A member of the Met/HGF-receptor family is expressed in a BMP-4-like pattern in the ectoderm of Xenopus gastrulae

The importance and involvement of growth factors and their corresponding receptors in embryonic induction has been more and more recognized during the past decade, in particular by loss-of-function experiments using dominant negative receptors. Here, we report the isolation of XHR, a Xenopus receptor-type tyrosine kinase, with homology to members of the Met/hepatocyte growth factor (HGF)-receptor family. Sequence comparison of XHR with other members of the Met/HGF-receptor family as well as in situ expression analyses suggest that XHR represents a novel member of this family of receptor-type tyrosine kinases. As could be shown by whole-mount in situ analysis, XHR transcripts are first expressed in the entire ectoderm at the onset of gastrulation. As gastrulation proceeds, XHR-transcription is turned off in cells induced by dorsal mesoderm to form neural tissue and thus, becomes predominantly confined to prospective epidermis. The strikingly similar expression patterns of XHR and Bone Morphogenetic Protein-4 (BMP-4), an inducer of epidermis and inhibitor of neural development, suggest an involvement of XHR signalling in the early cell-fate decision of ectodermal cells to form either neural derivatives or epidermis.

Neural induction in Xenopus laevis: evidence for the default model

At gastrulation, vertebrate ectoderm is competent to differentiate into either neural tissue or epidermis. Several soluble factors that can neuralize ectoderm in explant cultures have been isolated. Alternatively, neuralization can be achieved by dissociating the cells of the blastula ectoderm. These various treatments appear to neuralize by blocking or diluting out the action of an epidermal-inducing factor. Recent results demonstrate that bone morphogenetic protein 4 (BMP-4), a member of the transforming growth factor beta (TGF-beta) ligand superfamily, is a potent neural inhibitor and epidermal inducer and may represent the endogenous epidermal-inducing factor.

Ectopic application of recombinant BMP-2 and BMP-4 can change patterning of developing chick facial primordia

The facial primordia initially consist of buds of undifferentiated mesenchyme, which give rise to a variety of tissues including cartilage, muscle and nerve. These must be arranged in a precise spatial order for correct function. The signals that control facial outgrowth and patterning are largely unknown. The bone morphogenetic proteins Bmp-2 and Bmp-4 are expressed in discrete regions at the distal tips of the early facial primordia suggesting possible roles for BMP-2 and BMP-4 during chick facial development. We show that expression of Bmp-4 and Bmp-2 is correlated with the expression of Msx-1 and Msx-2 and that ectopic application of BMP-2 and BMP-4 can activate Msx-1 and Msx-2 gene expression in the developing facial primordia. We correlate this activation of gene expression with changes in skeletal development. For example, activation of Msx-1 gene expression across the distal tip of the mandibular primordium is associated with an extension of Fgf-4 expression in the epithelium and bifurcation of Meckel's cartilage. In the maxillary primordium, extension of the normal domain of Msx-1 gene expression is correlated with extended epithelial expression of shh and bifurcation of the palatine bone. We also show that application of BMP-2 can increase cell proliferation of the mandibular primordia. Our data suggest that BMP-2 and BMP-4 are part of a signalling cascade that controls outgrowth and patterning of the facial primordia.

Conservation of BMP signaling in zebrafish mesoderm patterning

To investigate the conservation of mechanisms for mesodermal patterning between zebrafish and Xenopus, we isolated two cDNA clones encoding bone morphogenetic protein (BMP)-related proteins from a zebrafish cDNA library. Based on their predicted amino acid sequences, these two clones were designated as zbmp-2 and zbmp-4. Whole-mount in situ hybridization analysis revealed that in gastrula embryo, both genes were localized in the ventral part of the embryo, consistent with the proposed function of Xenopus BMP-4 in ventral mesoderm specification. zbmp-4 expression, however, was also seen in the embryonic shield, the most dorsal mesodermal structure. To examine the ability of zbmp-2 to ventralize mesoderm, we injected synthetic mRNA into zebrafish embryos and found that overexpression of this gene eliminated dorsal structures including notochord at both morphological and molecular level. In contrast, expression of ventral marker gene eve1 was expanded to the dorsal side. These effects are analogous to the ventralization of embryos caused by ectopic xBMP-4 expression. Taken together, one may conclude that the developmental mechanisms for mesodermal patterning regulated by BMPs are evolutionarily conserved between amphibians and teleosts.

