The loss of ventral ectoderm identity correlates with the inability to form an AER in the legless hindlimb bud

The Geometry of Limb Motor Innervation is Controlled by the Dorsal-Ventral Compartment Boundary in the Chick Limbless Mutant

Precise control of limb muscles, and ultimately of limb movement, requires accurate motor innervation. Motor innervation of the vertebrate limb is established by sequential selection of trajectories at successive decision points. Motor axons of the lateral motor column (LMC) segregate at the base of the limb into two groups that execute a choice between dorsal and ventral tissue: medial LMC axons innervate the ventral limb, whereas lateral LMC axons innervate the dorsal limb. We investigated how LMC axons are targeted to the limb using the chick mutant limbless (ll), which has a dorsal transformation of the ventral limb mesenchyme. In ll the spatial pattern of motor projections to the limb is abnormal while their targeting is normal. While extensive, the dorsal transformation of the ll ventral limb mesenchyme is incomplete whereas the generation, specification and targeting of spinal motor neurons are apparently unaffected. Thus, the dorsal-ventral motor axon segregation is an active choice that is independent of the ratio between dorsal and ventral tissue but dependent on the presence of both tissues. Therefore, the fidelity of the motor projections to the limb depends on the presence of both dorsal and ventral compartments, while the geometry of motor projections is controlled by the position of limb dorsal-ventral compartment boundary.

Growth factors FGF8 and FGF2 and their receptor FGFR1, transcriptional factors Msx-1 and MSX-2, and apoptotic factors p19 and RIP5 participate in the early human limb development

The expression pattern of fibroblast growth factors FGF8 and FGF2 and their receptor FGFR1, transcription factors MSX-1 and MSX-2, as well as cell proliferation (Ki-67) and cell death associated caspase-3, p19 and RIP5 factors were analyzed in histological sections of eight 4th-9th-weeks developing human limbs by immunohistochemistry and semi-thin sectioning. Increasing expression of all analyzed factors (except FGF8) characterized both the multilayered human apical ectodermal ridge (AER), sub-ridge mesenchyme (progress zone) and chondrocytes in developing human limbs. While cytoplasmic co-expression of MSX-1 and MSX-2 was observed in both limb epithelium and mesenchyme, p19 displayed strong cytoplasmic expression in non-proliferating cells. Nuclear expression of Ki-67 proliferating cells, and partly of MSX-1 and MSX-2 was detected in the whole limb primordium. Strong expression of factors p19 and RIP5, both in the AER and mesenchyme of human developing limbs indicates their possible involvement in control of cell senescence and cell death. In contrast to animal studies, expression of FGFR1 in the surface ectoderm and p19 in the whole limb primordium might reflect interspecies differences in limb morphology. Expression of FGF2 and downstream RIP5 gene, and transcription factors Msx-1 and MSX-2 did not show human-specific changes in expression pattern. Based on their spatio-temporal expression during human limb development, our study indicates role of FGFs and Msx genes in stimulation of cell proliferation, limb outgrowth, digit elongation and separation, and additionally MSX-2 in control of vasculogenesis. The cascade of orchestrated gene expressions, including the analyzed developmental factors, jointly contribute to the complex human limb development.

