The formation of the mesoderm in urodelean amphibians

Summary Dorsal (D), lateral (L and R), and ventral (V) portions of the endoderm of blastulae ofAmbystoma mexicanum of different age (stages 8⁺ to 10^–) were combined with ectodermal caps of stage 8⁺ blastulae. All V and most L and R portions induced only ventrocaudal mesodermal structures — ventral type of mesoderm induction. Almost all D portions induced much more voluminous structures of predominantly axial character — dorsal type of mesoderm induction. The difference in mesoderm-inducing capacity of the dorsal as against the lateral and ventral endoderm is probably purely quantitative in character. The dorsal endoderm exhibits a pronounced dominance in mesoderm-inducing capacity. During the early symmetrization of the amphibian egg it is apparently especially the presumptive dorsal endoderm that becomes endowed with strong mesoderm-inducing properties.A comparison of the results obtained with endodermal portions of blastulae of different age showed that the mesoderm-inducing capacity first begins to decline in the dorsal endoderm (around stage 9), subsequently in the lateral, and finally in the ventral endoderm (at stage 10^–). At stage 10^– the dorsal endoderm no longer has mesoderm-inducing capacities.In the recombinates there is a striking correspondence between the regional differentiation of the mesoderm and that of the endoderm. The latter differs markedly from the presumptive significance of the various endodermal regions in the normal embryo.Primordial germ cells, which constitute a characteristic component of the ventral type of mesoderm induction, can be induced not only by ventral, but also by lateral and to some extent even by dorsal endoderm. The development of primordial germ cells from the ectodermal component of the various recombinates indicates that in the urodeles the origin of the primordial germ cells differs markedly from that in the anurans.The authors want to thank Miss A. de wit for expert technical assistance, Miss E. Bartová for making the drawings, and Dr. J. Faber for editorial help.     

The formation of the mesoderm in urodelean amphibians

Summary Strong Li treatment leads to auto-activation of blastula and gastrula ectoderm due to lethal and sublethal cytolysis, resulting in adirect meso- and endodermization of the ectoderm. In contrast, a mild Li treatment-0.04 M LiCl in Holtfreter solution for only two hours—which does not show cytolizing effects, has no or only a weak mesodermizing effect upon blastula ectoderm. However, when the explant contains endoderm as well the same treatment leads to a marked transformation of ectoderm into mesoderm and subsequently into endoderm (vegetalization). Thisindirect action of the Li ion under physiological conditions apparently represents an enhancement of the normally occurring mesoderm formation from the ectodermal component of the blastula under the inducing action of the endodermal component and its subsequent transformation into endodermal structures, probably due to a rise in competence. The question is raised in how far the direct meso-and endodermization of gastrula ectoderm by heterogeneous inductors is open to the same criticsm as in the case of auto-activation of the ectoderm by a strong Li treatment. Finally, the experiments of Ave, Kawakami and Sameshima (1968) are briefly discussed.

---

Zusammenfassung Eine starke Li-Behandlung führt im Blastula- und Gastrula-Ektoderm durch letale und subletale Zytolyse zu einer Selbstaktivierung, die sich auswirkt in einerdirekten Meso-und Entodermisierung des Ektoderms. Im Gegensatz dazu zeigt eine gelinde Li-Behandlung — 0,04 M LiCl in Holtfreter-Lösung während nur 2 Std —, die keine zytolyzierende Wirkung hat, keinen oder nur einen schwachen Einflu\ auf isoliertes Blastula-Ektoderm. Wenn dagegen das Explantat auch Entoderm enthlt, führt die gleiche Behandlung zu einer betrchlichen Umbildung von Ektoderm in Mesoderm und weiter in Entoderm (Vegetalisierung). Dieseindirekte Wirkung des Li-Ions unter physiologischen Verhltnissen stellt also offenbar eine Verstrkung dar der im normalen Keim vor sich gehenden Mesodermbildung aus der ektodermalen Komponente unter dem Einflu\ der induzierenden Wirkung der entodermalen Komponente und der darauffolgenden Transformierung in entodermale Strukturen, wsch. durch Kompetenzsteigerung. Es wird die Frage aufgeworfen, inwiefern die unmittelbare Meso- und Entodermisierung von Gastrula-Ektoderm durch heterogene Induktoren der gleichen Beurteilung zugnglich ist wie der Fall der Selbstaktivierung des Ektoderms durch starke Li-Behandlung. Schlie\lich werden die Experimente von Ave, Kawakami u. Sameshima (1968) kurz diskutiert.

