Early mouse embryos produce and release factors with transforming growth factor activity

Les cellules souches embryonnaires : Du développement myocardique à la médecine régénératrice

Embryonic stem cells are capable to recapitulate the first stages of myocardial development. Using mouse embryonic stem cells, transcriptional networks specifying the cardiac fate can be delineated. Furthermore, using members of the TGFbeta superfamily to commit mouse ES cells toward a cardiac lineage, recent studies showed that ESC-derived cardiomyocytes were capable to repair post-infarcted myocardium of small and large animals. The next challenges are to validate such results using human ESCs in order to better comprehend cardiac congenital diseases and to foresee a cell therapy of heart failure. double dagger.

Transforming growth factor beta and mouse development

Transforming growth factor beta has many biological effects including the control of cellular growth, differentiation, migration and extracellular matrix production; these are all processes essential for normal development. Although mice first generated more than eight years ago, bearing mutations in TGF beta ligands demonstrated the importance of TGF beta-induced signal transduction pathways for development in mammals but complete functional analysis is still lacking. Here, the current state-of-the-art in mouse development is reviewed. As a basis for understanding function, the principle features of mouse development over the 21 days of pregnancy are described and illustrated, from fertilization of the egg to mid-gestation when organogenesis is largely complete. This is completed with a description of when and where TGF beta ligands, receptors and downstream signalling molecules are expressed as the mouse embryo develops. The functions of TGF beta in preimplantation development, in implantation of the embryos in the uterine wall and in postimplantation development are then described through a review of the literature on gene ablation of the ligands, receptors and downstream molecules, or the ectopic expression of dominant negative forms of the receptors in vivo, which interfere with normal signal transduction. The evidence confirms multifunctional roles at all stages of development.

Transcriptional activation of the type II transforming growth factor-beta receptor gene upon differentiation of embryonal carcinoma cells

Previously, it has been shown that differentiation of embryonal carcinoma (EC) cells turns on the expression of functional transforming growth factor type-beta receptors. Here, we show that the type II receptor (TbetaR-II) gene is activated at the transcriptional level when EC cells differentiate. We show that the differentiated cells, but not the parental EC cells, express transcripts for TbetaR-II. In addition, the expression of TbetaR-II promoter/reporter gene constructs are elevated dramatically when EC cells differentiate and we identify at least two positive and two negative regulatory regions in the 5' flanking region of the TbetaR-II gene. Moreover, we identify a cAMP response element/activating transcription factor site that acts as a positive cis-regulatory element in the TbetaR-II promoter, and we demonstrate that the transcription factor ATF-1 binds to this site and strongly stimulates the expression of the TbetaR-II promoter/reporter gene constructs when ATF-1 is overexpressed in EC-derived differentiated cells. Equally important, we identify a negative regulatory element in a 53-base pair region that had previously been shown to inhibit strongly the expression of TbetaR-II promoter/reporter gene constructs. Specifically, we demonstrate that this region, which contains an inverted CCAAT box motif, binds the transcription factor complex NF-Y (also referred to as CBF) in vitro. Furthermore, expression of a dominant-negative NF-YA mutant protein, which prevents DNA binding by NF-Y, enhances TbetaR-II promoter expression. Together, these studies suggest that the transcription factors ATF-1 and NF-Y play important roles in the regulation of the TbetaR-II gene.

Autocrine effects of transforming growth factor-alpha on the development of preimplantation mouse embryos

PURPOSE

We wished to explore the role of transforming growth factor (TGF)-alpha in mouse embryonic development.

METHODS

We examined the gene expression of TGF-alpha and epidermal growth factor receptor (EGFR) in mouse blastocysts by the reverse transcription-polymerase chain reaction and evaluated the effects of TGF-alpha on the development of preimplantation mouse embryos using TGF-alpha antisense oligodeoxynucleotide. Mouse teratocarcinoma F9 cells were also a subject of this study.

RESULTS

Gene transcripts of TGF-alpha and EGFR were present in both blastocysts and F9 cells. TGF-alpha significantly stimulated the rate of blastocoel expansion in early-cavitating blastocysts and the proliferation of F9 cells. Northern blot analysis showed that TGF-alpha gene expression in F9 cells was markedly suppressed in the presence of TGF-alpha antisense oligodeoxynucleotide. TGF-alpha antisense oligonucleotide significantly reduced the rate of blastocoel expansion and the growth of F9 cells. The inhibitory effects of TGF-alpha antisense oligonucleotide on blastocysts and F9 cells were reversed by the addition of TGF-alpha.

