Embryonic induction

Hilde Mangold: Original microscope slides and records of the gastrula organizer experiments

The discovery of the amphibian gastrula organizer and its publication by Hans Spemann and Hilde Mangold in 1924 is a foundation of experimental embryology, and has shaped our understanding of embryonic induction and pattern formation in vertebrates until today. The original publication is a piece of scientific art, characterized by the meticulous hand drawings by Hilde Mangold, as well as the text that develops mechanistic concepts of modern embryology. While historic microphotographs of specimens got lost, the original microscope slides and Hilde Mangold's laboratory notebook have been secured in embryological collections until today. Here, we make the original data of the six embryonic specimens reported in 1924, as well as the laboratory notebook, available in an accessible digital format. Together, these data shed light on the scientific process that led to the discovery, and should help to make the experiments on the most important signalling center in early vertebrate development transparent for generations of embryologists to come.

Microphase separation of living cells

Self-organization of cells is central to a variety of biological systems and physical concepts of condensed matter have proven instrumental in deciphering some of their properties. Here we show that microphase separation, long studied in polymeric materials and other inert systems, has a natural counterpart in living cells. When placed below a millimetric film of liquid nutritive medium, a quasi two-dimensional, high-density population of Dictyostelium discoideum cells spontaneously assembles into compact domains. Their typical size of 100 μm is governed by a balance between competing interactions: an adhesion acting as a short-range attraction and promoting aggregation, and an effective long-range repulsion stemming from aerotaxis in near anoxic condition. Experimental data, a simple model and cell-based simulations all support this scenario. Our findings establish a generic mechanism for self-organization of living cells and highlight oxygen regulation as an emergent organizing principle for biological matter.

From embryos to embryoids: How external signals and self-organization drive embryonic development

Embryonic development has been traditionally seen as an inductive process directed by exogenous maternal inputs and extra-embryonic signals. Increasing evidence, however, is showing that, in addition to exogenous signals, the development of the embryo involves endogenous self-organization. Recently, this self-organizing potential has been highlighted by a number of stem cell models known as embryoids that can recapitulate different aspects of embryogenesis in vitro. Here, we review the self-organizing behaviors observed in different embryoid models and seek to reconcile this new evidence with classical knowledge of developmental biology. This analysis leads to reexamine embryonic development as a guided self-organizing process, where patterning and morphogenesis are controlled by a combination of exogenous signals and endogenous self-organization. Finally, we discuss the multidisciplinary approach required to investigate the genetic and cellular basis of self-organization.

Roles of Xenopus chemokine ligand CXCLh (XCXCLh) in early embryogenesis

Several chemokine molecules control cell movements during early morphogenesis. However, it is unclear whether chemokine molecules affect cell fate. Here, we identified and characterized the CXC-type chemokine ligand in Xenopus laevis, Xenopus CXCLh (XCXCLh), during early embryogenesis. XCXCLh is expressed in the dorsal vegetal region at the gastrula stage. Both overexpression and knockdown of XCXCLh in the dorsal region inhibited gastrulation. XCXCLh contributed to the attraction of mesendodermal cells and accelerated the reassembly of scratched culture cells. Also, XCXCLh contributed to early endodermal induction. Overexpression of VegTmRNA or high concentrations of calcium ions induced XCXCLh expression. XCXCLh may play roles in both cell movements and differentiation during early Xenopus embryogenesis.

Kidney development: an overview

BACKGROUND

Kidney diseases are worldwide public health problems with a high cost and increasing incidence. By revealing the genetic and cellular mechanism behind mammalian kidney development, better diagnostic methods and novel therapies can be expected to be developed. The mammalian kidney is a typical organ that develops on the basis of sequential and reciprocal cell and tissue interactions. Functional genetic analysis has identified that genes from different classes are involved in the construction of the kidney and the same genes are also connected to the development of diseases.

SUMMARY

This review gives an overview of the basics of kidney ontogeny, from identification of the primary kidney cell to inductive signals of ureter budding and formation of the segmented nephron. We also go through some of the key factors involved in the control of morphogenesis.

KEY MESSAGE

Despite the wealth of accumulated data on nephron development, including progenitor cell control factors and inductive signals, many of the detailed mechanisms remain to be revealed.

Evolutionary development of neural systems in vertebrates and beyond

The emerging field of "neuro-evo-devo" is beginning to reveal how the molecular and neural substrates that underlie brain function are based on variations in evolutionarily ancient and conserved neurochemical and neural circuit themes. Comparative work across bilaterians is reviewed to highlight how early neural patterning specifies modularity of the embryonic brain, which lays a foundation on which manipulation of neurogenesis creates adjustments in brain size. Small variation within these developmental mechanisms contributes to the evolution of brain diversity. Comparing the specification and spatial distribution of neural phenotypes across bilaterians has also suggested some major brain evolution trends, although much more work on profiling neural connections with neurochemical specificity across a wide diversity of organisms is needed. These comparative approaches investigating the evolution of brain form and function hold great promise for facilitating a mechanistic understanding of how variation in brain morphology, neural phenotypes, and neural networks influences brain function and behavioral diversity across organisms.