     

The formation of the mesoderm in urodelean amphibians

Summary Experiments are described in which in early to late blastulae ofAmbystoma mexicanum (stages 7–8/9 Harrison) the animal, ectodermal half (zones I.II) was combined with the vegetative, endodermal yolk mass (zone IV) in various orientations, viz. in random orientation or with the dorso-ventral axes of the two components in identical, opposite or perpendicular orientation (0°, 180°, or 90° translocation respectively). The results demonstrate unequivocally that the dorso-ventral polarity of the induced mesoderm, and thus of the embryo, depends exclusively upon the inherent dorso-ventral polarity of the endoderm, whereas the grey crescent, a considerable part of which is located in the animal, ectodermal half, plays no causal role whatsoever.The results also show that the dorso-ventral polarity is inherent in the entire endodermal mass, but that the subsequent regional differentiation of the endoderm depends upon stimulating influences emanating from the surrounding mesoderm, the later nutritive yolk representing that part of the endoderm which normally does not come under the influence of the mesoderm, and therefore fails to receive the necessary stimulus for further differentiation.On the basis of these findings Schultze's Umkehrexperiment as studied byPenners andSchleip, Penners, andPasteels are reinterpreted, whileDalcq andPasteels' general developmental theory as well asCurtis' cortical grafting experiments are critically discussed.

---

Zusammenfassung Es werden Experimente beschrieben, in denen in frühen bis späten Blastulae vonAmbystoma mexicanum (Stadien 7–8/9 Harrison) die animale, ektodermale Hälfte (Zonen I.II) mit der vegetativen, entodermalen Dottermasse (Zone IV) kombiniert wurde, und zwar in verschiedener Orientierung, d. h. in willkürlicher Orientierung oder mit den Dorsoventralachsen der beiden Komponenten identisch, entgegengesetzt oder senkrecht zueinander orientiert (0°, bzw. 180° oder 90° transloziert). Die Ergebnisse zeigen eindeutig, daß die Dorsoventralpolarität des induzierten Mesoderms, und damit die des Embryos, ausschließlich von der inhärenten Dorsoventralpolarität des Entoderms bestimmt wird, während der graue Halbmond, der zu einem beträchtlichen Teil in der animalen, ektodermalen Hälfte liegt, überhaupt keine kausale Rolle spielt.Außerdem zeigen die Ergebnisse, daß die Dorsoventralpolarität der ganzen Entodermmasse inhärent ist, daß aber die spätere regionale Differenzierung des Entoderms von stimulierenden Einflüssen seitens des umgebenden Mesoderms abhängig ist; der spätere Nährdotter ist derjenige Teil des Entoderms der normalerweise außerhalb des Wirkungsbereiches des Mesoderms liegt, und infolgedessen den für seine weitere Differenzierung benötigten Reiz nicht erhält.Angesichts dieser Befunde wird das Schultzesche Umkehrexperiment, welches vonPenners undSchleip, Penners, undPasteels näher untersucht worden ist, neu interpretiert, während die allgemeine Entwicklungstheorie vonDalcq u.Pasteels sowie die Cortextransplantationen vonCurtis kritisch diskutiert werden.

