Experimental control of axial pattern in the chick blastoderm by local expression of Wnt and activin: the role of HNK-1 positive cells

Pluripotent stem cells for skeletal tissue engineering

Here, we review the use of human pluripotent stem cells for skeletal tissue engineering. A number of approaches have been used for generating cartilage and bone from both human embryonic stem cells and induced pluripotent stem cells. These range from protocols relying on intrinsic cell interactions and signals from co-cultured cells to those attempting to recapitulate the series of steps occurring during mammalian skeletal development. The importance of generating authentic tissues rather than just differentiated cells is emphasized and enabling technologies for doing this are reported. We also review the different methods for characterization of skeletal cells and constructs at the tissue and single-cell level, and indicate newer resources not yet fully utilized in this field. There have been many challenges in this research area but the technologies to overcome these are beginning to appear, often adopted from related fields. This makes it more likely that cost-effective and efficacious human pluripotent stem cell-engineered constructs may become available for skeletal repair in the near future.

Establishment of the Vertebrate Germ Layers

The process of germ layer formation is a universal feature of animal development. The germ layers separate the cells that produce the internal organs and tissues from those that produce the nervous system and outer tissues. Their discovery in the early nineteenth century transformed embryology from a purely descriptive field into a rigorous scientific discipline, in which hypotheses could be tested by observation and experimentation. By systematically addressing the questions of how the germ layers are formed and how they generate overall body plan, scientists have made fundamental contributions to the fields of evolution, cell signaling, morphogenesis, and stem cell biology. At each step, this work was advanced by the development of innovative methods of observing cell behavior in vivo and in culture. Here, we take an historical approach to describe our current understanding of vertebrate germ layer formation as it relates to the long-standing questions of developmental biology. By comparing how germ layers form in distantly related vertebrate species, we find that highly conserved molecular pathways can be adapted to perform the same function in dramatically different embryonic environments.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Left-right patterning from the inside out: widespread evidence for intracellular control

The field of left-right (LR) patterning--the study of molecular mechanisms that yield directed morphological asymmetries in otherwise symmetrical organisms--is in disarray. On one hand is the undeniably elegant hypothesis that rotary beating of inclined cilia is the primary symmetry-breaking step: they create an asymmetric extracellular flow across the embryonic midline. On the other hand lurk many early symmetry-breaking steps that, even in some vertebrates, precede the onset of ciliary flow. We highlight an intracellular model of LR patterning where gene expression is initiated by physiological asymmetries that arise from subcellular asymmetries (e.g. motor-protein function along oriented cytoskeletal tracks). A survey of symmetry breaking in eukaryotes ranging from protists to vertebrates suggests that intracellular cytoskeletal elements are ancient and primary LR cues. Evolutionarily, quirky effectors like ciliary motion were likely added later in vertebrates. In some species (like mice), developmentally earlier cues may have been abandoned entirely. Late-developing asymmetries pose a challenge to the intracellular model, but early mid-plane determination in many groups increases its plausibility. Multiple experimental tests are possible.

Sulfated HNK-1 Epitope in Developing and Mature Kidney: A New Marker for Thin Ascending Loop of Henle and Tubular Injury in Acute Tubular Necrosis

The HNK-1 carbohydrate epitope is a 3-sulfo-glucuronyl residue attached to lactosamine structures on glycoproteins, proteoglycans, or glycolipids mostly expressed in the nervous system. Here, using monoclonal antibodies against the sulfated HNK-1 carbohydrate epitope, we first examined its distribution in developing and adult kidneys, then its expression in kidneys with tubular necrosis and renal neoplasms. This HNK-1 epitope was expressed in the human, rabbit, and rat, but not mouse kidney. It was detected within a subset of epithelial cells in the renal vesicle and in comma- and S-shaped bodies during early stages of nephrogenesis. In ureteral bud derivatives, the epitope was present transiently in the area where the collecting duct fused with the nephron. In the adult kidney, expression of the HNK-1 epitope became mainly restricted to the thin ascending loop of Henle where this epitope was carried by heparan- and chondro-proteoglycan. In pathological conditions, HNK-1 epitope expression increased dramatically in proximal epithelial tubule cells in kidneys with acute tubular necrosis. In tumors, the HNK-1 epitope was expressed in the epithelial component of nephroblastomas and in a subgroup of papillary renal cell carcinomas. These data suggest that molecules carrying the sulfated HNK-1 carbohydrate epitope may play an important role in critical stages of renal development and in the physiology of thin ascending loop of Henle. (J Histochem Cytochem 54:575-584, 2006)

Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development

Epithelial-mesenchymal transitions (EMTs) are an important mechanism for reorganizing germ layers and tissues during embryonic development. They have both a morphogenic function in shaping the embryo and a patterning function in bringing about new juxtapositions of tissues, which allow further inductive patterning events to occur [Genesis 28 (2000) 23]. Whereas the mechanics of EMT in cultured cells is relatively well understood [reviewed in Biochem. Pharmacol. 60 (2000) 1091; Cell 105 (2001) 425; Bioessays 23 (2001) 912], surprisingly little is known about EMTs during embryonic development [reviewed in Acta Anat. 154 (1995) 8], and nowhere is the entire process well characterized within a single species. Embryonic (developmental) EMTs have properties that are not seen or are not obvious in culture systems or cancer cells. Developmental EMTs are part of a specific differentiative path and occur at a particular time and place. In some types of embryos, a relatively intact epithelium must be maintained while some of its cells de-epithelialize during EMT. In most cases de-epithelialization (loss of apical junctions) must occur in an orderly, patterned fashion in order that the proper morphogenesis results. Interestingly, we find that de-epithelialization is not always necessarily tightly coupled to the expression of mesenchymal phenotypes.Developmental EMTs are multi-step processes, though the interdependence and obligate order of the steps is not clear. The particulars of the process vary between tissues, species, and specific embryonic context. We will focus on 'primary' developmental EMTs, which are those occurring in the initial epiblast or embryonic epithelium. 'Secondary' developmental EMT events are those occurring in epithelial tissues that have reassembled within the embryo from mesenchymal cells. We will review and compare a number of primary EMT events from across the metazoans, and point out some of the many open questions that remain in this field.

Peri-gastrulation development of the dasyurid marsupial Sminthopsis macroura (stripe-faced dunnart) in vitro and evidence for patterning of the epiblast prior to gastrulation

Marsupials are potentially excellent models for the study of gastrulation because of their superficial embryonic area (EA), post-gastrulation implantation and their potential to provide information about the evolution of gastrulation. Very few studies have examined this developmental period in marsupials. Using an established developmental timetable, peri-gastrula stage Sminthopsis macroura blastocysts were collected and described in detail by observations on live blastocysts and by the use of histological and immunohistochemical techniques on fixed blastocysts. Gastrulation in S. macroura shares several aspects common to that of both eutherian mammals and birds, but in terms of tissue arrangement and conceptus form, is more similar to the chick than other mammals. Two methods of culturing EA explants flat without their shell were devised. The techniques will markedly increase the number of possible experimental manipulations, which previously were limited by the presence of blastocyst investments. Exposure of fractions of explants of round, morphologically uniform pre-gastrula stage EA to growth factors or signaling molecules implicated in vertebrate gastrulation suggests that like the chick and mouse, the marsupial epiblast is patterned prior to gastrulation. Of all factors tested, basic fibroblast growth factor (bFGF) had the most prominent effect, promoting cell differentiation, and possible mesoderm formation. Data from explant culture suggests that similar to the chick and mouse, limited specification precedes the onset of gastrulation.

Positional information by Rauber's sickle and a new look at the mechanisms of primitive streak initiation in avian blastoderms

The present experimental in vitro study suggests that a primitive streak (PS) in avian blastoderms is induced by diffusion of morphogenetic substances emanating from Rauber's sickle. Indeed, even without direct contact between a quail Rauber's sickle and the reacting upper layer (by interposition of a vitelline membrane), a PS can be induced in the isolated area centralis or antisickle region of unincubated chicken blastoderms. The so-formed PSs are localized below the vitelline membrane in the immediate neighborhood of the apposed Rauber's sickle material. This seems to indicate that Rauber's sickle organizes the formation of the avian PS according to the basic concept of "positional information." The morphogenetic substances seem to have an effect only on the formation of a PS. Each part of Rauber's sickle seems to have, point by point, the same thickening and PS-inducing effect on each corresponding part of the underlying upper layer (UL). By a mechanism of sliding over the basement membrane and fusion, this finally results in the formation of one single median PS. Our study shows that a PS can be induced in the total absence of hypoblast (sickle endoblast) or caudal marginal zone, by only the presence of Rauber's sickle material. In contrast, the differentiation of mesoblast into blood islands under the influence of Rauber's sickle and neural tissue development are impaired by the interposition of a vitelline membrane. The latter could be due to the absence of a normal interaction of Rauber's sickle-derived sickle endoblast with endophyll and/or upper layer and the absence of cranial migration of the mesoblast. Thus, earlier studies and the present study indicate the existence of a temporospatially bound cascade of gastrulation and neurulation phenomena and blood island formation in the avian blastoderm, starting from Rauber's sickle, the primary major organizer with inducing, inhibiting, and dominating potencies. The latter not only plays a role by secretion of signaling molecules, but also influences development by its cell lineages (junctional endoblast and sickle endoblast).