Lmx1b-targeted cis-regulatory modules involved in limb dorsalization

Lmx1b is a homeodomain transcription factor responsible for limb dorsalization. Despite striking double-ventral (loss-of-function) and double-dorsal (gain-of-function) limb phenotypes, no direct gene targets in the limb have been confirmed. To determine direct targets, we performed a chromatin immunoprecipitation against Lmx1b at E12.5 followed by next generation sequencing (ChIP-seq). Nearly 84% (n=617) of the Lmx1b-bound genomic intervals (LBIs) identified overlap with chromatin regulatory marks indicative of potential cis-regulatory modules (PCRMs). In addition, 73 LBIs mapped to known CRMs active during limb development. We compared Lmx1b-bound PCRMs to genes differentially expressed by Lmx1b and found 292 PCRMs within 1 Mb of 254 Lmx1b-regulated genes. Gene ontologic analysis suggests that Lmx1b targets extracellular matrix production, bone/joint formation, axonal guidance, vascular development, cell proliferation and cell movement. We validated the functional activity of a PCRM associated with joint-related Gdf5 that provides a mechanism for Lmx1b-mediated joint modification and a PCRM associated with Lmx1b that suggests a role in autoregulation. This is the first report to describe genome-wide Lmx1b binding during limb development, directly linking Lmx1b to targets that accomplish limb dorsalization.

Anisotropic stress orients remodelling of mammalian limb bud ectoderm

The physical forces that drive morphogenesis are not well characterized in vivo, especially among vertebrates. In the early limb bud, dorsal and ventral ectoderm converge to form the apical ectodermal ridge (AER), although the underlying mechanisms are unclear. By live imaging mouse embryos, we show that prospective AER progenitors intercalate at the dorsoventral boundary and that ectoderm remodels by concomitant cell division and neighbour exchange. Mesodermal expansion and ectodermal tension together generate a dorsoventrally biased stress pattern that orients ectodermal remodelling. Polarized distribution of cortical actin reflects this stress pattern in a β-catenin- and Fgfr2-dependent manner. Intercalation of AER progenitors generates a tensile gradient that reorients resolution of multicellular rosettes on adjacent surfaces, a process facilitated by β-catenin-dependent attachment of cortex to membrane. Therefore, feedback between tissue stress pattern and cell intercalations remodels mammalian ectoderm.

Sp6 and Sp8 transcription factors control AER formation and dorsal-ventral patterning in limb development

The formation and maintenance of the apical ectodermal ridge (AER) is critical for the outgrowth and patterning of the vertebrate limb. The induction of the AER is a complex process that relies on integrated interactions among the Fgf, Wnt, and Bmp signaling pathways that operate within the ectoderm and between the ectoderm and the mesoderm of the early limb bud. The transcription factors Sp6 and Sp8 are expressed in the limb ectoderm and AER during limb development. Sp6 mutant mice display a mild syndactyly phenotype while Sp8 mutants exhibit severe limb truncations. Both mutants show defects in AER maturation and in dorsal-ventral patterning. To gain further insights into the role Sp6 and Sp8 play in limb development, we have produced mice lacking both Sp6 and Sp8 activity in the limb ectoderm. Remarkably, the elimination or significant reduction in Sp6;Sp8 gene dosage leads to tetra-amelia; initial budding occurs, but neither Fgf8 nor En1 are activated. Mutants bearing a single functional allele of Sp8 (Sp6^−/−;Sp8^+/−) exhibit a split-hand/foot malformation phenotype with double dorsal digit tips probably due to an irregular and immature AER that is not maintained in the center of the bud and on the abnormal expansion of Wnt7a expression to the ventral ectoderm. Our data are compatible with Sp6 and Sp8 working together and in a dose-dependent manner as indispensable mediators of Wnt/βcatenin and Bmp signaling in the limb ectoderm. We suggest that the function of these factors links proximal-distal and dorsal-ventral patterning.

In this report we examined the functional roles of Sp6 and Sp8 during limb development using compound loss-of-function mutants. Sp6 and Sp8, two members of the Sp gene family, are expressed in the limb bud ectoderm and function downstream of WNT/βcatenin signaling for Fgf8 induction. The analysis of the allelic series shows that the progressive reduction in the dose of Sp6 and Sp8 gene products leads to predictable morphology, from syndactyly, to split hand/foot malformation, oligodactyly, truncation and finally amelia, indicating that these two factors act in a complementary manner. The molecular characterization of the mutant limbs reveal that Sp6/Sp8 are required in a dose-dependent manner for Fgf8 and En1 induction, thereby placing them as an important link between the induction of the AER and the establishment of dorsal-ventral patterning during limb development.

