Downregulation of Grem1 expression in the distal limb mesoderm is a necessary precondition for phalanx development.

Background: The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth.

Results: As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges.

The dynamic spatial and temporal relationships between the phalanx-forming region and the interdigits determine digit identity in the chick limb autopod.

Background: Interdigits (IDs) determine digit identity in chick limbs. They are located between the digital rays and act as secondary signaling centers downstream of sonic hedgehog to provide positional information for determining digit identity in the phalanx-forming region (PFR). We examined the dynamic developmental mechanism by which PFR cells obtain positional information from IDs to determine the identity of individual digits in the chick hindlimb.

Identification of spontaneous mutations within the long-range limb-specific Sonic hedgehog enhancer (ZRS) that alter Sonic hedgehog expression in the chicken limb mutants oligozeugodactyly and silkie breed.

The evolutionarily conserved, non-coding ~800-base-pair (bp) zone of polarizing activity (ZPA) regulatory sequence (ZRS) controls Shh expression in the posterior limb. We report that the chicken mutant oligozeugodactyly (ozd), which lacks limb Shh expression, has a large deletion within the ZRS. Furthermore, the preaxial polydactylous, Silkie Breed chicken, which develops ectopic anterior limb Shh expression, has a single bp change within the ZRS.

The evolution of HoxD-11 expression in the bird wing: insights from Alligator mississippiensis.

Background: Comparative morphology identifies the digits of the wing of birds as 1,2 and 3, but they develop at embryological positions that become digits 2, 3 and 4 in other amniotes. A hypothesis to explain this is that a homeotic frame shift of digital identity occurred in the evolution of the bird wing, such that digits 1,2 and 3 are developing from embryological positions 2, 3 and 4. Digit 1 of the mouse is the only digit that shows no late expression of HoxD-11.

Unique SMAD1/5/8 activity at the phalanx-forming region determines digit identity.

The zone of polarizing activity is the primary signaling center controlling anterior-posterior patterning of the amniote limb bud. The autopodial interdigits (IDs) are secondary signaling centers proposed to determine digit identity by acting on the cells of the digital ray. Here, we focus on events accompanying digital fate determination and define a region of the digital ray that expresses Sox9 and Bmpr1b and is phosphorylated-SMAD1/5/8 (p-SMAD1/5/8) positive.

An antiangiogenic neurokinin-B/thromboxane A2 regulatory axis.

Establishment of angiogenic circuits that orchestrate blood vessel development and remodeling requires an exquisite balance between the activities of pro- and antiangiogenic factors. However, the logic that permits complex signal integration by vascular endothelium is poorly understood. We demonstrate that a "neuropeptide," neurokinin-B (NK-B), reversibly inhibits endothelial cell vascular network assembly and opposes angiogenesis in the chicken chorioallantoic membrane.

Regulation of Gremlin expression in the posterior limb bud.

Proper outgrowth of the limb bud requires a positive feedback loop between Sonic hedgehog (Shh) in the zone of polarizing activity (ZPA) and Fgfs in the overlying apical ectodermal ridge. The Bmp antagonist Gremlin is expressed in a domain anterior to the ZPA and is thought to act as a signaling intermediate between Shh and Fgf. It is currently unclear whether Shh acts directly or indirectly to initiate and maintain Gremlin.

The development of archosaurian first-generation teeth in a chicken mutant.

Modern birds do not have teeth. Rather, they develop a specialized keratinized structure, called the rhamphotheca, that covers the mandible, maxillae, and premaxillae. Although recombination studies have shown that the avian epidermis can respond to tooth-inductive cues from mouse or lizard oral mesenchyme and participate in tooth formation, attempts to initiate tooth development de novo in birds have failed.

Differential susceptibility of midbrain and spinal cord patterning to floor plate defects in the talpid2 mutant.

The chick talpid2 mutant displays polydactylous digits attributed to defects of the Hedgehog (HH) signaling pathway. We examined the talpid2 neural tube and show that patterning defects in the spinal cord and the midbrain are distinct from each other and from the limb. Unlike the Sonic Hedgehog (SHH) source in the limb, the SHH-rich floor plate (FP) is reduced in the talpid2 midbrain.

Analysis of the regulation of lin-41 during chick and mouse limb development.

We have cloned the chicken and mouse orthologues of the Caenorhabditis elegans heterochronic gene lin-41. During limb development, lin-41 is expressed in three phases over developmental time and most notably is associated with the developing autopod. Using chicken and mouse mutants and bead implantations, we report that lin-41 is genetically and biochemically downstream of both the Shh and Fgf signaling pathways.

Molecular evidence for an activator-inhibitor mechanism in development of embryonic feather branching.

The developmental basis of morphological complexity remains a central question in developmental and evolutionary biology. Feathers provide a unique system to analyze the development of complex morphological novelties. Here, we describe the interactions between Sonic hedgehog (Shh) and bone morphogenetic protein 2 (Bmp2) signaling during feather barb ridge morphogenesis.

The digits of the wing of birds are 1, 2, and 3. A review.

Fossil evidence documenting the evolutionary transition from theropod dinosaurs to birds indicates unambiguously that the digits of the wing of birds are digits 1, 2, and 3. However, some embryological evidence suggests that these digits are 2, 3, and 4. This apparent lack of correspondence has been described as the greatest challenge to the widely accepted theropod-bird link (Zhou 2004.