CONCLUSIONS

The present observations suggest that TGF-alpha acts as an autocrine factor in the development of preimplantation mouse embryos.

Expression of transforming growth factor alpha (TGF-alpha) gene in mouse embryonic development

PURPOSE

The expression of genes for TGF-alpha, epidermal growth factor (EGF), and the EGF receptor (EGFR) in mouse blastocysts was evaluated by the reverse transcription-polymerase chain reaction (RT-PCR). We evaluated the effects of TGF-alpha and EGF on the development of mouse embryo prior to implantation.

RESULTS

The results revealed the presence of transcripts of TGF-alpha and EGFR. However, EGF mRNA was not observed in repeated experiments. None of these growth factors influenced the rate of development from the two-cell stage to the blastocyst stage when added to the culture medium. These effects were further examined on measuring the incorporation of tritiated thymidine and leucine, providing indices of the synthesis of DNA and protein, respectively. A concentration of only 0.1 ng/ml of TGF-alpha, which shares a cell surface receptor with EGF, stimulated the synthesis of both DNA and protein. EGF at a concentration of 10 ng/ml stimulated the synthesis of DNA and protein by blastocysts. To explore autocrine effects of TGF-alpha on the rate of blastocoel expansion, TGF-alpha antisense oligodeoxynucleotides was used to reduce expression of the TGF-alpha gene. TGF-alpha at a concentration of 0.1 ng/ml stimulates the rate of blastocoel expansion in early cavitating mouse blastocysts. In contrast, TGF-alpha antisense oligonucleotides significantly reduced the rate of expansion.

CONCLUSIONS

Our present observations suggest that TGF-alpha/EGF and the EGFR may be involved in regulating embryonic development. In particular, TGF-alpha may serve as an autocrine factor in the regulation of embryonic development.

Cis-regulatory elements and transcription factors involved in the regulation of the transforming growth factor-beta 2 gene

Embryonal carcinoma (EC) cells and embryonic stem (ES) cells provide useful model systems for studying differentiation during early mammalian development. Previous studies have demonstrated that differentiation of two restricted mouse EC cell lines is accompanied by activation of the TGF-beta 2 gene. Moreover, one negative and two positive regulatory regions upstream of the transcription start site were identified, which appear to play key roles in the transcriptional regulation of the human TGF-beta 2 gene. In this report, we demonstrate that the same three regulatory regions strongly influence the activity of the TGF-beta 2 promoter in differentiated cells derived from the multipotent human EC cell line, NT2/D1, and from the murine totipotent ES cell line, CCE. We also determined that the same three regions are active in the regulation of the TGF-beta 2 gene in the murine parietal endoderm-like cell line, PYS-2. However, an additional negative regulatory region appears to contribute to the regulation of the TGF-beta 2 gene in PYS-2 cells. Last, mutation of a CRE/ATF element located just upstream of the transcription start site of the TGF-beta 2 gene reduces significantly the activity of the TGF-beta 2 promoter in the differentiated cells. However, in contrast to our previous findings, our gel mobility shift analyses demonstrate that this CRE/ATF element is bound by similar proteins in nuclear extracts prepared from undifferentiated and differentiated mouse EC cells as well as from undifferentiated human EC cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Transforming growth factor-beta 1 in reproduction and development

Expression patterns of TGF-beta s during embryogenesis and in adult reproductive organs, as well as the activities of these molecules in in vitro assays of biological processes relating to reproduction and development, have suggested that TGF-beta s may play a role in both reproductive function and embryonic development. To investigate the function of TGF-beta 1 in vivo, the murine TGF-beta 1 gene was disrupted by gene targeting, and animals that lacked TGF-beta 1 activity were generated. Homozygous mutant animals were obtained which exhibited a multifocal inflammatory disease. However, the observed numbers of homozygous mutant offspring were less than expected, suggesting the occurrence of some type of prenatal lethality. This paper reviews the proposed role of the TGF-beta s in reproductive and developmental processes and discusses observations obtained from the TGF-beta 1 gene-targeting experiments as they relate to these processes.