Lhx1 is required for specification of the renal progenitor cell field

In the vertebrate embryo, the kidney is derived from the intermediate mesoderm. The LIM-class homeobox transcription factor lhx1 is expressed early in the intermediate mesoderm and is one of the first genes to be expressed in the nephric mesenchyme. In this study, we investigated the role of Lhx1 in specification of the kidney field by either overexpressing or depleting lhx1 in Xenopus embryos or depleting lhx1 in an explant culture system. By overexpressing a constitutively-active form of Lhx1, we established its capacity to expand the kidney field during the specification stage of kidney organogenesis. In addition, the ability of Lhx1 to expand the kidney field diminishes as kidney organogenesis transitions to the morphogenesis stage. In a complimentary set of experiments, we determined that embryos depleted of lhx1, show an almost complete loss of the kidney field. Using an explant culture system to induce kidney tissue, we confirmed that expression of genes from both proximal and distal kidney structures is affected by the absence of lhx1. Taken together our results demonstrate an essential role for Lhx1 in driving specification of the entire kidney field from the intermediate mesoderm.

Morphogenesis by coupled regulatory networks: reliable control of positional information and proportion regulation

Based on a non-equilibrium mechanism for spatial pattern formation we study how position information can be controlled by locally coupled discrete dynamical networks, similar to gene regulation networks of cells in a developing multicellular organism. As an example we study the developmental problems of domain formation and proportion regulation in the presence of noise, as well as in the presence of cell flow. We find that networks that solve this task exhibit a hierarchical structure of information processing and are of similar complexity as developmental circuits of living cells. Proportion regulation is scalable with system size and leads to sharp, precisely localized boundaries of gene expression domains, even for large numbers of cells. A detailed analysis of noise-induced dynamics, using a mean-field approximation, shows that noise in gene expression states stabilizes (rather than disrupts) the spatial pattern in the presence of cell movements, both for stationary as well as growing systems. Finally, we discuss how this mechanism could be realized in the highly dynamic environment of growing tissues in multicellular organisms.

SOX7 is an immediate-early target of VegT and regulates Nodal-related gene expression in Xenopus

In zebrafish, the divergent F-type SOX casanova acts downstream of Nodal signaling to specify endoderm. While no casanova orthologs have been identified in tetrapods, the F-type SOX, SOX7, is supplied maternally in Xenopus (Fawcett and Klymkowsky, 2004. GER 4, 29). Subsequent RT-PCR and section-based in situ hybridization analyses indicate that SOX7 mRNA is localized to the vegetal region of the blastula-stage embryo. Overexpression and maternal depletion studies reveal that the T-box transcription factor VegT, which initiates mesoendodermal differentiation, directly regulates SOX7 expression. SOX7, but not SOX17 (another F-type SOX), binds to sites within the Xnr5 promoter and SOX7, but not SOX17, induces expression of the Nodal-related genes Xnr1, Xnr2, Xnr4, Xnr5, and Xnr6, the homeodomain transcription factor Mixer, and the endodermal marker SOX17beta; both SOX7 and SOX17 induce expression of the pan-endodermal marker endodermin. SOX7's induction of Xnr expression in animal caps is independent of Mixer and Nodal signaling. In animal caps, VegT's ability to induce Mixer and Edd appears to depend upon SOX7 activity. Whole embryo experiments suggests that vegetal factors partially compensate for the absence of SOX7. Based on the antagonistic effects of SOX7 and SOX3 (Zhang et al., 2004. Dev. Biol. 273, 23) and their common binding sites in the Xnr5 promoter, we propose a model in which competitive interactions between these two proteins are involved in refining the domain of endodermal differentiation.

FGF-10 plays an essential role in the growth of the fetal prostate

Induction and branching morphogenesis of the prostate are dependent on androgens, which act via the mesenchyme to induce prostatic epithelial development. One mechanism by which the mesenchyme may regulate the epithelium is through secreted growth factors such as FGF-10. We have examined the male reproductive tract of FGF-10(-/-) mice, and at birth, most of the male secondary sex organs were absent or atrophic, including the prostate, seminal vesicle, bulbourethral gland, and caudal ductus deferens. Rudimentary prostatic buds were occasionally observed in the prostatic anlagen, the urogenital sinus (UGS) of FGF-10(-/-) mice. FGF-10(-/-) testes produced sufficient androgens to induce prostatic development in control UGS organ cultures. Prostatic rudiments from FGF-10(-/-) mice transplanted into intact male hosts grew very little, but showed some signs of prostatic differentiation. In cultures of UGS, the FGF-10 null phenotype was partially reversed by the addition of FGF-10 and testosterone, resulting in the formation of prostatic buds. FGF-10 alone did not stimulate prostatic bud formation in control or FGF-10(-/-) UGS. Thus, FGF-10 appears to act as a growth factor which is required for development of the prostate and several other accessory sex organs.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

