Spatial response to fibroblast growth factor signalling in Xenopus embryos

We have examined the spatial pattern of activation of the extracellular signal-regulated protein kinase (ERK) during Xenopus development, and show that it closely resembles the expression of various fibroblast growth factors (FGFs). Until the tailbud stage of development, all ERK activation domains are sensitive to the dominant negative FGF receptor, showing that activation is generated by endogenous FGF signalling. ERK is not activated by application of other growth factors like BMP4 or activin, nor is endogenous activation blocked by the respective dominant negative receptors. This shows that various domains of FGF expression, including the periblastoporal region and the midbrain-hindbrain boundary, are also sites of FGF signalling in vivo. Wounding induces a transient (<60 minutes) activation of ERK which is not significantly reduced by the dominant negative FGF receptor. An artificial FGF source, created by injection of eFGF mRNA into cleavage stage embryos, provokes ERK activation outside of its injection site over a range of several cell diameters. The range and extent of ERK activation outside the source region is unchanged by co-injection of a dominant negative form of Ras, which blocks ERK-activation within the source. This suggests that FGF protein can diffuse over several cell diameters.

Expression and functions of FGF-3 in Xenopus development

We have analyzed the expression pattern of the Xenopus FGF-3 gene during early development and examined its biological activity in three different bioassays using Xenopus embryos. We show that from the early gastrula stage there is a domain of expression around the blastopore which becomes a posterior domain as the blastopore closes. An anterior ectodermal domain becomes detectable from mid-gastrula stages in the prospective hind-brain, and there are several later domains of expression: the midbrain-hindbrain junction, the otocyst, the pharyngeal pouches and the tailbud region. By using double whole-mount in situ hybridizations we show that the XFGF-3 expression in the brain is dynamically regulated both in time and space during development. The anterior domain of early neurula stage embryos corresponds to the prospective rhombomeres 3-5. By the time the neural tube is closed, XFGF-3 expression is restricted to r4 and later a new domain of expression is established at the midbrain/hindbrain junction. In addition, we show that, despite its difference in receptor specificity, XFGF-3 can induce the formation of mesoderm from animal caps similarly to other FGFs. It also displays a posteriorizing activity on whole embryos similar to other FGFs. Although the absence of maternal expression makes it unlikely that XFGF-3 is involved in mesoderm induction in vivo, its posterior domain of expression during gastrulation and its posteriorizing activity suggests that it participates in the maintenance of mesodermal gene expression and in the FGF mediated patterning of the anteroposterior axis during gastrulation.

Development of the gut in Xenopus laevis

The lining of the gut, together with the pancreas, liver, gall bladder, and respiratory system, is formed from the endoderm. The gut also contains smooth muscle and connective tissue of mesodermal origin. The amphibian Xenopus laevis is potentially an excellent model organism for studying how the cells of the endoderm and mesoderm become programmed to produce these internal organs. However, the anatomical complexity of the coiled gut presents a problem in studying its development. In order to overcome this problem we here present a comprehensive guide to the anatomy and histology of the developing Xenopus gut. We use a simple dissection to display its anatomy and the expression of four endodermal markers (alkaline phosphatase, IFABP, XlHbox8, and endodermin). We present schematic diagrams that show how the gut is arranged in three dimensions and how this organisation changes during development. We also present drawings of histological sections of the gut which allow any region to be identified and so represent an atlas for working with sections. Finally, we describe the histology of the cells of the various organs of the gut. This histological identification may be necessary for the identification of parts following experiments in which the normal pattern is disturbed.

