Wnt signaling activates Shh signaling in early postnatal intervertebral discs, and re-activates Shh signaling in old discs in the mouse

Intervertebral discs (IVDs) are strong fibrocartilaginous joints that connect adjacent vertebrae of the spine. As discs age they become prone to failure, with neurological consequences that are often severe. Surgical repair of discs treats the result of the disease, which affects as many as one in seven people, rather than its cause. An ideal solution would be to repair degenerating discs using the mechanisms of their normal differentiation. However, these mechanisms are poorly understood. Using the mouse as a model, we previously showed that Shh signaling produced by nucleus pulposus cells activates the expression of differentiation markers, and cell proliferation, in the postnatal IVD. In the present study, we show that canonical Wnt signaling is required for the expression of Shh signaling targets in the IVD. We also show that Shh and canonical Wnt signaling pathways are down-regulated in adult IVDs. Furthermore, this down-regulation is reversible, since re-activation of the Wnt or Shh pathways in older discs can re-activate molecular markers of the IVD that are lost with age. These data suggest that biological treatments targeting Wnt and Shh signaling pathways may be feasible as a therapeutic for degenerative disc disease.

What my mother told me: Examining the roles of maternal gene products in a vertebrate

In Xenopus, mRNAs synthesized during oocyte differentiation are inherited by the egg and direct all protein synthesis until the late-blastula stage. This provides an opportunity to study the roles of maternally expressed genes in embryonic development of a vertebrate. Oocytes can be depleted of specific mRNAs by the injection of antisense deoxyoligonucleotides and then fertilized to assay for developmental abnormalities. The ease of experimental manipulation of early Xenopus embryos in culture gives considerable opportunity for the analysis of the abnormalities seen.

The establishment of regional identity in the Xenopus blastula

The molecules involved in the commitment of Xenopus cells to particular germ layers are unknown. The question has been investigated for the cells of the blastula in in vivo cell transplantation assays and in vitro aggregation assays. Using the former technique, we have shown that vegetal cells become committed before gastrulation, even when placed in inappropriate sites. We could find no evidence of regional determination within the endoderm germ layer at the early gastrula stage. In aggregation assays, animal and vegetal cells sorted incompletely at the mid-blastula stage and more efficiently at the gastrula stage. Their behaviour may be mediated by differential expression of adhesion molecules, which so far remain unidentified.

FoxI1e activates ectoderm formation and controls cell position in the Xenopus blastula

The segregation of the vertebrate embryo into three primary germ layers is one of the earliest developmental decisions. In Xenopus, where the process is best understood, the endoderm is specified by a vegetally localized transcription factor, VegT, which releases nodal signals that specify the adjacent marginal zone of the blastula to become mesoderm. However, little is known about how the ectoderm becomes specified. In this paper, we show that the forkhead protein FoxI1e (also known as Xema) is required at the blastula stage for normal formation of both the central nervous system and epidermis, the two early derivatives of the ectoderm. In addition, FoxI1e is required to maintain the regional identity of the animal cells of the blastula, the cells that are precursors of ectodermal structures. In its absence, they lose contact with the animal cap, mix with cells of other germ layers and differentiate according to their new positions. Because FoxI1e is initially expressed in the animal region of the embryo and is rapidly downregulated in the neural plate, its role in neural and epidermal gene expression must precede the division of the ectoderm into neural and epidermal. The work also shows that FoxI1e plays a role in the embryo in the poorly understood process of differential adhesion, which limits cell mixing as primary germ layers become specified.

Maternal Wnt11 Activates the Canonical Wnt Signaling Pathway Required for Axis Formation in Xenopus Embryos

Wnt signaling pathways play essential roles in patterning and proliferation of embryonic and adult tissues. In many organisms, this signaling pathway directs axis formation. Although the importance of intracellular components of the pathway, including beta-catenin and Tcf3, has been established, the mechanism of their activation is uncertain. In Xenopus, the initiating signal that localizes beta-catenin to dorsal nuclei has been suggested to be intracellular and Wnt independent. Here, we provide three lines of evidence that the pathway specifying the dorsal axis is activated extracellularly in Xenopus embryos. First, we identify Wnt11 as the initiating signal. Second, we show that activation requires the glycosyl transferase X.EXT1. Third, we find that the EGF-CFC protein, FRL1, is also essential and interacts with Wnt11 to activate canonical Wnt signaling.

XPACE4 is a localized pro-protein convertase required for mesoderm induction and the cleavage of specific TGFbeta proteins in Xenopus development

XPACE4 is a member of the subtilisin/kexin family of pro-protein convertases. It cleaves many pro-proteins to release their active proteins, including members of the TGFbeta family of signaling molecules. Studies in mouse suggest it may have important roles in regulating embryonic tissue specification. Here, we examine the role of XPACE4 in Xenopus development and make three novel observations: first, XPACE4 is stored as maternal mRNA localized to the mitochondrial cloud and vegetal hemisphere of the oocyte; second, it is required for the endogenous mesoderm inducing activity of vegetal cells before gastrulation; and third, it has substrate-specific activity, cleaving Xnr1, Xnr2, Xnr3 and Vg1, but not Xnr5, Derriere or ActivinB pro-proteins. We conclude that maternal XPACE4 plays an important role in embryonic patterning by regulating the production of a subset of active mature TGFbeta proteins in specific sites.

