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PubMed1998

Bix1, a direct target of Xenopus T-box genes, causes formation of ventral mesoderm and endoderm.

Brachyury, a member of the T-box gene family, is required for posterior mesoderm and notochord differentiation in vertebrate development, and mis-expression of Xenopus Brachyury causes ectopic mesoderm formation. Brachyury is a transcription activator, and its ability to activate transcription is essential for its biological function, but Brachyury target genes have proved difficult to identify. Here we employ a hormone-inducible Brachyury construct and subtractive hybridization to search for such targets. Using this approach we have isolated Bix1, a homeobox gene expressed both in the marginal zone of Xenopus and in the vegetal hemisphere. Expression of Bix1 is induced in an immediate-early fashion by mesoderm-inducing factors such as activin as well as by the products of the T-box genes Xbra and VegT (also known as Antipodean, Brat and Xombi). Activation of Bix1 in response to Xbra is direct in the sense that it does not require protein synthesis, and both Xbra and VegT activate expression of a reporter gene driven by the Bix 5' regulatory region, which contains an Xbra/VegT binding site. Mis-expression of low levels of Bix1 causes formation of ventral mesoderm, while high levels induce endodermal differentiation. These results suggest that Bix1 acts downstream of both VegT and Xbra to induce formation of mesoderm and endoderm.

Activins↗
PubMed1999

Functions for Drosophila brachyenteron and forkhead in mesoderm specification and cell signalling.

The visceral musculature of the larval midgut of Drosophila has a lattice-type structure and consists of an inner stratum of circular fibers and an outer stratum of longitudinal fibers. The longitudinal fibers originate from the posterior tip of the mesoderm anlage, which has been termed the caudal visceral mesoderm (CVM). In this study, we investigate the specification of the CVM and particularly the role of the Drosophila Brachyury-homologue brachyenteron. Supported by fork head, brachyenteron mediates the early specification of the CVM along with zinc-finger homeodomain protein-1. This is the first function described for brachyenteron or fork head in the mesoderm of Drosophila. The mode of cooperation resembles the interaction of the Xenopus homologues Xbra and Pintallavis. Another function of brachyenteron is to establish the surface properties of the CVM cells, which are essential for their orderly migration along the trunk-derived visceral mesoderm. During this movement, the CVM cells, under the control of brachyenteron, induce the formation of one muscle/pericardial precursor cell in each parasegment. We propose that the functions of brachyenteron in mesodermal development of Drosophila are comparable to the roles of the vertebrate Brachyury genes during gastrulation.

Animals↗
PubMed1999

Antivin, a novel and divergent member of the TGFbeta superfamily, negatively regulates mesoderm induction.

Mesoderm induction and patterning are mediated by members of the TGFbeta superfamily. We have isolated a novel zebrafish member, antivin, that structurally is highly related to mouse lefty. Overexpression of antivin completely abolishes mesoderm induction at blastula stage, yet resultant embryos develop well-patterned epidermal and neural derivatives. The mesoderm-inhibiting activity of antivin can be mimicked by lefty and is suppressed by increasing levels of the mesodermal inducer Activin or its receptors. On the basis of its expression and activity, we propose that Antivin normally functions as a competitive inhibitor of Activin to limit mesoderm induction in the early embryo.

Activin Receptors↗
PubMed1999

The surface ectoderm is essential for nephric duct formation in intermediate mesoderm.

The nephric duct is the first epithelial tubule to differentiate from intermediate mesoderm that is essential for all further urogenital development. In this study we identify the domain of intermediate mesoderm that gives rise to the nephric duct and demonstrate that the surface ectoderm is required for its differentiation. Removal of the surface ectoderm resulted in decreased levels of Sim-1 and Pax-2 mRNA expression in mesenchymal nephric duct progenitors, and caused inhibition of nephric duct formation and subsequent kidney development. The surface ectoderm expresses BMP-4 and we show that it is required for the maintenance of high-level BMP-4 expression in lateral plate mesoderm. Addition of a BMP-4-coated bead to embryos lacking the surface ectoderm restored normal levels of Sim-1 and Pax-2 mRNA expression in nephric duct progenitors, nephric duct formation and the initiation of nephrogenesis. Thus, BMP-4 signaling can substitute for the surface ectoderm in supporting nephric duct morphogenesis. Collectively, these data suggest that inductive interactions between the surface ectoderm, lateral mesoderm and intermediate mesoderm are essential for nephric duct formation and the initiation of urogenital development.

