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J M Slack

Publications and source records attributed to J M Slack.

At least 91 records · Page 5Linked to original sources
PubMed1988

Inductive effects of fibroblast growth factor and lithium ion on Xenopus blastula ectoderm.

We have studied the response of Xenopus blastula ectoderm to fibroblast growth factor and to lithium ion. The properties of acidic and basic FGF are very similar showing a 50% induction level at 1-2 ng ml-1 and a progressive increase of muscle formation up to concentrations of 100-200 ng ml-1. The elongation of explants also shows a dose-response relationship. The minimum contact requirement for induction of ectoderm explants is about 90 min and the stage range of ectodermal competence extends from midblastula to early gastrula, both these figures resembling those obtained in embryological experiments with vegetal tissue as the inducer. Lithium chloride concentrations which produce anteriorization of whole embryos have no effect on isolated ectoderms unless accompanied by FGF. Simultaneous treatment with FGF and Li lead to a marked enhancement of both elongation and muscle formation over that produced by FGF alone. By contrast, ventral marginal explants show increased elongation and muscle formation if treated with lithium alone suggesting that they have already received a low-dose FGF treatment within the embryo. It is concluded that endogenous FGF may be solely responsible for inducing the ventral mesoderm and that dorsalization of ventral mesoderm to the level of somitic muscle might be achieved either by a very high local concentration of FGF in the dorsal region, or by the action of a second, synergistic, agent in the dorsal region.

Animals↗
PubMed1987

Regional specification within the mesoderm of early embryos of Xenopus laevis.

We have further analysed the roles of mesoderm induction and dorsalization in the formation of a regionally specified mesoderm in early embryos of Xenopus laevis. First, we have examined the regional specificity of mesoderm induction by isolating single blastomeres from the vegetalmost tier of the 32-cell embryo and combining each with a lineage-labelled (FDA) animal blastomere tier. Whereas dorsovegetal (D1) blastomeres induce 'dorsal-type' mesoderm (notochord and muscle), laterovegetal and ventrovegetal blastomeres (D2-4) induce either 'intermediate-type' (muscle, mesothelium, mesenchyme and blood) or 'ventral-type' (mesothelium, mesenchyme and blood) mesoderm. No significant difference in inductive specificity between blastomeres D2, 3 and 4 could be detected. We also show that laterovegetal and ventrovegetal blastomeres from early cleavage stages can have a dorsal inductive potency partially activated by operative procedures, resulting in the induction of intermediate-type mesoderm. Second, we have determined the state of specification of ventral blastomeres by isolating and culturing them in vitro between the 4-cell stage and the early gastrula stage. The majority of isolates from the ventral half of the embryo gave extreme ventral types of differentiation at all stages tested. Although a minority of cases formed intermediate-type and dorsal-type mesoderms we believe these to result from either errors in our assessment of the prospective DV axis or from an enhancement, provoked by microsurgery, of some dorsal inductive specificity. The results of induction and isolation experiments suggest that only two states of specification exist in the mesoderm of the pregastrula embryo, a dorsal type and a ventral type. Finally we have made a comprehensive series of combinations between different regions of the marginal zone using FDA to distinguish the components. We show that, in combination with dorsal-type mesoderm, ventral-type mesoderm becomes dorsalized to the level of intermediate-type mesoderm. Dorsal-type mesoderm is not ventralized in these combinations. Dorsalizing activity is confined to a restricted sector of the dorsal marginal zone, it is wider than the prospective notochord and seems to be graded from a high point at the dorsal midline. The results of these experiments strengthen the case for the three-signal model proposed previously, i.e. dorsal and ventral mesoderm inductions followed by dorsalization, as the simplest explanation capable of accounting for regional specification within the mesoderm of early Xenopus embryos.

Animals↗
PubMed1987

Fate map for the 32-cell stage of Xenopus laevis.

