A gene regulatory network controls the balance between mesendoderm and ectoderm at pluripotency exit

During embryogenesis, differentiation of pluripotent cells into somatic cell types depends both on signaling cues and intrinsic gene expression programs. While the molecular underpinnings of pluripotency are well mapped, much less is known on how mouse embryonic stem cells (mESCs) differentiate. Using RNA-Seq profiling during specification to the three germ layers, we showed that mESCs switched on condition-specific gene expression programs from the onset of the differentiation procedure and that primed pluripotency did not constitute an obligatory intermediate state. After inferring the gene network controlling mESC differentiation, we tested the role of the highly connected nodes by deleting them in a triple knock-in Sox1-Brachyury-Eomes mESC line reporting on ectoderm, mesoderm, and endoderm fates. This led to the identification of regulators of mESC differentiation that acted at several levels: Sp1 as a global break on differentiation, Nr5a2 controlling ectoderm specification, and notably Fos:Jun and Zfp354c as opposite switches between ectoderm and mesendoderm fate.

Melanotropin as a potential regulator of pigment pattern formation in embryonic skin

Frozen tissue sections of developing axolotl embryos were labeled by indirect immunofluorescence with anti-alpha-MSH. Anti-MSH immunoreactivity is first detectable in embryos when neural crest cells are migrating from the neural tube. Antibody labeling is visible around the lateral and ventral edges of the neural tube and in the embryonic ectoderm. As development progresses, the amount of labeling increases greatly, particularly in developing ectoderm. Western blots of soluble proteins extracted from various developmental stages of axolotl embryo ectoderm reveal that MSH activity is associated directly with several high molecular weight components that may be part of the embryonic extracellular matrix. Thus, we suggest that melanotropin activity is present in embryonic axolotl skin, is associated with the extracellular matrix, and is thereby in a position to play a supportive and/or directive role in the establishment of embryonic pigment patterns.

Expression of Wise in chick embryos

We have performed in situ hybridization to study the expression of Wise in early chick embryos. Wise expression is first detectable in the ectoderm at posterior levels of late neurula. As development proceeds, Wise expression is seen in specific patterns in the ectoderm of the trunk region, pharyngeal arches, limb buds, and feather buds. In addition to these areas, particular cartilages such as the ones in the maxillary process and limbs start to express Wise at the late pharyngula stage, and the expression in these cartilages becomes stronger than that in epidermal components at later stages. Importantly, Wise is expressed in regions where other signaling molecules such as Wnt, Bmp, and Shh are known to function in morphogenesis and differentiation. Direct comparisons of the expression of Wise and these genes are also demonstrated.

Negative regulation of Activin/Nodal signaling by SRF during Xenopus gastrulation

Activin/Nodal signaling is essential for germ-layer formation and axial patterning during embryogenesis. Recent evidence has demonstrated that the intra- or extracellular inhibition of this signaling is crucial for ectoderm specification and correct positioning of mesoderm and endoderm. Here, we analyzed the function of Xenopus serum response factor (XSRF) in establishing germ layers during early development. XSRF transcripts are restricted to the animal pole ectoderm in Xenopus early embryos. Ectopic expression of XSRF RNA suppresses mesoderm induction, both in the marginal zone in vivo and caused by Activin/Nodal signals in animal caps. Dominant-negative mutant or antisense morpholino oligonucleotide-mediated inhibition of XSRF function expands the expression of mesendodermal genes toward the ectodermal territory and enhances the inducing activity of the Activin signal. SRF interacts with Smad2 and FAST-1, and inhibits the formation of the Smad2-FAST-1 complex induced by Activin. These results suggest that XSRF might act to ensure proper mesoderm induction in the appropriate region by inhibiting the mesoderm-inducing signals during early embryogenesis.

An essential role for zebrafish Fgfrl1 during gill cartilage development

The vertebrate craniofacial skeleton develops via a complex process involving signaling cascades in all three germ layers. Fibroblast growth factor (FGF) signaling is essential for several steps in pharyngeal arch development. In zebrafish, Fgf3 and Fgf8 in the mesoderm and hindbrain have an early role to pattern the pouch endoderm, influencing craniofacial integrity. Endodermal FGF signaling is required for the differentiation and survival of postmigratory neural crest cells that form the pharyngeal skeleton. We identify a novel role for zebrafish Fgf receptor-like 1a (Fgfrl1a) that is indispensable during gill cartilage development. We show that depletion of Fgfrl1a is sufficient to abolish cartilage derivatives of the ceratobranchials. Using an Fgfrl1a-deficient model, we analyzed expression of genes critical for chondrogenesis in the different compartments of the developing pharyngeal arch. Fgfrl1a-depleted animals demonstrate typical neural crest specification and migration to populate the arch primordia as well as normal pouch segmentation. However, in the absence of Fgfrl1a, larvae fail to express the transcription factor glial cells missing 2 (gcm2), a gene necessary for cartilage and gill filament formation, in the ectodermal lining of the branchial arches. In addition, two transcription factors essential for chondrogenesis, sox9a and runx2b, fail to express within the mesenchymal condensations of the branchial arches. A duplicate zebrafish gene, fgfrl1b, has now been identified. We show that Fgfrl1b is also required for proper formation of all ventral cartilage elements and acts cooperatively with Fgfrl1a during gill cartilage formation.

Basement membrane heterogeneity during chick development as shown by tomato (Lycopersicon esculentum) lectin binding

Basement membranes (BMs) constitute a distinct compartment of the extracellular matrix (ECM). All BMs show a similar structural appearance but differ in molecular composition. These variations have critical functional implications. The aim of this study is to establish the pattern of the tomato lectin (Lycopersicon esculentum agglutinin--LEA) binding sites in the BMs of the developing chick embryo (stages 4-21, Hamburger and Hamilton, 1951) in order to achieve a better understanding of the molecular heterogeneity of BMs. The study was performed with transmission electron microscopy (TEM) histochemistry, and confocal laser microscopy. TEM showed that LEA bound to the lamina densa and to the lamina fibroreticularis of the BMs. Through the period studied, most of the LEA binding appeared in the ectodermal BM and its derivatives. In the limb bud, LEA binding to the ectoderm BM was more intense in the ventral half than in the dorsal half. Furthermore, LEA allowed the early (HH16) detection of the transverse fibrillar tracts. In the lens and in the inner ear primordium, the BMs were LEA positive through the placode and cup stages. The binding was progressively reduced through the vesicle stage. The BMs of the olfactory primordium, and of the Rathke's pouch were positive. In contrast, the BMs of the developing central nervous system were negative. The BMs of both the paraxial and the lateral plates of the mesoderm were negative, whereas the notochord and the BM of the Wolffian duct were positive. The endodermal BM and its derivatives were negative. The ECM located between the fusing endocardial tubes, and the BM of the fusion zone of the paired aortae, were positive. This suggested an active role of the LEA-positive glycoproteins in the fusion of endothelia. Our results show the heterogeneity of the chick embryo BMs during development. In addition, LEA constitutes an excellent marker for the primordial germ cells.

Sp5l is a mediator of Fgf signals in anteroposterior patterning of the neuroectoderm in zebrafish embryo

The neuroectoderm is patterned along the anterior-posterior axis in vertebrate embryos. Fgf signals are required to induce the posterior neuroectodermal fates, but they repress the anterior fate. Sp5l/Spr2, an Sp1-like transcription factor family member, has been shown to be required for development of mesoderm and posterior neuroectoderm. We demonstrate here that repression of the anterior neuroectodermal markers fez and otx1 by fgf17b or fgf3 coincides with induction of sp5l in the anterior neuroectoderm, and that this repression is efficiently rescued by simultaneous sp5l knockdown. On the other hand, sp5l knockdown is able to inhibit inductive activity of ectopic Fgf signals on the expression of the posterior neuroectodermal markers gbx2, hoxb1b, and krox20. Furthermore, effect of overexpression of a dominant negative Fgf receptor on anteroposterior patterning of the neuroectoderm is rescued by sp5l overexpression. Taken together, these data suggest that sp5l mediates the functions of Fgf signals in anteroposterior patterning of the neuroectoderm during zebrafish embryogenesis.