Integration of the transcriptional networks regulating limb morphogenesis

The developing limb is one of the best described vertebrate systems for understanding how coordinated gene expression during embryogenesis leads to the structures present in the mature organism. This knowledge, derived from decades of research, is largely based upon gain- and loss-of-function experiments. These studies have provided limited information about how the key signaling pathways interact with each other and the downstream effectors of these pathways. We summarize our current understanding of known genetic interactions in the context of three temporally defined gene regulatory networks. These networks crystallize our current knowledge, depicting a dynamic process involving multiple feedback loops between the ectoderm and mesoderm. At the same time, they highlight the fact that many essential processes are still largely undescribed. Much of the dynamic transcriptional activity occurring during development is regulated by distal cis-regulatory elements. Modern genomic tools have provided new approaches for studying the function of cis-regulatory elements and we discuss the results of these studies in regard to understanding limb development. Ultimately, these genomic techniques will allow scientists to understand how multiple signaling pathways are integrated in space and time to drive gene expression and regulate the formation of the limb.

Lmx1b-expressing cells in the mouse limb bud define a dorsal mesenchymal lineage compartment

The LIM-homeodomain transcription factor lmx1b is a critical regulator of vertebrate dorsal-ventral limb patterning. In the mouse embryo, lmx1b is initially transcribed throughout most, if not all, limb bud cells at early stages, but then rapidly becomes restricted specifically to dorsal mesenchymal cells with a sharp boundary between the dorsal-positive and ventral-negative expression domains. How this expression pattern is initially established is not well understood, nor are mechanism(s) that maintain a sharp dorsal-ventral boundary between lmx1b expressing and nonexpressing cells. Here, we employ a genetic fate mapping approach to establish that the transition from a broad expression domain of lmx1b to a restricted dorsal domain of expression involves selective loss of lmx1b expression in presumptive ventral cells. In addition, we show that once lmx1b expression becomes restricted to dorsal mesenchyme cells, these cells form a lineage-based compartment that prevents mixing between dorsal and ventral cells, consistent with recent fate mapping experiments carried out the chick and mouse (Pearse et al.,2007, Dev Biol 310:388-400; Arques et al.,2007, Development 134:3713-3722). Moreover, lmx1b activity is required to maintain, but not to establish the dorsal mesenchymal compartment likely through a mechanism involving differential cell adhesion. Taken together, our results indicate that lmx1b expressing cell define a dorsal limb bud mesenchymal lineage compartment and that maintenance of this compartment depends on lmx1b function.

Shh signaling in limb bud ectoderm: Potential role in teratogen-induced postaxial ectrodactyly

A variety of teratogens induce the loss of postaxial forelimb structures when administered during mid-gestation to the mouse. Previous studies demonstrated that teratogen exposure is associated with a reduction in zone of polarizing activity (ZPA) -related polarizing activity without a noticeable loss of Shh expression. Herein, we quantitatively confirm that expression of Shh, Ptch1, and Gli3 are unaltered by teratogen exposure and demonstrate that sonic hedgehog (Shh) translation is unaffected. Examination of the polarizing response of host chick wings to teratogen-exposed ZPA tissue revealed an induced growth response and ectopic induction of Fgf4, Bmp2, Ptch1, and Gli1 expression similar to control ZPA tissue. Control ZPA tissue altered the fate of cells destined to die in the anterior necrotic zone, whereas cell death ensued in hosts receiving teratogen-exposed grafts. Immunohistochemical studies localized Shh protein in the mouse limb to the posterior mesoderm and overlying ectoderm. We postulate that teratogen exposure alters the ability of Shh to signal to the ectoderm and present microarray and reverse transcriptase-polymerase chain reaction data, indicating that Shh signaling could occur in the limb bud ectoderm.