Regulation of Hox gene expression and posterior development by the Xenopus caudal homologue Xcad3

The caudal gene codes for a homeodomain transcription factor that is required for normal posterior development in Drosophila. In this study the biological activities of the Xenopus caudal (Cdx) family member Xcad3 are examined. A series of domain-swapping experiments demonstrate that the N-terminus of Xcad3 is necessary for it to activate Hox gene expression and that this function can be replaced by the activation domain from the viral protein VP16. In addition, experiments using an Xcad3 repressor mutant (XcadEn-R), which potently blocks the activity of wild-type Xcad3, are reported. Overexpression of XcadEn-R in embryos inhibits the activation of the same subset of Hox genes that are activated by wild-type Xcad3 and leads to a dramatic disruption of posterior development. We show that Xcad3 is an immediate early target of the FGF signalling pathway and that Xcad3 posteriorizes anterior neural tissue in a similar way to FGF. Furthermore, Xcad3 is required for the activation of Hox genes by FGFs. These data provide strong evidence that Xcad3 is required for normal posterior development and that it regulates the expression of the Hox genes downstream of FGF signalling.

Two phases of Hox gene regulation during early Xenopus development

We have shown previously that fibroblast growth factor (FGF) signalling in posterior regions of the Xenopus embryo is required for the development of the trunk and tail via a molecular pathway that includes the caudal-related gene Xcad3 and the posterior Hox genes [1]. These results have been contested by the work of Kroll and Amaya [2], which shows that Xenopus embryos transgenic for a dominant-negative form of the FGF receptor (FGF-RI) express posterior Hox genes normally, leading these authors to suggest that the FGFs are not required for anteroposterior (A-P) patterning of the dorsal axis. In order to investigate the apparent discrepancy between these studies, we have produced Xenopus embryos transgenic for two inhibitors of the FGF/Caudal pathway: a kinase-deficient dominant-negative FGF receptor (XFD) [3]; and a domain-swapped form of Xcad3 (Xcad-EnR) in which the activation domain of Xcad3 is replaced by the repression domain of the Drosophila Engrailed protein. Both of these were introduced as fusions with the green fluorescent protein (GFP), which allows identification of non-mosaic transgenic embryos at early gastrula stages by simply looking for GFP fluorescence. Analysis of gene expression in embryos transgenic for these constructs indicated that the activation of posterior Hox genes during early neurula stages absolutely requires FGF signalling and transcriptional activation by Xcad3, while the maintenance of Hox gene expression in the trunk and tail during later development is independent of both FGF and Xcad.

Postgastrulation effects of fibroblast growth factor on Xenopus development

We have investigated postgastrulation functions of FGFs in Xenopus development by the implantation of heparin beads soaked in FGF2 to various positions at various stages. Anterior implantations show different effects depending on whether they are made to early neurulae or to later stages. At stage 13-14 there is a total or partial suppression of anterior structures including the forebrain, eyes, and midbrain. From stage 15 onwards there is no loss of anterior parts but there is a change in the structure of the eye such that the neural retina remains continuous with the wall of the diencephalon and the territories normally forming the optic stalk and pigment epithelium instead become neural retina. Posterior implantations cause a disruption of somite segmentation without affecting the differentiation of muscle cells. This is associated with a prolongation of the uniform expression of X-Delta-2 during the phase of segmental determination. There is also an induction of ectopic otocysts, which can lie either ipsilateral or contralateral to the FGF-bead. The results are discussed in terms of the known late expression domains of the various Xenopus FGFs, and of the late functions of FGFs in higher vertebrates. They provide new evidence for a role of endogenous FGFs in the development of the eye, somites, and otocysts.

Analysis of the developing Xenopus tail bud reveals separate phases of gene expression during determination and outgrowth

We have studied Xenopus tail development from the end of gastrulation to the commencement of outgrowth at the tail bud stage. We show that an early group of genes are expressed at the stage of tail bud determination, at the end of gastrulation, and a late group are expressed at around stage 27 just before tail bud outgrowth. Together, these genes define seven distinct regions of the tail bud as outgrowth commences. We have previously shown that formation of a tail bud depends on the interaction of three tissue regions, called N, M and C, at stage 13. Here we show that expression of the late group of genes is dependent on this NMC interaction. We describe molecular correlates of two of these regions, M and C, which were formerly unobservable and whose existence was inferred from embryological experiments.