Single base pair change in the long-range Sonic hedgehog limb-specific enhancer is a genetic basis for preaxial polydactyly.

In most instances of preaxial polydactyly (PPD), Sonic Hedgehog (Shh), an essential limb patterning signal, is ectopically expressed in an anterior region of the developing limb in addition to the normal posterior domain. It is thought that this anterior Shh expression leads directly to the development of the extra digits. Recent reports have identified a conserved limb-specific Shh enhancer approximately 1 megabase upstream of the Shh transcription initiation site, and individual base pair changes within this region are associated with PPD.

Birds have dinosaur wings: The molecular evidence.

Within developmental biology, the digits of the wing of birds are considered on embryological grounds to be digits 2, 3 and 4. In contrast, within paleontology, wing digits are named 1, 2, 3 as a result of phylogenetic analysis of fossil taxa indicating that birds descended from theropod dinosaurs that had lost digits 4 and 5. It has been argued that the development of the wing does not support the conclusion that birds are theropods, and that birds must have descended from ancestors that had lost digits 1 and 5.

Levels of Gli3 repressor correlate with Bmp4 expression and apoptosis during limb development.

Removal of the posterior wing bud leads to massive apoptosis of the remaining anterior wing bud mesoderm. We show here that this finding correlates with an increase in the level of the repressor form of the Gli3 protein, due to the absence of the Sonic hedgehog (Shh) protein signaling. Therefore, we used the anterior wing bud mesoderm as a model system to analyze the relationship between the repressor form of Gli3 and apoptosis in the developing limb.

Bmp7 mediates early signaling events during induction of chick epidermal organs.

The induction and specification of a large number of vertebrate organs require reciprocal signaling between an epithelium and subjacent mesenchyme. In the formation of integumentary organs, the initial inductive signaling events leading to the formation of the organ primordia stem from the mesenchyme. However, the epithelium must have the capacity to respond to these signals.

Isolation of the chicken Lmbr1 coding sequence and characterization of its role during chick limb development.

In the developing amniote limb, anteroposterior (A/P) patterning is controlled through secretion of the Sonic Hedgehog (SHH) protein by cells in the zone of polarizing activity (ZPA) located in the posterior mesoderm. In the chicken mutant oligozeugodactyly (ozd), Shh is expressed normally in the entire embryo with the exception that it is undetectable in the developing limbs; this results in the loss of specific bones in wings and legs. The ozd phenotype is similar to that of humans affected with acheiropodia (ACHR), and the ACHR mutation has been mapped to a deletion of exon 4 and portions of introns 3 and 4 in the LMBR1 gene.

Fibroblast growth factor (FGF)-4 can induce proliferation of cardiac cushion mesenchymal cells during early valve leaflet formation.

While much has been learned about how endothelial cells transform to mesenchyme during cardiac cushion formation, there remain fundamental questions about the developmental fate of cushions. In the present work, we focus on the growth and development of cushion mesenchyme. We hypothesize that proliferative expansion and distal elongation of cushion mesenchyme mediated by growth factors are the basis of early valve leaflet formation.

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb.

We have analyzed a new limb mutant in the chicken that we name oligozeugodactyly (ozd). The limbs of this mutant have a longitudinal postaxial defect, lacking the posterior element in the zeugopod (ulna/fibula) and all digits except digit 1 in the leg. Classical recombination experiments show that the limb mesoderm is the defective tissue layer in ozd limb buds.

Shh-Bmp2 signaling module and the evolutionary origin and diversification of feathers.

To examine the role of development in the origin of evolutionary novelties, we investigated the developmental mechanisms involved in the formation of a complex morphological novelty-branched feathers. We demonstrate that the anterior-posterior expression polarity of Sonic hedgehog (Shh) and Bone morphogenetic protein 2 (Bmp2) in the primordia of feathers, avian scales, and alligator scales is conserved and phylogenetically primitive to archosaurian integumentary appendages. In feather development, derived patterns of Shh-Bmp2 signaling are associated with the development of evolutionarily novel feather structures.

Shh and Gli3 are dispensable for limb skeleton formation but regulate digit number and identity.

Most current models propose Sonic hedgehog (Shh) as the primary determinant of anteroposterior development of amniote limbs. Shh protein is said to be required to direct the formation of skeletal elements and to specify digit identity through dose-dependent activation of target gene expression. However, the identity of genes targeted by Shh, and the regulatory mechanisms controlling their expression, remain poorly understood.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs.

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration.

Development of an evolutionarily novel structure: fibroblast growth factor expression in the carapacial ridge of turtle embryos.

The turtle shell, an evolutionarily novel structure, contains a bony exoskeleton that includes a dorsal carapace and a ventral plastron. The development of the carapace is dependent on the carapacial ridge (CR), a bulge in the dorsal flank that contains an ectodermal structure analogous to the apical ectodermal ridge (AER) of the developing limb (Burke. 1989a.

Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function.

The secreted protein encoded by the Sonic hedgehog (Shh) gene is localized to the posterior margin of vertebrate limb buds and is thought to be a key signal in establishing anterior-posterior limb polarity. In the Shh(-/-) mutant mouse, the development of many embryonic structures, including the limb, is severely compromised. In this study, we report the analysis of Shh(-/-) mutant limbs in detail.