Morphogen gradients, positional information, and Xenopus: interplay of theory and experiment

The idea of morphogen gradients has long been an important one in developmental biology. Studies with amphibians and with Xenopus in particular have made significant contributions to demonstrating the existence, identity, and mechanisms of action of morphogens. Mesoderm induction and patterning by activin, nodals, bone morphogenetic proteins, and fibroblast growth factors have been analyzed thoroughly and reveal recurrent and combinatorial roles for these protein growth factor morphogens and their antagonists. The dynamics of nodal-type signaling and the intersection of VegT and beta-catenin intracellular gradients reveal detailed steps in early long-range patterning. Interpretation of gradients requires sophisticated mechanisms for sharpening thresholds, and the activin-Xbra-Gsc system provides an example of this. The understanding of growth factor signal transduction has elucidated growth factor morphogen action and provided tools for dissecting their direct long-range action and distribution. The physical mechanisms of morphogen gradient establishment are the focus of new interest at both the experimental and theoretical level. General themes and emerging trends in morphogen gradient studies are discussed.

Regulated expression of the MADS-box transcription factor SrfA mediates activation of gene expression by protein kinase A during Dictyostelium sporulation

Cell differentiation and morphogenesis are tightly regulated during sporulation in the lower eukaryote Dictyostelium discoideum. The control of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is essential to coordinate these processes. Several signal transduction pathways are being recognized that lead to the regulation of intracellular cAMP levels. However, very little is known about the events lying downstream of PKA that are essential to activate late gene expression and terminal differentiation of the spores. We have studied the relationship between PKA and the MADS-box transcription factor SrfA, essential for spore differentiation. Constitutive activation of PKA was not able to rescue sporulation in a strain that lacks srfA suggesting the possibility that srfA functions downstream of PKA in a signal transduction pathway leading to spore maturation. A distal promoter region regulates the induction of srfA expression in the prespore region during culmination. We found that this promoter can be induced precociously by activating PKA with 8-Br-cAMP suggesting a transcriptional regulation by PKA. Moreover, precocious sporulation and expression of the spore marker spiA in a strain that overexpresses PKA, correlates with a precocious induction of srfA expression. The temporal and spatial pattern of expression was also studied in a mutant strain lacking the main adenylyl cyclase that functions during culmination, ACR. This strain is expected to have lower PKA activity and consistently, the level of srfA expression was reduced. Moreover, the temporal induction of srfA in the prespore region was also delayed during culmination. Our results strongly suggest that PKA activation during culmination leads to the induction of the expression of srfA. The correct temporal and spatial pattern of srfA expression appears to be part of a mechanism that ensures the adequate coordination of gene expression and morphogenesis.

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

Intracellular and extracellular regulation of ureteric bud morphogenesis

The urinary collecting duct system of the permanent kidney develops by growth and branching of an initially unbranched epithelial tubule, the ureteric bud. Formation of the ureteric bud as an outgrowth of the wolffian duct is induced by signalling molecules (such as GDNF) that emanate from the adjacent metanephrogenic mesenchyme. Once it has invaded the mesenchyme, growth and branching of the bud is controlled by a variety of molecules, such as the growth factors GDNF, HGF, TGFbeta, activin, BMP-2, BMP-7, and matrix molecules such as heparan sulphate proteoglycans and laminins. These various influences are integrated by signal transduction systems inside ureteric bud cells, with the MAP kinase, protein kinase A and protein kinase C pathways appearing to play major roles. The mechanisms of morphogenetic change that produce branching remain largely obscure, but matrix metalloproteinases are known to be necessary for the process, and there is preliminary evidence for the involvement of the actin/myosin contractile cytoskeleton in creating branch points.

Amphibian choroid plexus lipocalin, Cpl1

Choroid plexus lipocalin 1 (Cpl1) has been isolated from the African clawed toad (Xenopus laevis) and the cane toad (Bufo marinus). Xcpl1 has been used as a marker for studying early neural development. Due to its retinoid binding properties and the fact that it causes dysmorphogenesis when overexpressed in the early embryo, the protein product is considered to be part of the retinoic acid signalling pathway. Later in development and during adulthood, the epithelial cell sheet of the choroid plexus which forms the blood-cerebrospinal fluid barrier expresses cpl1 as the predominant secretory protein. These data, the similarity of Cpl1 to prostaglandin D(2) synthase and its functional homology to transthyretin will be discussed.

Endogenous and ectopic gland induction by FGF-10

FGF-10, a member of the fibroblast growth factor family, is expressed in mesodermally derived cell populations during embryogenesis. During normal ocular development, FGF-10 is expressed in the perioptic mesenchyme adjacent to the Harderian and lacrimal gland primordia. In this report, we provide evidence that FGF-10 is both necessary and sufficient to initiate glandular morphogenesis. Lens-specific expression of FGF-10 was sufficient to induce ectopic ocular glands within the cornea. In addition, lacrimal and Harderian glands were not seen in FGF-10 null fetuses. Based on these results we propose that FGF-10 is an inductive signal that initiates ocular gland morphogenesis.