     

The formation of the mesoderm in urodelean amphibians

Summary Xenoplastic recombinations of animal and vegetative parts ofAmbystoma mexicanum and Triturus alpestris blastulae, and similar recombinations of parts of³H-thymidinelabelled and unlabelledAmbystoma mexicanum blastulae demonstrate convincingly that the vegetative part (zone IV, see Nieuwkoop, 1969a) of such a recombinate does not contribute to mesoderm formation, but exclusively forms endodermal derivatives. In contrast, the animal cap of the blastula (zones I.II)—which only gives rise to atypical ectoderm if isolated—not only furnishesall the ecto-neurodermal derivatives, butall the mesodermal structures of the developing recombinate as well, and finally to a varying extent forms additional endodermal structures in the recombinate.In the recombinates endodermization of the ectodermal cap occurred at the anterior end of the invaginated archenteron—corresponding to the presumptive pharyngeal endoderm —, and along the dorsal side of the endodermal tube, while an endoderm-like epithelium is formed at the boundary between the caudal endoderm and the ectoderm (proctodaeum formation). These results suggest that in normal development also endodermization occurs in the ectodermal half of the egg. This occurs particularly on the dorsal side, leading to the formation of the presumptive pharyngeal endoderm situated above the dorsal blastoporal groove.These experiments show that the vegetative half of the amphibian blastula is firmly determined as the future endoderm, whereas the animal half is still virtually undetermined and pluripotent.     

Wnt 6 regulates the epithelialisation process of the segmental plate mesoderm leading to somite formation

In higher vertebrates, the paraxial mesoderm undergoes a mesenchymal to epithelial transformation to form segmentally organised structures called somites. Experiments have shown that signals originating from the ectoderm overlying the somites or from midline structures are required for the formation of the somites, but their identity has yet to be determined. Wnt6 is a good candidate as a somite epithelialisation factor from the ectoderm since it is expressed in this tissue. In this study, we show that injection of Wnt6-producing cells beneath the ectoderm at the level of the segmental plate or lateral to the segmental plate leads to the formation of numerous small epithelial somites. Ectopic expression of Wnt6 leads to sustained expression of markers associated with the epithelial somites and reduced or delayed expression of markers associated with mesenchymally organised somitic tissue. More importantly, we show that Wnt6-producing cells are able to rescue somite formation after ectoderm ablation. Furthermore, injection of Wnt6-producing cells following the isolation of the neural tube/notochord from the segmental plate was able to rescue somite formation at both the structural (epithelialisation) and molecular level, as determined by the expression of marker genes like Paraxis or Pax-3. We show that Wnts are indeed responsible for the epithelialisation of somites by applying Wnt antagonists, which result in the segmental plate being unable to form somites. These results show that Wnt6, the only known member of this family to be localised to the chick paraxial ectoderm, is able to regulate the development of epithelial somites and that cellular organisation is pivotal in the execution of the differentiation programmes. We propose a model in which the localisation of Wnt6 and its antagonists regulates the process of epithelialisation in the paraxial mesoderm.     

Inhibition of mesoderm formation by follistatin

 Mesoderm induction requires interaction between cells of the animal and vegetal hemispheres of the embryo. Several molecules have been proposed as candidates for mesoderm-inducing signals, with activin a particularly strong candidate. However, it has not been possible to inhibit mesoderm formation in vivo by specifically blocking activin action. Follistatin is able to inhibit the action of activin but not that of the mature region of Vg1, a member of the transforming growth factor β family. Follistatin therefore provides a useful tool for distinguishing between signalling by these two factors. We have overexpressed Xenopus follistatin mRNA and analysed the expression of several mesodermal markers. Our results show an inhibition of mesodermal formation by follistatin in a concentration-dependent manner, showing the requirement of activin for mesodermal induction. Received: 22 August 1997 / Accepted: 16 January 1998     

Molecular approaches to the study of mesoderm formation in amphibians

The mesoderm is the region of the embryo that gives rise to muscle, blood and connective tissues; it becomes segregated from the ectoderm and endoderm at gastrulation. Embryological studies have revealed, however, that the potential for certain embryonic cells to become part of the mesoderm is established well before gastrulation, most likely through an extracellular signalling process termed ‘induction’. The recent characterization of mesoderm-specific mRNAs and proteins now permits an analysis of the very earliest events involved in the specification of the mesoderm at the molecular level. Such experiments should contribute to our understanding of the mechanisms involved in controlling cell differentiation.     