Animals↗
PubMed2000

bozozok and squint act in parallel to specify dorsal mesoderm and anterior neuroectoderm in zebrafish.

In vertebrate embryos, maternal (beta)-catenin protein activates the expression of zygotic genes that establish the dorsal axial structures. Among the zygotically acting genes with key roles in the specification of dorsal axial structures are the homeobox gene bozozok (boz) and the nodal-related (TGF-(beta) family) gene squint (sqt). Both genes are expressed in the dorsal yolk syncytial layer, a source of dorsal mesoderm inducing signals, and mutational analysis has indicated that boz and sqt are required for dorsal mesoderm development. Here we examine the regulatory interactions among boz, sqt and a second nodal-related gene, cyclops (cyc). Three lines of evidence indicate that boz and sqt act in parallel to specify dorsal mesoderm and anterior neuroectoderm. First, boz requires sqt function to induce high levels of ectopic dorsal mesoderm, consistent with sqt acting either downstream or in parallel to boz. Second, sqt mRNA is expressed in blastula stage boz mutants, indicating that boz is not essential for activation of sqt transcription, and conversely, boz mRNA is expressed in blastula stage sqt mutants. Third, boz;sqt double mutants have a much more severe phenotype than boz and sqt single mutants. Double mutants consistently lack the anterior neural tube and axial mesoderm, and ventral fates are markedly expanded. Expression of chordin and noggin1 is greatly reduced in boz;sqt mutants, indicating that the boz and sqt pathways have overlapping roles in activating secreted BMP antagonists. In striking contrast to boz;sqt double mutants, anterior neural fates are specified in boz;sqt;cyc triple mutants. This indicates that cyc represses anterior neural development, and that boz and sqt counteract this repressive function. Our results support a model in which boz and sqt act in parallel to induce dorsalizing BMP-antagonists and to counteract the repressive function of cyc in neural patterning.

Animals↗
PubMed2000

HNF1(beta) is required for mesoderm induction in the Xenopus embryo.

XHNF1(&bgr;) is a homeobox-containing gene initially expressed at the blastula stage in the vegetal part of the Xenopus embryo. We investigated its early role by functional ablation, through mRNA injection of an XHNF1(beta)/engrailed repressor fusion construct (XHNF1(beta)/EngR). Dorsal injections of XHNF1(beta)/EngR mRNA abolish dorsal mesoderm formation, leading to axial deficiencies; ventral injections disrupt ventral mesoderm formation without affecting axial development. XHNF1(beta)/EngR phenotypic effects specifically depend on the DNA-binding activity of its homeodomain and are fully rescued by coinjection of XHNF1(beta) mRNA. Vegetal injection of XHNF1(beta)/EngR mRNA blocks the mesoderm-inducing ability of vegetal explants. Both B-Vg1 and VegT maternal determinants trigger XHNF1(beta) expression in animal caps. XHNF1(beta)/EngR mRNA blocks B-Vg1-mediated, but not by eFGF-mediated, mesoderm induction in animals caps. However, wild-type XHNF1(beta) mRNA does not trigger Xbra expression in animal caps. We conclude that XHNF1(beta) function is essential, though not sufficient, for mesoderm induction in the Xenopus embryo.

Animals↗
PubMed2001

Xenopus Dishevelled signaling regulates both neural and mesodermal convergent extension: parallel forces elongating the body axis.

During amphibian development, non-canonical Wnt signals regulate the polarity of intercalating dorsal mesoderm cells during convergent extension. Cells of the overlying posterior neural ectoderm engage in similar morphogenetic cell movements. Important differences have been discerned in the cell behaviors associated with neural and mesodermal cell intercalation, raising the possibility that different mechanisms may control intercalations in these two tissues. In this report, targeted expression of mutants of Xenopus Dishevelled (Xdsh) to neural or mesodermal tissues elicited different defects that were consistent with inhibition of either neural or mesodermal convergent extension. Expression of mutant Xdsh also inhibited elongation of neural tissues in vitro in Keller sandwich explants and in vivo in neural plate grafts. Targeted expression of other Wnt signaling antagonists also inhibited neural convergent extension in whole embryos. In situ hybridization indicated that these defects were not due to changes in cell fate. Examination of embryonic phenotypes after inhibition of convergent extension in different tissues reveals a primary role for mesodermal convergent extension in axial elongation, and a role for neural convergent extension as an equalizing force to produce a straight axis. This study demonstrates that non-canonical Wnt signaling is a common mechanism controlling convergent extension in two very different tissues in the Xenopus embryo and may reflect a general conservation of control mechanisms in vertebrate convergent extension.