Separation of neural induction and neurulation in Xenopus

Cellular interactions with laminin are important for numerous morphogenetic events. In Xenopus, the first of these is neurulation. The integrin alpha6 subunit mediates an attachment of the cells of the neural plate to the underlying basal lamina. A disruption of this interaction results in embryos that fail to neurulate (T. E. Lallier et al., 1996, Development 122, 2539-2554). Here we provide evidence supporting the specificity of this phenomenon and characterize developmental events as either disrupted or unaffected by a perturbation of alpha6 integrin expression. First, reduction of alpha6 integrin expression does not halt mitotic division throughout the embryo, indicating that the neural defects observed are not simply a global perturbation of all developmental processes. Second, a gene associated with dorsal mesoderm formation, brachyury, is expressed normally in alpha6 integrin-perturbed embryos. Third, the expression of BMP4, noggin, chordin, and follistatin, all of which are critical for neural induction, are at near normal levels. In addition, several genes expressed shortly after neural induction (N-CAM, nrp1, and Xanf1) are not perturbed in nonneurulating embryos. Interestingly, expression of one neural-specific gene (synaptobrevin), which is normally detectable late in neurulation, is abolished in these alpha6 integrin-perturbed embryos. Furthermore, the spatial expression of several transcripts is expanded in alpha6 integrin-perturbed embryos (orthodenticle and engrailed). Taken together, these data indicate that while alpha6 integrin-mediated interactions with laminin are required for neurulation, they are not required for the initial processes of neural induction. However, these cell-extracellular matrix interactions appear to be important in later inductive events and rostrocaudal patterning of the neural tube.

FGF10 is an inducer and Pax6 a competence factor for lacrimal gland development

We investigated the mechanism of tissue induction and specification using the lacrimal gland as a model system. This structure begins its morphogenesis as a bud-like outgrowth of the conjunctival epithelium and ultimately forms a branched structure with secretory function. Using a reporter transgene as a specific marker for gland epithelium, we show that the transcription factor Pax6 is required for normal development of the gland and is probably an important competence factor. In investigating the cell-cell signaling required, we show that fibroblast growth factor (FGF) 10 is sufficient to stimulate ectopic lacrimal bud formation in ocular explants. Expression of FGF10 in the mesenchyme adjacent to the presumptive lacrimal bud and absence of lacrimal gland development in FGF10-null mice strongly suggest that it is an endogenous inducer. This was supported by the observation that inhibition of signaling by a receptor for FGF10 (receptor 2 IIIb) suppressed development of the endogenous lacrimal bud. In explants of mesenchyme-free gland epithelium, FGF10 stimulated growth but not branching morphogenesis. This suggested that its role in induction is to stimulate proliferation and, in turn, that FGF10 combines with other factors to provide the instructive signals required for lacrimal gland development.

Endoderm patterning by the notochord: development of the hypochord in Xenopus

The patterning and differentiation of the vertebrate endoderm requires signaling from adjacent tissues. In this report, we demonstrate that signals from the notochord are critical for the development of the hypochord, which is a transient, endodermally derived structure that lies immediately ventral to the notochord in the amphibian and fish embryo. It appears likely that the hypochord is required for the formation of the dorsal aorta in these organisms. We show that removal of the notochord during early neurulation leads to the complete failure of hypochord development and to the elimination of expression of the hypochord marker, VEGF. Removal of the notochord during late neurulation, however, does not interfere with hypochord formation. These results suggest that signals arising in the notochord instruct cells in the underlying endoderm to take on a hypochord fate during early neural stages, and that the hypochord does not depend on further notochord signals for maintenance. In reciprocal experiments, when the endoderm receives excess notochord signaling, a significantly enlarged hypochord develops. Overall, these results demonstrate that, in addition to patterning neural and mesodermal tissues, the notochord plays an important role in patterning of the endoderm.

casanova plays an early and essential role in endoderm formation in zebrafish

The cellular and molecular mechanisms that regulate endoderm development in vertebrates have only recently begun to be explored. Here we show that the zebrafish locus casanova plays an early and essential role in this process. casanova mutants lack a gut tube and do not express any molecular markers of endoderm differentiation. The early endodermal expression of genes such as axial, gata5, and fkd2 does not initiate in casanova mutants, indicating that the endoderm is defective from the onset of gastrulation. Mosaic analysis demonstrates that casanova functions cell autonomously within the endodermal progenitors. We also report the isolation of a zebrafish homologue of Mixer, a gene important for early endoderm formation in Xenopus. casanova does not encode zebrafish Mixer, and mixer expression is normal in casanova mutants, indicating that casanova acts downstream of, or parallel to, mixer to promote endoderm formation. We further find that the forerunner cells, a specialized group of noninvoluting dorsal mesendodermal cells, do not form in casanova mutants. Studies of casanova mutants do not support an important role for the forerunner cells in either dorsal axis or tail development, as has been previously proposed. In addition, although different populations of mesodermal precursors are generated normally in casanova mutants, morphogenetic defects in the heart, vasculature, blood, and kidney are apparent, suggesting a possible role for the endoderm in morphogenesis of these organs.

