polarity of the tetrapod limb is determined by the confined expression of Sonic hedgehog (Shh) at the posterior margin of developing early limb buds, under the control of HOX proteins encoded by gene members of both the HoxA and HoxD clusters. Here, we use a set of partial deletions to show that only the last four Hox paralogy groups can elicit this response: that is, precisely those genes whose expression is excluded from most anterior limb bud cells owing to their collinear transcriptional activation. We propose that the limb A-P polarity is produced as a collateral effect of Hox gene collinearity, a process highly constrained by its crucial importance during trunk development. In this view, the co-option of the trunk collinear mechanism, along with the emergence of limbs, imposed an A-P polarity to these structures as the most parsimonious solution. This in turn further contributed to stabilize the architecture and operational mode of this genetic system.

BMP action in skeletogenesis involves attenuation of retinoid signaling

The bone morphogenetic protein (BMP) and growth and differentiation factor (GDF) signaling pathways have well-established and essential roles within the developing skeleton in coordinating the formation of cartilaginous anlagen. However, the identification of bona fide targets that underlie the action of these signaling molecules in chondrogenesis has remained elusive. We have identified the gene for the retinoic acid (RA) synthesis enzyme Aldh1a2 as a principal target of BMP signaling; prochondrogenic BMPs or GDFs lead to attenuation of Aldh1a2 expression and, consequently, to reduced activation of the retinoid signaling pathway. Consistent with this, antagonism of retinoid signaling phenocopies BMP4 action, whereas RA inhibits the chondrogenic stimulatory activity of BMP4. BMP4 also down-regulates Aldh1a2 expression in organ culture and, consistent with this, Aldh1a2 is actively excluded from the developing cartilage anlagens. Collectively, these findings provide novel insights into BMP action and demonstrate that BMP signaling governs the fate of prechondrogenic mesenchyme, at least in part, through regulation of retinoid signaling.

Blimp-1 is an essential component of the genetic program controlling development of the pectoral limb bud

Formation of paired limbs in vertebrate embryos has long been a particularly useful paradigm for the study of pattern formation. Here, we show that Blimp-1, a SET domain and zinc finger-containing transcriptional factor, plays an important role in the development of the pectoral fins of the zebrafish structures that are homologous to forelimbs of amniotes. The blimp-1 gene is expressed dynamically in the mesenchyme as well as the ectodermal cells of the early fin bud, and later, in the cells of the apical ectodermal ridge (AER) of the outgrowing fin. Consistent with this expression profile, loss of Blimp-1 activity severely impairs fin outgrowth and patterning. We present evidence that blimp-1 functions downstream of tbx5 and fgf24 and therefore is not required for the initial specification of the fin bud primordia. Subsequently, however, its function is necessary for the induction of fgf10 and sonic hedgehog in the mesenchyme. In addition, Blimp-1 activity is absolutely critical for the proper induction of gene expression in the ectoderm and establishment of the AER. Taken together, these results identify an additional layer of control in the genetic pathway that operates in the developing limb and provides novel insights into regulatory mechanisms that organize its pattern.

Notch1 signals through Jagged2 to regulate apoptosis in the apical ectodermal ridge of the developing limb bud

The Notch family of receptors is involved in a wide variety of developmental processes, including cell fate specification, cell proliferation, and cell survival decisions during cell differentiation and tissue morphogenesis. Notch1 and Notch ligands are expressed in the developing limbs, and Notch signalling has been implicated in the formation of a variety of tissues that comprise the limb, such as the skeleton, musculature, and vasculature. Notch signalling has also been implicated in regulating overall limb size. We have used a conditional allele of Notch1 in combination with two different Cre transgenic lines to delete Notch1 function either in the limb mesenchyme or in the apical ectodermal ridge (AER) and limb ectoderm. We demonstrate that Notch signalling, involving Notch1 and Jagged2, is required to regulate the number of Fgf8-expressing cells that comprise the AER and that regulation of the levels of fibroblast growth factor signalling is important for the freeing of the digits during normal limb formation. Regulation of the extent of the AER is achieved by Notch signalling positively regulating apoptosis in the AER. We also demonstrate that Notch1 is not required for proper formation of all the derivatives of the limb mesenchyme.

Digital-image analysis of the angiographic patterns of the popliteal artery in patients with aorto-iliac occlusive disease (Leriche syndrome)

Angiographic patterns of the popliteal artery are of great clinical relevance in vascular surgery below the knee. Using radiological, digital and statistical methods the variants and Luminal diameters of the popliteal artery branching in 46 men and 30 women with Lerich syndrome were studied. Statistical analysis did not reveal any gender or syntopic dimorphisms (P > or = 0.05). In subtype IA (87.5%) the anterior tibial artery and the short type of posterior tibioperoneal trunk were found. In subtype IB (2.63%) an arterial trifurcation was observed. In subtype IC (1.97%) the posterior tibial artery and the short type of anterior tibioperoneal trunk were seen. In two subtypes: IIA-1 (1.32%) and IIA-2 (0.66%) the anterior tibial artery and the long type of posterior tibioperoneal trunk were found. In subtype II B (5.92%) the long type of anterior tibioperoneal trunk and the posterior tibial artery were observed. The symmetry of the left and right poptiteal patterns was seen in two most frequent subtypes: I A (r1 = 0.80) and II B (r2 = 0.83). Either the anterior or posterior tibial artery had a smaller diameter than the coexisting tibioperoneal trunk (P<0.01). In a trifurcation the luminal diameters formed a decreasing sequence of the following arteries: anterior tibial, posterior tibial and peroneal. The angiometric analysis of luminal diameters showed the predominant vessel in each subtype: anterior tibioperoneal trunk (IC, IIB), posterior tibioperoneal trunk (IA, IIA-1, IIA-2) and anterior tibial artery (IB).

BMP receptor type IA in limb bud mesenchyme regulates distal outgrowth and patterning

The mesenchyme of the developing vertebrate limb responds to inductive signals, giving rise to skeletal elements that define limb shape and size. Several signals emanate from the limb ectoderm and in particular from the specialized epithelium of the apical ectodermal ridge (AER), including three members of the bone morphogenetic protein (BMP) family of signaling molecules, BMP2, BMP4 and BMP7. Using the Cre/loxP system in mice, we rendered limb bud mesenchyme insensitive to BMP signals through the type I receptor, BMPR-IA. Conditional mutants had shortened limbs and almost complete agenesis of the autopod because of reduced cell proliferation. Reduced expression of downstream BMP signaling targets, Msx1, Msx2 and gremlin in the distal mesenchyme (progress zone) correlated with decreased levels of cyclin D1 and Wnt5a. Ectopic anterior activation of sonic hedgehog (SHH) signaling and Hox expression revealed alterations in anterior-posterior (AP) patterning. Abnormal localization of Lmx1b-expressing cells in the ventral mesenchyme, along with histological alterations and an abnormal melanization pattern of the limb, indicate altered dorsal-ventral (DV) boundaries. These findings suggest that signaling through BMPR-IA in limb mesenchyme is essential for distal outgrowth and also influences AP and DV patterning.