Adaptor Proteins, Signal Transducing↗
PubMed2001

The organizer of the mouse gastrula is composed of a dynamic population of progenitor cells for the axial mesoderm.

An organizer population has been identified in the anterior end of the primitive streak of the mid-streak stage embryo, by the expression of Hnf3beta, Gsc(lacZ) and Chrd, and the ability of these cells to induce a second neural axis in the host embryo. This cell population can therefore be regarded as the mid-gastrula organizer and, together with the early-gastrula organizer and the node, constitute the organizer of the mouse embryo at successive stages of development. The profile of genetic activity and the tissue contribution by cells in the organizer change during gastrulation, suggesting that the organizer may be populated by a succession of cell populations with different fates. Fine mapping of the epiblast in the posterior region of the early-streak stage embryo reveals that although the early-gastrula organizer contains cells that give rise to the axial mesoderm, the bulk of the progenitors of the head process and the notochord are localized outside the early gastrula organizer. In the mid-gastrula organizer, early gastrula organizer derived cells that are fated for the prechordal mesoderm are joined by the progenitors of the head process that are recruited from the epiblast previously anterior to the early gastrula organizer. Cells that are fated for the head process move anteriorly from the mid-gastrula organizer in a tight column along the midline of the embryo. Other mid-gastrula organizer cells join the expanding mesodermal layer and colonize the cranial and heart mesoderm. Progenitors of the trunk notochord that are localized in the anterior primitive streak of the mid-streak stage embryo are later incorporated into the node. The gastrula organizer is therefore composed of a constantly changing population of cells that are allocated to different parts of the axial mesoderm.

Animals↗
PubMed1987

The midblastula cell cycle transition and the character of mesoderm in u.v.-induced nonaxial Xenopus development.

Xenopus embryos (UV embryos) resulting from u.v. (254 nm) irradiation to the vegetal egg hemisphere and thus developing little or no axial pattern (UV5-Scharf & Gerhart, 1983), have been compared histologically with synchronous normal siblings at each of three stages. In addition, the relative amounts of blood-forming tissue produced in normal and in UV embryos have been studied by Western blotting total protein from larval stages and by immunofluorescence on sections. The observations on midblastulae (around 5000 cells) were aimed at detecting any systematic retardation, due to u.v., of the slowing of the cell cycle that normally commences at the 2-4000 cell stage and makes possible zygotic transcription and the preparation for gastrulation. No such retardation was apparent. Observations on postgastrular stages gave an assessment of the size and character of the population of mesoderm founder cells, in relation to the control, for embryos visibly undergoing entirely nonaxial development. Little deficit in total mesodermal cell number was found, though the entire mesoderm adopted the histological character proper to only some 40% of that in the normal pattern i.e. trunk lateral plate. Blood-forming capacity appears to be enhanced out of all proportion to the size of the mesoderm as a whole. The results are discussed in terms of the probable nature of the primary positional system for axial pattern and the later mechanisms of mesodermal patterning.

Animals↗
PubMed2001

Real-time imaging of cell-cell adherens junctions reveals that Drosophila mesoderm invagination begins with two phases of apical constriction of cells.

Invagination of the epithelial cell sheet of the prospective mesoderm in Drosophila gastrulation is a well-studied, relatively simple morphogenetic event that results from dynamic cell shape changes and cell movements. However, these cell behaviors have not been followed at a sufficiently short time resolution. We examined mesoderm invagination in living wild-type embryos by real-time imaging of fluorescently labeled cell-cell adherens junctions, which are located at the apical zones of cell-cell contact. Low-light fluorescence video microscopy directly visualized the onset and progression of invagination. In an initial period of approximately 2 minutes, cells around the ventral midline reduced their apical surface areas slowly in a rather synchronous manner. Next, the central and more lateral cells stochastically accelerated or initiated their apical constriction, giving rise to random arrangements of cells with small and relatively large apices. Thus, we found that mesoderm invagination began with slow synchronous and subsequent fast stochastic phases of cell apex constriction. Furthermore, we showed that the mesoderm invagination of folded gastrulation mutant embryos lacked the normal two constriction phases, and instead began with asynchronous, feeble cell shape changes. Our observations suggested that Folded gastrulation-mediated signaling enabled synchronous activation of the contractile cortex, causing competition among the individual mesodermal cells for apical constriction. Movies available on-line: http://www.biologists.com/JCS/movies/jcs2073.html