Neurotrophin receptor homolog (NRH1) proteins regulate mesoderm formation and apoptosis during early Xenopus development

Recent experiments suggest that Xenopus Neurotrophin Receptor Homolog 1 (NRH1) proteins act through the planar cell polarity pathway to regulate convergent extension movements during gastrulation and neurulation. We show in this paper that NRH1 proteins are also required for the proper expression of mesodermally expressed genes such as Xbra and Chordin, and to a lesser extent, of Xwnt11. Loss of NRH1 function is followed, during gastrula and neurula stages, by a dramatic increase in apoptosis. Apoptosis is delayed by injection of Xbra RNA, suggesting that cell death is a consequence, at least in part, of the down-regulation of this gene, and it is also delayed by expression of activated forms of Rho, Rac and Cdc42. These small GTPases have previously been implicated in the planar cell polarity pathway in Xenopus and, in other systems, in the regulation of apoptosis. We conclude that the effects of NRH1 proteins include the regulation of mesodermal gene expression and that the disruption of gastrulation that is caused by their loss of function is a consequence of the down-regulation of Xbra and other genes, in addition to direct interference with the planar cell polarity pathway. The apoptosis observed in embryos lacking NRH1 function is not an indirect consequence of the disruption of gastrulation, and indeed it may contribute to the observed morphological defects.     

Regulated expression of the retinoblastoma gene product by fibroblast growth factor but not by activin during mesoderm induction in Xenopus

 The retinoblastoma (RB) gene is a tumor suppressor gene that plays an important role in cell cycle arrest and in the terminal differentiation of skeletal myoblasts. Differentiation into muscle occurs in Xenopus embryo explants during mesoderm induction by fibroblast growth factor (FGF) or activin A. We examined expression of the RB gene product (pRB) during mesoderm induction in vivo and in vitro. We show that hypo- and hyper-phosphorylated forms of pRB are present during early development and that expression of both forms increases significantly during the blastula stage, concomitant with mesoderm induction. Further investigation revealed that pRB is enriched in the presumptive mesoderm of the blastula stage embryo. In animal cap explants induced by Xenopus bFGF (XbFGF), pRB expression levels increased approximately tenfold while no increase was observed in explants induced by activin. However, when explants were induced by XbFGF in the presence of sodium orthovanadate, a compound previously shown to synergize with FGF to produce more dorsal ”activin-like” inductions than FGF alone, only a slight increase in pRB expression was observed. Furthermore, upregulation of pRB during mesoderm induction in vitro displayed an inverse correlation with expression of XFKH1, a marker for notochord. These results suggest that pRB may be important for patterning along the dorsoventral axis. Received: 22 February 1996 / Accepted: 20 September 1996     

Regionalisation of cell fate and morphogenetic movement of the mesoderm during mouse gastrulation

The developmental fate of cells in the epiblast of early-primitive-streak-stage mouse embryos was assessed by studying the pattern of tissue colonisation displayed by lac Z-expressing cells grafted orthotopically to nontransgenic embryos. Results of these fate-mapping experiments revealed that the lateral and posterior epiblast contain cells that will give rise predominantly to mesodermal derivatives. The various mesodermal populations are distributed in overlapping domains in the lateral and posterior epiblast, with the embryonic mesoderm such as heart, lateral, and paraxial mesoderm occupying a more distal position than the extraembryonic mesoderm. Heterotopic grafting of presumptive mesodermal cells results in the grafted cells adopting the fate appropriate to the new site, reflecting a plasticity of cell fate determination before ingression. The first wave of epiblast cells that ingress through the primitive streak are those giving rise to extraembryonic mesoderm. Cells that will form the mesoderm of the yolk sac and the amnion make up a major part of the mesodermal layer of the midprimitive-streak-stage embryo. Cells that are destined for embryonic mesoderm are still found within the epiblast, but some have been recruited to the distal portion of the mesoderm. By the late-primitive-streak-stage, the mesodermal layer contains only the precursors of embryonic mesoderm. This suggests that there has been a progressive displacement of the midstreak mesoderm to extraembryonic sites, which is reminiscent of that occurring in the overlying endodermal tissue. The regionalisation of cell fate in the late-primitive-streak mesoderm bears the same spatial relationship as their ancestors in the epiblast prior to cell ingression. This implies that both the position of the cells in the proximal-distal axis and their proximity to the primitive streak are major determinants for the patterning of the embryonic mesoderm. © 1995 Wiley-Liss, Inc.     