Regulatory targets for transcription factor AP2 in Xenopus embryos

The transcription factor AP2 (TFAP2) has an important role in regulating gene expression in both epidermis and neural crest cells. In order to further characterize these functions we have used a hormone inducible TFAP2alpha fusion protein in a Xenopus animal cap assay to identify downstream targets of this factor. The most common pattern comprised genes predominantly expressed in the epidermis. A second group was expressed at high levels in the neural crest, but all of these were also expressed in the epidermis as well as in other tissues in which TFAP2alpha has not been detected, suggesting modular control involving both TFAP2-dependent and TFAP2-independent components. In addition, a few strongly induced genes did not overlap at all in expression pattern with that of TFAP2alpha in the early embryo, and were also activated precociously in the experimentally manipulated ectoderm, and thus likely represent inappropriate regulatory interactions. A final group was identified that were repressed by TFAP2alpha and were expressed in the neural plate. These results provide further support for the importance of TFAP2alpha in ectoderm development, and also highlight the molecular linkage between the epidermis and neural crest in the Xenopus embryo.

An essential role of Xenopus Foxi1a for ventral specification of the cephalic ectoderm during gastrulation

During gastrulation in Xenopus, the head ectoderm is subdivided into the central nervous system (CNS) anlage (neural plate) and the non-CNS ectoderm (i.e. epidermis, placodes and neural crest). The winged-helix transcription factor Xfoxi1a is one of the earliest markers for the preplacodal region at the mid-neurula stage. Interestingly, before the establishment of the preplacodal region, Xfoxi1a expression is detected in the entire cephalic non-neural ectoderm at the mid- and late gastrula stages. The present study focuses on the role of Xfoxi1a particularly at the gastrula stages. The early Xfoxi1a expression in the anteroventral ectoderm is dependent on Bmp signals and suppressed by Wnt signals. Inhibition of Xfoxi1a activities by injection of antisense oligonucleotides leads to suppression of non-CNS ectodermal markers (e.g. keratin) and expansion of the anterior expression domain of the CNS marker Sox2. Conversely, misexpression of Xfoxi1a suppresses Sox2 and induces keratin in the anterior neural plate. In the animal cap, Xfoxi1a overexpression antagonizes the neuralizing activity of Chordin (Chd). Studies using an inducible Xfoxi1a construct (GR-Xfoxi1a) show that the ventralizing function of Xfoxi1a is confined to the gastrula stage. Thus, Xfoxi1a is an essential regulator of ventral specification of the early head ectoderm during gastrulation.

In vitro attachment of bovine hatched blastocysts on fibronectin is mediated by integrin in a RGD dependent manner

We investigated the effect of extracellular matrix protein on in vitro attachment and outgrowth of bovine hatched blastocysts. In vitro produced bovine hatched blastocysts were cultured on a fibronectin- or laminin-coated Petri dishes. Hatched blastocysts adhered and outgrew on the fibronectin-coated dish whereas no attachment was observed on the laminin-coated dish. The attachment and outgrowth on fibronectin were significantly inhibited in the presence of synthetic peptides containing the Arg-Gly-Asp (RGD) sequence, which interacts with the fibronectin receptor (integrin alpha5beta1), but were not inhibited by the control peptides containing the Arg-Gly-Glu (RGE) sequence. Addition of anti-fibronectin receptor (integrin alpha5beta1) antibody to the culture medium also inhibited the attachment and outgrowth on fibronectin-coated Petri dishes. Subsequently we examined mRNA expression and protein expression of alpha5 and beta1 integrin subunit in the hatched blastocyst by reverse transcription-polymerase chain reaction (RT-PCR) and immunostaining, respectively. Expression of both mRNA and protein were detected in blastocysts. These results indicate that trophectoderm cells of bovine hatched blastocysts have already acquired the ability to adhere and outgrow on fibronectin in vitro by an integrin- mediated manner.

High-throughput functional screen of mouse gastrula cDNA libraries reveals new components of endoderm and mesoderm specification

This study describes a cross-species functional screen of mouse gastrula cDNA libraries for components of endoderm and mesoderm specification. Pools of 96 cDNAs from arrayed mouse gastrula cDNA libraries were transcribed into mRNA and injected into either the presumptive mesoderm or the ectoderm of one-cell Xenopus laevis embryos. Injected embryos were examined at gastrula stage by in situ hybridization with endoderm or mesoderm markers. Using this approach, we screened over 700 pools or approximately 60,000 cDNAs. We identified 17 unique cDNAs that function during mesoderm and/or endoderm specification and 16 that cause general morphology changes. Identified molecules fall into eight general functional groups as follows: cell cycle components (seven), transcription factors (four), extracellular secreted molecules (seven), transmembrane receptors (one), intracellular signaling components (five), microtubule components (two), metabolism molecules (three), and unknown (four). Several of the genes we identified would not have been predicted to be involved in endoderm or mesoderm specification, highlighting the usefulness of nonbiased screening approaches. This includes Otx2, which we show is a downstream target of Xsox17beta. The speed, low cost, and high efficiency of this cross-species screen makes it an ideal method for examining cDNAs from difficult-to-obtain sources. Therefore, this approach complements the current mouse molecular genetics systems and provides a powerful means for the genome-wide examination of mammalian gene function.

Expression patterns of engrailed-like proteins in the chick embryo

The protein products of both of the identified chick engrailed-like (En) genes, chick En-1 and chick En-2, are localized in cells of the developing brain, mandibular arch, spinal cord, dermatome, and ventral limb bud ectoderm, as demonstrated by labeling with the polyclonal antiserum alpha Enhb-1 developed by Davis et al. (Development 111:281-298, 1991). A subpopulation of cephalic neural crest cells is also En-protein-positive. The monoclonal antibody 4D9 recognizes the chick En-2 gene product exclusively (Patel et al.: Cell 58:955-968, 1989; Davis et al., 1991) and colocalizes with chick En-2 mRNA in the developing head region of the chick embryo as shown by in situ hybridization (Gardner et al.: J. Neurosci. Res. 21:426-437, 1988). In the present study we examine the pattern of alpha Enhb-1 and 4D9 localization throughout the chick embryo from the first appearance of antibody (Ab)-positive cells at stage 8 (Hamburger and Hamilton: J. Morphol. 88:49-92, 1951) through stage 28 (1-5.5 days). We compare the localization patterns of the two Abs to each other, as well as to the localization of the monoclonal Ab, HNK-1, which recognizes many neural crest cells, using double- and triple-label fluorescence immunohistochemistry. Most En protein-positive cells in the path of neural crest cell migration are not HNK-1 positive. In detailed examination of alpha Enhb-1 and 4D9 localization, we find previously undetected patterns of En protein localization in the prechordal plate, hindbrain, myotome, ventral body-wall mesoderm, and extraembryonic membranes. Based upon these observations we propose: 1) that En expression in the mesoderm may be induced through interaction with En expressing cells in the neuroectoderm; 2) that En expression in the head mesenchyme is associated with somitomere 4; and 3) that En expression may be involved in epithelial-mesenchymal cell transformations.

Derivation of human embryonic germ cells: an alternative source of pluripotent stem cells

Based on evidence suggesting similarities to human embryonic stem cells, human embryonic germ (hEG) cells have been advocated as an alternative pluripotent stem cell resource but have so far received limited attention. To redress this imbalance, human fetal gonads were collected for the isolation and culture of primordial germ cells at 7-9 weeks postconception. We provide evidence for the derivation, culture, and differentiation of hEG cells in vitro. This evidence includes the expression of markers characteristic of pluripotent cells, the retention of normal XX or XY karyotypes, and the demonstration of pluripotency, as suggested by the expression of markers indicative of differentiation along the three germ lineages (ectoderm, mesoderm, and endoderm) and an associated loss of pluripotent markers. In assessing this differentiation, however, we also demonstrate a hitherto unacknowledged overlap in gene expression profiles between undifferentiated and differentiated cell types, highlighting the difficulty in ascribing cell lineage by gene expression analyses. Furthermore, we draw attention to the problems inherent in the management of these cells in prolonged culture, chiefly the difficulty in preventing spontaneous differentiation, which hinders the isolation of pure, undifferentiated clonal lines. While these data advocate the pursuit of pluripotent hEG cell studies with relevance to early human embryonic development, culture limitations carry implications for their potential applicability to ambitious cell replacement therapies.