Plakoglobin is required for maintenance of the cortical actin skeleton in early Xenopus embryos and for cdc42-mediated wound healing

Early Xenopus embryos are large, and during the egg to gastrula stages, when there is little extracellular matrix, the cytoskeletons of the individual blastomeres are thought to maintain their spherical architecture and provide scaffolding for the cellular movements of gastrulation. We showed previously that depletion of plakoglobin protein during the egg to gastrula stages caused collapse of embryonic architecture. Here, we show that this is due to loss of the cortical actin skeleton after depletion of plakoglobin, whereas the microtubule and cytokeratin skeletons are still present. As a functional assay for the actin skeleton, we show that wound healing, an actin-based behavior in embryos, is also abrogated by plakoglobin depletion. Both wound healing and the amount of cortical actin are enhanced by overexpression of plakoglobin. To begin to identify links between plakoglobin and the cortical actin polymerization machinery, we show here that the Rho family GTPase cdc42, is required for wound healing in the Xenopus blastula. Myc-tagged cdc42 colocalizes with actin in purse-strings surrounding wounds. Overexpression of cdc42 dramatically enhances wound healing, whereas depletion of maternal cdc42 mRNA blocks it. In combinatorial experiments we show that cdc42 cannot rescue the effects of plakoglobin depletion, showing that plakoglobin is required for cdc42-mediated cortical actin assembly during wound healing. However, plakoglobin does rescue the effect of cdc42 depletion, suggesting that cdc42 somehow mediates the distribution or function of plakoglobin. Depletion of alpha-catenin does not remove the cortical actin skeleton, showing that plakoglobin does not mediate its effect by its known linkage through alpha-catenin to the actin skeleton. We conclude that in Xenopus, the actin skeleton is a major determinant of cell shape and overall architecture in the early embryo, and that plakoglobin plays an essential role in the assembly, maintenance, or organization of this cortical actin.

Bix4 is activated directly by VegT and mediates endoderm formation in Xenopus development

The maternal T-box gene VegT, whose transcripts are restricted to the vegetal hemisphere of the Xenopus embryo, plays an essential role in early development. Depletion of maternal VegT transcripts causes embryos to develop with no endoderm, while vegetal blastomeres lose the ability to induce mesoderm (Zhang, J., Houston, D. W., King, M. L., Payne, C., Wylie, C. and Heasman, J. (1998) Cell 94, 515–524). The targets of VegT, a transcription activator, must therefore include genes involved both in the specification of endoderm and in the production of mesoderm-inducing signals. We recently reported that the upstream regulatory region of the homeobox-containing gene Bix4 contains T-box binding sites. Here we show that expression of Bix4 requires maternal VegT and that two T-box binding sites are necessary and sufficient for mesodermal and endodermal expression of reporter genes driven by the Bix4 promoter in transgenic Xenopus embryos. Remarkably, a single T-box binding site is able to act as a mesoderm-specific enhancer when placed upstream of a minimal promoter. Finally, we show that Bix4 rescues the formation of endodermal markers in embryos in which VegT transcripts have been ablated but does not restore the ability of vegetal pole blastomeres to induce mesoderm. These results demonstrate that Bix4 acts directly downstream of VegT to specify endodermal differentiation in Xenopus embryos.

Evidence that dorsal-ventral differences in gap junctional communication in the early Xenopus embryo are generated by beta-catenin independent of cell adhesion effects

Gap junctional communication (GJC) is regulated in the early Xenopus embryo and quantitative differences in junctional communication correlate with the specification of the dorsal-ventral axis. To address the mechanism that is responsible for regulating this differential communication, we investigated the function of beta-catenin during the formation of the dorsal-ventral axis in Xenopus embryos by blocking its synthesis with antisense oligodeoxynucleotides. This method has previously been shown to reduce the level of beta-catenin in the early embryo, prior to zygotic transcription, and to inhibit the formation of the dorsal axis (Heasman et al., 1994, Cell 79, 791-803). We show here that antisense inhibition of beta-catenin synthesis also reduces GJC among cells in the dorsal hemisphere of 32-cell embryos to levels similar to those observed among ventral cells. Full-length beta-catenin mRNA can restore elevated levels of dorsal GJC when injected into beta-catenin-deficient oocytes, demonstrating the specificity of the beta-catenin depletion with the antisense oligonucleotides. Thus, endogenous beta-catenin is required for the observed differential GJC. This regulation of GJC is the earliest known action of the dorsal regulator, beta-catenin, in Xenopus development. Two lines of evidence, presented here, indicate that beta-catenin acts within the cytoplasm to regulate GJC, rather than through an effect on cell adhesion. First, when EP-cadherin is overexpressed and increased adhesion is observed, embryos display both a ventralized phenotype and reduced dye transfer. Second, a truncated form of beta-catenin (i.e., the ARM region), that lacks the cadherin-binding domain, restores dorsal GJC to beta-catenin-depleted embryos. Thus, beta-catenin appears to regulate GJC independent of its role in cell-cell adhesion, by acting within the cytoplasm through a signaling mechanism.

The origin and biology of CIS cells: general discussion

Participants at the 4th Copenhagen Workshop on Carcinoma in situ and Cancer of the Testis, representing cell biologists and tumour biologists, met together to discuss the similarities and differences between primordial germ cells (PGCs) of the embryo, and the carcinoma in situ (CIS) stem cell of human testicular germ cell tumours (GCTs). Much has been discovered about PGCs in the last 10 years and we still do not know the exact nature of CIS cells. Knowledge of PGCs comes mainly from mouse experiments and knowledge of CIS comes from the study of human tumours. A mouse model of human GCT would help to investigate the nature of CIS cells. Grafting mouse male genital ridges into mouse fetal testes results in the development of testicular tissue and the formation of teratomatous tumour components. Amplification of PGCs in culture is possible but this results in their transformation into embryonic germ (EG) cells. CIS cells die by apoptosis if they are isolated, and short-term culture is only possible if the CIS cells are cultured in their normal environment within seminiferous tubules. It may be possible for CIS cells to differentiate in culture although they cannot be maintained in culture as isolated cells. Human CIS cells are likely to be formed as a result of in utero factors rather than agents acting on normal adult testicular germ cells. EG cells stimulate feeder cells by paracrine factors but it is not known if these cells produce autocrine factors.