Inhibition of FGF signaling converts dorsal mesoderm to ventral mesoderm in early Xenopus embryos

In early vertebrate development, mesoderm induction is a crucial event regulated by several factors including the activin, BMP and FGF signaling pathways. While the requirement of FGF in Nodal/activin-induced mesoderm formation has been reported, the fate of the tissue modulated by these signals is not fully understood. Here, we examined the fate of tissues when exogenous activin was added and FGF signaling was inhibited in animal cap explants of Xenopus embryos. Activin-induced dorsal mesoderm was converted to ventral mesoderm by inhibition of FGF signaling. We also found that inhibiting FGF signaling in the dorsal marginal zone, in vegetal-animal cap conjugates or in the presence of the activin signaling component Smad2, converted dorsal mesoderm to ventral mesoderm. The expression and promoter activities of a BMP responsive molecule, PV.1 and a Spemann organizer, noggin, were investigated while FGF signaling was inhibited. PV.1 expression increased, while noggin decreased. In addition, inhibiting BMP-4 signaling abolished ventral mesoderm formation induced by exogenous activin and FGF inhibition. Taken together, these results suggest that the formation of dorso-ventral mesoderm in early Xenopus embryos is regulated by a combination of FGF, activin and BMP signaling.     

Role of fibroblast growth factors as inducing agents in early embryonic development

To assess the potential role of a molecule in development we need to know three things: 1) what are the biological activities of the molecule, 2) what is its expression pattern, and 3) what are the consequences of removing it from the embryo? In the case of the FGF family in Xenopus embryos we have quite a lot of information about all three questions. Most members of the family can induce mesoderm from isolated animal caps, thus mimicking the natural “ventral vegetal” inducing signal operative in the blastula. This activity can be exerted on isolated, disaggregated cells and does not involve a change in division rate. When overexpressed from injected mRNA, the activity of FGFs depends largely on whether or not they possess a signal sequence, showing the importance of secretion in the inductive process. In addition to the mesoderm-inducing activity, there are effects of overexpression on whole embryos which lead to a suppression of anterior structures. Three types of FGF have so far been cloned from Xenopus: direct homologs of each of the mammalian types FGF-2 and FGF-3, and eFGF (“embryonic FGF”), which is equidistant in sequence from mammalian FGF-4 and FGF-6. Attempts to find homologs of mammalian FGF-5 and FGF-7 in Xenopus have proved unsuccessful. All three types of Xenopus FGF are expressed in early development. FGF-2 and eFGF are present in the oocyte and fertilized egg, and are thus both available at the time of mesoderm induction. FGF-3 and eFGF are both expressed from the embryonic genome during gastrulation and concentrated in the forming mesoderm. FGF-2 is expressed from the embryonic genome during neurulation in the brain, and a little later in the branchial arch mesenchyme and in the forming myotomes. These expression patterns suggest that there are several functions for the FGFs. The most successful strategy for inhibition of the FGF system has been the use of a dominant negative receptor construct introduced by Kirschner and colleagues. Overexpression of this construct can abolish the FGF responsiveness of animal caps. In whole embryos, the absence of FGF signaling causes a reduction, although not a total ablation, of mesoderm formation. There is also a severe effect on axis formation in which formation of the posterior parts is reduced consequent on an inhibition of invagination and elongation of the dorsal mesoderm. Thus, the present evidence suggests that the FGF system contributes to, although is not solely responsible for, mesoderm induction in vivo. It is also necessary for normal gastrulation movements, particularly in the dorsal mesoderm, and is likely to have several later functions, particularly in development of the central nervous system and the myotomes. © 1994 Wiley-Liss, Inc.     