Evidence for antagonism of BMP-4 signals by MAP kinase during Xenopus axis determination and neural specification

We have previously shown that mitogen-activated protein (MAP) kinase activity is required for neural specification in Xenopus. In mammalian cells, the BMP-4 effector Smad1 is inhibited by phosphorylation at MAP kinase sites (Kretzschmar et al., 1997). To test the hypothesis that MAP kinase inhibits the BMP-4/Smad1 pathway during early Xenopus development, we have generated a Smad1 mutant lacking the MAP kinase phosphorylation sites (M4A-Smad1) and compared the effects of wild-type (WT)- and M4A-Smad1 on axial pattern and neural specification in Xenopus embryos. Although overexpression of either WT- or M4A-Smad1 produced ventralized embryos, at each mRNA concentration, M4A-Smad1 had a greater ventralizing effect than WT-Smad1. Interestingly, overexpression of either form of Smad1 in ventral blastomeres disrupted posterior pattern and morphogenesis; again, more severe defects were produced by expression of M4A-Smad1 than by equal amounts of WT-Smad1. Ectodermal expression of M4A-Smad1 disrupted expression of the anterior neural gene otx2 in vivo and inhibited neural specification in response to endogenous signals in mesoderm-ectoderm recombinates. In contrast, overexpression of WT-Smad1 at identical levels had little effect on either neural specification or otx2 expression. Comparisons of protein levels following overexpression of either WT- or M4A-Smad1 indicate that WT-Smad1 may be slightly more stable than M4A-Smad1; thus, differences in stability cannot account for the increased effectiveness of M4A-Smad1. Our results demonstrate that mutations disrupting the MAPK phosphorylation sites act collectively as a gain-of-function mutation in Smad1 and that inhibitory phosphorylation of Smad1 may be a significant mechanism for the regulation of BMP-4/Smad1 signals during Xenopus development.

Immunohistochemical and ultrastructural characterization of the initial post-hatching development of bovine embryos

The problems of sustaining placenta formation in embryos produced by nuclear transfer have emphasized the need for basic knowledge about epiblast formation and gastrulation in bovine embryos. The aims of this study were to define stages of bovine post-hatching embryonic development and to analyse functional mechanisms of germ-layer formation. Embryos developed in vivo were collected after slaughter from superovulated cows on days 9, 11, 14 and 21 after insemination and processed for transmission electron microscopy (n = 26) or immunohistochemistry (n = 27) for potential germ-layer characterization (cytokeratin 8 for potential ectoderm; alpha-1-fetoprotein for potential endoderm; and vimentin for potential mesoderm). On day 9, the embryos were devoid of zona pellucida and presented a well-defined inner cell mass (ICM), which was covered by a thin layer of trophoblast cells (the Rauber's layer). Formation of the hypoblast from the inside of the ICM was ongoing. On day 11, the Rauber's layer was focally interrupted and adjacent underlying ICM cells formed tight junctions. The hypoblast, which formed a thin confluent cell layer, was separated from the ICM and the tropho-blast by intercellular matrix. The embryos were ovoid to tubular and displayed a confluent hypoblast on day 14. The epiblast was inserted into the trophoblast epithelium and tight junctions and desmosomes were present between adjacent epiblast cells as well as between peripheral epiblast and trophoblast cells. In some embryos, the epiblast was more or less covered by foldings of trophoblast in the process of forming the amniotic cavity. Cytokeratin 8 was localized to the trophoblast and the hypoblast underlying the epiblast; alpha-1-fetoprotein was localized to most hypoblast cells underlying the trophoblast; and vimentin was localized to most epiblast cells. On day 21, the smallest embryos displayed a primitive streak and formation of the neural groove, whereas the largest embryos presented a neural tube, up to 14 somites and allantois development. These embryos depicted the gradual formation of the endoderm, mesoderm and ectoderm as well as differentiation of paraxial, intermediate and lateral plate mesoderm. Cytokeratin 8 was localized to the trophoblast, the hypoblast and the surface and neural ectoderm; and alpha-1-fetoprotein was localized to the hypoblast, but not the definitive endoderm, the intensity increasing with development. Vimentin was initially localized to some, but not all, cells positioned particularly in the ventral region of the primitive streak, to presumptive definitive endoderm cells inserted into the hypoblast, and to mesoderm. In conclusion, within 2 weeks of hatching, bovine embryos complete formation of the hypoblast and the epiblast, establishment of the amniotic cavity, ingression of epiblast cells for primitive streak formation, involution of cells through the node and the streak for endoderm and mesoderm fomation, neurulation and differentiation of the mesoderm. The recruitment of cells from the epiblast to form the primitive streak as well as the endoderm and mesoderm is associated with expression of the intermediate filament vimentin.

Ber-EP4 immunoreactivity depends on the germ layer origin and maturity of the squamous epithelium

AIM

To map the expression of Ber-EP4 in well-differentiated squamous epithelia, metaplastic squamous epithelia and dysplastic squamous epithelia of different origins.

METHODS AND RESULTS

Squamous epithelium of different origin was stained using a standard immunohistochemistry method applied to paraffin sections. We found that normal squamous epithelium of the oral cavity, oesophagus, uterine cervix, vagina, anal canal, and branchial cysts are Ber-Ep4-negative, as are the mature squamous metaplasia of bronchial mucosa, urinary bladder mucosa and uterine cervical mucosa. In contrast, immature squamous metaplasia of bronchial mucosa, or uterine cervical mucosa, and squamous dysplasia of oral mucosa of endodermal origin, or uterine cervical mucosa in most cases expressed Ber-EP4.

CONCLUSION

Squamous epithelia of ectodermal origin never express Ber-EP4, whether normal, hyperplastic, dysplastic or neoplastic. In contrast, squamous epithelium of endodermal origin sometimes contains the target glycoproteins of Ber-EP4 when immature, metaplastic, dysplastic or neoplastic. The results indicate that the differences in expression of Ber-EP4 in squamous epithelium depend primarily on germ layer origin, and on the maturity of the epithelium.

Opioid growth factor and organ development in rat and human embryos

In addition to neurotransmission, the native opioid peptide, [Met5]enkephalin, is a tonically active inhibitory growth molecule that is termed opioid growth factor (OGF). OGF interacts with the zeta (zeta) opioid receptor to influence cell proliferation and tissue organization. We now identify OGF and the zeta receptor in embryonic derivatives including ectoderm, mesoderm, and endoderm of the rat on gestation day 20. Messenger RNA for preproenkephalin (PPE), the precursor of OGF, was detected in the developing cells, suggesting an autocrine production of this peptide. Acute exposure of the pregnant female to OGF resulted in a decrease in DNA synthesis in cells of organs representing all three germ layers, and did so in a receptor-mediated fashion. The influence of OGF was direct, as evidenced in organ culture studies. Blockade of endogenous opioid interaction using naltrexone (NTX) produced an increase in DNA synthesis, indicating the constitutive and functional nature of opioid activity on growth during prenatal life. Human fetal cells contained OGF and the zeta receptor. These data support the hypothesis that endogenous opioid modulation of organ development is a fundamental principle of mammalian embryogenesis, and that OGF has a profound influence on ontogeny. Irregularities in the role of opioids as growth regulators in relationship to the more than 500,000 newborns suffering from birth defects each year in the US needs to be examined.

Absence of the tight junctional protein AF-6 disrupts epithelial cell-cell junctions and cell polarity during mouse development

BACKGROUND

The establishment, maintenance and rearrangement of junctions between epithelial cells are extremely important in many developmental, physiological and pathological processes. AF-6 is a putative Ras effector; it is also a component of tight and adherens junctions, and has been shown to bind both Ras and the tight-junction protein ZO-1. In the mouse, AF-6 is encoded by the Af6 gene. As cell-cell junctions are important in morphogenesis, we generated a null mutation in the murine Af6 locus to test the hypothesis that lack of AF-6 function would cause epithelial abnormalities.

RESULTS

Although cell-cell junctions are thought to be important in early embryogenesis, homozygous mutant embryos were morphologically indistinguishable from wild-type embryos through 6.5 days post coitum (dpc) and were able to establish all three germ layers. The earliest morphological abnormalities were observed in the embryonic ectoderm of mutant embryos at 7.5 dpc. The length of the most apical cell-cell junctions was reduced, and basolateral surfaces of those cells were separated by multiple gaps. Cells of the embryonic ectoderm were less polarized as assessed by histological criteria and lateral localization of an apical marker. Mutant embryos died by 10 dpc, probably as a result of placental failure.