A complete fate map has been produced for the 32-cell stage of Xenopus laevis. Embryos with a regular cleavage pattern were selected and individual blastomeres were injected with the lineage label fluorescein-dextran-amine (FDA). The spatial location of the clones was deduced from three-dimensional (3D) reconstructions of later stages and the volume of each tissue colonized by labelled cells in each tissue was measured. The results from 107 cases were pooled to give a fate map which shows the fate of each blastomere in terms of tissue types, the composition of each tissue by blastomere, the location of each prospective region on the embryo and the fate of each blastomere in terms of spatial localization. Morphogenetic movements up to stage 10 (early gastrula) were assessed by carrying out a number of orthotopic grafts at blastula and gastrula stages using donor embryos uniformly labelled with FDA. Although there is a regular topographic projection from the 32-cell stage this varies a little between individuals because of variability of positions of cleavage planes and because of short-range cell mixing during gastrulation. The cell mixing means that the topographic projection fails for anteroposterior segments of the dorsal axial structures and it is not possible to include short segments of notochord or neural tube or individual somites on the pregastrulation fate map.

Animals↗
PubMed1985

Homoeotic transformations in man: implications for the mechanism of embryonic development and for the organization of epithelia.

Homoeotic transformations are substitutions of one body part for another which arise during embryogenesis or regeneration. They are well known among the Arthropoda but are not generally thought to occur in Man or other vertebrates. In this paper the occurrence and characteristics of 21 types of epithelial heterotopia and metaplasia are reviewed and it is concluded that they are fully comparable with the homoeotic transformations of the arthropods.. The transformations are concentrated in the gastrointestinal, urinary and female reproductive systems and typically appear as foci of ectopic epithelium with a sharp discontinuity of cell type at the edges of the patches. Most of the transformations occur in renewal tissues and must therefore be interpreted as changes in the states of determination (epigenetic codings) of the stem cells rather than changes between already differentiated cells. Most, but not all, of the transformations are between tissues whose precursors are neighbouring regions of a common cell sheet during early embryogenesis and which are therefore likely to have neighbouring epigenetic codings. Following the Cairns hypothesis for epithelial organization it is proposed that stem cells themselves are protected against changes in epigenetic coding but their daughter cells, normally destined to differentiate and die, are not. Homoeotic transformations may thus occur in situations in which daughter cells become promoted to stem cells which happens either during the growth phase of the organism or during tissue regeneration in the adult.

Animals↗
PubMed1985

Peanut lectin receptors in the early amphibian embryo: regional markers for the study of embryonic induction.

The regional and temporal specificity of peanut agglutinin binding was determined for early amphibian embryos. With the onset of neurulation, a receptor appears on the epidermis, but remains absent from the neural plate. A second type of receptor, largely masked by sialic acid, appears throughout the extracellular matrix. In the axolotl, the epidermal receptor is epimucin and the matrix receptor is fibronectin plus other components. Both receptors are autonomously expressed, on schedule, by appropriate explants of gastrula tissue. Expression of the epidermal receptor is suppressed after exposure to a neural inducing signal. This shows that the epidermal PNA receptor is a reliable marker of epidermal character and that neural induction affects the program of macromolecular synthesis within hours of the graft.

Ambystoma↗
PubMed1985

Normal fates and states of specification of different regions in the axolotl gastrula.