Antagonistic actions of activin A and BMP-2/4 control dorsal lip-specific activation of the early response gene XFD-1' in Xenopus laevis embryos

Transcription of the early response gene XFD-1' (XFKH1) in the dorsal lip (Spemann organizer) of Xenopus embryos is activated by dorsal mesoderm inducing factors. Promoter studies revealed the presence of an activin A response element (ARE) which is both necessary and sufficient for transcriptional activation of reporter genes in animal cap explants incubated with activin A. Surprisingly, this ARE is also active within vegetal explants in the absence of exogenously added inducers, but an additional inhibitory response element prevents transcription of the XFD-1' gene in the ventral/vegetal region of the embryo in vivo. This element is located upstream of the ARE, it responds to bone morphogenic proteins 2 and 4 (BMP-2/4) triggered signals and it overrides the activating properties of the ARE. Expression patterns of BMP-2 and BMP-4 in the late blastula stage embryo and, especially, their absence from the dorsal blastopore lip may thus control the spatial transcription of the XFD-1' gene. Accordingly, the temporal activation and the spatial restriction of XFD-1' gene activity to the Spemann organizer is regulated by antagonistic actions of two distinct members of the TGF-beta family (activin and BMP) which act on different promoter elements.

The ectodermal control in chick limb development: Wnt-7a, Shh, Bmp-2 and Bmp-4 expression and the effect of FGF-4 on gene expression

We have manipulated the chick limb bud by dorsoventrally inverting the ectoderm, by grafting the AER to the dorsal or ventral ectoderm and by insertion of an FGF-4 soaked heparin bead into the mesoderm. After dorso-ventral reversal of the ectoderm, Wnt-7a expression is autonomous from an early stage of limb development in the original dorsal ectoderm. Exogenous FGF-4 causes ectopic Wnt-7a expression and induces ectopic Shh. In addition, exogenous FGF-4 increases the thickness of cartilages and also shortens them, and both Bmp-2 and Bmp-4 may mediate this effect. The ectoderm outside the AER can regulate not only the dorso-ventral polarity of the underlying mesenchyme cells but also the cartilage formation, and both Bmp-2 and Bmp-4 may mediate this control.

BMP-2/-4 mediate programmed cell death in chicken limb buds

During limb development, the mesenchymal cells in restricted areas of limb bud, anterior necrotic zone, posterior necrotic zone, opaque zone and interdigital necrotic zones, are eliminated by programmed cell death. The transcripts of bone morphogenetic protein (Bmp)-2 and −4 were first detected in the areas where cell death was observed, then showed overlapping expression with the programmed cell death zones except the opaque zone. To investigate the function of BMP-2 and BMP-4 during limb pattern formation, the dominant negative form of BMP receptor was overexpressed in chick leg bud via a replication-competent retrovirus to block the endogenous BMP-2/-4 signaling pathway. This resulted in excess web formation at the anterior and posterior regions of limb buds in addition to marked suppression of the regression of webbing at the interdigital regions. Significant reductions in the number of apoptotic cells in these three necrotic zones were found in the limb buds which received the virus carrying dominant negative BMP receptor. This indicates that extra tissue formation is due to suppression of programmed cell death in the three necrotic zones. Moreover, BMP-2/-4 protein induced apoptosis of mesenchymal cells isolated from the interdigital region in vitro. Other TGFbeta family proteins as TGFbeta1 and Activin did not show this effect. These results suggest that BMP-2 and BMP-4 are the apoptotic signal molecules of the programmed cell death process in the chick limb buds.

Bone morphogenic protein-4

Bone morphogenic protein-4 (BMP-4) is one of nine structurally related BMPs belonging to the transforming growth factor-beta (TGF-beta) superfamily of secreted proteins. Mature BMP-4 is a dimer that binds to a multimeric transmembrane receptor with serine/threonine kinase activity. Although discovered because it stimulates bone formation in adult mammals, BMP-4 has important roles as a signalling molecule in embryonic tissues, including the developing central and peripheral nervous system, musculature and skeleton. It participates in an ancient signalling pathway also found in insects and worms. Nevertheless, the main practical application of BMPs is for stimulating repair of bone, and their use in humans is currently being assessed.

