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

Vertebrate limb development relies on the activity of signaling centers that promote growth and control patterning along three orthogonal axes of the limb bud. The apical ectodermal ridge, at the distal rim of the limb bud ectoderm, produces WNT and FGF signals, which promote limb bud growth and progressive distalization. The zone of polarizing activity, a discrete postero-distal mesenchymal domain, produces SHH, which stimulates growth and organizes patterning along the antero-posterior axis. The dorsal and ventral ectoderms produce, respectively, WNT7A and BMPs, which induce dorso-ventral limb fates. Interestingly, these signaling centers and the mechanisms they instruct interact with each other to coordinate events along the three axes. We review here the main interactions described between the three axial systems of the developing limb and discuss their relevance to proper limb growth and patterning., (Copyright © 2017 Elsevier Inc. All rights reserved.)

More than sixty years ago, while studying feather tracks on the shoulder of the chick embryo, Dr. John Saunders used Nile Blue dye to stain the tissue. There, he noticed a darkly stained line of cells that neatly rims the tip of the growing limb bud. Rather than ignoring this observation, he followed it up by removing this tissue and found that it led to a striking truncation of the limb skeletons. This landmark experiment marks the serendipitous discovery of the apical ectodermal ridge (AER), the quintessential embryonic structure that drives the outgrowth of the limb. Dr. Saunders continued to lead the limb field for the next fifty years, not just through his own work, but also by inspiring the next generation of researchers through his infectious love of science. Together, he and those who followed ushered in the discovery of fibroblast growth factor (FGF) as the AER molecule. The seamless marriage of embryology and molecular biology that led to the decoding of the AER serves as a shining example of how discoveries are made for the rest of the developmental biology field., (Copyright © 2017 Elsevier Inc. All rights reserved.)

The vertebrate limb with its complex anatomy develops from a small bud of undifferentiated mesoderm cells encased in ectoderm. The bud has its own intrinsic polarity and can develop autonomously into a limb without reference to the rest of the embryo. In this review, recent advances are integrated with classical embryology, carried out mainly in chick embryos, to present an overview of how the embryo makes a limb bud. We will focus on how mesoderm cells in precise locations in the embryo become determined to form a limb and express the key transcription factors Tbx4 (leg/hindlimb) or Tbx5 (wing/forelimb). These Tbx transcription factors have equivalent functions in the control of bud formation by initiating a signalling cascade involving Wnts and fibroblast growth factors (FGFs) and by regulating recruitment of mesenchymal cells from the coelomic epithelium into the bud. The mesoderm that will form limb buds and the polarity of the buds is determined with respect to both antero-posterior and dorso-ventral axes of the body. The position in which a bud develops along the antero-posterior axis of the body will also determine its identity - wing/forelimb or leg/hindlimb. Hox gene activity, under the influence of retinoic acid signalling, is directly linked with the initiation of Tbx5 gene expression in the region along the antero-posterior axis of the body that will form wings/forelimbs and determines antero-posterior polarity of the buds. In contrast, Tbx4 expression in the regions that will form legs/hindlimbs is regulated by the homeoprotein Pitx1 and there is no evidence that Hox genes determine antero-posterior polarity of the buds. Bone morphogenetic protein (BMP) signalling determines the region along the dorso-ventral axis of the body in which both wings/forelimbs and legs/hindlimbs develop and dorso-ventral polarity of the buds. The polarity of the buds leads to the establishment of signalling regions - the dorsal and ventral ectoderm, producing Wnts and BMPs, respectively, the apical ectodermal ridge producing fibroblast growth factors and the polarizing region, Sonic hedgehog (Shh). These signals are the same in both wings/forelimbs and legs/hindlimbs and control growth and pattern formation by providing the mesoderm cells of the limb bud as it develops with positional information. The precise anatomy of the limb depends on the mesoderm cells in the developing bud interpreting positional information according to their identity - determined by Pitx1 in hindlimbs - and genotype. The competence to form a limb extends along the entire antero-posterior axis of the trunk - with Hox gene activity inhibiting the formation of forelimbs in the interlimb region - and also along the dorso-ventral axis., (© 2015 Anatomical Society.)

Ptch1 and Ptch2 are highly conserved vertebrate homologs of Drosophila ptc, the receptor of the Hedgehog (Hh) signaling pathway. The vertebrate Ptch1 gene encodes a potent tumor suppressor and is well established for its role in embryonic development. In contrast, Ptch2 is poorly characterized and dispensable for embryogenesis. In flies and mice, ptc/Ptch1 controls Hh signaling through the regulation of Smoothened (Smo). In addition, Hh pathway activation also up-regulates ptc/Ptch1 expression to restrict the diffusion of the ligand. Recent studies have implicated Ptch2 in this ligand dependent antagonism, however whether Ptch2 encodes a functional Shh receptor remains unclear. In this report, we demonstrate that Ptch2 is a functional Shh receptor, which regulates Smo localization and activity in vitro. We also show that Ptch1 and Ptch2 are co-expressed in the developing mouse limb bud and loss of Ptch2 exacerbates the outgrowth defect in the limb-specific Ptch1 knockout mutants, demonstrating that Ptch1 and Ptch2 co-operate in regulating cellular responses to Shh in vivo., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Background: Avian forelimb (wing) contains only three digits, and the three-digit formation in the bird forelimb is one of the avian-specific limb characteristics that have been evolutionarily inherited from the common ancestral form in dinosaurs. Despite many studies on digit formation in the chick limb bud, the developmental mechanisms giving rise to the three-digit forelimb in birds have not been completely clarified., Results: To identify which cell populations of the early limb bud contribute to digit formation in the late limb bud, fate maps of the early fore- and hindlimb buds were prepared. Based on these fate maps, we found that the digit-forming region in the forelimb bud is narrower than that in the hindlimb bud, suggesting that some developmental mechanisms on the anterior-most region lead to a reduced number of digits in the forelimb. We also found temporal differences in the onset of appearance of the ANZ (anterior necrotic zone) as well as differences in the position of the anterior edge of the AER., Conclusions: Forelimb-specific events in the anterior limb bud are possible developmental mechanisms that might generate the different cell fates in the fore- and hindlimb buds, regulating the number of digits in birds., (© 2014 Wiley Periodicals, Inc.)