The role of interleukin-4 in the regulation of mouse primordial germ cell numbers

Interleukin-4 (IL-4), a pleiotropic cytokine, stimulates a dose-dependent increase in the number of mouse primordial germ cells in culture. Results from bromodeoxyuridine incorporation assays suggest that IL-4 acts as a survival factor rather than as a mitogen for primordial germ cells in this system. Studies on the embryonic expression patterns of IL-4 and its receptors, using RT-PCR and ELISA, show that IL-4 and its receptors are present at the correct time and place to influence PGC numbers in vivo.

Early embryonic expression of XLPOU-60, a Xenopus POU-domain protein

XLPOU-60 is a Xenopus POU-domain gene whose expression is tightly controlled during early development at transcriptional, post-transcriptional, translational, and post-translational levels. We report the expression pattern of the XLPOU-60 protein; it is first detectable in the stage V oocyte and accumulates rapidly following fertilisation, reaching a peak at the time of the mid-blastula transition. In the blastula, XLPOU-60 protein translated from injected synthetic mRNA enters nuclei. During gastrulation, both transcript and protein are rapidly down-regulated in a cell-autonomous manner; down-regulation is not dependent on cell-cell contact or induction by activin in an animal cap assay. For the mRNA, this down-regulation correlates with changes in the length of its poly(A) tail and is dependent on sequences in the untranslated regions of the transcript. On the basis of its protein expression pattern and known DNA-binding properties, we speculate that XLPOU-60 may play a role in the control of early transcriptional events in the Xenopus embryo.

A Xenopus c-kit-related receptor tyrosine kinase expressed in migrating stem cells of the lateral line system

The mammalian c-kit receptor tyrosine kinase gene is required during embryogenesis for the survival and/or proliferation of three migrating stem cell populations: primordial germ cells, haematopoietic stem cells and neural crest-derived melanoblasts. We have cloned a Xenopus gene, XKrk1, whose closest relative is c-kit. Differences in the expression pattern suggest that XKrk1 is not the Xenopus homologue of c-kit; however, it is expressed in a migrating stem cell population, the precursor cells for the mechanosensory lateral line system. XKrk1 is the first reported marker for lateral line stem cells.

Nonsense-mediated mRNA decay in Xenopus oocytes and embryos

Mutant mRNAs carrying a premature stop codon have a reduced half-life in the cells of many species, probably due to the presence of "surveillance" pathways, which selectively target such mRNAs for degradation. It is reported here that this phenomenon may also occur in Xenopus. In vitro-synthesised transcripts encoding a Xenopus POU-domain protein, XLPOU-60, are stable after injection into the oocyte and embryo. However, introduction of a premature stop codon into these transcripts results in their rapid degradation following injection. In contrast, mutant transcripts with additional or deleted codons but retaining a correct reading frame are stable. These results suggest that RNA stability should be considered when designing control mRNAs for Xenopus injection experiments.

The biology of primordial germ cells

Primordial germ cells are the founder cells of the gametes. They arise at the earliest stages of development, and migrate to the genital ridges, where they join the somatic cells of the future gonad. The factors that determine their formation and migration are largely unknown. Primary culture of PGCs isolated at different times during their migration has allowed the analysis of some aspects of the control of their behaviour. The effects of culture medium conditioned by genital ridges suggested that both proliferation and migration of PGCs may be controlled in part by diffusible factors. Several purified growth factors are now known which affect PGC numbers in culture. These include positive and negative regulators of proliferation, survival factors, and chemotropic factors. During migration, PGCS must change their affinities for surrounding cells, and some aspects of this can be analysed in culture. For example, PGCs isolated at different stages in their migration show different capacities to adhere to fibronectin.

Involvement of a neutral glycolipid in differential cell adhesion in the Xenopus blastula

Many different molecular species mediate cell adhesion during embryonic development. These can have either protein or carbohydrate functional groups, which can act in either a homophilic or a heterophilic manner, and often in concert. We report here that a monoclonal antibody, M4B, raised against Xenopus blastomere membranes, inhibits the calcium-dependent adhesion of dissociated blastomeres. M4B maintains its inhibitory effect on adhesion when converted into univalent fragments, and specifically affects calcium-dependent adhesion. The antigen is regulated in both space and time during early development. It is found on cell surfaces throughout the egg to blastula stages, but is more concentrated on cells in the animal and marginal zones of the blastula. It is dramatically downregulated during gastrulation, and becomes largely restricted to gut epithelium by the larval stages. We show also that M4B function is spatially differentiated at the blastula stage, since it inhibits the aggregation of dissociated animal cells to a greater extent than vegetal cells. This membrane antigen may therefore play a role in the differential adhesion observed between different regions of the blastula, and which we presume to underlie the segregation of the primary germ layers during gastrulation. M4B recognizes a complex of plasma membrane glycolipids. Periodate treatment destroys the ability of these glycolipids to react with the antibody, indicating that the epitope resides in the carbohydrate moiety of the glycolipids. Chemical characterization shows that it is a neutral glycolipid, and that the major component is of the glycoglycerolipid, rather than the more common glycosphingolipid class. Blocking experiments with oligosaccharides of defined structure, and antibody crossreactivity show that the M4B antibody does not recognize several known embryonic carbohydrate antigens. These results demonstrate that M4B antibody recognizes a novel group of developmentally regulated glycolipids which function in calcium-dependent cell--cell adhesion in the Xenopus blastula.