The Drosophila short gastrulation gene prevents Dpp from autoactivating and suppressing neurogenesis in the neuroectoderm

The short gastrulation (sog) gene is expressed in broad lateral stripes comprising the neuroectoderm of the Drosophila blastoderm embryo. sog encodes a predicted secreted protein that functions nonautonomously to antagonize the activity of the TGF-beta-like Decapentaplegic (Dpp) signaling pathway in the dorsal region of the embryo. Recently, it has been shown that sog and dpp are functionally equivalent to their respective Xenopus homologs chordin and BMP-4. In this report we provide the first direct evidence that sog plays a local role in the lateral region of the blastoderm embryo to oppose Dpp activity in the neuroectoderm. In the dorsal region, Dpp signaling both suppresses neurogenesis and maintains expression of genes that promote dorsal cell fates (dorsalization). We show that Dpp also can perform both of these functions in the neuroectoderm. In wild-type embryos, the ability of Dpp to induce expression of dorsal markers including itself (autoactivation) in the neuroectoderm is blocked by sog. We propose that Sog protects the neuroectoderm from an invasive positive feedback loop created by Dpp diffusion and autoactivation. We show that the two functions of Dpp signaling, neural suppression and dorsalization, are triggered by distinct thresholds of Dpp activity. Epistasis experiments reveal that all observed sog activity can be accounted for by Sog functioning as a dedicated Dpp antagonist. Finally, we provide evidence that Sog functions as a diffusible morphogen in the blastoderm embryo. These data strongly support the view that the primary phylogenetically conserved function of the Drosophila sog and dpp genes and the homologous Xenopus chordin and BMP-4 genes is to subdivide the primitive embryonic ectoderm into neural versus non-neural domains.

In vivo implantation of human osteosarcoma cells in nude mice induces bones with human-derived osteoblasts and mouse-derived osteocytes

Two human osteosarcoma cell lines, Hu09 and OST, were suspended in Matrigel (Becton Dickinson Labware, Bedford, Massachusetts) and implanted subcutaneously in the backs of nude mice. To study phenotypic changes of tumor cells and host cells, expression of mRNA for osteopontin (OPN), osteocalcin (OC), and osteonectin (ON) was analyzed by in situ hybridization. Bone tissue was formed in the tumors derived from Hu09 cells. OPN mRNA was transcribed predominantly in osteocyte-like cells within the bone, whereas OC mRNA was transcribed in osteoblast-like cells that surrounded the bone. ON mRNA was detected in both types of cells. The similarity of the expression pattern of OPN, OC, and ON during osteogenesis of Hu09 cells to that of normal skeletal development suggests that the bone formed in Hu09-implanted mice is the same as normal bone tissue. By DNA-DNA in situ hybridization using a human-specific Alu probe and a mouse-specific m-L1 probe, osteoblast-like cells in Hu09 tumorous bone were, however, of human origin, whereas osteocyte-like cells were of mouse origin. In the tumors derived from OST cells, no osteogenesis was observed during the experimental period, and the expression of OPN, OC, and ON was not detected in tumor cells. An endochondral bone formation was not evident when these cells were simply implanted into muscle tissue. An endochondral bone was, however, reactively induced in the host mUscle tissue either when 1 alpha-hydroxyvitamin D3 and all-transretinoic acid were administered to OST-implanted mice or when Hu09 cells were pretreated with dexamethasone before implantation. Hu09 implantation seems to be a useful tool not only for the study of the differentiation of osteosarcoma cells but also for the investigation of the mechanism of bone formation. This system, using Hu09 and OST, may provide us with a new tool for the isolation of the unidentified factors that induce or inhibit osteogenesis in vivo.

The role of bone morphogenetic proteins in vertebral development

This study first shows a striking parallel between the expression patterns of the Bmp4, Msx1 and Msx2 genes in the lateral ridges of the neural plate before neural tube closure and later on, in the dorsal neural tube and superficial midline ectoderm. We have previously shown that the spinous process of the vertebra is formed from Msx1- and 2-expressing mesenchyme and that the dorsal neural tube can induce the differentiation of subcutaneous cartilage from the somitic mesenchyme. We show here that mouse BMP4- or human BMP2-producing cells grafted dorsally to the neural tube at E2 or E3 increase considerably the amount of Msx-expressing mesenchymal cells which are normally recruited from the somite to form the spinous process of the vertebra. Later on, the dorsal part of the vertebra is enlarged, resulting in vertebral fusion and, in some cases (e.g. grafts made at E3), in the formation of a ‘giant’ spinous process-like structure dorsally. In strong contrast, BMP-producing cells grafted laterally to the neural tube at E2 exerted a negative effect on the expression of Pax1 and Pax3 genes in the somitic mesenchyme, which then turned on Msx genes. Moreover, sclerotomal cell growth and differentiation into cartilage were then inhibited. Dorsalization of the neural tube, manifested by expression of Msx and Pax3 genes in the basal plate contacting the BMP-producing cells, was also observed. In conclusion, this study demonstrates that differentiation of the ventrolateral and dorsal parts of the vertebral cartilage is controlled by different molecular mechanisms. The former develops under the influence of signals arising from the floor plate-notochord complex. These signals inhibit the development of dorsal subcutaneous cartilage forming the spinous process, which requires the influence of BMP4 to differentiate.