Type V collagen controls the initiation of collagen fibril assembly

Vertebrate collagen fibrils are heterotypically composed of a quantitatively major and minor fibril collagen. In non-cartilaginous tissues, type I collagen accounts for the majority of the collagen mass, and collagen type V, the functions of which are poorly understood, is a minor component. Type V collagen has been implicated in the regulation of fibril diameter, and we reported recently preliminary evidence that type V collagen is required for collagen fibril nucleation (Wenstrup, R. J., Florer, J. B., Cole, W. G., Willing, M. C., and Birk, D. E. (2004) J. Cell. Biochem. 92, 113-124). The purpose of this study was to define the roles of type V collagen in the regulation of collagen fibrillogenesis and matrix assembly. Mouse embryos completely deficient in pro-alpha1(V) chains were created by homologous recombination. The col5a1-/- animals die in early embryogenesis, at approximately embryonic day 10. The type V collagen-deficient mice demonstrate a virtual lack of collagen fibril formation. In contrast, the col5a1+/- animals are viable. The reduced type V collagen content is associated with a 50% reduction in fibril number and dermal collagen content. In addition, relatively normal, cylindrical fibrils are assembled with a second population of large, structurally abnormal collagen fibrils. The structural properties of the abnormal matrix are decreased relative to the wild type control animals. These data indicate a central role for the evolutionary, ancient type V collagen in the regulation of fibrillogenesis. The complete dependence of fibril formation on type V collagen is indicative of the critical role of the latter in early fibril initiation. In addition, this fibril collagen is important in the determination of fibril structure and matrix organization.

Function of BMPs in the apical ectoderm of the developing mouse limb

Several bone morphogenetic proteins (BMPs) are expressed in the apical ectodermal ridge (AER), a critical signaling center that directs the outgrowth and patterning of limb mesoderm, but little is known about their function. To study the functions of apical ectodermal BMPs, an AER-specific promoter element from the Msx2 gene was used to target expression of the potent BMP antagonist noggin to the apical ectoderm of the limbs of transgenic mice. Msx2-noggin mutant mice have severely malformed limbs characterized by syndactyly, postaxial polydactyly, and dorsal transformations of ventral structures indicated by absence of ventral footpads and presence of supernumerary ventral nails. Mutant limb buds exhibit a dorsoventral (DV) and anteroposterior (AP) expansion in the extent of the AER. AER activity persists longer than normal and is maintained in regions of the apical ectoderm where its activity normally ceases. Mutant limbs possess a broad band of mesodermal tissue along the distal periphery that is absent from normal limbs and which fails to undergo the apoptosis that normally occurs in the subectodermal mesoderm. Taken together, our results suggest that apical ectodermal BMPs may delimit the boundaries of the AER by preventing adjacent nonridge ectodermal cells from becoming AER cells; negatively modulate AER activity and thus fine-tune the strength of AER signaling; and regulate the apoptosis of the distal subectodermal mesoderm that occurs as AER activity attenuates, an event that is essential for normal limb development. Our results also confirm that ectodermal BMP signaling regulates DV patterning.

Wnts and wing: Wnt signaling in vertebrate limb development and musculoskeletal morphogenesis

In the past twenty years, secreted signaling molecules of the Wnt family have been found to play a central role in controlling embryonic development from hydra to human. In the developing vertebrate limb, Wnt signaling is required for limb bud initiation, early limb patterning (which is governed by several well-characterized signaling centers), and, finally, late limb morphogenesis events. Wnt ligands are unique, in that they can activate several different receptor-mediated signal transduction pathways. The most extensively studied Wnt pathway is the canonical Wnt pathway, which controls gene expression by stabilizing beta-catenin in regulating a diverse array of biological processes. Recently, more attention has been given to the noncanonical Wnt pathway, which is beta-catenin-independent. The noncanonical Wnt pathway signals through activating Ca(2+) flux, JNK activation, and both small and heterotrimeric G proteins, to induce changes in gene expression, cell adhesion, migration, and polarity. Abnormal Wnt signaling leads to developmental defects and human diseases affecting either tissue development or homeostasis. Further understanding of the biological function and signaling mechanism of Wnt signaling is essential for the development of novel preventive and therapeutic approaches of human diseases. This review provides a critical perspective on how Wnt signaling regulates different developmental processes. As Wnt signaling in tumor formation has been reviewed extensively elsewhere, this part is not included in the review of the clinical significance of Wnt signaling.