Function of maternal cytokeratin in Xenopus development

Intermediate filaments are ubiquitous in eukaryotic cells, but their functions are poorly understood. The Xenopus oocyte contains both messenger RNA and protein products of cytokeratin and vimentin genes in non-overlapping arrays. The cytokeratin filaments contain dimers of the type I (acidic) subunit XLK3a(19), and the type II (basic) subunit XCK1(8), polymerized to form a cortical network. These are homologues of the human simple epithelial keratins 19 and 8, respectively. After the first few cell cycles following fertilization these filaments become restricted to the superficial cells of the blastula. We have depleted the oocyte's store of the type II cytokeratin mRNA by injecting antisense oligodeoxynucleotides (oligos) and studied the effect on embryonic development. As zygotic transcription does not commence until the late blastula stage, there are at least 9 hours in which to see the effect of loss of function of this mRNA. We report here that the cytokeratin filaments become depleted in the cortical cells of the embryo. As a result, there is a loss of the 'compacted' epithelial surface of the blastula, an inability to close a wounded surface and defective gastrulation.

Distinct distribution of vimentin and cytokeratin in Xenopus oocytes and early embryos

We report the identity of a major component of Triton-insoluble extracts from Xenopus oocytes and early embryos. In a previous paper we showed that an antibody, Z9, cross-reacts with two polypeptides from such extracts (Mr 56,000 and 57,000) as well as Xenopus vimentin. Direct microsequencing of the Mr 57,000 protein shows near identity of three tryptic fragments with regions of the predicted amino acid sequence of XCK1(8), a basic cytokeratin whose mRNA is known to be expressed in Xenopus oocytes. We have raised an antibody, CK7, against a fusion protein generated from this cDNA. The specificity of this antibody has been tested using 1- and 2-dimensional immunoblotting, which show that it is specific for the Mr 56,000 and 57,000 proteins, suggesting that these two proteins may be the products of two non-allelic XCK1(8) genes. The antibody does not cross-react with vimentin. We have used CK7 to follow the distribution of XCK1(8) throughout development by immunoblotting and immunocytochemistry. In larval stages, strong staining is seen in the notocord, the apical epithelia of the gut, the mesentery, and a few cells in the spinal cord. In oocytes and early embryos, two distinct intermediate filament (IF) networks can be distinguished: a cortical cytokeratin network, and a deeper vimentin one. In addition, the oocyte germ plasm stains with Z9 but not CK7. We propose that such distinct distributions of each IF protein reflect functional differences during early development.

Response to fibronectin of mouse primordial germ cells before, during and after migration

The adhesive extracellular matrix glycoprotein fibronectin is thought to play a central role in cell migration during embryogenesis. In order to define this role, we have examined the response to fibronectin in cell culture of mouse primordial germ cells (PGCs) before, during and after their migration from the hindgut into their target tissue, the genital ridges. Using an explant culture system, we show that PGCs will emigrate from tissue fragments containing hindgut, and that fibronectin stimulates this migration. Adhesion assays show that the start of PGC migration is associated with a fall in adhesion to fibronectin. Double-labelling studies using in situ hybridization and histochemistry demonstrate that migrating PGCs do not contain detectable fibronectin mRNA, suggesting that they do not synthesize and secrete the fibronectin within their migratory substratum. Taken together, these findings are consistent with an important role for fibronectin in stimulating PGC migration. In addition, however, they suggest that the interaction between PGCs and fibronectin may be important in timing the start of migration, with the fall in adhesion allowing the PGCs to commence their migration towards the genital ridges.

TGF beta 1 inhibits proliferation and has a chemotropic effect on mouse primordial germ cells in culture

Primordial germ cells are the stem cells that provide the functional gametes of adult animals. In many animal groups they are set aside at the earliest stages of development, and migrate from their sites of first appearance to the sites where the gonad will form, the genital ridges. During this migration they proliferate. In the mouse embryo their numbers increase from less than one hundred to approximately four thousand during the period of their migration. In a previous paper we showed that both the proliferation and the direction of migration of mouse PGCs in culture were influenced by soluble factors released from their target tissue, the genital ridges. Studies on other stem cell populations have shown that complex combinations of growth factors control their proliferation, migration and differentiation. In this paper, we show that TGF beta 1 inhibits proliferation of PGCs taken from 8.5 day old embryos and cultured on embryonic fibroblast feeder layers. We also show that the previously reported chemotropic effect of genital ridges in this culture system is mediated by TGF beta 1, or a closely related molecule, released from the genital ridges.

Effects of the steel gene product on mouse primordial germ cells in culture

Mutations at the steel (sl) and dominant white spotting (W) loci in the mouse affect primordial germ cells (PGC), melanoblasts and haemopoietic stem cells. The W gene encodes a cell-surface receptor of the tyrosine kinase family, the proto-oncogene c-kit. In situ analysis has shown c-kit messenger RNA expression in PGC in the early genital ridges. The Sl gene encodes the ligand for this receptor, a peptide growth factor, called here stem cell factor (SCF). SCF mRNA is expressed in many regions of the early mouse embryo, including the areas of migration of these cell types. It is important now to identify the role of the Sl-W interaction in the development of these migratory embryonic stem cell populations. Using an in vitro assay system, we show that SCF increases both the overall numbers and colony sizes of migratory PGC isolated from wild-type mouse embryos, and cultured on irradiated feeder layers of STO cells (a mouse embryonic fibroblast line). In the absence of feeder cells, SCF causes a large increase in the initial survival and apparent motility of PGC in culture. But labelling with bromodeoxyuridine shows that SCF is not, by itself, a mitogen for PGC. SCF does not exert a chemotropic effect on PGC in in vitro assays. These results suggest that SCF in vivo is an essential requirement for PGC survival. This demonstrates the control of the early germ-line population by a specific trophic factor.