Interactions between Wnt and Vg1 signalling pathways initiate primitive streak formation in the chick embryo

The posterior marginal zone (PMZ) of the chick embryo has Nieuwkoop centre-like properties: when transplanted to another part of the marginal zone, it induces a complete embryonic axis, without making a cellular contribution to the induced structures. However, when the PMZ is removed, the embryo can initiate axis formation from another part of the remaining marginal zone. Chick Vg1 can mimic the axis-inducing ability of the PMZ, but only when misexpressed somewhere within the marginal zone. We have investigated the properties that define the marginal zone as a distinct region. We show that the competence of the marginal zone to initiate ectopic primitive streak formation in response to cVg1 is dependent on Wnt activity. First, within the Wnt family, only Wnt8C is expressed in the marginal zone, in a gradient decreasing from posterior to anterior. Second, misexpression of Wnt1 in the area pellucida enables this region to form a primitive streak in response to cVg1. Third, the Wnt antagonists Crescent and Dkk-1 block the primitive streak-inducing ability of cVg1 in the marginal zone. These findings suggest that Wnt activity defines the marginal zone and allows cVg1 to induce an axis. We also present data suggesting some additional complexity: first, the Vg1 and Wnt pathways appear to regulate the expression of downstream components of each other’s pathway; and second, misexpression of different Wnt antagonists suggests that different classes of Wnts may cooperate with each other to regulate axis formation in the normal embryo.

New insights into critical events of avian gastrulation

The formation and progression of the primitive streak are key events of avian gastrulation. We examine these processes in detail, using various morphological approaches. We show that formation of the primitive streak occurs locally at the caudal midline of the area pellucida, as cells in the caudal midline undergo an epithelial-to-mesenchymal transformation, and that extensive migration of delaminated cells arising from more rostral or peripheral areas of the blastoderm is not involved in streak formation. Instead, such delamination occurs earlier and is restricted to the process of hypoblast formation. Moreover, we provide evidence that progression of the primitive streak involves two processes: convergent-extension movements within the streak per se, and progressive delamination of midline epiblast cells in a caudal-to-rostral sequence. We have identified a subpopulation of primitive-streak cells located at its dorsal midline surface that undergoes extensive rostral displacement concomitant with streak progression. The fact that these cells are located only dorsally and do not elongate ventrally as do adjacent ingressing cells, suggests that these cells retain their residency within the primitive streak, at least until regression of the primitive streak occurs. Finally, by following labeled cells over time we establish the timing of movement of epiblast cells toward and into the primitive streak, providing direct evidence that cell-cell intercalation occurs within the primitive streak during its progression. Collectively, our results provide new insight into complex and central events of avian gastrulation.

A model of primitive streak initiation in the chick embryo

Initiation of the primitive streak in avian embryos provides a well-studied example of a pattern-forming event that displays a striking capacity for regulation. The mechanisms underlying the regulative properties are, however, poorly understood and are not easily accounted for by traditional models of pattern formation, such as reaction-diffusion models. In this paper, we propose a new activator-inhibitor model for streak initiation. We show that the model is consistent with experimental observations, both in its pattern-forming properties and in its ability to form these patterns on the correct time-scales for biologically realistic parameter values. A key component of the model is a travelling wave of inhibition. We present a mathematical analysis of the speed of such waves in both diffusive and juxtacrine relay systems. We use the streak initiation model to make testable predictions. By varying parameters of the model, two very different types of patterning can be obtained, suggesting that our model may be applicable to other processes in addition to streak initiation.