A fate map was constructed for four regions of the early gastrula of Ambystoma mexicanum using orthotopic grafts from donors labelled with FLDx (fluoresceinated-lysinated-dextran). The region around the animal pole gave rise to epidermis only and did not include prospective neural plate. The dorsal marginal zone contributed to cephalic endoderm and to the whole length of the axial mesoderm (notochord and somites), the lateral marginal zone to lateroventral and somitic mesoderm, and the ventral marginal zone to lateroventral mesoderm. It was found that the dorsal marginal zone contributed relatively more to the anterior regions of the mesodermal mantle and the ventral marginal zone more to its posterior parts. The same regions of the gastrula and also vegetal yolky tissue were cultured as explants and labelled with tritiated mannose. Their glycoprotein synthesis pattern was compared to those of the neurula tissues to which they contribute in vivo. Animal pole explants synthesized large amounts of the epidermis-specific marker epimucin. Dorsal marginal zone explants did not synthesize epimucin but did make amounts of S2 and S6 indicative of mesoderm, as well as the notochord-specific markers S2.2 and S3.2. Lateral marginal zone explants showed the same pattern as the dorsal marginal zone including the two notochord-specific markers, although they do not contribute to notochord in vivo. Ventral marginal zone explants were more variable in their behaviour. Yolky tissue from the vegetal hemisphere of the gastrula or the archenteron floor of the neurula synthesized mainly polydisperse material of high molecular weight rather than discrete glycoproteins. The results indicate that at the early gastrula stage states of specification exist which correspond to the three germ layers, ecto-, meso- and endoderm. The ectodermal specification of animal pole explants is quite robust and cannot easily be changed by variation of the culture conditions. However treatment with a concentrated pellet of vegetalizing factor does induce a change to mesodermal specification, which is clearly detectable in the pattern of glycoprotein synthesis. Similar inductive interactions between different regions of the early embryo are thought to occur during normal development.

Ambystoma mexicanum↗
PubMed1985

Cell lineage labels and region-specific markers in the analysis of inductive interactions.

This paper reviews work with cell lineage labels and cell-type specific markers in the analysis of inductive interactions in early amphibian development. Our results provide clear evidence for the existence of three such interactions. Mesodermal induction occurs in the early blastula and results from the action of vegetal pole cells on the animal hemisphere. At least two mesodermal rudiments are formed, one dorsal and one ventral. During the next interaction, which we call dorsalization, the ventral mesodermal rudiment becomes subdivided into several territories under the influence of the dorsal marginal zone, or organizer. Finally, during gastrulation, the involuting organizer induces neural tissue from the overlying ectoderm. This interaction is called neural induction. Although these phenomena can readily be demonstrated under experimental conditions, direct evidence that they occur in normal development awaits an understanding of the molecular basis of induction.

Amphibians↗
PubMed1985

Mesoderm induction in Xenopus laevis: a quantitative study using a cell lineage label and tissue-specific antibodies.

We have compared the development of the animal pole (AP) region of early Xenopus embryos in normal development, in isolation, and in combination with explants of tissue from the vegetal pole (VP) region. For the grafts and the combinations the animal pole tissue was lineage labelled with FLDx in order to ascertain the provenance of the structures formed. The normal fate of the AP region was determined by orthotopic grafts at stages 7 1/2 (early blastula), 8 (mid blastula) and 10 (early gastrula). At later stages most of the labelled cells were found in ectodermal tissues such as epidermis, head mesenchyme and neural tube (the last from stages 7 1/2 and 8 only). However, in stage-7 1/2 and stage-8 grafts some of the labelled cells were also found in the myotomes and lateral mesoderm. In isolated explants the AP region of all three stages differentiated only as epidermis assessed both histologically and by immunofluorescence using an antibody to epidermal keratin. The fate of labelled cells in AP-VP combinations was quite different and confirms the reality of mesoderm induction. In combinations made at stages 7 1/2 and 8 the proportion of AP-derived mesoderm is substantially greater than the proportion of labelled mesoderm in the equivalent fate mapping experiments. This shows that the formation of mesoderm in such combinations is the result of an instructive rather than a permissive interaction. The formation of mesodermal tissues in stage-7 1/2 combinations was confirmed by using a panel of antibodies which react with particular tissues in normal tailbud-stage embryos: anti-keratan sulphate for the notochord, anti-myosin for the muscle and anti-keratin for epidermis and notochord. Combinations made at stage 10 gave no positive cases and reciprocal heterochronic combinations between stages 7 1/2 and 10 showed that this is the result of a loss of competence by the stage-10 AP tissue. Whereas stage-7 1/2 AP tissue combined with stage-10 VP tissue gave many positive cases, the reciprocal experiment gave only a few. We have also tested the regional specificity of the induction. Stage-7 1/2 vegetal pole explants were divided into dorsal and ventral regions and then combined, separately, with stage-7 1/2 animal poles. The dorsovegetal tissue induces 'dorsal-type' mesoderm (notochord and large muscle masses) while ventrovegetal tissue induces 'ventral-type' mesoderm (blood, mesothelium and a little muscle). We conclude that mesoderm formation in combinations is an instructive event and propose a double gradient model to explain the complex character of the response.