Transforming growth factor-beta in the early mouse embryo: implications for the regulation of muscle formation and implantation

In a search for functions of transforming growth factor-beta during early embryonic development we used two different experimental approaches. In the first we made use of embryonic stem (ES) cells. ES cells in culture differentiate to derivatives of all three germ layers and mimic some aspects of organogenesis when grown as aggregates in suspension to form embryoid bodies. Differentiation proceeds further when the embryoid bodies attach to suitable substrates. Muscle and neuronal cells are among the most readily identified cell types then formed. We examined the effect of all-trans retinoic acid (RA) and members of the transforming growth factor-beta family (TGF-beta 1, TGF-beta 2) under these conditions in an assay where single aggregates formed in hanging microdrops in medium supplemented with serum depleted of lipophilic substances which would include retinoids. Endoderm-like cells formed under all conditions tested. RA at concentrations of 10(-8) M and 10(-7) M induced the formation of neurons but in the absence of RA or at concentrations up to 10(-9) M, neurons were not observed. Instead, beating muscle formed in about one-third of the plated aggregates; this was greatly reduced when RA concentrations increased above 10(-9) M. Immunofluorescent staining for muscle specific myosin showed that two muscle cell types could be distinguished: elongated, non-contractile myoblasts and mononucleate flat cells. The mononucleate flat cells appeared to correspond with rhythmically contracting muscle. The number of non-contractile myoblasts increased 3-fold over controls in the presence of 10(-9) M RA. TGF-beta s increased the number of contractile and non-contractile muscle cells by a factor 3 to 7 over controls, depending on the TGF-beta isoform added and the muscle cell type formed. TGF-beta 2 also invariably increased the rate at which contracting muscle cells were first observed in replated aggregates. The stimulatory effect of TGF-beta s on the formation of mononucleate flat cells was completely abrogated by RA at 10(-9) M while the number of myoblasts under similar conditions was unchanged. These data suggest that a complex interplay between retinoids and TGF-beta isoforms may be involved in regulation of differentiation in early myogenesis. In the second approach, neutralizing polyclonal rabbit antibodies specific for TGF-beta 2 were injected into the cavity of mouse blastocysts 3.5 days post coîtum (pc). After 1 day in culture, embryos were transferred to pseudopregnant females. The number of decidua, embryos and resorptions were counted at day 8.5-9.5 pc.(ABSTRACT TRUNCATED AT 400 WORDS)

Colocalization of transforming growth factor-alpha and a functional epidermal growth factor receptor (EGFR) to the inner cell mass and preferential localization of the EGFR on the basolateral surface of the trophectoderm in the mouse blastocyst

Results of previous studies suggested that responses of mouse blastocysts to TGF-alpha/EGF treatment are mediated by EGF receptors (EGFR) located on the apical surface of the trophectoderm (TE). We report here results of experiments using gold-labeled EGF that confirm the presence of these apically located EGFRs. In addition, immunoelectron microscopy (IEM) studies using anti-EGFR antibodies indicate that the receptor is preferentially distributed on the basolateral surface of the TE. Furthermore, the receptor is also present on the inner cell mass (ICM) and is likely to be functional, since treatment of isolated ICMs with TGF-alpha affects [35S]methionine uptake and incorporation into acid-insoluble material. IEM was also used to demonstrate that EGF, which is not synthesized by the mouse preimplantation embryo, is present in both the oviduct and the uterus. Maternally derived EGF is present in both ICM and TE cells in freshly isolated blastocysts, but is present in greatly reduced amounts following overnight culture of blastocysts in vitro. Last, IEM was also used to demonstrate that TGF-alpha is preferentially localized to the ICM and polar TE. The co-localization of TGF-alpha and functional EGFRs to the ICM and polar TE suggests potential autocrine, juxtacrine, and paracrine roles for TGF-alpha in blastocyst development.

