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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. Key Findings: Transcriptome analysis revealed seven patterns of differential expression across stage 51 Xenopus hindlimb buds, with the distal region most transcriptionally distinct.Two distal (capn8.3, zic5) and two proximal (gata5 and tnn) genes with no previously described role in limb development were identified.Genes linked to Wnt, Fgf and retinoic acid (RA) signaling, cell adhesion and proliferation are differentially expressed across the proximodistal axis.All genes associated with RA signaling are proximal, and genes previously shown to be down‐regulated by RA are distal.Our findings support the presence of patterning gradients of RA across the early limb proximodistal axis in Xenopus. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Developmental Dynamics 243:741-752, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The article discusses non-traditional models in understanding mechanisms that regulate limb development among mammals. Research in non-traditional mammals have been done on organisms including opossum Monodelphis domestica, bats Carollia perspicillata, and pigs Sus scrofa, to understand the modified evolution of their limb development, as well as generate adult morphologies. Knowledge of differing limb development among mammals will improve understanding of human development.

Cognitive-behavioral practices such as meditation and yoga have long been viewed as methods of reaching states of peace and relaxation, but recent research has focused on the role of these practices in reducing endogenous mediators of stress and inflammation that would otherwise be harmful to our bodies. Further, these stress-related factors play major roles in inflammation, acting as barriers to wound healing and tissue regeneration. Fractures, denervation, tendon and ligament rupture, and cartilage degradation are morbidities associated with injury and often act as an impediment for healing. Studies of human fingertip regeneration exist; however, the underlying molecular and environmental changes have yet to be completely elucidated. Studying the regenerative capabilities of lower organisms and fetal wound healing has allowed scientists to understand the mechanisms behind regeneration, coming closer to a human application. Much research relies on the idea that the developing embryo shares a great deal in common with regenerating appendages of organisms such as the salamander. This review will cover historical perspectives of regeneration biology and current topics in limb regeneration, with particular interest given to the upper extremity, including the commonalities between human embryological development and amphibian regeneration, growth factors and pathways that show correlation with development and regeneration, recently discovered differences in fetal and adult wound healing, and current research and knowledge regarding human extremity tissue regeneration. With a greater understanding of the mechanisms and mediators involved in regeneration, the application of cognitive-behavioral practices may assist in seeing the future goals of regeneration come to fruition. [ABSTRACT FROM AUTHOR]

The apical ectodermal ridge (AER) in Neoceratodus develops after an initial period of mesenchymal proliferation and outgrowth of the fin bud and persists until chondrogenesis of the stylopod and zeugopod is initiated. At this time, the lateral margins of the AER convert to the fin fold leading to subsequent development of the dermal fin skeleton. Thorogood's (1991 ) fin fold model predicts that the AER should persist longer in Neoceratodus than it does in actinopterygians because of the comparatively extensive endochondral skeleton in lungfish. While the AER does persist into early chondrogenesis and is extended compared to actinopterygians (lost before fin radial chondrogenesis) it does not persist into further stages of chondrogenesis, providing partial support for Thorogood's model. Fgf8 appears in the lungfish fin epithelium during the initial period of fin outgrowth before a physical AER forms, when Fgf8 is restricted to the AER plus the preaxial and postaxial epithelium immediately adjacent to the AER. Fgf8 is no longer detected after the AER is replaced by a fin fold. Neoceratodus appears to provide a halfway point between ray fins and limbs during very early development as Thorogood proposed, but not precisely for the reasons his model suggests. [ABSTRACT FROM AUTHOR]

Patterning of the developing vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) via the expression of Sonic hedgehog ( Shh) and along the proximal-distal axis by the apical ectodermal ridge (AER) through the production of fibroblast growth factors (FGFs). ZPA grafting, as well as ectopic application of SHH to the anterior chick limb bud, demonstrate that digit patterning is largely influenced by these secreted factors. Although signal transduction pathways have been well characterized for SHH and for FGFs, little is known of how these signals are regulated extracellularly in the limb. The present study shows that alteration of the extracellular environment through trypsin treatment can have profound effects on digit patterning. These effects appear to be mediated by the induction of Shh in host tissues and by ectopic AER formation, implicating the extracellular matrix in regulating the signaling activities of key patterning genes in the limb. [ABSTRACT FROM AUTHOR]