Fertilization of cultured Xenopus oocytes and use in studies of maternally inherited molecules

The methods described here of fertilizing stage VI oocytes are lengthy and quite difficult techniques. They would become more attractive if the success rate (i.e., the number of fertilizations compared to the numbers of matured oocytes) could be improved. An important step toward this for the host transfer technique would be to monitor carefully the status of mature Xenopus females ovaries in relation to cyclical HCG stimulation, so that we could predict more accurately whether stage VI oocytes are fertilizable. The in vitro technique would obviously be improved if oocytes could be fertilized without removing their membranes, perhaps by using oviduct extracts. So far, this approach has had only limited success. It seems that the rewards of using these techniques could be great, in terms of understanding the maternal contribution to development. Although our experiments have not yet shown that oocyte injection of DNA has any advantage over egg injection, it is clear that it is possible to make "mRNA-minus mutants" by this approach. In the message depletion experiments mentioned here, we targeted the cleavage of an mRNA which is of low abundance in the full grown oocyte, but preliminary experiments have shown that we can deplete more abundant messages and produce specific phenotypes. Of course such experiments need to be controlled to show that the effect is specific, and the best proof that this is the case is to rescue the effect with injection of the appropriate mRNA. Finally, it seems likely that the method can be used to study the function of both localized molecules, such as the putative primordial germ cell (PGC) or dorsal determinants, and more ubiquitous molecules such as cytoskeletal elements.

Identification of vimentin and novel vimentin-related proteins in Xenopus oocytes and early embryos

We have made antibodies against fusion proteins of Xenopus vimentin. We show for the first time the distribution of vimentin in larval stages, where it is found in cells of mesenchymal origin, and in radial glial cells. In sections of Xenopus oocytes and early embryos, immunocytochemistry reveals the presence of an extensive cytoplasmic network, distributed in an animal-vegetal gradient. Germ plasm stains particularly strongly. The form of the IF proteins in this network is unusual. In immunoblot experiments the anti-vimentin antibodies detect a number of distinct proteins. We have identified those that are the products of the two known vimentin genes, by injection of synthetic mRNA transcribed from cloned vimentin cDNAs into oocytes, followed by two-dimensional Western blotting. This has demonstrated unambiguously that one Xenopus vimentin, Vim1, is present in oocytes and early embryos. However, two other immunoreactive proteins detected in Triton extracts of oocytes and early embryos are not the products of Vim1, since depletion of vimentin mRNA by antisense oligonucleotide injection has no effect on the synthesis of these proteins. These results suggest that novel IF-like proteins are expressed in Xenopus oocytes and early embryos.

Regional identity is established before gastrulation in the Xenopus embryo

Regional differences in cell recognition properties along the animal-vegetal axis of Xenopus laevis embryos were investigated by using an in vitro cell sorting assay. Dissociated cells were obtained from defined regions of blastula- and early gastrula-stage embryos. Binary combinations of cells from different regions, or from the same region at different ages, were aggregated in stationary culture. Labelling of one population in each pair with a cell-autonomous dye allowed the degree of sorting out in the resulting aggregates to be scored. In combinations of cells from animal caps (prospective ectoderm) and vegetal masses (prospective endoderm), sorting was detectable at the equivalent of late blastula stage and increased with developmental age to the gastrula stage. In addition, cells from the same region but different stages sorted from each other, indicating temporal changes in regional identity. Marginal cells (prospective mesoderm) sorted strongly from vegetal cells but only weakly from animal cells. These results indicate the presence of regional identity in the form of specific recognition properties in cells of the Xenopus blastula. We suggest that these properties act to establish and maintain coherent cell populations, corresponding to the primary germ layers, prior to gastrulation. These results account for the gradual restriction in developmental capacity of blastomeres seen previously in single-cell transplantation experiments from this laboratory.

Vimentin expression in oocytes, eggs and early embryos of Xenopus laevis

Immunocytochemical studies using a monoclonal anti-porcine vimentin antibody reveal a well-organized pattern of staining in Xenopus laevis oocytes, eggs and early embryos. The positions of Xenopus vimentin and desmin in two-dimensional (2D) polyacrylamide gels were first established by immunoblotting of muscle Triton extracts with anti-intermediate filament antibodies (anti-IFA), which cross-react with all intermediate filament proteins (IFPs). The anti-porcine vimentin reacts with vimentin and desmin in muscle 2D immunoblots, but only reacts with one polypeptide in oocyte blots in the position predicted for vimentin (Mr 55 × 10(3), pI 5.6). Using an anti-sense probe derived from a Xenopus vimentin genomic clone in RNase protection assays, we show that expression of vimentin begins in previtellogenic oocytes. The level of expression remains constant throughout oogenesis and in unfertilized eggs. These data suggest that vimentin is expressed in oocytes and eggs. Most interestingly, the immunocytochemical results also show that vimentin is present in the germ plasma of oocytes, eggs and early embryos. It is therefore possible that vimentin has an important role in the formation or behaviour of early germ line cells.

Studies on the migration of mouse germ cells

Primordial germ cells in the mouse embryo migrate from their site of origin to the gonad where they differentiate, giving rise eventually to the gametes of the mature adult animal. The migratory phase is transient and therefore permits analysis of factors regulating the motile activity of cells in tissues. Germ cells can be isolated during migration and cultured on feeder cells of an established cell line (STO). In this system the molecular interactions mediating germ cell adhesion can be probed using antibodies to both germ cell surface determinants and to extracellular matrix components.