Specific induction of apoptosis in P19 embryonal carcinoma cells by retinoic acid and BMP2 or BMP4

Retinoic acid (RA) affects the response of many cells to growth factors, including the bone morphogenetic proteins (BMPs). The BMPs are members of the TGF-beta, family of growth factors, originally identified by their bone-inducing activities. Their widespread expression suggests many roles other than that in osteogenesis. Because RA modulates the cell's response to growth factors, this may be a means by which the retinoids exert some of their known teratogenic effects. One such cellular response may be apoptosis. While apoptosis is required for normal development, the location and timing of its induction must be carefully controlled. Recently, several TGF-beta family members have been implicated in the induction of apoptosis in certain cell types. We show here, using P19 embryonal carcinoma cells, that the combination of RA and BMP2 or BMP4 synergistically induces apoptosis in 40% of the population within 24 hr. In contrast, RA alone induces apoptosis in only 10-15% of the population and each of the BMPs alone minimally induces apoptosis. Apoptosis depends on the dose of both the RA and the BMP as well as on new protein synthesis. Further, the induction of apoptosis prevents the formation of fully differentiated neurons and glial cells and instead leads to primarily smooth muscle cell differentiation. These results suggest that some of the malformations caused by retinoids may be due to the induction of inappropriate apoptosis in cells exposed to BMPs.

Signalling by TGF-beta family members: short-range effects of Xnr-2 and BMP-4 contrast with the long-range effects of activin

BACKGROUND

One way of establishing a morphogen gradient in a developing embryo involves the localized synthesis of an inducing molecule followed by its diffusion into surrounding tissues. The morphogen-like effects of the mesoderm-inducing factor activin provide support for this idea in amphibian development. The questions remain, however, of how activin exerts its long-range effects, and whether long-range signalling is a property of all transforming growth factor beta (TGF-beta) family members.

RESULTS

We compare the signalling ranges of activin and two other TGF-beta family members, Xnr-2 and BMP-4. Unlike activin, Xnr-2 and BMP-4 act over short distances. Furthermore, the effects of constitutively active activin receptors are strictly cell-autonomous. These observations suggest that the long-range effects of activin occur through protein diffusion and that "relay' mechanisms are not initiated by any of these TGF-beta family members. Mechanisms limiting the signalling range of Xnr-2 were addressed by studying Xnr-2 processing and secretion. An activin-Xnr-2 fusion protein signals over many cell diameters, suggesting that regulated processing or secretion is one limiting factor. Disaggregation and reaggregation of Xnr-2-producing tissues also extends the range of Xnr-2, suggesting that components of intact tissue restrict spread of the protein.

CONCLUSIONS

The long-range effects of activin are likely to occur through the diffusion of activin protein. The short-range effects of Xnr-2 and BMP-4 emphasize that long-range diffusion is not a general property of TGF-beta-related molecules. Finally, signalling ranges may be regulated by constraints on processing or secretion and by interactions with extracellular components of embryonic tissues.

Expression of genes for bone morphogenetic proteins and receptors in human dental pulp

Bone morphogenetic proteins (BMP) have been shown to induce reparative dentine formation experimentally but the cells responsible, which respond to BMPs, have not been identified. The BMP signal is probably mediated by interaction of type I and II BMP receptors (R). Here, the RNA of human adult dental pulp and pulp cells in culture was examined by reverse transcription (RT) polymerase chain reaction (PCR) for evidence of mRNA for BMPs. mRNAs for BMP-2, -4, osteogenic protein-1, ActR-1 (activin-like kinase receptor), BMPR-IA, -IB and -II were detected by RT-PCR. The 698-bp PCR fragment for BMPR-IB was used to probe pulp cells for expression of that receptor. Cell expression of BMPR-IB was detected by the hybridization probe. The findings suggest that resident pulp cells may be able to respond to BMPs to initiate tissue formation.