The presence of an extracellular matrix between cells involved in the determination of the mesoderm bands in embryos ofPatella vulgata (Mollusca,gastropoda)

Summary In 32-cell stage embryos ofPatella vulgata one of the macromeres contacts the animal micromeres, and as a result is induced to differentiate into the stem cell of the mesodermal cell line. In this study we show the presence of an extracellular matrix (ECM) between these two interacting cell types. The ECM appears to be formed by the micromeres during the 32-cell stage. Staining experiments with alcian blue and tannic acid indicate that in contains glycoconjugates, possibly in the form of proteoglycans. The characteristics of the ECM were examined further by fluorescein isothiocyanate (FITC)-lectin labelling. Of 17 lectins tested, concanavalin A (ConA), succinyl-ConA, LCH-B (Lens culinaris) and PEA (Pisum sativum) showed a positive labelling of the ECM. These results are in accordance with the electron microscopic data. The appearance of the ECM at this specific stage and place suggests that it might play an important role in the induction of the mesodermal cell line.     

Identification of novel genes affecting mesoderm formation and morphogenesis through an enhanced large scale functional screen in Xenopus

The formation of mesoderm is an important developmental process of vertebrate embryos, which can be broken down into several steps; mesoderm induction, patterning, morphogenesis and differentiation. Although mesoderm formation in Xenopus has been intensively studied, much remains to be learned about the molecular events responsible for each of these steps. Furthermore, the interplay between mesoderm induction, patterning and morphogenesis remains obscure. Here, we describe an enhanced functional screen in Xenopus designed for large-scale identification of genes controlling mesoderm formation. In order to improve the efficiency of the screen, we used a Xenopus tropicalis unique set of cDNAs, highly enriched in full-length clones. The screening strategy incorporates two mesodermal markers, Xbra and Xmyf-5, to assay for cell fate specification and patterning, respectively. In addition we looked for phenotypes that would suggest effects in morphogenesis, such as gastrulation defects and shortened anterior-posterior axis. Out of 1728 full-length clones we isolated 82 for their ability to alter the phenotype of tadpoles and/or the expression of Xbra and Xmyf-5. Many of the clones gave rise to similar misexpression phenotypes (synphenotypes) and many of the genes within each synphenotype group appeared to be involved in similar pathways. We determined the expression pattern of the 82 genes and found that most of the genes were regionalized and expressed in mesoderm. We expect that many of the genes identified in this screen will be important in mesoderm formation.     

Blastopore formation and dorsal mesoderm induction are independent events in early Cynops embryogenesis

The independent roles of blastopore formation and dorsal mesoderm induction in dorsal axis formation of the Cynops pyrrhogaster embryo were attempted to be clarified. The blastopore-forming (bottle) cells originated mainly from the progeny of the mid-dorsal C and/or D blastomeres of the 32-cell embryo, but were not defined to a fixed blastomere. It was confirmed that the isolated dorsal C and D blastomeres autonomously formed a blastopore. Ultraviolet-irradiated eggs formed an abnormal blastopore and then did not form a dorsal axis, although the lower dorsal marginal zone (LDMZ) still had dorsal mesoderm-inducing activity. Involution of the dorsal marginal zone was disturbed by the abnormal blastopore. These embryos were rescued by artificially facilitating involution of the dorsal marginal zone. Suramin-injected and nocodazole-treated blastulae did not have involution of the dorsal marginal zone, although the blastopore was formed. Neither embryos formed the dorsal axis. The dorsal mesoderm-inducing activity of the LDMZ in the nocodazole-treated gastrulae was still active. In contrast, the LDMZ of the suramin-injected embryos lost its dorsal mesoderm-inducing activity. bra expression was activated in the nocodazole-treated embryos but not in the suramin-injected embryos. The present study suggested that (i) the dorsal determinants consist of blastopore-forming and dorsal mesoderm-inducing factors, which are not always mutually dependent; (ii) both factors are activated during the late blastula stage; (iii) the dorsal marginal zone cannot specify to an organized notochord and muscle without the involution that blastopore formation leads to; and (iv) the localization of both factors in the same place is prerequisite for dorsal axis formation.     