Forebrain induction, retinoic acid, and vulnerability to schizophrenia: insights from molecular and genetic analysis in developing mice

Schizophrenia is thought to be a disease of early development that ultimately affects forebrain neurons and circuits. There may be a relationship between disrupted forebrain development; malformations of the limb, face, and heart; and signaling via the steroid-like hormone retinoic acid (RA) in some schizophrenic patients. The limbs, face, heart, and forebrain all develop from sites where neural crest-derived, RA-producing mesenchyme contributes to induction and differentiation of adjacent epithelia. Induction between neural crest-derived, RA-producing mesenchyme, the anterior neural tube, and the anterior surface epithelium of the embryo guides regional differentiation and pathway formation during forebrain development. Furthermore, there are at least two mouse mutations--in the Pax-6 and Gli-3 genes--that cause peripheral malformations and specifically disrupt neural crest mediated, RA-dependent induction and differentiation in the forebrain. These observations suggest that induction might provide a common target for genes that alter morphogenesis of peripheral structures, disrupt RA-signaling, and compromise forebrain development. In the forebrain, some of these disruptions might influence the numbers or cellular properties of neurons and circuits. Such changes might be reflected in the aberrant forebrain function that characterizes schizophrenia.

Reciprocal EGF signaling back to the uterus from the induced C. elegans vulva coordinates morphogenesis of epithelia

BACKGROUND

Reciprocal signaling between distinct tissues is a general feature of organogenesis. Despite the identification of developmental processes in which coordination requires reciprocal signaling, little is known regarding the underlying molecular details. Here, we use the development of the uterine-vulval connection in the nematode Caenorhabditis elegans as a model system to study reciprocal signaling.

RESULTS

In C. elegans, development of the uterine-vulval connection requires the specification of uterine uv1 cells and morphogenesis of 1 degrees -derived vulval cells. LIN-3, an epidermal growth factor (EGF) family protein, is first produced by the gonadal anchor cell to induce vulval precursor cells to generate vulval tissue. We have shown that lin-3 is also expressed in the 1 degrees vulval lineage after vulval induction and that the 1 degrees vulva is necessary to induce the uv1 uterine cell fate. Using genetic and cell biological analyses, we found that the specification of uterine uv1 cells is dependent on EGF signaling from cells of the 1 degrees vulval lineages to a subset of ventral uterine cells of the gonad. RAS and RAF are necessary for this signaling. We also found that EGL-38, a member of the PAX family of proteins, is necessary for transcription of lin-3 in the vulva but not in the anchor cell. A let-23 mutation that confers ligand-independent activity bypasses the requirement for EGL-38 in specification of the uv1 cell fate.

CONCLUSIONS

We have shown how relatively simple EGF signals can be used reciprocally to specify the uterine-vulval connection during C. elegans development.

Mixer, a homeobox gene required for endoderm development

An expression cloning strategy in Xenopus laevis was used to isolate a homeobox-containing gene, Mixer, that can cause embryonic cells to form endoderm. Mixer transcripts are found specifically in the prospective endoderm of gastrula, which coincides with the time and place that endodermal cells become histologically distinct and irreversibly determined. Loss-of-function studies with a dominant inhibitory mutant demonstrate that Mixer activity is required for endoderm development. In particular, the expression of Sox17alpha and Sox17beta, two previously identified endodermal determinants, require Mixer function. Together, these data suggest that Mixer is an embryonic transcription factor involved in specifying the endodermal germ layer.

The development of the kidney

This chapter describes the earlier stages of development of the vertebrate metanephric kidney. It focuses on the mouse and descriptive morphology is used for considering both molecular mechanisms, underpinning kidney morphogenesis and differentiation, and the ways in which these processes can go awry and lead to congenital kidney disorders—particularly in humans. The mature kidney is a fairly complex organ attached to an arterial input vessel and two output vessels, the vein and the ureter. Inside, the artery and vein are connected by a complex network of capillaries that invade a large number of glomeruli, the proximal entrance to nephrons, which are filtration units that link to an arborized collecting-duct system that drains into the ureter. The ability of the kidney and isolated metanephrogenic mesenchyme, to develop in culture means that the developing tissues can be subjected to a wide variety of experimental procedures designed to investigate their molecular and cellular properties and to test hypotheses about developmental mechanisms.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