Regulation of epiblast cell movements by chondroitin sulfate during gastrulation in the chick

Avian gastrulation is dependent on the ingression of outer layer cells into the interior of the embryo by means of a transient structure referred to as the primitive streak. As the growing streak progresses through the central area pellucida of the blastoderm, selective de-epithelialization of epiblast cells results in the initial migratory cells of the primitive mesoderm and endoderm. Here, we have examined the possibility that extracellular matrix molecules of the epiblast basal lamina influence the selection of streak-specific epiblast cells. By using whole embryo culture, we have found that removal of chondroitin sulfate glycosaminoglycans at gastrulation stages leads to defective streak formation. In situ hybridization with streak-specific markers in these embryos reveals ectopic patterns of gene expression, suggesting that differentiation of primitive streak precursors in the pregastrula epiblast is independent of normal streak morphogenesis. In addition, in vitro assays with chondroitin sulfate containing matrices suggest that specific cells of the epiblast are inhibited from joining the streak during gastrulation. Taken together, these results indicate that the presence of chondroitin sulfate in the epiblast basal lamina facilitates the allocation of cells to the primary germ layers by preventing ectopic axis formation.

Hepatocyte growth factor/scatter factor promotes a switch from E- to N-cadherin in chick embryo epiblast cells

Epiblast cells downregulate E-cadherin and upregulate N-cadherin as they ingress through the primitive streak and when placed in culture. The factors that promote the alteration in cadherin expression during gastrulation are unknown. The effects of hepatocyte growth factor/scatter factor (HGF/SF) on cadherin expression were tested in cultures of prestreak epiblast cells. HGF/SF decreased the expression of E-cadherin and increased the percentage of cells with N-cadherin and sarcomeric myosin. Cells with N-cadherin but not E-cadherin differentiated into skeletal muscle. HGF/SF also stimulated proliferation and the formation of cellular aggregates. Sensitivity to HGF/SF in vitro depended on the original position of cells within the epiblast. More cells from the lateral epiblast switched cadherins and proliferated in response to HGF/SF than medial epiblast cells. HGF/SF may affect gastrulation by altering cadherin expression, modulating cell adhesion, and stimulating proliferation within the epiblast.

Establishment of anterior-posterior polarity in avian embryos

In pre-streak chick embryos, the extraembryonic posterior marginal zone is able to induce an embryonic axis at an ectopic site without contributing cells to the induced primitive streak. This region expresses mesoderm-inducing factors that are capable of inducing an ectopic streak. Downstream of these events, chordin and bone morphogenetic protein acting within the central disc may play mutually opposing roles influencing streak formation. Although extraembryonic regions are important in establishing the embryonic axis, there does not appear to be an anterior region with head-inducing activity similar to that of the anterior visceral endoderm of the mammalian embryo.

Rearranging gastrulation in the name of yolk: evolution of gastrulation in yolk-rich amniote eggs

Gastrulating birds and mammals form a primitive streak in lieu of a circular blastopore, and a conspicuous underlying tissue layer, the hypoblast. In an attempt to understand the evolution of these amniote characteristics, pregastrula and gastrulation stages in selected amniotes are compared with the more ancestral situation in amphibians. At blastula/blastoderm stages, the overall fate maps and the arrangement of tissues around the organizer are rather similar, as is exemplified by a comparison of gene expression and fate maps in the frog and chick. Compared with amphibians, however, the eggs of reptiles, birds and monotreme mammals have a disproportionately large yolk that alters gastrulation morphology. During amphibian gastrulation, the organizer moves from anterior to posterior, to lay down the dorsal axis around the vegetal hemisphere (Arendt, D., Nübler-Jung, K., 1997. Dorsal or ventral: similarities in fate maps and gastrulation patterns in annelids, arthropods and chordates. Mech. Dev. 61, 1-15). In contrast, in amniote eggs, the large yolk impedes the organizer from moving around the entire vegetal hemisphere so that axis formation begins and ends at the same side of the egg. This has apparently provoked an evolutionary transformation of an amphibian-like blastopore, first into the 'blastoporal canal' of reptiles, and then into the birds' and mammals' primitive streak. The blastopore divides into two functionally divergent parts, one as the site of mesoderm internalization ('intraembryonic blastopore') and the other as the site of ectodermal epiboly ('extraembryonic blastopore'). The hypoblast is proposed to derive from the 'endodermal wedge' that is seen already in the amphibian gastrula. Hypoblast formation would then represent a special kind of gastrulation movement that also exists in the amphibians, and for which the term 'hypoboly' is introduced.