CONCLUSIONS

AF-6 is a critical regulator of cell-cell junctions during mouse development. The loss of neuroepithelial polarity in mutants is consistent with a loss of efficacy of the cell-cell junctions that have a critical role in establishing apical/basolateral asymmetry.

Rhabdomyosarcoma metastasizing as a malignant ectomesenchymoma

Rhabdomyosarcoma is a common childhood malignancy that may occasionally occur as a component of a mixed mesenchymal tumor, e.g., a triton tumor or malignant ectomesenchymoma. A case is reported of a 13-year-old boy who had resection of a paratesticular embryonal rhabdomyosarcoma with subsequent radiation and chemotherapy. Two years later, a retroperitoneal metastasis was resected. Histology showed a mixture of rhabdomyoblasts, ganglion cells, and a third population of cells with combined features of these two distinct cell types. Electron microscopy confirmed the presence of rhabdomyoblasts with characteristic bundles of myofilaments and Z-band material, and ganglion cells with prominent nuclei and nucleoli, rough endoplasmic reticulum, dense core granules, filaments, and tubules. Notably, the third cell population showed features of both rhabdomyoblasts and ganglion cells. Immunohistochemistry confirmed the mixed population of rhabdomyoblasts (positive for vimentin, desmin, negative for S-100, NSE), ganglion cells (positive for S-100 and NSE, negative for vimentin and desmin), and the third population expressing all test antigens. The features of this metastatic lesion are those of a malignant ectomesenchymoma with combined rhabdomyosarcoma and ganglioneuroma components. It is postulated that this lesion results from biphenotypic expression of tumor cells that previously expressed only rhabdomyoblastic differentiation. The role of prior chemotherapy and radiotherapy in this particular case is unclear.

[Immunohistochemical characterisation of surgically excised choroidal neovascularisation in age-related macular degeneration]

AIM

The objective of the study was to gather further information about the pathogenesis of choroidal neovascularisations (CNV), which is still not clearly understood, and to establish criteria for making decisions on a appropriate therapy. Immunohistochemical characteristation should allow a more comprehensive evaluation of cellular components of the membranes and their functional role.

PATIENTS AND METHODS

In 29 patients (16 women, 13 men) with age-related macular degeneration ranging in age from 46 to 91 years (mean age, 76.4 years), CNV were excised by pars-plana vitrectomy. Sections were stained with hematoxylin-eosin (HE) and periodic acid-Schiff (PAS) and examined by light microscopy. For the immunohistochemical characterisation of the surgical specimens the following anti-genetic determinants were used: glial fibrillary acid protein (GFAP) for glial cells, synaptophysin for neuronal cells, neuron-specific enolase (NSE) for neuronal and neuroectodermal cells, CD 31 for endothelial cells and pancytokeratin (KL1) for cells of the retinal pigment epithelium (RPE). Cells undergoing apoptosis were labeled with the TUNEL technique.

RESULTS

22 (76%) surgical specimens showed TUNEL positive cells in the connective tissue, vascular endothelium and retinal pigment epithelium. Positive immunostaining of neuronal antigenetic determinants was found for glial fibrillary acid protein in 22 patients (76%), for synaptophysin in 28 patients (97%) and for neuron-specific enolase in 21 patients (72%) CNV. The epithelial marker KL1 was positive in 28 patients (97%) and the endothelial marker CD 31 in 20 patients (69%).

CONCLUSION

The immunohistochemical analyses of CNV showed that in the majority of cases during the excision of choroidal neovascularizations in addition to scar tissue and connective tissue also parts of the native retinal pigment epithelium and of the neurosensory retina are removed which is only partly visible with standard staining techniques. These findings suggest that the mostly not satisfying postoperative results are partly due to the damage of neuronal cells and a partial loss of the retinal pigment epithelium. Apoptosis as a regulating mechanism in choroidal neovascularization. The variable appearence of apoptosis suggests that it is possibly related to the degree of activity of CNV.

Reliability of delta-crystallin as a marker for studies of chick lens induction

Induction of a lens by the optic vesicle of the brain was the first demonstration of how tissue interactions could influence cell fate during development. However, recent work with amphibians has shown that the optic vesicle is not the primary inducer of lens formation. Rather, an earlier interaction between anterior neural plate and presumptive lens ectoderm appears to direct lens formation. One problem with many early experiments was the absence of an unambiguous assay for lens formation. Before being able to test whether the revised model of lens induction applies to chicken embryos, we examined the suitability of using delta-crystallin as a marker of lens formation. Although delta-crystallin is the major protein synthesized in the chick lens, one or both of the two delta-crystallin genes found in chickens is transcribed in many non-lens tissues as well. In studies of lens formation where appearance of the delta-crystallin protein is used as a positive assay, synthesis of delta-crystallin outside of the lens could make experiments difficult to interpret. Therefore, polyacrylamide gel electrophoresis, immunoblotting, and immunofluorescence were used to determine whether the delta-crystallin messenger RNA detected in non-lens tissues is translated into protein, as it is in the lens. On Coomassie-blue-stained gels of several tissues from stage-22 embryos, a prominent protein was observed that co-migrated with delta-crystallin. However, on immunoblots, none of the nonlens tissues tested contained detectable levels of delta-crystallin at this stage. By imunofluorescence, delta-crystallin was observed in Rathke's pouch and in a large area of oral ectoderm near Rathke's pouch, yet none of the cells in these non-lens tissues showed the typical elongated morphology of lens fiber cells. When presumptive lens ectoderm or other regions of ectoderm from stage-10 embryos were cultured and tested for lens differentiation, both cell elongation and delta-crystallin synthesis were observed, or neither were observed. The results suggest that delta-crystallin synthesis and cell elongation together serve as useful criteria for assessing a positive lens response.

Roles of transforming growth factor-alpha and epidermal growth factor in chick limb development

We have examined the distribution of transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and the chicken EGF receptor (c-erbB), in embryonic chick limbs. Prior to limb budding, TGF-alpha is present in prospective limb-forming mesoderm and in prospective apical ectodermal ridge (AER)-forming ectoderm, but is not detected in non-limb-forming flank mesoderm or ectoderm, nor in presumptive non-AER-forming limb ectoderm, suggesting possible roles in initial limb formation and AER induction. Consistent with this possibility, TGF-alpha is present in the mesoderm of the wing buds of the amelic chick mutants limbless and wingless, which form and bud normally, but is absent from limbless and wingless ectoderm, which fails to form an AER. TGF-alpha and EGF are present in the AER of the developing limb, and TGF-alpha, EGF, and c-erbB are present in the underlying subridge mesoderm, suggesting possible roles in reciprocal AER/subridge mesoderm interactions required for limb outgrowth. We found that exogenous TGF-alpha and EGF can promote the outgrowth of limb mesoderm in the absence of the AER in vitro and can also promote the outgrowth of limbless and wingless wing bud explants. EGF is present in ventral but not dorsal limb ectoderm, suggesting a role for EGF in specification of ventral ectoderm. TGF-alpha and EGF are not detected in the differentiating cartilaginous elements or muscle primordia of the limb, suggesting that cessation of TGF-alpha and EGF expression may be required for cartilage and muscle formation. We have found that exogenous TGF-alpha and EGF inhibit chondrogenesis and myogenesis of limb mesenchyme in vitro. Together these results indicate that signaling through the EGF receptor via endogenous TGF-alpha and EGF may be important for initial limb formation, AER induction, outgrowth of limb mesoderm, and regulation of limb chondrogenic and myogenic differentiation.

The expression of XIF3 in undifferentiated anterior neuroectoderm, but not in primary neurons, is induced by the neuralizing agent noggin

The gene XIF3 encodes a neural-specific type-III intermediate filament protein whose expression in the embryo precedes that of the neurofilaments by several hours. We now show, by in situ hybridization, that it is expressed at the neurula stage in primary neurons and, to a lesser extent, in undifferentiated anterior neuroectoderm. At the swimming tadpole stage, strong expression is restricted to the midbrain-hindbrain boundary, even-numbered rhombomeres of the hindbrain and the Vth and VIIth cranial ganglia. XIF3 gene expression can be induced in ectodermal cells (animal caps) derived from blastula when grown to the neurula stage in the presence of the neuralizing agent noggin. In agreement with the proposed ability of noggin to neuralize, but not to promote neuronal differentiation, we find that the pattern of noggin-inducible XIF3 expression in animal caps is consistent with expression in undifferentiated anterior neuroectoderm but not in primary neurons.