Organizer induction determines left-right asymmetry in Xenopus

Vertebrates appear bilaterally symmetrical but have considerable left-right (LR) asymmetry in the anatomy and placement of internal organs such as the heart. Although a number of asymmetrically expressed genes are known to affect LR patterning, both the initial source of asymmetry and the mechanism that correctly orients the LR axis remain controversial. In this study, we show that the induction of dorsal organizing centers in the embryo can orient LR asymmetry. Ectopic organizing centers were induced by microinjection of mRNA encoding a variety of body axis duplicating proteins, including members of the Wnt signal transduction pathway. The ectopic and primary body axes form side-by-side conjoined twins, with the secondary axis developing as either the left or right sibling. In all cases, correct LR asymmetry was observed in the left twin, regardless of whether it was derived from the primary axis or induced de novo by injection of Xwnt-8, beta-catenin, or Siamois mRNA. In contrast, the right twin was generally unbiased, regardless of the origin of the left body axis, as seen in many instances of experimentally induced and spontaneous conjoined twins. An unanticipated exception was that right twins induced by beta-catenin and Siamois, two downstream effectors of Wnt signaling, exhibited predominately normal heart looping, even when they formed the right twin. Taken together, these results indicate that LR asymmetry is locally oriented as a consequence of Wnt signaling through beta-catenin and Siamois. We discuss the possibility that signals upstream of beta-catenin and Siamois might be required in order for a right sibling to be randomized.

Hematolymphopoietic and inflammatory cytokines in neural development

It is now clear that cytokines traditionally viewed as immune modulators participate in inflammatory responses within the adult nervous system. However, in the developing nervous system hematolymphopoietic cytokines also play a role unrelated to neural-immune interactions. Instead, many of these factors subserve primary regulatory functions related both to the morphogenesis and to the cellular maturation of the central and peripheral nervous systems. This article focuses specifically on cytokine actions in neural development.

A vegetally localized T-box transcription factor in Xenopus eggs specifies mesoderm and endoderm and is essential for embryonic mesoderm formation

Pattern formation in early embryogenesis is guided by maternal, localized determinants and by inductive interactions between cells. In Xenopus eggs, localized molecules have been identified and some, such as Vg1 and Xwnt-11, can specify cell fates by functioning as inducers or patterning agents. We have used differential screening to identify new Xenopus genes that regulate mesodermal patterning, and we have isolated a new member of the T-box family of transcription factors. This gene, named Brat, is expressed maternally and its transcripts are localized to the vegetal hemisphere of the egg. During early embryonic cleavage, Brat mRNA becomes partitioned primarily within vegetal cells that are fated to form the endoderm. Zygotic expression of Brat begins at the onset of gastrulation within the presumptive mesoderm of the marginal zone. Consistent with its zygotic expression pattern, Brat induces, in a dose-dependent manner, a full spectrum of mesodermal genes that mark tissues across the dorsal-ventral axis, from the blood through the Spemann organizer. Brat also induces endoderm, consistent with its vegetal localization, making Brat a good candidate for a maternal determinant of the endoderm. We tested whether endogenous Brat is required for mesoderm formation by expressing a dominant-negative, transcriptional repressor form of Brat in embryos. This treatment inhibited mesoderm formation and severely disrupted normal development, thereby establishing that Brat plays a critical role in embryonic mesoderm formation and body patterning.

Direct neural induction and selective inhibition of mesoderm and epidermis inducers by Xnr3

During gastrulation in amphibians, secreted factors from Spemann's organizer act on dorsal ectoderm to induce the central nervous system. A number of secreted factors produced by Spemann's organizer have recently been identified. The TGFbeta family member Xnr3 is similar in amino acid sequence to the mouse factor nodal and is expressed in a restricted group of cells in the superficial layer of Spemann's organizer. Xnr3, unlike the related factors nodal, Xnr1 and Xnr2, lacks mesoderm-inducing activity. We report here that Xnr3 can directly induce neural tissue in Xenopus ectoderm explants (animal caps). Injection of animal caps with either Xnr3 RNA or plasmids induces the expression of the pan-neural genes NCAM and nrp1, as well as the anterior neural marker Cpl1. A growing body of evidence suggests that neural induction in Xenopus proceeds as the default in the absence of epidermis inducers. The best candidates for the endogenous epidermis inducers are BMP-4 and BMP-7. The neural inducing activity of Xnr3 can be inhibited by overexpression of BMP-4, as has been observed with the neural inducers noggin, chordin and follistatin. Furthermore, Xnr3 can block mesoderm induction by BMP-4 and activin, but not by Xnr2. The structural basis underlying the divergent activities of Xnr2 and Xnr3 was analyzed using site-directed mutagenesis. Mutations introduced to the conserved cysteine residues characteristic of the TGFbeta family were found to inactivate Xnr2, but not Xnr3. The most unique feature of Xnr3 is the absence of a conserved cysteine at the C terminus of the protein. This feature distinguishes Xnr3 from other TGFbeta family members, including Xnr2. However, we observed that changing the C terminus of Xnr3 to more closely resemble other TGFbeta family members did not significantly alter its activity, suggesting that other structural features of Xnr3 distinguish its biological activity from Xnr2.