Animals↗
PubMed1985

Regional specificity of glycoconjugates in Xenopus and axolotl embryos.

This paper reviews work on the presence, synthesis and developmental regulation of glycoconjugates (proteoglycans, glycoproteins and glycolipids) in the early amphibian embryo. In the axolotl there is little regional specificity of protein synthesis until the tailbud stage, but substantial regional specificity of glycoprotein synthesis from the end of gastrulation. Glycolipid synthesis is more uniform although a number of unique species are made in the epidermis. Isolated explants from axolotl early gastrulae show three types of behaviour in terms of glycoprotein synthesis, corresponding to the classical germ layers. Xenopus embryos at this stage show a higher degree of mosaicism. Changes of glycoprotein synthesis in response to mesodermal or neural induction follow the predicted course depending on the regional character of the induced tissue. The regional binding patterns of a number of lectins and monoclonal antibodies specific for particular carbohydrate determinants are presented and their significance discussed.

Ambystoma↗
PubMed1984

Analysis of embryonic induction by using cell lineage markers.

Three distinct inductive interactions have been demonstrated in early embryos of Xenopus laevis: mesoderm induction, dorsalization and neural induction. The experiments were done with grafts from embryos uniformly labelled with passive cell lineage markers, either FITC-lysine-dextran (FLDx) or horseradish peroxidase (HRP), which allow the provenance of regions to be determined down to the single cell level. In each case the fate of the target tissue in the presence of the appropriate inductor was quite different from the fate in normal development.

Animals↗
PubMed1984

Regional biosynthetic markers in the early amphibian embryo.

A search has been made for regional differences in macromolecular synthesis in the axolotl embryo at a stage when the principal regions have become determined but terminal differentiation has not yet begun. The epidermis of the neurula makes a number of abundant proteins which are not made elsewhere. Some of these are identified by immunoprecipitation as cytokeratins (relative molecular masses (Mr) 62,59,54,51 and 46 X 10(3)). At the same stage a network of tonofilaments becomes visible by electron microscopy and is also confined to the epidermis. There is no significant incorporation of 3H-sugars into glycoproteins before neurulation. During neurulation specific species begin to be made by the notochord (Mr 265 and 185 X 10(3)), the mesoderm (315 X 10(3)) and the epidermis (170 X 10(3)). The latter species is water soluble, has a native Mr of 470 X 10(3), is somewhat resistant to trypsin and has a high sugar content. Since these characteristics do not relate to any known glycoprotein it has been given a new name: epimucin. Several neutral glycolipids and gangliosides are present in the early embryo, the most abundant of which is identified as galactocerebroside. Synthesis occurs from fertilization onwards, however even by the neurula stage the qualitative pattern is the same in all parts except for the epidermis which shows two substantial enhancements and one reduction compared to other regions. The differences reported probably relate to physiological functions rather than to the mechanism of determination itself, so their chief importance is as markers of early embryo regions in experiments on induction.

Ambystoma↗
PubMed1984

In vitro development of isolated ectoderm from axolotl gastrulae.