X-twi is expressed prior to gastrulation in presumptive neurectodermal and mesodermal cells in dorsalized and ventralized Xenopus laevis embryos

Early X-twi expression has been now investigated from egg laying to the early neurulation stages in Xenopus embryos, using both in situ hydridization and the more sensitive techniques of RT-PCR. We show that in unfertilized eggs, a decreasing gradient of X-twi transcript distribution is observed from animal to vegetative caps. X-twi RNA can be weakly detected at stages prior to gastrulation, and with increased intensity from stage 8 onwards. At blastula, X-twi transcripts are located towards the animal pole, and as gastrulation begins, they are detected in the developing axial mesoderm and then they accumulate in the sensorial layer of the neurectoderm, the mesodermal layer and in neural crest cells up to late neurula stages. We show, in addition, that in lithium-chloride- and UV-treated Xenopus embryos (that are respectively both "anteriorized/dorsalized" and in "posteriorized/ventralized"), X-twi RNA is detected in cells in similar positions to those that express X-twi in normal embryos. As a whole, our results show that X-twi is expressed even when regionalization of the mesoderm is disturbed and raises the question of a putative function of X-twi prior to gastrulation.

Patterns of gene expression in the core of Spemann's organizer and activin-treated ectoderm in Cynops pyrrhogaster

The presumptive pharyngeal endoderm region of the Cynops early gastrula induces head or trunk-tail structures in sandwich culture. Activin-treated ectoderm can mimic this phenomenon at least at the histological level. The patterns of expression of organizer-specific genes were examined to compare these two inductive materials at the molecular level. A chordin cDNA clone from Cynops pyrrhogaster (Cychd) was isolated by reverse transcription-polymerase chain reaction (RT-PCR). Cychd mRNA was first detected in the presumptive pharyngeal endoderm and prechordal plate regions of stage 11 embryos, and was expressed continuously until stage 20. The spatiotemporal expression pattern of Cychd was similar to that of Xenopus chordin. The patterns of expression of organizer-related genes in the pharyngeal endoderm and activin-treated ectoderm were compared by RT-PCR analysis. Expression of Cychd in these two materials peaked at the time when they can induce head structures in sandwich culture. Expression of fork head and goosecoid did not change in the presumptive pharyngeal endoderm over this period. Cychd may play a key role in head formation in the Cynops embryo.

A role for SOX1 in neural determination

In vertebrates, the delineation of the neural plate from a region of the primitive ectoderm is accompanied by the onset of specific gene expression which in turn promotes the formation of the nervous system. Here we show that SOX1, an HMG-box protein related to SRY, is one of the earliest transcription factors to be expressed in ectodermal cells committed to the neural fate: the onset of expression of SOX1 appears to coincide with the induction of neural ectoderm. We demonstrate a role for SOX1 in neural determination and differentiation using an inducible expression P19 cell system as an in vitro model of neurogenesis. Misexpression of SOX1 can substitute for the requirement of retinoic acid to impart neural fate to competent ectodermal P19 cells. Using a series of antigenic markers which identify early neural cell types in combination with BrdU labeling, we demonstrate a temporal and spatial correlation between the differentiation of cell types along the dorsoventral axis of the neural tube and the downregulation of SOX1 expression. SOX1, therefore, defines the dividing neural precursors of the embryonic central nervous system (CNS).

prdl-a, a gene marker for hydra apical differentiation related to triploblastic paired-like head-specific genes

Two homeobox genes, prdl-a and prdl-b, which were isolated from a Hydra vulgaris cDNA library, encode paired-like class homeodomains highly related to that of the aristaless-related genes. In adult polyps, prdl-b is a marker for synchronously dividing nematoblasts while prdl-a displays an expression restricted to the the nerve cell lineage of the head region. During budding and apical regeneration, an early and transient prdl-a expression was observed in endodermal cells of the stump at a time when the head organizer is established. When apical regeneration was delayed upon concomittant budding, prdl-a expression was found to be altered in the stump. Furthermore, a specific anti-prdl-a protein immunoserum revealed that prdl-a was overexpressed in adult polyps of the Chlorohydra viridissima multiheaded mutant, with an expression domain extending below the tentacle ring towards the body column. Accordingly, prdl-a DNA-binding activity was enhanced in nuclear extracts from this mutant. These results suggest that prdl-a responds to apical forming signals and might thus be involved in apical specification. When a marine hydrozoan (Podocorynae carnea) was used, the anti-prdl-a antibody showed cross-reactivity with cells located around the oral region, indicating that prdl-a function is shared by other cnidaria. The ancestral role for prdl-a-related genes in the molecular definition of the head (or oral-surrounding region) is discussed.

Regulation of bHLH-PAS protein subcellular localization during Drosophila embryogenesis

The Drosophila Single-minded and Tango basic-helix-loop-helix-PAS protein heterodimer controls transcription and embryonic development of the CNS midline cells, while the Trachealess and Tango heterodimer controls tracheal cell and salivary duct transcription and development. Expression of both single-minded and trachealess is highly restricted to their respective cell lineages, however tango is broadly expressed. The developmental control of subcellular localization of these proteins is investigated because of their similarity to the mammalian basic-helix-loop-helix-PAS Aromatic hydrocarbon receptor whose nuclear localization is dependent on ligand binding. Confocal imaging of Single-minded and Trachealess protein localization indicate that they accumulate in cell nuclei when initially synthesized in their respective cell lineages and remain nuclear throughout embryogenesis. Ectopic expression experiments show that Single-minded and Trachealess are localized to nuclei in cells throughout the ectoderm and mesoderm, indicating that nuclear accumulation is not regulated in a cell-specific fashion and unlikely to be ligand dependent. In contrast, nuclear localization of Tango is developmentally regulated; it is localized to the cytoplasm in most cells except the CNS midline, salivary duct, and tracheal cells where it accumulates in nuclei. Genetic and ectopic expression experiments indicate that Tango nuclear localization is dependent on the presence of a basic-helix-loop-helix-PAS protein such as Single-minded or Trachealess. Conversely, Drosophila cell culture experiments show that Single-minded and Trachealess nuclear localization is dependent on Tango since they are cytoplasmic in the absence of Tango. These results suggest a model in which Single-minded and Trachealess dimerize with Tango in the cytoplasm of the CNS midline cells and trachea, respectively, and the dimeric complex accumulates in nuclei in a ligand-independent mode and regulates lineage-specific transcription. The lineage-specific action of Single-minded and Trachealess derives from transcriptional activation of their genes in their respective lineages, not from extracellular signaling.

Domains of retinoid signalling and neurectodermal expression of zebrafish otx1 and goosecoid are mutually exclusive

Retinoid signalling plays an important role in embryonic pattern formation. Excess of retinoic acid during gastrulation results in axial defects in vertebrate embryos, suggesting that retinoids are involved in early anteroposterior patterning. To study retinoid signalling in zebrafish embryos, we developed a novel method to detect endogenous retinoids in situ in embryos, using a fusion protein of the ligand inducible transactivation domain of a retinoic acid receptor and a heterologous DNA binding domain. Using this method, we show that retinoid signalling is localized in zebrafish embryos in the region of the embryonic shield, and towards the end of gastrulation in a posterior dorsal domain. To investigate the relationships between the spatial distribution of retinoid signalling and the regulation of retinoid target genes, we studied the downregulation by retinoic acid of two genes expressed in anterior regions of the embryo, goosecoid and otx1. These experiments show that expression of both genes is strongly downregulated in the anterior neurectoderm of zebrafish embryos treated with retinoic acid, whereas mesendodermal expression is only mildly affected. Interestingly, a significant downregulation of goosecoid expression by retinoic acid was observed only during midgastrulation but not in earlier stages. In agreement with these results, spatial expression of goosecoid and otx1 does not overlap with the region of retinoid signalling in the late gastrula. Our data support the hypothesis that a localized retinoid signal is involved in axial patterning during early development, at least in part through the repression of anterior genes in posterior regions of the embryo. Furthermore, our data suggest that the action of retinoids is spatially as well as temporally regulated in the developing embryo.