Animals↗
PubMed1986

Presumptive mesoderm cells from Xenopus laevis gastrulae attach to and migrate on substrata coated with fibronectin or laminin.

During amphibian gastrulation, presumptive mesoderm cells migrate from the blastopore towards the animal pole along the inner surface of the ectodermal layer. Their natural substratum is a network of anastomosing extracellular matrix fibrils, which contains fibronectin and laminin, as shown by immunostaining. If the fibril network is transferred onto a coverslip from the ectodermal layer, dissociated mesodermal cells readily attach to such conditioned surfaces and show active migration in a medium of high pH and low calcium ion concentration. In the present study, the surface of tissue culture dishes was coated with fibronectin, laminin, collagen type IV or heparan sulphate, to examine the effects on cell attachment and movement. The presumptive mesoderm cells from Xenopus laevis gastrulae showed rapid adhesion and active movement on the fibronectin- or laminin-coated surfaces. Cell adhesion was stronger and the mean rate of movement was higher on the fibronectin-coated surface than on the laminin-coated surface. The dissociated ectodermal cells did not attach to the fibronectin- or laminin-coated surfaces. The mesodermal cells did not attach to the collagen-, or heparan sulphate-coated surfaces, showing that these components of the basement membrane cannot serve as an adequate substratum for the mesoderm cells, at least by themselves.

Animals↗
PubMed2006

Mesodermal anatomies in cnidarian polyps and medusae.

The cellular and developmental analysis of evolutionary-conserved genes directing bilaterian mesodermal and myogenic cell fate previously identified the hydromedusan entocodon and its differentiation product, the striated muscle, as mesodermal derivatives. In view of these findings we presented a hypothesis disputing the diploblast classification of cnidarians without providing further explanations for the apparent diploblasty of the polyp stage and the formation of the subepidermal striated muscle in those Medusozoa lacking the entocodon nodule (Seipel and Schmid, 2005). Hence we carried out a systematic review of the histological and experimental evidence for mesodermal differentiations in cnidarians. In anthozoan and scyphozoan but not in hydrozoan polyps the presumptive mesodermal elements include amoeboid cells, the mesentery retractor muscles and scleroblasts, all of which are embedded or deeply rooted in the extracellular matrix (mesoglea) and derive from the ectoblastemal cells invading the extracellular matrix from the gastrulation site during or shortly after endoderm formation. These data lend further support to the cnidarian mesodermate hypothesis, whereby cnidarians and bilaterians share a common triploblast ancestor, the Urtriploblast, a small, motile, possibly medusa-like organism that did not feature a sessile polyp stage in its life cycle. As a consequence the diploblasty of the hydrozoan polyps may represent a derived morphology resulting from heterochronic modulations of the gastrulation process after endoderm formation.

Animals↗
PubMed2000

Local changes in gap junctional permeability accompany regionalization of the mesoderm in early fish (loach, Misgurnus fossilis) embryos.

The marginal area of early loach (Misgurnus fossilis) embryos in the proximity of the yolk cell (YC), the zone of early activation of mesoderm-specific genes, was revealed to be characterized by regional changes in the permeability and selectivity of gap junctional (GJ) channels. Comparison of the efficiency of GJ transfer of negatively (Lucifer Yellow, LY and fluorescein, F) and positively (4',6-diamidino-2-phenylindole, DAPI) charged dyes showed that, at the early gastrula stage, GJs formed between blastomeres of the presumptive mesoderm layer and the YC acquired a higher permeability for LY and F and became virtually impenetrable for DAPI over most of the germ ring area, except for its dorsal side. GJs in the dorsal-anterior region were permeable for both LY and DAPI. The dorsal-posterior area was labelled with neither of the dyes. Observations of the fate of embryonic shield blastomeres stained with both LY and DAPI showed that their progenies were confined to the head mesoderm, whereas the paraxial somite mesoderm and the ventral mesoderm contained the cells labelled only with LY or F. The presumptive notochord region was not labelled at all. These data support the hypothesis that GJs between the embryonic cells may be involved in the spatial organization of development due to their ability to transfer different regulatory factors in different directions, filtering molecules in accordance with the electric charge, conformation and size.