The developing forelimb is patterned along the proximal-distal and anterior-posterior axes by opposing gradients of retinoic acid and fibroblast growth factors and by graded sonic hedgehog signaling, respectively. However, how coordinated patterning along both axes is accomplished with temporal precision remains unknown. The limb molecular oscillator hairy2 was recently shown to be a direct readout of the combined signaling activities of retinoic acid, fibroblast growth factor and sonic hedgehog in the limb mesenchyme. Herein, an integrated time-space model is presented to conciliate the progress zone and two-signal models for limb patterning. We propose that the limb clock may allow temporal information to be decoded into positional information when the distance between opposing signaling gradients is no longer sufficient to provide distinct cell fate specification., (Copyright © 2013 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Split-hand/split-foot malformation (SHFM), representing variable degree of median clefts of hands and feet, is a genetically heterogeneous group of limb malformations with seven loci mapped on different human chromosomes. However, only 3 genes (TP63, WNT10B, DLX5) for the seven loci have been identified. The study, presented here, described three consanguineous Pakistani families segregating SHFM in autosomal recessive manner. Linkage in the families was searched by genotyping microsatellite markers and mutation screening of candidate gene was performed by Sanger DNA sequencing. Clinical features of affected members of these families exhibited SHFM phenotype with involvement of hands and feet. Genotyping using microsatellite markers mapped the families to WNT10B gene at SHFM6 on chromosome 12q13.11-q13. Subsequently, sequence analysis of WNT10B gene revealed a novel 4-bp deletion mutation (c.1165_1168delAAGT) in one family and 7-bp duplication (c.300_306dupAGGGCGG) in two other families. Structure-based analysis showed a significant conformational shift in the active binding site of mutated WNT10B (p.Lys388Glufs*36), influencing binding with Fzd8. The mutations identified in the WNT10B gene extend the body of evidence implicating it in the pathogenesis of SHFM., (Copyright © 2013 Elsevier B.V. All rights reserved.)

BMP signaling is pivotal for normal limb bud development in vertebrate embryos and genetic analysis of receptors and ligands in the mouse revealed their requirement in both mesenchymal and ectodermal limb bud compartments. In this study, we genetically assessed the potential essential functions of SMAD4, a mediator of canonical BMP/TGFß signal transduction, in the mouse limb bud ectoderm. Msx2-Cre was used to conditionally inactivate Smad4 in the ectoderm of fore- and hindlimb buds. In hindlimb buds, the Smad4 inactivation disrupts the establishment and signaling by the apical ectodermal ridge (AER) from early limb bud stages onwards, which results in severe hypoplasia and/or aplasia of zeugo- and autopodal skeletal elements. In contrast, the developmentally later inactivation of Smad4 in forelimb buds does not alter AER formation and signaling, but prolongs epithelial-mesenchymal feedback signaling in advanced limb buds. The late termination of SHH and AER-FGF signaling delays distal progression of digit ray formation and inhibits interdigit apoptosis. In summary, our genetic analysis reveals the temporally and functionally distinct dual requirement of ectodermal Smad4 during initiation and termination of AER signaling., (Copyright © 2013 Wiley Periodicals, Inc.)

Cell proliferation has been studied in the pectoral fin bud of the brown trout (Salmo trutta fario L.). A single injection of tritiated thymidine into the coelom was performed at stages 160, 180 or 200. Fixations were made at increasing intervals between 30 min and 12 days after the injection. The incubation temperature was 7° C, stage 244 being attained 12 days after injection at stage 160.Graphs of the variations of the labelling index, the mitotic index and the percentage of labelled mitoses as a function of the time after injection of tritiated thymidine at stage 160 have been drawn for the periderm, the basal layer of the ectoderm and the mesoderm. The duration of the cell cycles and of their phases were deduced from the graphs. Results are summarized in the table.Knowledge of the parameters of cell proliferation in the mesoderm and the ectoderm of the fin bud is of great interest for the analysis of morphogenetic mechanisms leading to the formation of the apical ectodermal ridge. The dorsal ectoderm of the bud proliferates intensely between stages 158 and 167, and this might lead to the initiation of the formation of the apical ectodermal ridge. Following that first period, the ventral ectoderm, in turn, proliferates more actively than the dorsal one between stages 170 and 209. In the subjacent mesoderm two proliferative crises occur at stages 161.75 and 170, thus probably causing or permitting the extension of the dorsal and ventral parts of the ectoderm. This alternative proliferative activity of dorsal and ventral ectoderm finally results in the formation of an apical ectodermal ridge that becomes elongated into an apical fold, which itself will give rise to the swimming paddle of the fin including its skeletal elements (lepidotrichia andactinotrichia).