The muscleblind gene participates in the organization of Z-bands and epidermal attachments of Drosophila muscles and is regulated by Dmef2

We report the embryonic phenotype of muscleblind (mbl), a recently described Drosophila gene involved in terminal differentiation of adult ommatidia. mbl is a nuclear protein expressed late in the embryo in pharyngeal, visceral, and somatic muscles, the ventral nerve cord, and the larval photoreceptor system. All three mbl alleles studied exhibit a lethal phenotype and die as stage 17 embryos or first instar larvae. These larvae are partially paralyzed, show a characteristically contracted abdomen, and lack striation of muscles. Our analysis of the somatic musculature shows that the pattern of muscles is established correctly, and they form morphologically normal synapses. Ultrastructural analysis, however, reveals two defects in the terminal differentiation of the muscles: inability to differentiate Z-bands in the sarcomeric apparatus and reduction of extracellular tendon matrix at attachment sites to the epidermis. Failure to differentiate both structures could explain the partial paralysis and contracted abdomen phenotype. Analysis of mbl expression in embryos that are either mutant for Dmef2 or ectopically express Dmef2 places mbl downstream of Dmef2 function in the myogenic differentiation program. mbl, therefore, may act as a critical element in the execution of two Dmef2-dependent processes in the terminal differentiation of muscles.

Hox group 3 paralogs regulate the development and migration of the thymus, thyroid, and parathyroid glands

The thymus, thyroid, and parathyroid glands in vertebrates develop from the pharyngeal region, with contributions both from pharyngeal endoderm and from neural crest cells in the pharyngeal arches. Hoxa3 mutant homozygotes have defects in the development of all three organs. Roles for the Hoxa3 paralogs, Hoxb3 and Hoxd3, were investigated by examining various mutant combinations. The thyroid defects seen in Hoxa3 single mutants are exacerbated in double mutants with either of its paralogs, although none of the double-mutant combinations resulted in thyroid agenesis. The results indicate that the primary role of these genes in thyroid development is their effect on the development and migration of the ultimobranchial bodies, which contribute the parafollicular or C-cells to the thyroid. Hoxb3, Hoxd3 double mutants show no obvious defects in the thymus or parathyroids. However, the removal of one functional copy of Hoxa3 from the Hoxb3, Hoxd3 double mutants (Hoxa3 +/-, Hoxb3-/-, Hoxd3-/-) results in the failure of the thymus and parathyroid glands to migrate to their normal positions in the throat. Very little is known about the molecular mechanisms used to mediate the movement of tissues during development. These results indicate that Hoxa3, Hoxb3, and Hoxd3 have highly overlapping functions in mediating the migration of pharyngeal organ primordia. In addition, Hoxa3 has a unique function with respect to its paralogs in thymus, parathyroid, and thyroid development. This unique function may be conferred by the expression of Hoxa3, but not Hoxb3 nor Hoxd3, in the pharyngeal pouch endoderm.

Regulation of ectodermal differentiation in Xenopus laevis animal caps treated with TPA and ammonium chloride

Animal caps isolated from Xenopus laevis embryos at the blastula stage were treated sequentially with NH4Cl, a known cement gland inducer, and with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a known neural inducer. The two artificial inducers were also used in reverse order to see if they can mimic the natural inducers acting during the progressive determination of the ectodermal organ. Immunofluorescence and whole-mount in situ hybridization were used to study the expression of tubulin, taken to indicate an early step on the pathway of cell elongation, and neural cell adhesion molecule (N-CAM) taken to indicate an early step in the determination of the nervous system. The expression of XCG-1, a marker of early specification of the cement gland, was also studied. The results showed that the two artificial inducers can mimic the effects of the natural inducers in animal cap explants. The TPA behaves like a neural inducer, reducing the number and the extension of the cement gland when added to the medium in addition to NH4Cl, before or after NH4Cl treatment. In the process of cement gland/neural induction, it is possible to redirect the ectoderm already specified as cement gland to neural tissue, but it does not seem possible to respecify the neural tissue as cement gland. Moreover, the animal caps were also cut into dorsal and ventral parts and the two halves were treated separately. The results were similar to those obtained with treatment of the entire animal cap, suggesting that a dorsal-ventral pattern is not yet established before the gastrula stage, and that in normal embryos there are boundaries between the effects of different inducers.

Multiple delta genes and lateral inhibition in zebrafish primary neurogenesis

In Drosophila, cells are thought to be singled out for a neural fate through a competitive mechanism based on lateral inhibition mediated by Delta-Notch signalling. In tetrapod vertebrates, nascent neurons express the Delta1 gene and thereby deliver lateral inhibition to their neighbours, but it is not clear how these cells are singled out within the neurectoderm in the first place. We have found four Delta homologues in the zebrafish--twice as many as reported in any tetrapod vertebrate. Three of these--deltaA, deltaB and deltaD--are involved in primary neurogenesis, while two--deltaC and deltaD--appear to be involved in somite development. In the neural plate, deltaA and deltaD, unlike Delta1 in tetrapods, are expressed in large patches of contiguous cells, within which scattered individuals expressing deltaB become singled out as primary neurons. By gene misexpression experiments, we show: (1) that the singling-out of primary neurons, including the unique Mauthner cell on each side of the hindbrain, depends on Delta-Notch-mediated lateral inhibition, (2) that deltaA, deltaB and deltaD all have products that can deliver lateral inhibition and (3) that all three of these genes are themselves subject to negative regulation by lateral inhibition. These properties imply that competitive lateral inhibition, mediated by coordinated activities of deltaA, deltaB and deltaD, is sufficient to explain how primary neurons emerge from proneural clusters of neuroepithelial cells in the zebrafish.

Embryonic morphogenesis signaling pathway mediated by JNK targets the transcription factor JUN and the TGF-beta homologue decapentaplegic

The dorsal surface of the Drosophila embryo is formed by the migration of the lateral epithelial cells to cover the amnioserosa. The Drosophila cJun-N-terminal kinase (DJNK) is essential for this process. Mutations in DJNK or the DJNK activator hemipterous (HEP) lead to incomplete dorsal closure, resulting in a hole in the dorsal cuticle. The molecules downstream of DJNK in this signaling pathway have not been established. Here we demonstrate that the basket1 (bsk1) mutation of DJNK causes decreased interaction with DJUN. Expression of decapentaplegic (DPP), a TGF-beta homologue, in the leading edge of the dorsal epithelium, is identified as a genetic target of the JNK pathway. A constitutive allele of JUN is able to rescue the dorsal closure defect of bsk1 and restores DPP expression. Furthermore, ectopic DPP rescues the defects in dorsal closure caused by bsk1. These data indicate that the interaction of DJNK with DJUN contributes to the dorsal closure signaling pathway and targets DPP expression.

Xenopus msx1 mediates epidermal induction and neural inhibition by BMP4

Epidermal fate in Xenopus ectoderm has been shown to be induced by a secreted growth factor, Bone Morphogenetic Protein 4 (BMP4). However, the molecular mechanism mediating this response is poorly understood. Here, we show that the expression of the homeobox gene, msx1, is an immediate early response to BMP4 in Xenopus embryos. The timing of expression and embryonic distribution of msx1 parallel those described for BMP4. Moreover, overexpression of msx1 in early Xenopus embryos leads to their ventralization as described for BMP4. Consistent with mediating a BMP type of signaling, overexpression of msx1 is sufficient to induce epidermis in dissociated ectoderm cells, which would otherwise form neural tissue. Finally, msx1 can also rescue neuralization imposed by a dominant negative BMP receptor (tBR) in ectodermal explants. We propose that Xenopus msx1 acts as a mediator of BMP signaling in epidermal induction and inhibition of neural differentiation.