Expression of fibroblast growth factor receptors by embryonal carcinoma cells and early mouse embryos

We have previously shown that differentiation of embryonal carcinoma (EC) cells leads to both increased binding of FGF (fibroblast growth factor) and suppression of k-FGF expression. In the current study, we examined the expression of FGF receptors by EC cells, EC-derived differentiated cells and early mammalian embryos using the technique of reverse transcription-polymerase chain reaction (RT-PCR). We determined that both mouse, F9, and human, NT2/D1, EC cells as well as their differentiated counterparts express transcripts for two forms of FGF receptors, bek (bacterially expressed kinase) and flg (fms-like gene). In addition, we determined that mouse blastocysts express flg transcripts. The presence of FGF receptor transcripts in early embryos and the previous finding of FGF-related activity in medium conditioned by mouse blastocysts argue that the FGF family plays important roles during early mammalian development.

Induction of c-fos transcripts in early postimplantation mouse embryos by TGF-alpha, EGF, PDGF, and FGF

The activity of growth factor receptors in the early postimplantation mouse embryo was studied by analyzing changes in expression of mRNA transcripts of an early response gene, c-fos, after binding of specific ligands. Reverse transcription of mRNA coupled with the polymerase chain reaction was used to detect gene transcription in single embryos after exposure to growth factors. Postimplantation embryos (at 7.5 days of gestation) had physiologically active receptors for transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), human platelet-derived growth factor (PDGF), recombinant PDGF-AA homodimer, and basic fibroblast growth factor (FGF), as indicated by induced expression of c-fos mRNA, c-fos expression was not induced in untreated embryos or in embryos incubated with active recombinant PDGF-BB homodimer. These results show that growth factor receptors are functional during early mammalian embryogenesis.

Differential localization of TGF-beta 2 in mouse preimplantation and early postimplantation development

The localization of transforming growth factor type beta 2 (TGF-beta 2) has been followed during preimplantation and early postimplantation murine development using an anti-peptide antibody that specifically recognizes TGF-beta 2. The staining pattern showed that TGF-beta 2 is expressed from the four-cell stage onward and is differentially regulated as cells diverge to various lineages. High levels of staining were found in the trophectoderm of the blastocyst but no staining was observed in the inner cell mass. During postimplantation development the primitive and embryonic ectoderm also lacked detectable staining while visceral endoderm stained well. Parietal endoderm cells also showed positive staining reaction although to a lesser extent than visceral endoderm cells. These findings were confirmed in model systems of the embryo, namely, embryonal carcinoma and embryonic stem cells differentiated to to cells with either visceral or parietal endoderm characteristics. The possible regulatory role of this factor in early embryogenesis is discussed.

Expression of transforming growth factor-beta 3 by embryonal carcinoma cells, parietal endoderm-like cells and early mouse embryos

Utilizing the technique of reverse transcription-polymerase chain reaction (RT-PCR), we have examined the expression of transforming growth factor-beta 3 (TGF-beta 3) by embryonal carcinoma (EC) cells, EC-derived differentiated cells and early mammalian embryos. Using a TGF-beta bioassay, we determined that PYS-2 cells express considerable TGF-beta activity that cannot be completely neutralized by antibodies specific for TGF-beta 1 and TGF-beta 2. We also have determined that PYS-2 cells, as well as F9 EC cells and their differentiated cells, express transcripts for TGF-beta 3. In addition, we have determined that blastocysts, cultured for three days in serum-containing medium, express TGF-beta 3 transcripts. Thus, our data suggest that expression of TGF-beta 3 is initiated during early stages of mammalian development.

Regulation and expression of transforming growth factor type-� during early mammalian development

We have examined the effect of differentiation on the expression of different members of the transforming growth factor type-beta (TGF-beta) family using embryonal carcinoma (EC) cells and early mammalian embryos. We determined that TGF-beta activity increases approximately 25-100% when the mouse EC cell line, F9, is induced to differentiate with retinoic acid (RA). Interestingly, the increased TGF-beta activity reflects the induction of TGF-beta 2 secretion following differentiation of both F9 EC cells and the human EC cell line, NT2/D1. Using the technique of reverse transcription-polymerase chain reaction (RT-PCR), we have verified that differentiation induces the expression of TGF-beta 2 as well as a distant member of the TGF-beta family, Vgr-1. Transcripts for TGF-beta 2 and Vgr-1 were readily detected in the differentiated cells of F9 and PC-13 but not in their undifferentiated counterparts. Moreover, TGF-beta 2 mRNA was readily detected in NT2/D1 cells following differentiation. In addition, transcripts for TGF-beta 2 were detected by RT-PCR in mouse morulae, preimplantation blastocysts and cultured blastocysts. Based on the data presented, it appears that the expression of both TGF-beta 2 and Vgr-1 is closely associated with the induction of differentiation during early development.