Animals↗
PubMed1995

Early endocardial formation originates from precardiac mesoderm as revealed by QH-1 antibody staining.

The formation of endocardial endothelium in quail embryos was investigated using in vivo and in vitro systems. At stage 7+ (2 somite), the initial emergence of endothelial cells within the bilateral heart forming region (HFR) was detected in quail embryos by immunohistochemistry with QH-1 (an anti-quail endothelial cell marker) and confocal microscopy. We consistently observed more QH-1 positive cells in the right HFR than the left. At stage 8 (4 somite), the HFR, including QH-1 positive cells, were located in the splanchnic mesoderm after formation of the coelom. During stage 8, the HFR migrated along the margin of anterior intestinal portal in association with the endoderm. By stage 8+ (5 somite), the two HFR had fused at the midline and formed a plexus of QH-1 positive endothelial precursor cells. The definitive endocardium developed as a single, hollow, tube within this plexus. Posteriorly, QH-1 positive cells of the HFR established vascular-like connections with QH-1 positive cells that had formed outside (peripheral to) the HFR. During migration and subsequent determination, the precardiac mesoderm is continuously associated with the basement membrane of the anterior endoderm. To determine the role of endoderm on endocardial endothelial cell formation and development, precardiac mesoderm from stage 5 embryos, which does not express QH-1 antigen, was explanted onto the surface of collagen gels. When co-cultured with endoderm, the outgrowth of free cells from the mesoderm was much more extensive, many of which invaded the gel and expressed the QH-1 antigen; mesoderm cultured without endoderm did not seed nor express QH-1 antigen. These findings suggest that the segregation of endothelial and myocardial lineages may occur by an endoderm-mediated, mesenchymal formation.

Animals↗
PubMed1975

[Role of the somitic mesoderm in the development of the rib cage of bird embryos. I. Origin of the sternal component and conditions for the development of the ribs (author's transl)].

The developmental origin of the sternal component of the ribs and the conditions for the development of the rib cage have been elucidated in bird embryos. Experiments consisted in homo- or heterospecific transplantations of 2-day-old quail or chick embryo somitic mesoderm into chick hosts of the same age in ortho- or heterotopic position along the cephalo-caudal axis. Results show that vertebral halves, ribs (not only their vertebral segment, but also their sternal component when they possess one), the trunk and intercostal muscles, as well as at least part of the scapula originate exclusively from the somitic material, while the sternum, ventral muscles and the other parts of the pectoral girdle are derived from the somatopleural mesoderm. The development of the rib basket is subjected to following rules: -only the somitic mesoderm of the prospective thoracic region (somites 19-26) is able to give rise to ribs. -only the somitic mesoderm of the posterior thoacic region (somites 22-26) is able to develop ribs with a sternal component. -the vertebral component of ribs can develop outside the thoracic region. -contrariwise, the sternal component can form only in the vicinity of the sternal anlage, i.e. within about three somites in front and rear of the normal limits of the thoracic region. It is concluded that the somitic mesoderm is already regionalized at a stage slightly preceding its metamerization and that the somatopleural territory of the sternum plays a morphogenetic role in the development of the sternal component of ribs, although it does not make a cellular contribution to their construction.

Animals↗
PubMed2001

FGF signaling is essential for the early events in the development of the chick nervous system and mesoderm.