FGF-8 is associated with anteroposterior patterning and limb regeneration in Xenopus

FGF-8 has attracted attention particularly because of its importance for limb development in the chick and mouse, although it also has a number of earlier expression domains in these species. We have now cloned an FGF-8 homologue from Xenopus in which it is easier to do functional studies on early development. There is no maternal expression, while zygotic expression is highest in the gastrula and neurula stages. XFGF-8 is expressed as a ring around the blastopore and subsequently in the tail bud. There are several domains in the head including the hatching gland, the branchial clefts, and the midbrain-hindbrain border. At later stages there is a prominent band of expression in the limb bud epidermis. Although there is no morphological apical ridge, this band of expression suggests that the Xenopus limb bud contains a cryptic region with a similar ability to stimulate mesenchymal outgrowth. The mesoderm-inducing activity of XFGF-8 is somewhat lower than that of other FGFs, while the posteriorizing activity is similar. These differences are probably due to the different receptor specificity. The posterior expression and high posteriorizing activity suggest that XFGF-8 contributes to the patterning of the anterior-posterior axis by FGF family members during gastrulation. In contrast to the amniotes, Xenopus limb buds can regenerate following damage. We show that regeneration is correlated with the reexpression of XFGF-8 in the distal epidermis, suggesting that this ability is critical for successful limb regeneration.

Identification of two independent transcriptional activation domains in the Autographa californica multicapsid nuclear polyhedrosis virus IE1 protein

The Autographa californica multicapsid nuclear polyhedrosis virus immediate-early protein, IE1, is a 582-amino-acid phosphoprotein that regulates the transcription of early viral genes. Deletion of N-terminal regions of IE1 in previous studies (G. R. Kovacs, J. Choi, L. A. Guarino, and M. D. Summers, J. Virol. 66:7429-7437, 1992) resulted in the loss of transcriptional activation, suggesting that this region may contain an acidic activation domain. To identify independently functional transcriptional activation domains, we developed a heterologous system in which potential regulatory domains were fused with a modified Escherichia coli Lac repressor protein that contains a nuclear localization signal (NLacR). Transcriptional activation by the resulting NLacR-IE1 chimeras was measured with a basal baculovirus early promoter containing optimized Lac repressor binding sites (lac operators). Chimeras containing IE1 peptides dramatically activated transcription of the basal promoter only when lac operator sequences were present. In addition, transcriptional activation by NLacR-IE1 chimeras was allosterically regulated by the lactose analog, isopropyl-beta-D-thiogalactopyranoside (IPTG). For a more detailed analysis of IE1 regulatory domains, the M1 to T266 N-terminal portion of IE1 was subdivided (on the basis of average amino acid charge) into five smaller regions which were fused in various combinations to NLacR. Regions M1 to N125 and A168 to G222 were identified as independent transcriptional activation domains. Some NLacR-IE1 chimeras exhibited retarded migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels. As with wild-type IE1, this aberrant gel mobility was associated with phosphorylation. Mapping studies with the NLacR-IE1 chimeras indicate that the M1 to A168 region of IE1 is necessary for this phosphorylation-associated effect.

Growth control: action mouse

A recently described knockout mouse has abnormally large muscles. The phenotype suggests that the ablated product, growth differentiation factor 8 or 'myostatin', may be one of the long sought inhibitors that control the growth of individual tissues and organs.