Sp8 is crucial for limb outgrowth and neuropore closure

In this report we describe the developmental expression and function of Sp8, a member of the Sp family of zinc finger transcription factors, and provide evidence that the legless transgene insertional mutant is a hypomorphic allele of the Sp8 gene. Sp8 is expressed during embryogenesis in the forming apical ectodermal ridge (AER), restricted regions of the central nervous system, and tail bud. Targeted deletion of the Sp8 gene gives a striking phenotype, with severe truncation of both forelimbs and hindlimbs, absent tail, as well as defects in anterior and posterior neuropore closure leading to exencephaly and spina bifida. Outgrowth of the limb depends on formation of the AER, a signaling center that forms at the limb bud apex. In Sp8 mutants, the AER precursor cells are induced and initially express multiple appropriate marker genes, but expression of these genes is not maintained and progression to a mature AER is blocked. These observations indicate that Sp8 functions downstream of Wnt3, Fgf10, and Bmpr1a in the signaling cascade that mediates AER formation.

Asymmetric limb malformations in a new transgene insertional mutant, footless

Six to eight copies of a transgene integrated into mouse chromosome 15 resulting in a new transgene insertional mutant, Footless, presenting with malformations of the limbs, kidney, and soft palate. Homozygotes possess a unique asymmetric pattern of limb truncations. Posterior structures from the autopod and zeugopod of the hindlimbs are missing with left usually more severely affected than right. In contrast, anterior structures are missing from the right forelimbs. The left forelimb is usually normal except for the absence of the distal telephalanges and nails. These structures are absent on all formed digits. In situ hybridization assays examined the expression of Shh, dHand, Msx2, Fgf8, En1, and Lmx1b in mutant limb buds and indicated normal establishment of the anterior/posterior and dorsal/ventral axes of the developing limbs. However, dysmorphology of the apical ectodermal ridge was observed in the mutant limb buds.

Doubleridge, a mouse mutant with defective compaction of the apical ectodermal ridge and normal dorsal-ventral patterning of the limb

doubleridge is a transgene-induced mutation characterized by polydactyly and syndactyly of the forelimbs. The transgene insertion maps to the proximal region of chromosome 19. During embryonic development of the mutant forelimb, delayed elevation and compaction of the apical ectodermal ridge (AER) produces a ridge that is abnormally broad and flat. Fgf8 expression persists in the ventral forelimb ectoderm of the mutant until E10.5. Strong expression of Fgf8 and other markers at the borders of the AER at E11.5 gives the appearance of a double ridge. At E11.5, apoptotic cells are distributed across the broadened ridge, but at E13.5, there is reduced apoptosis in the interdigital regions. The Shh expression domain is widely spaced at the posterior margin of the AER. The doubleridge AER is morphologically similar to that of En1 null mice, but the expression of En1 and Wnt7a is properly restricted in doubleridge, and the dorsal and ventral structures are correctly determined. doubleridge thus exhibits an unusual limb phenotype combining abnormal compaction of the AER with normal dorsal/ventral patterning.