Gradient model describes the spatial-temporal expression pattern of Hoxa genes in the developing vertebrate limb

Pattern formation of the developing vertebrate limb is mainly controlled by the zone of polarizing activity (ZPA) and the apical ectodermal ridge (AER) which may act as sources of diffusing morphogens. These sources are tightly interconnected and maintained by positive feedback and, together with the established role of Wnt7 a on the dorsal side of the bud, they constitute a cartesian reference frame for the processes of patterning and growth of the limb bud. As an input to our model we have used the local extent and temporal activity of the AER source as it is reflected by Fgf-4 expression in the ridge. We have assumed that this source produces a morphogen which diffuses in the three-dimensional limb field and degradates by first-order kinetics. When in a cell the morphogen concentration exceeds a particular threshold value, a gene is switched on. To every threshold corresponds a specific gene. In the following we introduce an order of increasing concentration thresholds corresponding to the sequence of Hoxa-10, 11, and 13 genes (threshold collinearity). With this simple rule of correspondence we can reproduce both spatial and temporal collinearities of Hoxa gene expression. This outcome may be the first direct observable effect of a putative morphogen in the developing limb. The expression patterns are essentially transient, and they are followed by sequential refinements which lead to the final limb structures. Furthermore, the continuous flow of the morphogen through the progress zone guarantees the coherent course of patterning and limb growth. Several experiments are proposed for additional tests of the validity of the model and the eventual reversibility of Hoxa gene expression.

The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controlling [correction of controling] dorsoventral patterning of Xenopus mesoderm

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstuck pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.

Induction of nephrogenic mesenchyme by osteogenic protein 1 (bone morphogenetic protein 7)

The definitive mammalian kidney forms as the result of reciprocal interactions between the ureteric bud epithelium and metanephric mesenchyme. As osteogenic protein 1 (OP-1/bone morphogenetic protein 7), a member of the TGF-beta superfamily of proteins, is expressed predominantly in the kidney, we examined its involvement during metanephric induction and kidney differentiation. We found that OP-1 mRNA is expressed in the ureteric bud epithelium before mesenchymal condensation and is subsequently seen in the condensing mesenchyme and during glomerulogenesis. Mouse kidney metanephric rudiments cultured without ureteric bud epithelium failed to undergo mesenchymal condensation and further epithelialization, while exogenously added recombinant OP-1 was able to substitute for ureteric bud epithelium in restoring the induction of metanephric mesenchyme. This OP-1-induced nephrogenic mesenchyme differentiation follows a developmental pattern similar to that observed in the presence of the spinal cord, a metanephric inducer. Blocking OP-1 activity using either neutralizing antibodies or antisense oligonucleotides in mouse embryonic day 11.5 mesenchyme, cultured in the presence of metanephric inducers or in intact embryonic day 11.5 kidney rudiment, greatly reduced metanephric differentiation. These results demonstrate that OP-1 is required for metanephric mesenchyme differentiation and plays a functional role during kidney development.

An ascidian homologue of vertebrate BMPs-5-8 is expressed in the midline of the anterior neuroectoderm and in the midline of the ventral epidermis of the embryo

The ascidian tadpole larva is thought to be the prototype for the ancestral chordate. Although ascidians show a highly determinate mode of development, recent studies suggest significant roles of cell-cell interaction during embryogenesis. To elucidate the signaling molecules responsible for the cellular interaction, we investigated an ascidian homologue of the transforming growth factor beta (TGF-beta) superfamily. HrBMPa is an ascidian member of the 60A subclass of the BMP subfamily. Molecular phylogenetic analysis suggested that HrBMPa branched prior to further divergence of vertebrate BMPs-5-8. The zygotic expression of HrBMPa was initiated around gastrulation. HrBMPa transcripts were first evident in precursor cells of the spinal cord, notochord, epidermis and nervous system, although signals in the first two regions quickly disappeared. In neurulae and early tailbud embryos, transcripts were evident in the adhesive organ, midline of the anterior dorsal neuroectoderm and midline of both ventral and dorsal ectoderm, suggesting that HrBMPa plays a major role in neuroectodermal cell differentiation during embryogenesis. This HrBMPa expression profile resembled that of Xenopus BMP-7, implying a primordial function of BMP-7 among vertebrate BMPs-5-8.

Notochord signals control the transcriptional cascade of myogenic bHLH genes in somites of quail embryos

Microsurgical, tissue grafting and in situ hybridization techniques have been used to investigate the role of the neural tube and notochord in the control of the myogenic bHLH genes, QmyoD, Qmyf5, Qmyogenin and the cardiac alpha-actin gene, during somite formation in stage 12 quail embryos. Our results reveal that signals from the axial neural tube/notochord complex control both the activation and the maintenance of expression of QmyoD and Qmyf5 in myotomal progenitor cells during the period immediately following somite formation and prior to myotome differentiation. QmyoD and Qmyf5 expression becomes independent of axial signals during myotome differentiation when somites activate expression of Qmyogenin and alpha-actin. Ablation studies reveal that the notochord controls QmyoD activation and the initiation of the transcriptional cascade of myogenic bHLH genes as epithelial somites condense from segmental plate mesoderm. The dorsal medial neural tube then contributes to the maintenance of myogenic bHLH gene expression in newly formed somites. Notochord grafts can activate ectopic QmyoD expression during somite formation, establishing that the notochord is a necessary and sufficient source of diffusible signals to initiate QmyoD expression. Myogenic bHLH gene expression is localized to dorsal medial cells of the somite by inhibitory signals produced by the lateral plate and ventral neural tube. Signaling models for the activation and maintenance of myogenic gene expression and the determination of myotomal muscle in somites are discussed.