Induction of primitive streak and Hensen's node by the posterior marginal zone in the early chick embryo

In the preprimitive streak chick embryo, the search for a region capable of inducing the organizer, equivalent to the Nieuwkoop Center of the amphibian embryo, has focused on Koller's sickle, the hypoblast and the posterior marginal zone. However, no clear evidence for induction of an organizer without contribution from the inducing tissue has been provided for any of these structures. We have used DiI/DiO labeling to establish the fate of midline cells in and around Koller's sickle in the normal embryo. In the epiblast, the boundary between cells that contribute to the streak and those that do not lies at the posterior edge of Koller's sickle, except at stage X when it lies slightly more posteriorly in the epiblast. Hypoblast and endoblast (a second lower layer formed under the streak) have distinct origins in the lower layer, and goosecoid expression distinguishes between them. We then used anterior halves of chick prestreak embryos as recipients for grafts of quail posterior marginal zone; quail cells can be identified subsequently with a quail-specific antibody. Anterior halves alone usually formed a streak, most often from the posterior edge. Quail posterior marginal zones without Koller's sickle were grafted to the anterior side of anterior halves. These grafts were able to increase significantly the frequency of streaks arising from the anterior pole of stage X-XI anterior halves without contributing to the streak or node. Stage XII anterior halves no longer responded. A goosecoid-expressing hypoblast did not form under the induced streak, indicating that it is not required for streak formation. We conclude that the marginal zone posterior to Koller's sickle can induce a streak and node, without contributing cells to the induced streak.

Chordin regulates primitive streak development and the stability of induced neural cells, but is not sufficient for neural induction in the chick embryo

We have investigated the role of Bone Morphogenetic Protein 4 (BMP-4) and a BMP antagonist, chordin, in primitive streak formation and neural induction in amniote embryos. We show that both BMP-4 and chordin are expressed before primitive streak formation, and that BMP-4 expression is downregulated as the streak starts to form. When BMP-4 is misexpressed in the posterior area pellucida, primitive streak formation is inhibited. Misexpression of BMP-4 also arrests further development of Hensen's node and axial structures. In contrast, misexpression of chordin in the anterior area pellucida generates an ectopic primitive streak that expresses mesoderm and organizer markers. We also provide evidence that chordin is not sufficient to induce neural tissue in the chick. Misexpression of chordin in regions outside the future neural plate does not induce the early neural markers L5, Sox-3 or Sox-2. Furthermore, neither BMP-4 nor BMP-7 interfere with neural induction when misexpressed in the presumptive neural plate before or after primitive streak formation. However, chordin can stabilise the expression of early neural markers in cells that have already received neural inducing signals. These results suggest that the regulation of BMP signalling by chordin plays a role in primitive streak formation and that chordin is not sufficient to induce neural tissue.

Misexpression of chick Vg1 in the marginal zone induces primitive streak formation

In the chick embryo, the primitive streak is the first axial structure to develop. The initiation of primitive streak formation in the posterior area pellucida is influenced by the adjacent posterior marginal zone (PMZ). We show here that chick Vg1 (cVg1), a member of the TGFbeta family of signalling molecules whose homolog in Xenopus is implicated in mesoderm induction, is expressed in the PMZ of prestreak embryos. Ectopic expression of cVg1 protein in the marginal zone chick blastoderms directs the formation of a secondary primitive streak, which subsequently develops into an ectopic embryo. We have used cell marking techniques to show that cells that contribute to the ectopic primitive streak change fate, acquiring two distinct properties of primitive streak cells, defined by gene expression and cell movements. Furthermore, naive epiblast explants exposed to cVg1 protein in vitro acquire axial mesodermal properties. Together, these results show that cVg1 can mediate ectopic axis formation in the chick by inducing new cell fates and they permit the analysis of distinct events that occur during primitive streak formation.