Ectodermal Wnt3/beta-catenin signaling is required for the establishment and maintenance of the apical ectodermal ridge

The formation of the apical ectodermal ridge (AER) is critical for the distal outgrowth and patterning of the vertebrate limb. Recent work in the chick has demonstrated that interplay between the Wnt and Fgf signaling pathways is essential in the limb mesenchyme and ectoderm in the establishment and perhaps the maintenance of the AER. In the mouse, whereas a role for Fgfs for AER establishment and function has been clearly demonstrated, the role of Wnt/beta-catenin signaling, although known to be important, is obscure. In this study, we demonstrate that Wnt3, which is expressed ubiquitously throughout the limb ectoderm, is essential for normal limb development and plays a critical role in the establishment of the AER. We also show that the conditional removal of beta-catenin in the ventral ectodermal cells is sufficient to elicit the mutant limb phenotype. In addition, removing beta-catenin after the induction of the ridge results in the disappearance of the AER, demonstrating the requirement for continued beta-catenin signaling for the maintenance of this structure. Finally, we demonstrate that Wnt/beta-catenin signaling lies upstream of the Bmp signaling pathway in establishment of the AER and regulation of the dorsoventral polarity of the limb.

Initiation of dorso-ventral axis during chick limb development

We analysed spatio-temporal expression of dorso-ventral genes - Wnt-7a, En-1, Lmx-1 and Fgf-8 - during both normal and ectopic limb formation following fibroblast growth factor (FGF) application to the flank. We confirm that Wnt-7a is the first of these genes to be expressed in dorsal ectoderm in limb-forming regions. We also noticed patterns and kinetics of gene expression specific to chick that could account for differences observed in ridge formation between chick and mouse. We find that Wnt-7a expression, in dorsal ectoderm, is rapidly and locally induced by FGF application. In contrast, ectopic induction of Lmx-1 expression, in dorsal mesoderm, is much slower, occurs first at a distance from the FGF-2 bead and seems initially independent of direct Wnt-7a signalling during FGF-2 limb induction. Finally, we show that there is no contribution to extra-limb mesoderm from normal limb mesoderm and confirm that flank cells give rise to the extra limb. Furthermore, we suggest that an inhibitor present in the flank normally prevents Lmx-1 expression in this region and restricts its expression to limb-forming regions.

Patterning mechanisms controlling vertebrate limb development

Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anterior-posterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-beta/BMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.

BMP controls proximodistal outgrowth, via induction of the apical ectodermal ridge, and dorsoventral patterning in the vertebrate limb

Dorsoventral (DV) patterning of the vertebrate limb requires the function of the transcription factor Engrailed 1 (EN1) in the ventral ectoderm. EN1 restricts, to the dorsal half of the limb, the expression of the two genes known to specify dorsal pattern. Limb growth along the proximodistal (PD) axis is controlled by the apical ectodermal ridge (AER), a specialized epithelium that forms at the distal junction between dorsal and ventral ectoderm. Using retroviral-mediated misexpression of the bone morphogenetic protein (BMP) antagonist Noggin or an activated form of the BMP receptor in the chick limb, we demonstrate that BMP plays a key role in both DV patterning and AER induction. Thus, the DV and PD axes are linked by a common signal. Loss and gain of BMP function experiments show that BMP signaling is both necessary and sufficient to regulate EN1 expression, and consequently DV patterning. Our results also indicate that BMPs are required during induction of the AER. Manipulation of BMP signaling results in either disruptions in the endogenous AER, leading to absent or severely truncated limbs or the formation of ectopic AERs that can direct outgrowth. Moreover, BMP controls the expression of the MSX transcription factors, and our results suggest that MSX acts downstream of BMP in AER induction. We propose that the BMP signal bifurcates at the level of EN1 and MSX to mediate differentially DV patterning and AER induction, respectively.