The development of ectoderm isolated from the animal pole of axolotl gastrulae is monitored by light microscopy, electron microscopy and analysis of newly synthesized proteins, glycoproteins and glycolipids. When control embryos are undergoing neurulation it is shown that the explants autonomously begin to express epidermal markers and do not express mesodermal markers. However the results suggest that not all the cells become epidermal and electron microscope examination shows that only the outer layer does so, the inner cells remaining undifferentiated.

Ambystoma↗
PubMed1983

Positional information in the forelimb of the axolotl: properties of the posterior skin.

Two series of experiments were carried out to investigate the properties of the positional information carried by posterior skin of the axolotl forelimb. The skin was assayed by grafting it to the anterior side of a normal limb and then amputating through the graft region. The formation of a double posterior regenerate indicates that the grafted skin carried the posterior coding. In the first series, double posterior limbs were created by grafting posterior half limb rudiments to the flank of tailbud-stage embryos. The animals were reared for several months and then a half cuff of anterior skin, judged in relation to the whole body axes, was assayed by grafting to one of the host limbs. The results show that both sides of the double posterior limb carry the posterior coding and confirm our expectation that the visible anatomy is a good guide to the underlying codings. In the second series animals were prepared by embryonic grafts so that they bore an extra normal limb on the flank. This extra limb was marked by pigmentation and in some cases by triploidy. When the limbs had developed, posterior skin from the extra limb was grafted to the anterior side of a host limb. The host limbs were amputated at intervals ranging from 2 weeks to 1 year after the skin graft. The results show that the posterior coding carried by the graft is stable even in an anterior environment. Studies of the cellular composition of regenerates which had received triploid grafts showed that the graft epidermis was progressively replaced by that of the host. The dermis on the other hand retained triploid cells throughout.

Ambystoma↗
PubMed1983

Dorsalization and neural induction: properties of the organizer in Xenopus laevis.

We have studied the action of the organizer in Xenopus laevis using grafts labelled with horseradish peroxidase (HRP). Orthotropic grafts of the dorsal marginal zone (the organizer) from an HRP-labelled embryo into an unlabelled host showed that this region contributes to the anterior archenteron wall, to the entire craniocaudal extent of the notochord and to a few cells in the somites. Little or no contribution was made to the neural tube. Orthotopic grafts of the ventral marginal zone (the tissue that responds to a grafted organizer) indicated that it only contributes to the posterior half of the embryo. Within this region it spreads around the entire ventrolateral mesoderm, occasionally contributing a few cells to the somites. The posterior endoderm was also heavily labelled. When the dorsal marginal zone from an HRP-labelled embryo was inserted into a slit cut in the ventral marginal zone of an unlabelled host a mirror-symmetrical double-dorsal duplicated embryo resulted, in which only the notochord and a few cells in the somites of the secondary embryo were derived from the graft. The bulk of the secondary somites was, therefore, derived from host ventral marginal zone tissue which normally makes very little contribution to the somites. This indicates that host ventral marginal zone becomes dorsalized by the graft. The neural tube of the secondary embryo was also unlabelled, showing that it was induced by the influence of the graft on the overlying ectoderm, which normally forms ventral epidermis. We have also grafted ventral marginal zone tissue into a slit cut into the dorsal marginal zone of a host embryo. HRP-labelled tissue was grafted into an unlabelled embryo and vice versa. This graft did not produce a double ventral embryo and this reinforces the traditional view that the dorsal marginal zone is a special signalling region. Instead, the resulting embryos usually had a twinned notochord with the graft tissue in between, differentiated as somite. This confirms that juxtaposing ventral and dorsal marginal zone 'dorsalizes' the ventral tissue but does not affect the dorsal tissue which differentiates, as usual, as notochord. Thus, our results allow us to conclude that the organizer mediates two distinct interactions in bringing about the formation of duplicated embryos. The first is dorsalization of adjacent ventral mesoderm and the second is the induction of neuroepithelium from ectoderm overlying the new archenteron roof.

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