Left/right patterning signals and the independent regulation of different aspects of situs in the chick embryo

Recently, a pathway of genes which are part of a cascade regulating the side on which the heart forms during chick development was characterized (M. Levin et al., 1995, Cell 82, 1-20). Here we extend these previous studies, showing that manipulation of at least one member of the cascade, Sonic hedgehog (Shh), can affect the situs of embryonic rotation and of the gut, in addition to the heart. Bilateral expression of Shh, which is normally found exclusively on the left, does not result in left isomerism (a bilaterally symmetrical embryo having two left sides) nor in a complete situs inversus phenotype. Instead, misexpression of Shh on the right side of the node, which in turn leads to bilateral nodal expression, produces a heterotaxia-like condition, where different aspects of laterality are determined independently. Heart situs has previously been shown to be altered by ectopic Shh and activin. However, the most downstream gene identified in the LR pathway, nodal, had not been functionally linked to heart laterality. We show that ectopic (right-sided) nodal expression is able to affect heart situs, suggesting that the randomization of heart laterality observed in Shh and activin misexpression experiments is a result of changes in nodal expression and that nodal is likely to regulate heart situs endogenously. The first defined asymmetric signal in the left-right patterning pathway is Shh, which is initially expressed throughout Hensen's node but becomes restricted to the left side at stage 4(+). It has been hypothesized that the restriction of Shh expression may be due to repression by an upstream activin-like factor. The involvement of such an activin-like factor on the right side of Hensen's node was suggested because ectopic activin protein is able to repress Shh on the left side of the node, as well as to induce ectopic expression of a normally right-sided marker, the activin receptor cAct-RIIa. Here we provide further evidence in favor of this model. We find that a member of this family, Activin betaB, is indeed expressed asymmetrically, only on the right side of Hensen's node, at the correct time for it to be the endogenous asymmetric activin signal. Furthermore, we show that application of follistatin-loaded beads eliminates the asymmetry in Shh expression, consistent with an inhibition of an endogenous member of the activin-BMP superfamily. This combined with the previous data on exogenous activin supports the model that Activin betaB functions in the chick embryo to initiate Shh asymmetry. While these data extend our understanding of the early signals which establish left-right asymmetry, they leave unanswered the interesting question of how the bilateral symmetry of the embryo is initially broken to define a consistent left-right axis. Analysis of spontaneous chick twins suggests that, whatever the molecular mechanism, left-right patterning is unlikely to be due to a blastodermal prepattern but rather is initiated in a streak-autonomous manner.

Gastrulation and homeobox genes in chick embryos

We review the early stages of chick embryogenesis, in particular the formation of the hypoblast, and the ingression of endoderm and mesoderm through the primitive streak. The formation of a trilaminar embryo during gastrulation is accompanied by the specification of body axes. The first axis is already present in the unfertilized egg and runs from the cytoplasmatic animal to the yolk rich vegetal pole. Already within the uterus a second axis conveys bilateral symmetry to the embryo. It extends from a dorsal/anterior to a ventral/posterior position. These axial poles segregate during gastrulation to form the classical coordinates, a dorsal-ventral and an anterior-posterior axis. The establishment of axes is accompanied by the expression of specific combinations of homeobox genes during gastrulation in the chick, as in other metazoa. We review the avian specific information and compare it with findings in other species. A combinatorial homeobox code for the specification of identities during development is discussed.

Early developmental expression and experimental axis determination by the chicken Vg1 gene

BACKGROUND

Genes of the transforming growth factor beta (TGF beta) superfamily have been implicated in the earliest steps of developmental patterning in vertebrates. In Xenopus, the Vg1 gene is a candidate for the initiator of axis formation: its RNA and protein are broadly but appropriately localized at the start of development, and processed Vg1 protein is a powerful inducer of organized axial tissue in blastular animal caps in vitro and when locally produced in vivo after injection of Vg1 mRNA into blastomeres. Site-specific proteolytic processing occurs ubiquitously for most TGF beta members, producing the active peptide ligand, but is tightly restricted, by unknown mechanisms, for endogenous Vg protein in Xenopus and zebrafish embryos. This restriction may be involved in the spatial localization of activity required for an organizing role.

RESULTS

We have characterized an amniote (chick) orthologue of Vg1, cVg1, and examined its developmental expression. The early expression of cVg1 includes a phase broadly related to the known time and site of axis (primitive streak) initiation; the initial transcription of cVg1 is centred in the posterior marginal zone (PMZ), a region of the blastoderm known to contain the axial organizing activity at this stage. We also observed later neural and paraxial mesodermal expression of cVg1, which has not been described previously for Vg homologues in other vertebrates. We have grafted transfected COS cells, producing processed cVg1 protein, to peripheral positions around the chick early blastoderm. Such grafts initiate formation of morphologically complete primitive streaks, simulating the properties of grafts from the PMZ.

CONCLUSIONS

In vertebrate development, Vg genes may be required for an evolutionarily conserved early step in positioning or induction of the axis.