Changes in states of commitment of single animal pole blastomeres of Xenopus laevis

The experiments described in this paper were designed to compare the normal fates of animal pole blastomeres of Xenopus laevis with their state of commitment. Single animal pole blastomeres were labeled with a lineage marker and transplanted into the blastocoels of host embryos of different stages. The distribution of labeled daughter cells in the tadpole reflects the state of commitment of the parent cell at the time of transplantation. It is known that cells from the animal pole of the early blastula normally contribute predominantly to ectoderm with a small, but significant, contribution to the mesoderm. We show that on transplantation to the blastocoels of late blastula host embryos these blastomeres are pluripotent, contributing to all three germ layers. At later stages the normal fate of these cells becomes restricted solely to ectoderm and concomitantly the proportion of pluripotent cells is reduced, although the results depend upon the stage of the host embryo. Blastomeres from late blastula donors transplanted to mid gastrulae contribute solely to ectoderm in 34% of cases; however, in earlier hosts, when the vegetal hemisphere cells have "mesoderm inducing" or "vegetalizing" activity, late blastula animal pole blastomeres contribute to mesoderm and endoderm rather than ectoderm. Thus during the blastula stage animal pole cells pass from pluripotency to a labile state of commitment to ectoderm.

Vegetal pole cells and commitment to form endoderm in Xenopus laevis

In order to compare their states of commitment with their normal developmental fate, single vegetal pole cells from early Xenopus embryos were labeled and transplanted into the blastocoels of host embryos. In a previous study we showed, using this single cell transplantation assay, that vegetal pole cells become committed to endoderm by the early gastrula stage. In this paper we examine some properties of the commitment process. First, we show that it is gradual. When vegetal blastomeres are taken from progressively older embryos an increasing number of them enter only the endoderm, until by the early gastrula stage they all do. Second, we show that commitment can continue in vitro when an appropriate tissue mass is present. We suggest that commitment to form endoderm may be, in the right conditions, a cell autonomous process.

Invasive behaviour of mouse primordial germ cells in vitro

We have isolated migrating primordial germ cells (PGCs) from 10.5-day mouse embryos and studied their behaviour when cultured on a mouse embryo fibroblast (STO) cell line. Living and fixed PGCs were identified by fluorescent labelling with a monoclonal antibody specific for PGCs in the culture system used. The behaviour of the cells was studied using interference reflexion microscopy (IRM) and time-lapse video cinematography. The IRM pattern displayed by PGCs is typical of highly motile cell types, the cells lack focal contacts and possess large areas of close contacts indicative of weak membrane to substrate interaction. The PGCs exhibit relatively high rates of translocation and lack contact inhibition. They were observed to underlap STO cells in subconfluent monolayers and to penetrate between the cells of confluent monolayers, becoming located between the monolayer and its substrate. These observations support the hypothesis that migrating mouse PGCs are inherently motile and are able transiently to disrupt the adhesion of surrounding cells. These results suggest that PGCs actively migrate to the developing gonad in vivo.

The appearance and distribution of intermediate filament proteins during differentiation of the central nervous system, skin and notochord of Xenopus laevis

Antibodies against various intermediate filament proteins have been used to follow cell differentiation in the early Xenopus embryo. Three new monoclonal antibodies against Xenopus cytokeratins raised against Triton-insoluble material from tadpoles (RD35/2a, RD35/3a and D3/3a), two antibodies against mammalian cytokeratins (LE65 and LP3K), monoclonal anti-(rat 200 K neurofilament protein), rabbit anti-(rat glial filament acidic protein), and rabbit antibodies to hamster and calf vimentin were used. We show that cytokeratins are present in the early central nervous system (CNS) and persist in the ependymal cells of the adult CNS. We also show that the notochord contains cytokeratin. The ontogeny of intermediate filament protein appearance in the CNS, skin and notochord between neural fold stage and swimming tadpole stage are described. These results are discussed in particular with regard to the use of the antibodies as differentiation markers.

Cinemicrographic study of the cell movement in the primitive-streak-stage mouse embryo

Migration of the mesoderm cells in the primitive-streak-stage mouse embryo was directly studied by cinemicrography using whole embryo culture and Nomarski differential interference contrast optics. Relative transparency and small size of the early mouse embryos enabled direct observation of the individual cells and their cell processes. Seven-day-old mouse embryos were isolated and cultured in a small chamber in a medium consisting of 50% rat serum and 50% Dulbecco's modified minimum essential medium. The mesoderm cells move away from the primitive streak in both anterior and antimesometrial (distal) directions at a mean velocity of 46 micron h-1. They extend cell processes and constantly change cell shape. They do not translocate extensively as isolated single cells, but usually maintain attachment to other mesoderm cells. They show frequent cell division preceded by rounding up of the cell bodies, and accompanied by vigorous blebbing before and after cytokinesis. This study shows that it is possible to examine the motility of embryonic cells inside the mammalian embryo by direct observation if the embryo is small and transparent enough for the use of the Nomarski optics.

Migratory and postmigratory mouse primordial germ cells behave differently in culture

In all vertebrate groups, the progenitors of the germ line, the primordial germ cells (PGCs) arise extragonadally and move to the developing gonad early in embryonic development. We have examined the behavior of isolated pregonadal and gonadal PGCs in vitro on feeder layers of an embryo-derived cell line. Histochemically and serologically identified pregonadal germ cells are found to be actively motile in vitro and, furthermore, show behavior characteristic of invasive cells. PGCs isolated from the developing gonad, however, show little locomotory activity and are not invasive on the same cellular substrate. These observations suggest that PGCs undergo a major change in phenotype at the time of their entry into the gonad anlagen.