Chemical synthesis of bovine transforming growth factor-alpha: synthesis, characterization and biological activity

Bovine transforming growth factor-alpha (bTGF-alpha) is a 50 amino acid polypeptide with three disulfide linkages. In order to evaluate the biological function of this peptide, bTGF-alpha was synthesized via an automatic synthesizer and purified to homogeneity in high yield. The integrity of this synthetic peptide was confirmed by chemical analyses and bioassays. In a bovine liver radioreceptor assay, bTGF-alpha competes with radiolabeled EGF and has activity comparable to mEGF and hTGF-alpha. Compared to hEGF, bTGF-alpha elicits a greater response in a bovine mammary cell proliferation.

Evidence linking programmed cell death in the blastocyst to polyamine oxidation

Programmed cell death occurs in the inner cell mass during blastulation concomitant with the loss of its trophectodermal potential, and blastocele fluid kills malignant inner cell mass cells with trophectodermal potential (ECa 247) but spares those with embryonic potential (P19). A previous study had shown that blastocele-like fluid from embryoid bodies of the teratocarcinoma C44 contains a low-molecular-weight cytotoxin that exhibits the same target-cell selectivity as normal blastocele fluid. The current paper shows that the preferential killing of cells with trophectodermal potential is caused by hydrogen peroxide generated during the oxidation of polyamines in the cyst fluid by amine oxidases. The greater resistance of cells with embryonic potential to hydrogen peroxide is due to glutathione-dependent mechanisms. These data lead to the conclusion that an amine oxidase in the blastocyst oxidizes polyamines in blastocele fluid, generating hydrogen peroxide which causes programmed cell death of normal and malignant cells with trophectodermal potential.

Transforming growth factor betas in mammalian embryogenesis

Type beta transforming growth factors (TGF beta s) are members of a large superfamily of related proteins, each of which plays a pivotal role in embryonic processes. The TGF beta s per se are at least five in number, though only three isoforms have been identified in mammals. Here we will review the evidence, taken from in vitro studies on bioactivity and histochemical localization of RNAs and encoded proteins in vivo, that TGF beta 1, beta 2 and beta 3 are involved in several mammalian developmental processes, including control of growth, differentiation, tissue inductions and morphogenesis.

Transforming growth factor-beta and its receptor are differentially regulated in human embryonal carcinoma cells

The human embryonal carcinoma cell lines Tera-2 clone 13 and NTera-2 clone D1 can be induced by retinoic acid to differentiate in vitro into neuroectodermal derivatives. The undifferentiated cells are rapidly proliferating and tumorigenic, whereas retinoic-acid-treated cells possess a decreased growth rate, lose their transformed phenotype and show a finite lifespan. Differentiation is accompanied by a marked increase in the levels of mRNA for TGF-beta 1 and TGF-beta 2 and the production of TGF-beta activity. Just like murine embryonal carcinoma cells the growth of Tera-2 clone 13 cells is not affected by the addition of either TGF-beta 1 or TGF-beta 2 to the culture medium. In contrast to published data on murine embryonal carcinoma cells, Tera-2 clone 13 and NTera-2 clone D1 cells bind TGF-beta 1 with high affinity, which is due to the presence of type-III TGF-beta receptors. Furthermore, and again in contrast to murine embryonal carcinoma cells, treatment of the human embryonal carcinoma cells with retinoic acid causes a nearly complete loss of TGF-beta 1 binding sites. These results are discussed in the light of similarities and differences in the regulation of growth and differentiation of human and murine embryonal carcinoma cell lines.

Identification and characterization of polypeptide growth factors secreted by murine embryonal carcinoma cells

Undifferentiated P19 and PC13 murine embryonal carcinoma (EC) cells have been analyzed for their ability to secrete polypeptide growth factors. This has been carried out by a combination of specific bioassays and the use of biochemical and immunological detection methods. Both P19 and PC13 EC cells secrete a platelet-derived growth factor (PDGF)-like growth factor, a type beta transforming growth factor, and insulin-like growth factors. In addition, PC13 EC cells secrete a heparin-binding growth factor functionally related to fibroblast growth factor, while P19 EC cells secrete transforming growth factor-alpha. This is the first demonstration for secretion of transforming growth factor-alpha by an equivalent of early embryonic cells. The possible paracrine growth stimulating effects of these growth factors have been tested on differentiated derivatives of P19 EC cells, corresponding to all three germ layers. The differences in growth factor production by various embryonal carcinoma cells are discussed in relation to the developmental origin of these cell lines.