Mouse R-spondin2 (Rspo2) is a member of the R-spondin protein family, which is characterized by furin-like cysteine-rich domains and a thrombospondin type 1 repeat. R-spondin is a secreted molecule that activates Wnt/ β-catenin signaling. Rspo2-deficient mice were generated to investigate the function of mouse Rspo2 during embryonic development. The homozygous mutant forelimb showed defects in distal phalanges and nail structures, and the digits were anomalous in shape. The homozygous mutant hindlimb showed more severe malformations, including lack of digits and zeugopod components. Rspo2 is expressed in the apical ectodermal ridge (AER) of the developing limb. Fgf8 expression in the AER was significantly lower in the homozygous mutant forelimb than in the wild-type forelimb and it was disturbed along the dorsoventral axis. In the homozygous mutant hindlimb, Fgf8 and Fgf4 expression in the posterior AER and Sonic hedgehog expression in the zone of polarizing activity (ZPA) were reduced. The homozygous mutant hindlimb also showed expansion of Wnt7a expression in the dorsal ectoderm toward the ventral side. This study shows that Rspo2 is critical for maintenance of the AER and for growth and patterning in limb development. [ABSTRACT FROM AUTHOR]

Split hand foot malformation (SHFM) is a congenital limb malformation presenting with a median cleft of the hand and/or foot, syndactyly and polydactyly. SHFM is genetically heterogeneous with four loci mapped to date. Murine Dactylaplasia (Dac ) is phenotypically similar, and it has been mapped to a syntenic region of 10q24, where SHFM3 has been localized. Structural alterations of the gene-encoding dactylin, a constituent of the ubiquitinization pathway, leading to reduced levels of transcript have been identified in Dac . Here, we report a significant decrease of Dactylin transcript in several individuals affected by SHFM. This observation supports a central role for dactylin in the pathogenesis of SHFM. [ABSTRACT FROM AUTHOR]

Congenital malformations of the extremities are conspicuous and have been described through the ages. Over the past decade, a wealth of knowledge has been generated regarding the genetic regulation of limb development and the underlying molecular mechanisms. Recent studies have identified several of the signaling molecules, growth factors, and transcriptional regulators involved in the initiation and maintenance of the apical ectodermal ridge (AER) as well as the molecular markers defining the three axes of the developing limb. Studies of abnormal murine phenotypes have uncovered the role played by genes such as p63 and Dactylin in the maintenance of AER activity. These phenotypes resemble human malformations and in this review we describe the underlying mechanisms and clinical associations of split hand/foot malformation and ectrodactyly–ectodermal dysplasia–cleft lip/palate syndrome, which have both been associated with mutations in the p63 gene. [ABSTRACT FROM AUTHOR]

The fibroblast growth factor (FGF) and β-catenin-dependent Wnt signaling pathways are key regulators of vertebrate limb development. FGF10 induces expression of Wnt3a , which regulates the formation and FGF8 expression of the apical ectodermal ridge (AER). In amelic limbless limbs, an AER fails to form and FGF8 is not expressed, despite expression of FGF10 . It has been found that Wnt3a is initially expressed in limbless ectoderm, although subsequently is drastically reduced. In addition, changes in the expression pattern or level of several Frizzled receptors, Axin, Lef1/Tcf1 and β-catenin have been found in limbless limbs. Notably, while normal wing buds respond to LiCl-stimulated activation of β-catenin-dependent signaling by forming ectopic, FGF8 -expressing AER, LiCl was unable to induce an AER in limbless wing buds. The results of this study suggest that the limbless gene is required for β-catenin-dependent Wnt signaling in limb ectoderm leading to FGF8 expression and AER formation. [ABSTRACT FROM AUTHOR]