Macrophages in the urogenital ridge of the mid-gestation mouse fetus

Isolated urogenital ridges from mouse embryos 11 1/2-16 1/2 days post coitum were separated into genital ridge and mesonephric areas; the latter were then cut into small fragments and cultured as explants on glass or plastic. The explants soon became surrounded by flat outgrowths of cells, followed by a population of large round granular cells. The behaviour of these cells, first on the outgrowth and then on the glass is described. Evidence is presented that they are fetal macrophages.

Cytoskeletal changes during oogenesis and early development of Xenopus laevis

The frog oocyte is well known for studies on the control of gene expression, but has been used much less in studies on the cytoskeleton. However, frog oocytes are very large single cells, whose cytoplasmic movements and asymmetries are fundamental to the correct development of the subsequent embryo. One particular example of asymmetrically distributed cytoplasm is germ plasm, thought to be important in the formation of the germ line. Data are presented that show that germ plasm is highly concentrated mass of cytoskeletal elements, which include tubulin, and an intermediate filament protein of molecular weight 55 X 10(3). The distribution of these molecules has been studied during oogenesis and during early post-fertilization development. The implications of these findings are discussed.

The cytoskeleton of Xenopus oocytes and its role in development

Much is known about determinative events in early amphibian embryos, perhaps more than any other animal group. However, as yet, little attention has been focused on the cytoarchitecture of the oocyte, and the way in which this could regulate asymmetries in the egg, which in turn could lead to developmentally important interactions. The changing cytoarchitecture of the Xenopus oocyte is described with the emphasis on the following: -firstly the polarity; the oocyte is not radially symmetrical at early stages of oogenesis, but shows marked polarity. Secondly, several cytoskeletal elements change their distribution during oogenesis, and again during maturation to form a fertilizable egg. Thirdly, monoclonal antibody methods show that the oocyte develops several asymmetries which are retained in the egg and early embryo, and may be lineage related.

Single cell analysis of commitment in early embryogenesis

Fate maps of amphibian embryos tell us the destination of certain areas at later stages of development. After studying Vogt's fate maps, Spemann wrote (in 1938) that "the question which at once calls for an answer is whether this pattern of presumptive primordia in the beginning gastrula is the expression of a real difference of these parts, whether they are already more or less predestined or 'determined' for their ultimate fate, or whether they are still indifferent and will not receive their determination until a later time." Until recently answers to this question have relied upon explant experiments, which indicate that by the late blastula stage ectoderm, mesoderm and endoderm regions are distinctly determined. By using a method involving single cell labelling and transplantation, it is now possible to pinpoint more accurately the time during early embryogenesis at which individual blastomeres become committed. In the vegetal pole, determination towards endoderm is a gradual process beginning during the middle blastula stage (stage 8) and completed by the beginning of gastulation (stage 10). This method offers the possibilities of comparing the committed and the uncommitted state and studying at a molecular level the mechanisms of cell determination.

Single cell analysis of commitment in early embryogenesis

Fate maps of amphibian embryos tell us the destination of certain areas at later stages of development. After studying Vogt's fate maps, Spemann wrote (in 1938) that “the question which at once calls for an answer is whether this pattern of presumptive primordia in the beginning gastrula is the expression of a real difference of these parts, whether they are already more or less predestined or ‘determined’ for their ultimate fate, or whether they are still indifferent and will not receive their determination until a later time.” Until recently answers to this question have relied upon explant experiments, which indicate that by the late blastula stage ectoderm, mesoderm and endoderm regions are distinctly determined. By using a method involving single cell labelling and transplantation, it is now possible to pinpoint more accurately the time during early embryogenesis at which individual blastomeres become committed. In the vegetal pole, determination towards endoderm is a gradual process beginning during the middle blastula stage (stage 8) and completed by the beginning of gastulation (stage 10). This method offers the possibilities of comparing the committed and the uncommitted state and studying at a molecular level the mechanisms of cell determination.

Fibronectin visualized by scanning electron microscopy immunocytochemistry on the substratum for cell migration in Xenopus laevis gastrulae

In amphibian gastrulae, scanning electron microscopy (SEM) has shown the presence of a network of extracellular fibrils on the inner aspect of the ectoderm layer, which serves as the substratum for migration by the presumptive mesoderm cells. In vitro experiments have shown that the fibril network promotes attachment and migration by mesoderm cells, and probably guides the migration by contact guidance. Filopodia of the migrating cells showed preferential attachment to the fibrils. Use of a colloidal gold probe for SEM immunocytochemistry has shown that fibrils observed by SEM contain fibronectin, probably as a major component. This provides direct evidence that the extracellular matrix containing fibronectin provides the substratum and guides cell migration in morphogenetic movement.

Oocytes and early embryos of Xenopus laevis contain intermediate filaments which react with anti-mammalian vimentin antibodies

Previous studies have shown that Xenopus oocytes possess a cortical shell, which includes actin-containing microfilaments and cytokeratin-containing intermediate filaments. In this paper we show that oocytes of Xenopus laevis also contain filaments throughout their cytoplasm which are stained by several anti-vimentin antibody preparations. We also show that dramatic changes in pattern of these filaments occur during oocyte differentiation, first during vitellogenesis, and then during maturation of the oocyte to form an egg.

The mitochondrial cloud of Xenopus oocytes: The source of germinal granule material

The mitochondrial cloud is a prominent mass in the cytoplasm of previtellogenic oocytes of Xenopus laevis. It is shown here that the cloud contains both mitochondria and electron-dense granulofibrillar material (GFM). Using a combination of light microscopical, fluorescence, time-lapse filming, and electron microscopical techniques, the ontogeny of these components is reported and their fate is studied. It was found that the cloud is stationary in previtellogenic stages and fragments into islands of mitochondria and GFM during stage II (using the staging system of J. N. Dumont [1972) J. Morphol. 136, 153-180). These islands become localized in the peripheral cytoplasm at one pole of the stage III oocyte. By studying successive stages, GFM was followed through oogenesis and it was found localized only at the vegetal pole of stage IV and V oocytes. Furthermore, it was found that it bears a striking resemblance in position, appearance, and associations with mitochondria to the "germinal granules" of unfertilized eggs. Germinal granules have been shown by others to become incorporated into germ-line cells. It is concluded that the GFM is the precursor of this material and that the mitochondrial cloud is the site of its accumulation and localization in the previtellogenic oocyte.