Conservation of dorsal-ventral patterning in arthropods and chordates

Dorsal-ventral patterning within the ectodermal and mesodermal germ layers of Drosophila and Xenopus embryos is specified by a system of genes that has been conserved over 500 million years of evolution. In both organisms, the activity of the TGF-beta family member DPP/BMP4 is antagonized by SOG/CHORDIN. A second Xenopus gene, noggin, has a similar biological activity to chordin. Analysis of the action of these genes indicate that Spemann's organizer promotes dorsal cell fates in Xenopus by antagonizing a ventralizing signal encoded by the Bmp4 gene.

FGF-2, BMP-2, and BMP-4 regulate retinoid binding proteins and receptors in 3T3 cells

An Important part of intercellular signalling is the ability of responding cells to regulate multiple signal transduction pathways. Just as retinoic acid exposure alters expression of many peptide growth factors and their receptors, we have found that peptide growth factors alter the expression of cellular retinoic acid binding proteins (CRABPs) and retinoic acid receptors (RARs). FGF-2 (basic fibroblast growth factor) treatment of BALB 3T3 fibroblasts increased the level of CRABP I RNA, whereas bone morphogenetic protein (BMP)-2 and BMP-4 reduced this level as well as the levels of CRABP II and RAR beta 1/beta 3 transcripts. Regulation of the CRABP I gene by FGF-2 occurred posttranscriptionally by increasing RNA stability. However, BMP-2 down-regulated CRABP I message without affecting message stability. Neither of these mechanisms was dominant, with intermediate levels of CRABP I RNA occurring in the presence of both FGF-2 and BMP-2 or BMP-4. These two different modes of regulation thus allow different levels of CRABP I RNA accumulation in the presence of different ratios of FGF-2 and BMP-2 or BMP-4.

Bone morphogenetic proteins in development

The bone morphogenetic proteins (BMPs) constitute a large family of cytokines related to members of the transforming growth factor-beta superfamily. Recent evidence, in particular from gene targeting experiments in the mouse, indicates that BMPs are required for mesoderm formation and for the development and patterning of many different organ systems. Significant progress has also been made in understanding the role of BMPs in gastrulation and neurulation in Xenopus and in identifying genes regulating BMP expression and components of the downstream signaling pathways. Extracellular modifiers of BMP activity may constitute an opposing morphogenetic system.

Neural Crest Apoptosis and the Establishment of Craniofacial Pattern: An Honorable Death

During development of the vertebrate head neural crest cells emigrate from the hindbrain and populate the branchial arches, giving rise to distinct skeletal elements and muscle connective tissues in each arch. The production of neural crest from the hindbrain is discontinuous and crest cells destined for different arches, carrying different positional cues, are separated by regions of apoptosis centered on rhombomeres (r) 3 and r5. This cell death program is under the interactive control of the neighboring hindbrain segments. Both r3 and r5 produce large numbers of crest cells when freed from their flanking rhombomere, but when conjoined with their neighbor the cell death program is restored. Two key components of this program are Bmp 4 and msx-2, both of which are expressed in the apoptotic foci of r3 and r5 and which are also regulated by neighbor interactions. Importantly, the addition of recombinant Bmp 4 to isolated cultures of r3 and r5 induces the expression of Bmp 4 and msx-2 and restores the cell death program. This early neural crest segregation is maintained during development and it has profound effects upon the final craniofacial pattern. Even though crest cells from different axial origins will contribute to compound skeletal elements, these distinct populations do not intermingle. Furthermore head muscle connective tissues are exclusively anchored to skeletal domains arising from neural crest from the same axial level. Thus the discontinuous production of neural crest sculpts the crest into nonmixing streams and consequently ensures the fidelity of patterning.

BMP-4 mediates interacting signals between the neural tube and skin along the dorsal midline

BACKGROUND

The neural tube and its overlying tissues (skin and mesenchyme) interact along the dorsal midline during early development. This has been previously demonstrated experimentally in chicken embryos by the fact that the dorsal neural tube transplanted ectopically induced expression of Msx 2 in the adjacent tissues. It is important to identify the molecules responsible for these interactions.

RESULTS

We observed that BMP-4, a member of the TGFbeta-family, is expressed in the dorsal neural tube in a pattern closely correlated with that of Msx 2. In order to investigate whether BMP-4 mediates the signal between the neural tube and the skin/mesenchyme, BMP-4 was ectopically administered in ovo either by implantation of the recombinant protein or transplantation of COS cells producing BMP-4. Both manipulations resulted in ectopic induction of Msx 2 expression in the adjacent skin/mesenchyme. In addition, the activity of BMP-4 in inducing Msx 2 was counteracted by the floor plate.