Inhibition of eFGF expression in Xenopus embryos by antisense mRNA

We studied the effects in Xenopus embryos of overexpression of antisense RNA complementary to the messenger RNA of eFGF. We show that the expression of sense RNA can be severely depressed in the presence of an excess of antisense RNA. This occurs by both partial destruction of the message and by a depression of translation of the residual message. The diminution of inducing activity of eFGF, measured in animal cap assays either by activation of the Brachyury gene or by morphology, parallels the reduction of translation. Endogenous eFGF expression is reduced to a similar extent, again by a combination of mRNA destruction and inhibition of translation. This shows that the overexpression of antisense RNA is, contrary to general opinion, a potentially useful technique for studying gene function in Xenopus embryos. However, in the case of eFGF, there is little or no overall phenotypic effect on whole embryos. This is probably because of the presence of several other FGFs with overlapping expression domains in the early embryo.

Diagnostic leaf nutrient standards for low-chill peaches in subtropical Australia

Summary. A leaf nutrient survey was conducted of the low-chill peach cultivars, Flordaprince (October maturing) and Flordagold (mid November–early December maturing) at 3 commercial sites in both northern New South Wales and southern Queensland. Recently mature leaves from the middle third of a current season’s fruiting lateral (spring flush) were sampled at stone hardening and 2-weeks postharvest and of a non-fruiting lateral at maturity of the summer flush (after summer pruning) during the 1992–93 and 1993–94 seasons. At an additional site in New South Wales (Alstonville), leaf nutrient concentrations were also determined on cv. Flordagem (early November maturing) at 2-week intervals during both seasons. Soil (0–30 cm) chemical determinations were conducted at all sites at 2-weeks postharvest Seasonal trends in leaf nutrient composition were associated with a leaf age–maturity effect. As flush leaves matured during spring, and as mature leaves aged after hardening of the summer flush, nitrogen (N) concentration declined and calcium (Ca) concentration increased. Nitrogen and Ca concentrations increased when young leaves produced from the summer flush were sampled. Time of sampling produced the most consistently significant (P<0.05) main effects on leaf nutrient concentration. The 2-week postharvest period was selected as a convenient time to sample—when leaves were of a consistent age and maturity, and the effect of crop load on tree nutrient reserves was still present. Paclobutrazol, which reduces vegetative growth in stonefruit, was applied to all Queensland sites and, as a consequence, mid lateral leaves contained higher (P<0.05) Ca, magnesium (Mg) and chloride (Cl) and lower (P<0.05) N and phosphorus (P) concentrations than leaves from New South Wales sites. State effects can therefore be interpreted as paclobutrazol effects. Cultivar effects (P<0.05) occurred for many leaf nutrients, however, at the 2-week postharvest sampling, concentrations were sufficiently similar to combine as a narrow adequate concentration range for both cultivars. The diagnostic adequate leaf nutrient concentrations were within the range developed for high-chill peaches (Leece et al. 1971) with the exception of lower Ca, lower Mg for New South Wales (both cultivars), lower iron for Flordaprince (both states), higher P for Flordaprince in New South Wales and higher manganese values for Queensland (both cultivars). Regression analyses were conducted between leaf and fruit nutrient concentrations and soil chemical properties. The only consistent result demonstrated that as the soil Ca : Mg ratio increased, leaf Mg concentration decreased exponentially (P<0.001), indicating that the practice of heavy annual agricultural limestone or gypsum applications in the absence of Mg fertiliser, which had been adopted by several growers in the survey, is associated with lower leaf Mg concentrations.

eFGF, Xcad3 and Hox genes form a molecular pathway that establishes the anteroposterior axis in Xenopus