Intermediate filaments in the Xenopus oocyte: the appearance and distribution of cytokeratin-containing filaments

Cytokeratins have previously been shown to exist in the Xenopus oocyte cortex. Using three monoclonal antibodies against cytokeratins, we follow the appearance of cytokeratin-containing filaments, and their changes in distribution during oocyte differentiation and maturation. Cytokeratin-containing filaments are shown to change dramatically in distribution in the oocyte during its ontogeny.

Oocytes and early embryos of Xenopus laevis contain intermediate filaments which react with anti-mammalian vimentin antibodies

Previous studies have shown that Xenopus oocytes possess a cortical shell, which includes actin-containing microfilaments and cytokeratin-containing intermediate filaments. In this paper we show that oocytes of Xenopus laevis also contain filaments throughout their cytoplasm which are stained by several anti-vimentin antibody preparations. We also show that dramatic changes in pattern of these filaments occur during oocyte differentiation, first during vitellogenesis, and then during maturation of the oocyte to form an egg.

Fates and states of determination of single vegetal pole blastomeres of X. laevis

Vegetal pole cells of Xenopus morulae contribute progeny to all three germ layers, but from the midblastula stage onward they contribute only to the endoderm. We have investigated whether this restriction in fate reflects cell determination by implanting labeled vegetal pole cells into the blastocoels of host embryos and asking which structures later include labeled progeny. Single vegetal pole cells from the morula and also from the midblastula stage can contribute progeny to all germ layers. At the early gastrula stage the cells can contribute only to the endoderm. Thus the restriction of fate in the midblastula does not reflect cell determination. However, the cells do become determined by the beginning of gastrulation.

Alterations in the organisation of cytokeratin filaments in normal and malignant human colonic epithelial cells during mitosis

An ultrastructural analysis of the organisation of 10 nm intermediate filaments during mitosis, in normal and malignant human colonic epithelial cells present in resected material, was carried out. Similar changes in the integrity of the 10 nm filaments were seen both in normal mitotic cells (which have a highly specialised filamentous network during interphase), and malignant cells (which show a disorganised network during interphase). The 10 nm filaments were seen to shorten and 'fray' in early mitosis. This was associated with the formation of spheroidal aggregates of thin filaments and small granules, both free in the cytoplasm and associated with surface desmosomes. At mid-mitosis cells were essentially devoid of obvious filamentous material, (including the spheroidal aggregates). The filaments reformed in daughter cells after cytokinesis was completed.

The organization of intermediate filaments in normal human colonic epithelium and colonic carcinoma cells

A comparative ultrastructural study of the organization of intermediate filaments (tonofilaments) within normal colonic epithelium and colonic carcinoma cells was carried out. The pattern of development of the specialized intermediate filament (IF) network, occurring during the differentiation of normal colonic epithelial cells, was examined to allow a more complete comparison with tumour cells of different degrees of differentiation. Essentially the IF system becomes more organized as the cells differentiate, resulting in the formation of a three-dimensional network loosely surrounding the nucleus, extending through the sub-terminal web and basal cytoplasm, and inserting into lateral desmosomes. Another system of IFs also runs directly between lateral desmosomes, lying parallel to the plasma-membrane and desmosomal plaque, without inserting directly into the junction. Alterations in the organization of the IF system were seen in tumour cells with the type and extent being dependent upon their location and degree of differentiation. Cells within areas of the tumour where some degree of glandular structure was retained exhibited major alterations in their microfilament-containing structures. However, their IF system was essentially intact and lateral desmosomes were still present in similar numbers to those seen in normal, partially differentiated mid-crypt cells. Apolar tumour cells within areas of the tumour where a gross loss of cryptal architecture had occurred exhibited a striking disorganization of their IF system. No parallel bundles of interdesmosomal tonofilaments were present beneath the cell surface and the majority of the three-dimensional network appeared to have collapsed around the nucleus, although some extensions to randomly distributed surface desmosomes still occurred. It appears that this disorganization of the IF system is associated with a loss of cell polarity and may involve alterations in putative IF-associated proteins important in the interaction of IFs with surface desmosomes or other cytoskeletal elements. A study of this phenomenon may shed light on both the means of IF organization and assembly and their role in normal cells as well as the possible role of alterations in cytoskeletal elements in the expression or maintenance of the malignant phenotype.

Further analysis of the effect of ultra-violet irradiation on the formation of the germ line in Xenopus laevis

Ultra-violet (u.v.) irradiation of the vegetal pole of newly fertilized eggs has three documented effects: reduction of primordial germ cells (PGCs), cytological damage to the vegetal hemisphere and disruption of the normal mechanism by which the vegetal yolk mass induces the formation of the dorsal axis of the embryo. In this study, we find that 90 degrees rotation of the egg for various periods after irradiation rescues the dorsal axial structures but does not restore the number of PGCs found in the dorsal mesentery of the gut; neither is there any correlation between reduced numbers of PGCs and disruption of cleavage at the vegetal pole. We therefore conclude that the effect on the germ line is separate from the other two phenomena. Secondly, 90 degrees rotation of non-irradiated eggs was found to significantly reduce germ cell numbers migrating in the dorsal mesentery of the gut.