Fibroblast growth factor (FGF) belongs to a family of polypeptides with diverse biological functions. In the present study we have assessed the role of FGF signaling in the development of nervous system and mesodermal tissues in chick embryo. Treatment of in vitro cultured embryos with exogenous, human recombinant FGF led to abnormalities in neural induction and development, notochord formation and somitogenesis as studied by gross morphology and histology. Overall growth and development was also adversely affected as seen from the measurement of body axis length. Further, treatment of embryos with FGF resulted in differential modulation of expression of two genes important in normal development as studied by whole mount in situ hybridization using DIG-labeled riboprobes. The expression of Brachyury, which is necessary for mesoderm formation, was down-regulated in FGF-treated embryos. The expression of noggin, the product which participates in the patterning of the chick neural tube was, on the other hand, up-regulated within 2 h. We also studied development of neural and mesodermal tissues in conditions where FGF signaling was defective. This was achieved by culturing the embryos in the presence of suramin. In the presence of low doses of suramin (100-150 nmole/culture), abnormalities were detected mainly in the mesodermal structures while at higher doses (200-400 nmole/culture), the nervous system too was found to be abnormal in a large proportion of embryos. Treatment of chick embryos with suramin (200 nmole/culture) also modulated the expression of Brachyuryand noggin within a 2 h period. The results showthat FGF signaling plays an important role in the molecular events leading to the development of nervous system and mesodermal tissues in the chick embryo.

Animals↗
PubMed1991

Gene activation in the amphibian mesoderm.

Cell potency is progressively restricted in amphibian development by a series of cellular interactions called inductions. The mesoderm is believed to develop in response to the earliest known induction, in which vegetal cells of the blastula divert overlying animal hemisphere cells away from epidermal and towards mesodermal fates. We describe two early markers of mesodermal differentiation in Xenopus laevis, both mRNAs that encode DNA-binding proteins of the helix-loop-helix family. One is a frog homologue of MyoD, a gene that in transfection experiments can convert cultured fibroblasts into myoblasts. Xenopus MyoD (XMyoD) is expressed in the early myotomes, from which the axial musculature develops. The accumulation of XMyoD RNA precedes that of transcripts from the cardiac actin gene, until now the earliest known marker of the muscle lineage, this result indicating that XMyoD could play a role in initiating muscle differentiation in normal development. We show by microinjection of synthetic RNA that XMyoD can indeed activate muscle-specific gene expression in animal cap cells, which would normally form only ectoderm. However, the XMyoD-injected animal caps did not produce differentiated muscle, suggesting that additional specific factors are required for full myogenesis. The other mRNA is a relative of the twist gene of Drosophila, which is required for mesodermal differentiation in flies. This gene (Xtwi) is expressed widely in the early frog mesoderm, but not, however, in the myotomes, where XMyoD is expressed. Later, the Xtwi gene is activated, in response to a second induction, in the developing neural crest.

Animals↗
PubMed1979

Cell number in relation to primary pattern formation in the embryo of Xenopus laevis. II. Sequential cell recruitment, and control of the cell cycle, during mesoderm formation.

Morphological evidence is presented that definitive mesoderm formation in Xenopus is best understood as extending to the end of the neurula phase of development. A process of recruitment of cells from the deep neurectoderm layers into mesodermal position and behaviour, strictly comparable with that already agreed to occur around the internal blastoporal 'lip' during gastrula stage 20 (earliest tail bud). Spatial patterns of incidence of mitosis are described for the fifteen hours of development between the late gastrula and stage 20--22. These are related to the onset of new cell behaviours and overt cyto-differentiations characterizing the dorsal axial pattern, which occur in cranio-caudal and then medio-lateral spatial sequence as development proceeds. A relatively abrupt cessation of mitosis, among hitherto asynchronously cycling cells, precedes the other changes at each level in the presumptive axial pattern. The widespread incidence of cells still in DNA synthesis, anterior to the last mitoses in the posterior-to-anterior developmental sequence of axial tissue, strongly suggests that cells of notochord and somites in their prolonged, non-cycling phase are G2-arrested, and thus tetraploid. This is discussed in relation to what is known of cell-cycle control in other situations. Best estimates for cell-cycle time in the still-dividing, posterior mesoderm of the neurula lie between 10 and 15 h. The supposition of continuing recruitment from neurectoderm can resolve an apparent discrepancy whereby total mesodermal cell number nevertheless contrives to double over a period of approximately 12 h during neurulation when most of the cells are leaving the cycle. Because of pre-existing evidence that cells maintain their relative positions (despite distortion) during the movements that form the mesodermal mantle, the patterns presented in this paper can be understood in two ways: as a temporal sequence of developmental events undergone by individual, posteriorly recruited cells as they achieve their final positions in the body pattern, or alternatively as a succession of wavefronts with respect to changes of cell state, passing obliquely across the presumptive body pattern in antero-posterior direction. These concepts are discussed briefly in relation to recent ideas about pattern formation in growing systems.

Animals↗