Subdivision and developmental fate of the head mesoderm in Drosophila melanogaster

In this paper, we define temporal and spatial subdivisions of the embryonic head mesoderm and describe the fate of the main lineages derived from this tissue. During gastrulation, only a fraction of the head mesoderm (primary head mesoderm; PHM) invaginates as the anterior part of the ventral furrow. The PHM can be subdivided into four linearly arranged domains, based on the expression of different combinations of genetic markers (tinman, heartless, snail, serpent, mef-2, zfh-1). The anterior domain (PHMA) produces a variety of cell types, among them the neuroendocrine gland (corpus cardiacum). PHMB, forming much of the “T-bar” of the ventral furrow, migrates anteriorly and dorsally and gives rise to the dorsal pharyngeal musculature. PHMC is located behind the T-bar and forms part of the anterior endoderm, besides contributing to hemocytes. The most posterior domain, PHMD, belongs to the anterior gnathal segments and gives rise to a few somatic muscles, but also to hemocytes. The procephalic region flanking the ventral furrow also contributes to head mesoderm (secondary head mesoderm, SHM) that segregates from the surface after the ventral furrow has invaginated, indicating that gastrulation in the procephalon is much more protracted than in the trunk. We distinguish between an early SHM (eSHM) that is located on either side of the anterior endoderm and is the major source of hemocytes, including crystal cells. The eSHM is followed by the late SHM (lSHM), which consists of an anterior and posterior component (lSHMa, lSHMp). The lSHMa, flanking the stomodeum anteriorly and laterally, produces the visceral musculature of the esophagus, as well as a population of tinman-positive cells that we interpret as a rudimentary cephalic aorta (“cephalic vascular rudiment”). The lSHM contributes hemocytes, as well as the nephrocytes forming the subesophageal body, also called garland cells.     

The possible role of mesodermal growth factors in the formation of endoderm inXenopus laevis

We have raised a monoclonal antibody, 4G6, against gut manually isolated from stage 42Xenopus laevis embryos. It is specific for endoderm and recognises an epitope that is first expressed at stage 19 and which persists throughout subsequent development. The antibody maintains gut specificity through metamorphosis and into adulthood. The epitope is conserved in the mouse, where it is also found in the gut. Isolated vegetal poles fromXenopus blastula stage embryos express the epitope autonomously after culturing to the appropriate stage. This shows that certain aspects of endoderm differentiation do not require germ layer interactions. Animal cap cells from stage 9 blastulae cultured in the presence of the mesodermal growth factors FGF, XTC-MIF and PIF form both endodermal and mesodermal tissues, assessed by the binding of tissue-specific monoclonal antibodies. Endoderm is typically found in those caps which form intermediate and ventral forms of mesoderm, that is muscle and lateral plate. Correspondence to: E.A. Jones     

Heat induced developmental uncoupling of mesoderm from ectoderm and endoderm germ layer derivatives during Artemia postembryonic segmentation

Taking advantage of the fact than segmentation inArtemia is largely a postembryonic process making it more susceptible to environmental influences, heat treatments ofArtemia newly-hatched nauplii were shown to induce a severe inhibition of mesodermal structures, without apparently affecting the corresponding ectodermal and endodermal derivaives. This inhibition was reversible and with enough time the missing mesodermal structures developed. These results indicate that ectoderm and endoderm development can proceed without neccessarily a concomitant mesodermal differentiation, which in turn can be largely uncoupled from that of the rest of the germ layer derivatives.