TGF-beta signals and a pattern in Xenopus laevis endodermal development

We have analyzed two gene products expressed in the early endoderm of Xenopus laevis: Xlhbox-8, a pancreas-specific transcription factor and intestinal fatty acid binding protein (IFABP), a marker of small intestinal epithelium. Expression of the pancreas marker relies on cell signaling mediated by both the TGF-beta and FGF classes of secreted peptide growth factors, whereas, expression of the more posterior small intestinal marker does not. Endodermal explants devoid of mesoderm express both markers in a regionalized manner. Cortical rotation is required for the expression of the more anterior marker, Xlhbox-8, but not for the small intestinal marker, IFABP. These findings suggest that endodermal patterning is dependent, in part, on the same events and signals known to play important roles in mesodermal development. Furthermore, inhibition of TGF-beta signaling in the endoderm leads to ectopic expression of both mesodermal and ectodermal markers, suggesting the TGF-beta signaling may play a general role in the segregation of the three embryonic germ layers.

An extracellular matrix molecule that is selectively expressed during development is important for gastrulation in the sea urchin embryo

The extracellular matrix is important in the regulation of many cellular events of early development including migration, shape change, proliferation and gene expression. In the sea urchin embryo, disruption of the extracellular matrix results in selective defects in each of these events during gastrulation. Here we describe a new molecule of the extracellular matrix in Lytechinus variegatus, referred to as ECM 18, that has several important features. First, antibody interference of ECM 18 results in a profound but reversible inhibition of primary mesenchyme cell organization and endoderm morphogenesis during gastrulation. Second, during gastrulation, ECM 18 mRNA accumulates to highest levels in the invaginating endoderm and the ECM 18 protein deposited in the basal lamina surrounding the archenteron as well as in other areas of the blastocoel wall. Immunolocalization by fluorescence and electron microscopy demonstrates the selective accumulation of ECM 18 in the extracellular matrix. Third, although the mRNA encoding ECM 18 is present throughout development, the protein accumulates only during gastrulation. ECM 18 protein is not detected in eggs or early embryos and analysis of polysome-associated mRNA suggests that at least part of the translational regulation of ECM 18 is at the level of ECM 18 mRNA-polysome formation. Finally, sequence analysis of ECM 18 shows that the protein contains a repeat sequence with a conserved cysteine motif, suggestive of involvement in protein-protein interactions. Thus, ECM 18 appears to be important in mediating select morphogenetic changes during gastrulation and the pattern of its expression in the embryo is unique among the extracellular matrix molecules known in this embryo.

The Xenopus homologue of hepatocyte growth factor-like protein is specifically expressed in the presumptive neural plate during gastrulation

Using a RT-PCR approach, we were able to isolate a cDNA encoding the Xenopus homologue of hepatocyte growth factor-like protein, which we have termed accordingly Xhl. The deduced Xhl protein consists of 717 amino acids, contains four putative kringle domains and a serine protease-like domain characteristic for mammalian HGF and HGF-like protein. The mRNA of Xhl is exclusively expressed in the midline of the prospective neural plate during the period of neural induction, only. Ectopic expression of Xhl causes a 'spina bifida'-like phenotype with enlargement of neural tissue. Activation of Xhl mRNA transcription can be induced by delayed reaggregation of animal caps and appears to require vertical rather than planar signals from the organizer. These data suggest that Xhl is involved in the formation of the embryonic nervous system of Xenopus.

Antagonizing the Spemann organizer: role of the homeobox gene Xvent-1

We have identified a novel homeobox gene, Xvent-1, that is differentially expressed in the ventral marginal zone of the early Xenopus gastrula. Evidence is presented from mRNA microinjection experiments for a role for this gene in dorsoventral patterning of mesoderm. First, Xvent-1 is induced by BMP-4, a gene known to be a key regulator of ventral mesoderm development. Second, Xvent-1 and the organizer-specific gene goosecoid are able to interact, directly or indirectly, in a cross-regulatory loop suppressing each other's expression, consistent with their mutually exclusive expression in the marginal zone. Third, microinjection of Xvent-1 mRNA ventralizes dorsal mesoderm. The results suggest that Xvent-1 functions in a ventral signaling pathway that maintains the ventral mesodermal state and antagonizes the Spemann organizer.