Classical embryological experiments suggest that a posterior signal is required for patterning the developing anteroposterior axis. In this paper, we investigate a potential role for FGF signalling in this process. During normal development, embryonic fibroblast growth factor (eFGF) is expressed in the posterior of the Xenopus embryo. We have previously shown that overexpression of eFGF from the start of gastrulation results in a posteriorised phenotype of reduced head and enlarged proctodaeum. We have now determined the molecular basis of this phenotype and we propose a role for eFGF in normal anteroposterior patterning. In this study, we show that the overexpression of eFGF causes the up-regulation of a number of posteriorly expressed genes, and prominent among these are Xcad3, a caudal homologue, and the Hox genes, in particular HoxA7. There is both an increase of expression within the normal domains and an extension of expression towards the anterior. Application of eFGF-loaded beads to specific regions of gastrulae reveals that anterior truncations arise from an effect on the developing dorsal axis. Similar anterior truncations are caused by the dorsal overexpression of Xcad3 or HoxA7. This suggests that this aspect of the eFGF overexpression phenotype is caused by the ectopic activation of posterior genes in anterior regions. Further results using the dominant negative FGF receptor show that the normal expression of posterior Hox genes is dependent on FGF signalling and that this regulation is likely mediated by the activation of Xcad3. The biological activity of eFGF, together with its expression in the posterior of the embryo, make it a good candidate to fulfil the role of the ‘transforming’ activity proposed by Nieuwkoop in his ‘activation and transformation’ model for neural patterning.

XFGF-9: a new fibroblast growth factor from Xenopus embryos

We have identified the Xenopus homologue of mammalian FGF-9 (XFGF-9). Sequence comparison between Xenopus and mammals shows that they share 93% identity at the amino acid level, making FGF-9 the most highly conserved member within the family. The sequence shows that there is no N-terminal signal sequence but that there is an internal hydrophobic sequence resembling a transmembrane domain. By using an in vitro translation system, we demonstrate that XFGF-9 can be glycosylated by microsomes but shows no signal peptide cleavage. This suggests that it can be secreted using the internal hydrophobic domain to cross the endoplasmic reticulum membrane. Expression studies using RNAse protections and in situ hybridization show that XFGF-9 is expressed both maternally and zygotically. The maternal mRNA is detected at a higher level than other forms (XFGF-2 and eFGF), mainly in the animal hemisphere. A proportion of the maternal transcript persists until the early gastrula stage when it is joined by zygotic expression around the blastopore region, and thereafter the mRNA content shows some increase during further development. Zygotic XFGF-9 is expressed uniformly along the dorsal axis, as well as in the head region. We have expressed recombinant XFGF-9 protein in bacteria, and show that it has a mesoderm-inducing activity in the animal cap assay, with a similar specific activity to other fibroblast growth factor (FGFs). We have injected a synthetic mRNA into eggs, and show that it has both mesoderm-inducing activity in animal caps and also a posteriorizing activity in whole embryos. The levels of biological activity shown by the XFGF-9 mRNA injections compared to XFGF-2 and eFGF show that there is at least some extracellular function. This supports the biochemical results, suggesting that the protein has at least some capacity to be secreted.

Actin binding and proteolysis by the baculovirus AcMNPV: the role of virion-associated V-CATH

Infection of larvae by Autographa californica M nuclear polyhedrosis virus (AcMNPV) results in liquefaction of susceptible hosts, presumably due to the breakdown of cells and extracellular matrices. In Spodoptera frugiperda tissue culture cells, infection leads to dramatic rearrangement and eventual destruction of the actin cytoskeleton. The first of these rearrangements is the formation of actin cables in the cytoplasm of the cell. Cable formation requires release of the budded virus (BV) nucleocapsid from the endosome, but does not require new protein synthesis, suggesting that the nucleocapsid contains the activity necessary to induce cable formation. We have identified two distinct BV-associated actin-targeting activities. The first, a nucleocapsid-associated actin-binding activity, enabled actin copelleting and may also induce actin polymerization and cable formation. The second activity, associated with the nucleocapsid and envelope fractions of BV, was a protease that specifically degraded actin. This protease was identified as V-CATH, a cathepsin L-like protease that is a product of the AcMNPV v-cath gene.