Concentration-dependent patterning of the Xenopus ectoderm by BMP4 and its signal transducer Smad1

Morphogens are thought to establish pattern in early embryos by specifying several cell fates along a gradient of concentration; a well-studied example is the Drosophila protein decapentaplegic (DPP) acting in the wing disc. Recent work has established that bone morphogenetic protein 4 (BMP4), the vertebrate homologue of DPP, controls the fundamental choice between neural and epidermal fates in the vertebrate ectoderm, under the control of antagonists secreted by the organizer region of the mesoderm. We now show that BMP4 can act as a morphogen, evoking distinct responses in Xenopus ectodermal cells at high and low concentrations, in a pattern consistent with the positions of the corresponding cell types in the embryo. Moreover, this complex cellular response to extracellular BMP4 concentration does not require subsequent cell-cell communication and is thus direct, as required of a classical morphogen. We also show that the same series of cell types--epidermis, cement gland and neural tissue--can be produced by progressively inhibiting endogenous BMP signaling with specific antagonists, including the organizer factor noggin. Finally, expression of increasing doses of the signal transduction molecule Smad1 accurately reproduces the response to BMP4 protein. Since Smads have been shown to act in the nucleus, this finding implies a direct translation of extracellular morphogen concentration into transcription factor activity. We propose that a graded distribution of BMP activity controls the specification of several cell types in the gastrula ectoderm and that this extracellular gradient acts by establishing an intracellular and then nuclear gradient of Smad activity.

Mouse Dll3: a novel divergent Delta gene which may complement the function of other Delta homologues during early pattern formation in the mouse embryo

Mouse delta-like 3 (Dll3), a novel vertebrate homologue of the Drosophila gene Delta was isolated by a subtracted library screen. In Drosphila, the Delta/Notch signalling pathway functions in many situations in both embryonic and adult life where cell fate specification occurs. In addition, a patterning role has been described in the establishment of the dorsoventral compartment boundary in the wing imaginal disc. Dll3 is the most divergent Delta homologue identified to date. We confirm that Dll3 can inhibit primary neurogenesis when ectopically expressed in Xenopus, suggesting that it can activate the Notch receptor and therefore is a functional Delta homologue. An extensive expression study during gastrulation and early organogenesis in the mouse reveals a diverse and dynamic pattern of expression. The three major sites of expression implicate Dll3 in somitogenesis and neurogenesis and in the production of tissue from the primitive streak and tailbud. A careful comparison of Dll3 and Dll1 expression by double RNA in situ hybridisation demonstrates that these genes have distinct patterns of expression, but implies that together they operate in many of the same processes. We postulate that during somitogenesis Dll3 and Dll1 coordinate in establishing the intersomitic boundaries. We confirm that, during neurogenesis in the spinal cord, Dll1 and Dll3 are expressed by postmitotic cells and suggest that expression is sequential such that cells express Dll1 first followed by Dll3. We hypothesise that Dll1 is involved in the release of cells from the precursor population and that Dll3 is required later to divert neurons along a specific differentiation pathway.

The role of the msh homeobox gene during Drosophila neurogenesis: implication for the dorsoventral specification of the neuroectoderm

Development of the Drosophila central nervous system begins with the delamination of neural and glial precursors, called neuroblasts, from the neuroectoderm. An early and important step in the generation of neural diversity is the specification of individual neuroblasts according to their position. In this study, we describe the genetic analysis of the msh gene which is likely to play a role in this process. The msh/Msx genes are one of the most highly conserved families of homeobox genes. During vertebrate spinal cord development, Msx genes (Msx1-3) are regionally expressed in the dorsal portion of the developing neuroectoderm. Similarly in Drosophila, msh is expressed in two longitudinal bands that correspond to the dorsal half of the neuroectoderm, and subsequently in many dorsal neuroblasts and their progeny. We showed that Drosophila msh loss-of-function mutations led to cell fate alterations of neuroblasts formed in the dorsal aspect of the neuroectoderm, including a possible dorsal-to-ventral fate switch. Conversely, ectopic expression of msh in the entire neuroectoderm severely disrupted the proper development of the midline and ventral neuroblasts. The results provide the first in vivo evidence for the role of the msh/Msx genes in neural development, and support the notion that they may perform phylogenetically conserved functions in the dorsoventral patterning of the neuroectoderm.

The mesodermal expression of rolling stone (rost) is essential for myoblast fusion in Drosophila and encodes a potential transmembrane protein

In homozygous rolling stone embryos, the fusion of myoblasts to syncytial myotubes is diminished. Nevertheless, the visceral mesoderm, the heart mesoderm, and few somatic muscles are properly formed. Thus, we postulate a central role of rolling stone for the fusion process within the somatic mesoderm. We have cloned the rolling stone gene, and the deduced protein sequence is in accordance with a transmembrane protein, which agrees with the enrichment of Rost in the membrane fraction of Drosophila embryos. No homologous genes have been described so far. rolling stone is expressed in the embryonic nervous system and cells of the somatic mesoderm, most notable in muscle founder cells. To elucidate the function of rolling stone for myoblast fusion, we applied a knock-out strategy. The expression of an antisense rolling stone transcript specifically within the mesoderm of wild-type embryos results in fusion defects of myoblasts, proving that the rolling stone expression in the mesoderm is responsible for the rolling stone phenotype. We suggest that rolling stone is a member of a group of genes that are necessary for the fusion process during myogenesis.

Antagonistic interactions between FGF and BMP signaling pathways: a mechanism for positioning the sites of tooth formation

Vertebrate organogenesis is initiated at sites that are often morphologically indistinguishable from the surrounding region. Here we have identified Pax9 as a marker for prospective tooth mesenchyme prior to the first morphological manifestation of odontogenesis. We provide evidence that the sites of Pax9 expression in the mandibular arch are positioned by the combined activity of two signals, one (FGF8) that induces Pax9 expression and the other (BMP2 and BMP4) that prevents this induction. Thus it appears that the position of the teeth is determined by a combination of two different types of signaling molecules produced in wide but overlapping domains rather than by a single localized inducer. We suggest that a similar mechanism may be used for specifying the sites of development of other organs.

Cooperation of BMP7 and SHH in the induction of forebrain ventral midline cells by prechordal mesoderm

Ventral midline cells at different rostrocaudal levels of the central nervous system exhibit distinct properties but share the ability to pattern the dorsoventral axis of the neural tube. We show here that ventral midline cells acquire distinct identities in response to the different signaling activities of underlying mesoderm. Signals from prechordal mesoderm control the differentiation of rostral diencephalic ventral midline cells, whereas notochord induces floor plate cells caudally. Sonic hedgehog (SHH) is expressed throughout axial mesoderm and is required for the induction of both rostral diencephalic ventral midline cells and floor plate. However, prechordal mesoderm also expresses BMP7 whose function is required coordinately with SHH to induce rostral diencephalic ventral midline cells. BMP7 acts directly on neural cells, modifying their response to SHH so that they differentiate into rostral diencephalic ventral midline cells rather than floor plate cells. Our results suggest a model whereby axial mesoderm both induces the differentiation of overlying neural cells and controls the rostrocaudal character of the ventral midline of the neural tube.

Dynamic expression of chicken Sox2 and Sox3 genes in ectoderm induced to form neural tissue

The chick genes, cSox2 and cSox3, are members of a large family of genes that encode transcription factors. Previous studies have shown that these genes are predominantly expressed in the central nervous system during embryonic development. We show that cSox3 is expressed throughout the ectoderm that is competent to form nervous tissue before neural induction. The expression of cSox3 is lost from cells as they undergo gastrulation to form nonectodermal tissues; the transcription factor, Brachyury, appears in cells about to undergo gastrulation a short time before cSox3 transcripts are lost. Therefore, Brachyury expression may act functionally upstream of cSox3 downregulation. cSox3 expression is also lost from non-neuronal ectoderm shortly after the neural plate becomes morphologically apparent. cSox2 expression increases dramatically in the central nervous system as neural ectoderm is established. The appearance of cSox2 in neural ectoderm represents one of the earliest molecular responses to neural induction documented thus far.

Bone morphogenetic proteins (BMPs) as regulators of dorsal forebrain development

Bone Morphogenetic Proteins (BMPs) play crucial roles in a variety of developmental processes, but their functions during early vertebrate brain development are largely unknown. To investigate this problem, we have compared by in situ hybridization the expression of five Bmp genes belonging to the Drosophila Decapentaplegic (Bmp2 and Bmp4) and 60A subgroups (Bmp5, Bmp6 and Bmp7). Striking co-expression of these Bmps is observed within the dorsomedial telencephalon, coincident with a future site of choroid plexus development. Bmp co-expression overlaps that of Msx1 and Hfh4, and is complementary to that of Bf1. The domain of Bmp co-expression is also associated with limited growth of the neuroectoderm, as revealed by morphological observation, reduced cell proliferation, and increased local programmed cell death. In vitro experiments using explants from the embryonic lateral telencephalic neuroectoderm reveal that exogenous BMP proteins (BMP4 and BMP2) induce expression of Msx1 and inhibit Bf1 expression, a finding consistent with their specific expression patterns in vivo. Moreover, BMP proteins locally inhibit cell proliferation and increase apoptosis in the explants. These results provide evidence that BMPs function during regional morphogenesis of the dorsal telencephalon by regulating specific gene expression, cell proliferation and local cell death.