Developmental expression of PDGF, TGF-alpha, and TGF-beta genes in preimplantation mouse embryos

Control of growth and differentiation during mammalian embryogenesis may be regulated by growth factors from embryonic or maternal sources. With the use of single-cell messenger RNA phenotyping, the simultaneous expression of growth factor transcripts in single or small numbers of preimplantation mouse embryos was examined. Transcripts for platelet-derived growth factor A chain (PDGF-A), transforming growth factor (TGF)-alpha, and TGF-beta 1, but not for four other growth factors, were found in whole blastocysts. TGF-alpha, TGF-beta 1, and PDGF antigens were detected in blastocysts by immunocytochemistry. Both PDGF-A and TGF-alpha were detected as maternal transcripts in the unfertilized ovulated oocyte, and again in blastocysts. TGF-beta 1 transcripts appeared only after fertilization. The expression of a subset of growth factors in mouse blastocysts suggests a role for these factors in the growth and differentiation of early mammalian embryos.

Production and utilization of growth factors related to fibroblast growth factor by embryonal carcinoma cells and their differentiated cells

Previous studies have established that embryonal carcinoma (EC) cells produce several different growth factors, but express few, if any, receptors for epidermal growth factor, platelet-derived growth factor, or transforming growth factor type-beta. In this study, the production and utilization of fibroblast growth factor (FGF) by EC cells and their differentiated cells were investigated. We have determined that EC cells produce a heat-labile, heparin-binding factor that competes with FGF for binding to membrane receptors and appears to be immunologically related to FGF. The same or a similar factor is produced by three different EC cell lines, including a multipotent human EC cell line. However, production of this factor is apparently reduced when each EC cell line differentiates. Unlike the parental EC cells, the differentiated cells respond to FGF by growth stimulation and the growth responses to FGF correlate with increased binding of FGF. Although the binding data indicate that both the EC cells and their differentiated cells exhibit high affinity receptors for FGF, the differentiated cells express these receptors at levels approximately 10-fold higher. These findings suggest that the FGF-related growth factor could influence the growth of EC cells or their differentiated cells.

Autocrine growth induced by the insulin-related factor in the insulin-independent teratoma cell line 1246-3A

An insulin-independent teratoma-derived cell line, called 1246-3A, has been isolated from the adipogenic cell line 1246, which stringently requires insulin for proliferation. The 1246-3A cell line, which can proliferate in the absence of exogenous insulin, produces in its conditioned medium a growth factor similar to pancreatic insulin by its biological and immunological properties. This factor, called "insulin-related factor" (IRF), was purified and iodinated to study its binding to cell surface receptors. 125I-labeled IRF binding to intact 1246-3A cells is lower than to 1246 cells. Cell surface binding can be restored by culturing the 1246-3A cells in the presence of an anti-porcine insulin monoclonal antibody or by acid prewash of the cells prior to performing the binding. Scatchard analysis of binding indicates that IRF secreted by the 1246-3A cells partially occupies high-affinity binding sites on the producer cells. Moreover, insulin monoclonal antibody inhibits the proliferation of the IRF-producing 1246-3A cells, suggesting that these cells are dependent on the secreted IRF for growth in culture. We conclude that the insulin-related factor secreted by the insulin-independent 1246-3A cells stimulates their proliferation in an autocrine fashion.

The effects of epidermal growth factor on neural crest cells in tissue culture

Epidermal growth factor (EGF) stimulates the release of hyaluronic acid (HA) and chondroitin sulfate proteoglycan (CSPG) from quail trunk neural crest cultures in a dose-dependent fashion. It also promotes the expression of cell-associated heparan sulfate proteoglycan (HSPG) as detected by immunofluorescence and immunoprecipitation of the 3H-labeled proteoglycan. Furthermore, EGF stimulates [3H]thymidine incorporation into total cell DNA. These results raise the possibility that EGF or an analogous growth factor is involved in regulation of neural crest cell morphogenesis.