The role of fibroblast growth factors in early Xenopus development

In recent years we and others have been attempting to identify the molecular nature of the inducing signals in early Xenopus development. We have found that most members of the fibroblast growth factor (FGF) family are biologically active as mesoderm-inducing factors when applied to ectoderm from blastulae. In addition to this, they will support continued expression of the pan-mesodermal transcription factor Xbra in the mesoderm of gastrula stage embryos. We have studied the expression pattern of four types of FGF in early embryos. Two types (FGF-2 and FGF-9) are expressed maternally and are thus present at the time of natural mesoderm induction. The expression of two other types (FGF-3 and FGF-4) is activated in the newly formed mesoderm of the gastrula. If the activity of the FGF family is inhibited by overexpression of a dominant-negative FGF receptor, there is a reduction in mesoderm formation, there are abnormalities arising from an inhibition of normal gastrulation movements and there is a defect in formation of the posterior parts. We believe that the mesoderm formation and cell movement effects are attributable to loss of Xbra expression, and the posterior defects to lack of posterior HOX gene activity. Overexpression of eFGF gives rise to a posteriorized phenotype, in which posterior HOX genes are expressed in a more anterior position. We conclude that the FGF system has multiple functions in early development, including mesoderm formation, gastrulation movements and anteroposterior patterning.

eFGF is expressed in the dorsal midline of Xenopus laevis

A detailed study of the expression pattern of embryonic fibroblast growth factor (eFGF) during early Xenopus development has been undertaken using whole-mount DIG in situ hybridization. We show that the zygotic expression of eFGF is activated in the mesoderm of the early gastrula and is first visualized as a ring around the blastopore, with significantly higher levels of expression on the dorsal side of the embryo. As gastrulation proceeds, eFGF transcripts become increasingly abundant in the dorsal blastopore lip. In the early neurula eFGF expression can be detected in the extreme posterior of the embryo around the closed blastopore and in the cells of the notochord. This latter result is significant and represents the first report of a Xenopus FGF that is expressed in the notochord. In addition, we show that during gastrula and neurula stages, expression of eFGF closely follows the expression of the Xenopus brachyury (Xbra) gene. During later development eFGF expression is localized to the tail-bud region and a stripe at the mid-brain/hind-brain junction. These data provide further evidence that FGFs play an important role in regulating the expression of brachyury in the developing mesoderm.

Tail bud determination in the vertebrate embryo

BACKGROUND

Although as humans we lose our tails in the second month of embryonic development, a persistent tail is a prominent structural feature of most adult vertebrates. Indeed, the post-anal tail is part of the definition of a chordate. The internal organization of the developing tail--with neural tube, notochord and paired somites--is the same as that of the main body axis, so it can be expected that the mechanism of tail formation has a close relationship to that of the vertebrate body plan as a whole. Despite this, almost nothing is known about how tails arise.

RESULTS

We present evidence to show that the tail bud of Xenopus laevis arises as the result of interactions between distinct zones of tissue at the posterior of the embryo at the neurula stage. These tissue interactions were demonstrated by manipulations of exogastrulae, which normally form no tail, and by transplantation experiments performed on the neural plate of stage 13 neurulae, whereby embryos with supernumary tails were produced.

CONCLUSIONS

We propose a new model of tail bud determination, termed the NMC model, to explain the results we have obtained. In this model, the tail bud is initiated by an interaction between two territories in the neural plate and a posterior mesodermal territory.