CONCLUSION

These data suggest that BMP-4 positively mediates the signals from the neural tube to the adjacent tissues and that this signal may be an essential step for the establishment of the dorsal midline structures.

Overexpression of BMP-2 and BMP-4 alters the size and shape of developing skeletal elements in the chick limb

Bone morphogenetic proteins are members of the transforming growth factor beta (TGF beta) superfamily which are involved in a range of developmental processes including modelling of the skeleton. We show here that Bmp-2 is expressed in mesenchyme surrounding early cartilage condensations in the developing chick limb, and that Bmp-4 is expressed in the perichondrium of developing cartilage elements. To investigate their roles during cartilage development, BMP-2 and BMP-4 were expressed ectopically in developing chick limbs using retroviral vectors. Over-expression of BMP-2 or BMP-4 led to a dramatic increase in the volume of cartilage elements, altered their shapes and led to joint fusions. This increase in volume appeared to result from an increase in the amount of matrix and in the number of chondrocytes. The latter did not appear to be due to increased proliferation of chondrocytes, suggesting that it may result from increased recruitment of precursors. BMP-2 and BMP-4 also delayed hypertrophy of chondrocytes and formation of the osteogenic periosteum. These data provide insights into how BMP-2 and BMP-4 may model and control the growth of skeletal elements during normal embryonic development, suggesting roles for both molecules in recruiting non-chondrogenic precursors to chondrogenic fate.

Cloning of rat type I receptor cDNA for bone morphogenetic protein-2 and bone morphogenetic protein-4, and the localization compared with that of the ligands

A rat homologue cDNA of mouse (Koenig et al. [1994] Mol. Cell Biol. 14:5961-5974; Suzuki et al. [1994] Proc. Natl. Acad. Sci. USA 91: 10255-10259) and human (ten Dijke et al. [1994] J. Biol. Chem. 269:16985-16988) type I receptors for BMP-2 and BMP-4 was cloned. Tissue distribution of the receptor mRNA was studied by in situ hybridization using rats at embryonic days 9, 13, 15, and 18 as well as 1- and 5-day-old postnatal rats. In the rats at embryonic days 9, 13, and 15, the receptor mRNA was diffusely expressed over the embryonic bodies. At embryonic day 18, the receptor mRNA expression was high in the hair and whisker follicles, tooth bud, cartilage, bone, digestive organs, lung, kidney, heart, and meninges. The receptor mRNA was expressed over a much wider area than those of the ligands in many organs. In the lung and digestive organs, the receptor mRNA was diffusely expressed and most highly expressed in the bronchial epithelium and muscle layer, respectively, in both of which mRNA expression of the ligands was undetectable. The receptor mRNA was highly expressed in the meninges, although neither of the ligands was expressed in or near this region. These results suggest that this receptor participates in both mesoderm formation in early embryogenesis and differentiation of mesodermal cells during maturation of organs, and further suggest the presence of another factor(s) that binds the type I receptor.

A role for BMP-4 in the development of subcutaneous cartilage

BMP-4 is a transforming growth factor beta (TGF- beta) superfamily member which plays important roles in various developmental processes in vertebrate embryogenesis. In this study the expression pattern of BMP-4 was examined during early chick and quail embryonic development. Transcripts were found in the lateral ridges of the neural plate during neurulation and in dorsal regions of the neural tube after its closure along the whole length of the neural axis. To investigate the role of BMP-4 expressed in dorsal regions of the neural tube, cells producing BMP4 were grafted ectopically in the paraxial mesoderm in E2 chick embryos at the level of somites 16-22. Ectopic BMP4 induced Msx-1 and Msx-2 gene expression in superficial ectoderm and superficial mesodermal cells which normally do not express these transcription factors. Moreover, it could be seen that in the E9 operated embryos, additional cartilage had been induced at the level of the graft. These results suggest that BMP4 produced by dorsal regions of the neural tube acts as an endogenous inducing signal in the activation of the genes of the Msx family which are likely to be part of the cascade of molecular events leading to subcutaneous cartilage formation. Therefore, BMP-4 expression in the tissues located dorsally in the neural tube and superficial ectoderm is proposed to be involved in the development of the spinous process of the vertebra.