Transcriptional regulation of Notch and Delta: requirement for neuroblast segregation in Drosophila

Segregation of a single neural precursor from each proneural cluster in Drosophila relies on Notch-mediated lateral signalling. Studies concerning the spacing of precursors for the microchaetes of the peripheral nervous system suggested the existence of a regulatory loop between Notch and its ligand Delta within each cell that is under transcriptional control. Activation of Notch leads to repression of the achaete-scute genes which themselves regulate transcription of Delta, perhaps directly. Here we have tested a requirement for transcriptional regulation of Notch and/or Delta during neuroblast segregation in embryos, by providing Notch and Delta ubiquitously at uniform levels. Neuroblast segregation occurs normally under conditions of uniform Notch expression. Under conditions of uniform Delta expression, a single neuroblast segregates from each proneural group in 80% of the cases, more than one in the remaining 20%. Thus transcriptional regulation of Delta is largely dispensable. We discuss the possibility that segregation of single precursors in the central nervous system may rely on a heterogeneous distribution of neural potential between different cells of the proneural group. Notch signalling would enable all cells to mutually repress each other and only a cell with an elevated neural potential could overcome this repression.

Expression of heparan sulfate proteoglycan (perlecan) in the mouse blastocyst is regulated during normal and delayed implantation

Previous studies have shown that expression of the heparan sulfate proteoglycan, perlecan, on the external trophectodermal cell surfaces of mouse blastocysts increases during acquisition of attachment competence. However, it is not clear if this change in perlecan protein expression also is reflected at the level of perlecan mRNA expression. In the present investigation, the spatial and temporal patterns of perlecan mRNA expression in the mouse embryo during the periimplantation period were examined by in situ hybridization and reverse transcriptase-polymerase chain reaction. In addition, a delayed implantation model was used to determine the expression of perlecan mRNA and protein in dormant and estrogen-activated hatched blastocysts. The results demonstrate that perlecan mRNA expression is low in morulae, but increases in Day 4 blastocysts, attaining maximal expression in Day 4.5 attachment-competent blastocysts. In contrast, perlecan mRNA is detected in both the dormant and estrogen-activated delayed blastocysts; however, within 12 hr of blastocyst activation by estrogen, both perlecan protein and heparan sulfate chain expression markedly increase. Taken together, these results suggest that during normal development perlecan mRNA expression increases with the acquisition of attachment competence. Moreover, perlecan protein expression also is attenuated during delayed implantation and appears to increase in response to nidatory estrogen, perhaps via the increased translation of preexisting perlecan mRNA.

Evidence that absence of Wnt-3a signaling promotes neuralization instead of paraxial mesoderm development in the mouse

Wnt-3a mutant embryos show defects caudal to the forelimb level; somites are absent, the notochord is disrupted, and the central nervous system has a pronounced dysmorphology. Previous studies revealed that the primary defects of the mutant embryos are likely to be in the process of paraxial mesoderm formation. In this study, we analyzed the phenotype of Wnt-3a mutant embryos at early somite stages (8.0 days post coitum), when somite formation is initiated. In Wnt-3a mutants, cells which have ingressed through the primitive streak do not migrate laterally but remain under the streak and form an ectopic tubular structure. Several neural-specific molecular markers, but no paraxial mesoderm markers, are expressed in this structure, suggesting that the ectopic tube is an additional neural tube. In normal embryos, Wnt-3a is expressed in the primitive ectoderm, including the cells which are fated to give rise to the paraxial mesoderm and neurectoderm, but expression is absent in migrating mesoderm cells. These results suggest that Wnt-3a signaling may play a role in regulating paraxial mesodermal fates, at the expense of neurectodermal fates, within the primitive ectoderm of the gastrulating mouse embryo.

tbx6, a Brachyury-related gene expressed by ventral mesendodermal precursors in the zebrafish embryo

Classical embryology experiments have indicated the existence of dorsal-type and ventral-type mesoderms that arise as a consequence of mesoderm induction during vertebrate development. Here we report that the zebrafish tbx6 gene, a member of the Brachyury-related T-box family of genes, is exclusively expressed by ventral mesendoderm. Three observations link the expression of tbx6 to ventral mesoderm specification. First, the gene is initially expressed at the onset of gastrulation within a ventrolateral subpopulation of cells that express the pan-mesodermal gene, no tail (Brachyury). Second, the mesoderm-inducing factors activin and bFGF activate tbx6 expression in animal caps. Third, dorsalization of the mesendodermal precursor population following exposure of embryos to lithium ions causes down-regulation of tbx6 transcription. tbx6 is expressed transiently in the involuting derivatives of the ventral mesendoderm, which give rise to nonaxial mesodermal tissues; its expression is extinguished as tissue differentiation progresses. Transcription of tbx6 commences about an hour after initiation of expression of the pan-mesendodermal gene no tail and the organizer gene goosecoid. The dependence of tbx6 expression on no tail activity was examined in no tail mutant embryos. The activation of tbx6 transcription in ventral mesoderm does not depend on no tail gene activity. However, no tail appears to contribute to the maintenance of normal levels of tbx6 transcription and may be required for tbx6 transcription in the developing tail.

Each member of the Id gene family exhibits a unique expression pattern in mouse gastrulation and neurogenesis

We have performed a detailed comparative in situ hybridization analysis to examine the patterns of expression of all the members of the Id gene family (Id1-4) during murine gastrulation and neurogenesis. During gastrulation, both Id1 and Id3 are expressed in the tissues derived from the inner cell mass from 5.5 dpc onward, whereas Id2 is expressed in tissues derived from trophoblasts. Id4 expression is absent during this period of development. Embryonic Id1 messages are detected during gastrulation on the proximal side of the embryonic ectoderm, which is the border between the embryo proper and the extraembryonic tissues, and the expression of Id3 is found throughout the entire embryo proper. This unique pattern of expression of the different members of the Id family suggests a nonredundant role for these genes in antagonizing the activity of bHLH transcription factors during very early mouse development. During neurogenesis, the expression of each member of the Id family is present in an unique pattern along the dorsal-ventral axis of the neural tube: In the early stages of spinal cord development, both Id1 and Id2 are expressed in the roof plate, whereas Id3 is expressed both in the roof and the floor plates. As development progresses, the expression of both Id1 and Id3 is detected in the dividing neuroblasts, whereas Id2 and 4 are expressed in presumptive neurons which are undergoing maturation. The expression patterns of all the members of the Id gene family persist throughout the entire CNS, both in the spinal cord and in the brain. In addition, the characteristic expression of Id2 and Id4 in more mature neurons is reiterated both in the PNS and in the neurons of some of the sensory organs. These data suggest that the expression of different subgroups of the Id gene family may have different physiological consequences and thereby contributes in unique ways to specify the differentiation state of neuronal cells during development.

Isolation, characterization, and expression of Le-msx, a maternally expressed member of the msx gene family from the glossiphoniid leech, Helobdella

The msx gene family is one of the most highly conserved of the nonclustered homeobox-containing genes. We have isolated an msx homolog (Le-msx) from the glossiphoniid leech, Helobdella robusta, and characterized its pattern of expression by whole mount in situ hybridization. In situ expression and reverse transcription polymerase chain reaction (RT-PCR) data results show that Le-msx is a maternal transcript initially uniformly distributed in the cortex of immature oocytes that becomes asymmetrically localized to the polar regions of the uncleaved zygote. This is the earliest reported expression for the msx gene family and the first maternally expressed homeodomain-containing transcription factor reported in annelids. During embryonic development, Le-msx is expressed in all 10 embryonic stem cells and their segmental founder cell descendants. At midembryonic stages, Le-msx is expressed in the expanding germinal plate. Le-msx is confined to the central nervous system and nephridia at late (stage 9) stages and subsequently disappears from nephridia. In addition, we present a phylogenetic hypothesis for the evolution of the msx gene family, including the identification of a putative C. elegans msx homolog and the realignment of the sponge msx homolog to the NK class of homeodomain genes.