Developmental biology of the pancreas

The pancreas is an organ containing two distinct populations of cells, the exocrine cells that secrete enzymes into the digestive tract, and the endocrine cells that secrete hormones into the bloodstream. It arises from the endoderm as a dorsal and a ventral bud which fuse together to form the single organ. Mammals, birds, reptiles and amphibians have a pancreas with similar histology and mode of development, while in some fish, the islet cells are segregated as Brockmann bodies. Invertebrates do not have a pancreas, but comparable endocrine cells may be found in the gut or the brain. The early pancreatic bud shows uniform expression of the homeobox gene IPF-1 (also known as IDX-1, STF-1 or PDX), which when mutated to inactivity leads to total absence of the organ. The occurrence of heterotopic pancreas in the embryo, and also the metaplasias that can be displayed by a regenerating pancreas in the adult, both suggest that only a few gene products distinguish the pancreatic cell state from that of the surrounding tissues of duodenum, gall bladder and liver. In the developing pancreatic buds, the endocrine cells start to differentiate before the exocrine cells, and co-expression of different hormones by the same cell is often observed at early stages. Although pancreatic endocrine cells produce many gene products also characteristic of neurons, evidence from in vitro cultures and from quailchick grafts shows that they are of endogenous and not of neural crest origin. Observational studies suggest strongly that both endocrine and exocrine cells arise from the same endodermal rudiment. Development of the pancreas in embryonic life requires a trophic stimulus from the associated mesenchyme. In postnatal life, all cell types in the pancreas continue to grow. Destruction of acinar tissue by duct ligation or ethionine treatment is followed by rapid regeneration. Surgical removal of parts of the pancreas is followed by moderate but incomplete regeneration of both acini and islets. Poisoning with alloxan or streptozotocin can lead to permanent depletion of beta cells. Although the cell kinetics of the pancreas are not understood, it seems likely that there is a continuous slow turnover of cells, fed from a stem cells population in the ducts, and that the controls on the production rate of each cell type are local rather than systemic.

Characterization of v-cath, a cathepsin L-like proteinase expressed by the baculovirus Autographa californica multiple nuclear polyhedrosis virus

Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) contains a 966 bp ORF that encodes a papain type cysteine proteinase with cathepsin L-like characteristics. Using Western blot analysis of infected cell extracts we showed that v-cath proteinase has 35.5 kDa and 32 kDa precursor forms which are processed to a 27.5 kDa mature form in a manner characteristic of papain and cathepsin L. V-cath proteinase activity was greatest under acidic conditions (pH 5.0) and was reduced in the presence of the cysteine proteinase inhibitors, leupeptin and E64. Urea, a known enhancer of cathepsin L activity, also enhanced v-cath proteinase activity. AcMNPV v-cath proteinase was detected post-mortem in tissues of insects infected with wild-type (wt) virus. Insects infected with a v-cath deletion mutant did not become flaccid after death as is normally observed with wt AcMNPV infections. These findings indicate a link between v-cath activity and degradation of host tissues during virus pathogenesis.

Spatial and temporal expression of basic fibroblast growth factor (FGF-2) mRNA and protein in early Xenopus development

We have analyzed the expression pattern of bFGF (FGF-2) mRNA and protein in early Xenopus development using RNAse protections, in situ hybridization and immunocytochemical methods. We find that the maternal bFGF mRNA content is at its highest in the previtellogenic oocytes (stage I-II) but decreases during further oogenesis. This low maternal level persists into the early embryonic stages and is uniformly distributed on an RNA basis. Zygotic transcription is turned on both from anterior and posterior regions but not from the middle region in the mid-neurula stage, and the expression greatly increases during the late neurula and tailbud stages. In the tadpole stage, the expression is detected in the brain, eyes, ears and neural crest-derived mesenchyme of the head. Also, it is expressed along the mesodermal axis, the level falling as the myotomes differentiate. Immunocytochemical study shows abundant bFGF protein in early oocytes and much less in later oocytes. The localization is mainly to the nuclei of the early oocytes and to the cytoplasm of the later oocytes. Localization of maternal bFGF protein in the animal hemisphere is observed in the early embryonic stages and some reuptake into nuclei occurs by the early blastula stage. The zygotically synthesized protein starts to be expressed in the anterior region of the mid-neurula stage and soon also becomes detectable in the posterior region. By tailbud and pre-larval stages, this zygotic protein appears to be present along the entire neural and mesodermal axis. When these cases are sectioned it can be seen that bFGF protein is detected in most parts of the head. In the posterior domain, it is present in the mesoderm and clearly becomes localized in the muscle cells to the nuclei and to the cell termini adjoining the intermyotomal septa.