Two lineage boundaries coordinate vertebrate apical ectodermal ridge formation

Proximal–distal outgrowth of the vertebrate limb bud is regulated by the apical ectodermal ridge (AER), which forms at an invariant position along the dorsal–ventral (D/V) axis of the embryo. We have studied the genetic and cellular events that regulate AER formation in the mouse. In contrast to implications from previous studies in chick, we identified two distinct lineage boundaries in mouse ectoderm prior to limb bud outgrowth using a Cre/loxP-based fate-mapping approach and a novel retroviral cell-labeling technique. One border is transient and at the limit of expression of the ventral gene En1, which corresponds to the D/V midline of the AER, and the second border corresponds to the dorsal AER margin. Labeling of AER precursors using an inducible Cre showed that not all cells that initially express AER genes form the AER, indicating that signaling is required to maintain an AER phenotype. Misexpression of En1 at moderate levels specifically in the dorsal AER of transgenic mice was found to produce dorsally shifted AER fragments, whereas high levels ofEn1 abolished AER formation. In both cases, the dorsal geneWnt7a was repressed in cells adjacent to theEn1-expressing cells, demonstrating that signaling regulated by EN1 occurs across the D/V border. Finally, fate mapping of AER domains in these mutants showed that En1 plays a part in positioning and maintaining the two lineage borders.

Disrupting the establishment of polarizing activity by teratogen exposure

Between days 9.5 and 10, the forelimb buds of developing murine embryos progress from stage 1 which are just beginning to express shh and whose posterior mesoderm has only weak polarizing activity to stage 2 limbs with a distinguishable shh expression domain and full polarizing activity. We find that exposure on day 9.5 to teratogens that induce the loss of posterior skeletal elements disrupts the polarizing activity of the stage 2 postaxial mesoderm and polarizing activity is not subsequently restored. The ontogeny of expression of the mesodermal markers shh, ptc, bmp2, and hoxd-12 and 13, as well as the ectodermal markers wnt7a, fgf4, fgf8, cx43, and p21 occurred normally in day 9.5 teratogen-exposed limb buds. At stage 3, the treated limb apical ectodermal ridge usually possessed no detectable abnormalities, but with continued outgrowth postaxial deficiencies became evident. Recombining control, stage matched limb bud ectoderm with treated mesoderm prior to ZPA grafting restored the duplicating activity of treated ZPA tissue. We conclude that in addition to shh an early ectoderm-dependent signal is required for the establishment of the mouse ZPA and that this factor is dependent on the posterior ectoderm.

Wnt-7a in feather morphogenesis: involvement of anterior-posterior asymmetry and proximal-distal elongation demonstrated with an in vitro reconstitution model

How do vertebrate epithelial appendages form from the flat epithelia? Following the formation of feather placodes, the previously radially symmetrical primordia become anterior-posterior (A-P) asymmetrical and develop a proximo-distal (P-D) axis. Analysis of the molecular heterogeneity revealed a surprising parallel of molecular profiles in the A-P feather buds and the ventral-dorsal (V-D) Drosophila appendage imaginal discs. The functional significance was tested with an in vitro feather reconstitution model. Wnt-7a expression initiated all over the feather tract epithelium, intensifying as it became restricted first to the primordia domain, then to an accentuated ring pattern within the primordia border, and finally to the posterior bud. In contrast, sonic hedgehog expression was induced later as a dot within the primordia. RCAS was used to overexpress Wnt-7a in reconstituted feather explants derived from stage 29 dorsal skin to further test its function in feather formation. Control skin formed normal elongated, slender buds with A-P orientation, but Wnt-7a overexpression led to plateau-like skin appendages lacking an A-P axis. Feathers in the Wnt-7a overexpressing skin also had inhibited elongation of the P-D axes. This was not due to a lack of cell proliferation, which actually was increased although randomly distributed. While morphogenesis was perturbed, differentiation proceeded as indicated by the formation of barb ridges. Wnt-7a buds have reduced expression of anterior (Tenascin) bud markers. Middle (Notch-1) and posterior bud markers including Delta-1 and Serrate-1 were diffusely expressed. The results showed that ectopic Wnt-7a expression enhanced properties characteristic of the middle and posterior feather buds and suggest that P-D elongation of vertebrate skin appendages requires balanced interactions between the anterior and posterior buds.