Changes in the expression of the carbohydrate epitope HNK-1 associated with mesoderm induction in the chick embryo

Spatiotemporal expression pattern of the chicken glucocorticoid receptor during early embryonic development

Glucocorticoids - commonly known as stress hormones - belong to the family of steroid hormones and regulate numerous life essential physiological processes. As lipophilic molecules, glucocorticoids are known to cross the placental barrier in mammals, which - applied for therapeutic reasons or arising from environmental influences - illustrates the role of prenatal stress during embryonic developmental processes. The hormones employ their functions by binding to the glucocorticoid receptor (GR) and thus are involved in regulating the transcription of thousands of genes. Therefore, the aim of this study was to investigate the spatiotemporal expression pattern of the GR during early embryonic vertebrate development, using the chicken embryo as a model organism. The results should contribute to enhance and expand the current understanding of glucocorticoid signaling. By performing in-situ hybridization on whole mount chicken embryos from stage HH10 to HH29 and analyzing vibratome sections of hybridized embryos, we described the spatiotemporal expression pattern of the GR during early embryogenesis. Moreover, we compared the expression pattern of the GR with other developmental markers such as Pax7, Desmin, MyoD and HNK-1 using double in-situ hybridization and immunohistochemistry. We were able to determine the first emergence of GR expression in stage HH13 of chicken development in the cranial area, especially in the muscle anlagen of the branchial arches and of non-somitic neck muscles. Furthermore, we monitored the extension of GR expression pattern throughout later stages and found transcripts of GR during somitogenesis, limb development, myogenesis, neurulation and neural differentiation and moreover during organogenesis of the gastrointestinal organs, the heart, the kidneys and the lungs. Toward later stages, GR expression transitioned from more distinct areas of expression to an increasingly ubiquitous expression pattern. Our results support the notion of an enormous relevance of glucocorticoid signaling during vertebrate embryonic development and contribute to a better understanding of the consequences of prenatal stress and the clinical administration of prenatal glucocorticoids.

Cellular processes driving gastrulation in the avian embryo

Gastrulation consists in the dramatic reorganisation of the epiblast, a one-cell thick epithelial sheet, into a multilayered embryo. In chick, the formation of the internal layers requires the generation of a macroscopic convection-like flow, which involves up to 50,000 epithelial cells in the epiblast. These cell movements locate the mesendoderm precursors into the midline of the epiblast to form the primitive streak. There they acquire a mesenchymal phenotype, ingress into the embryo and migrate outward to populate the inner embryonic layers. This review covers what is currently understood about how cell behaviours ultimately cause these morphogenetic events and how they are regulated. We discuss 1) how the biochemical patterning of the embryo before gastrulation creates compartments of differential cell behaviours, 2) how the global epithelial flows arise from the coordinated actions of individual cells, 3) how the cells delaminate individually from the epiblast during the ingression, and 4) how cells move after the ingression following stereotypical migration routes. We conclude by exploring new technical advances that will facilitate future research in the chick model system.

Signaling events regulating embryonic polarity and formation of the primitive streak in the chick embryo

The avian embryo is a key experimental model system for early development of amniotes. One key difference with invertebrates and "lower" vertebrates like fish and amphibians is that amniotes do not rely so heavily on maternal messages because the zygotic genome is activated very early. Early development also involves considerable growth in volume and mass of the embryo, with cell cycles that include G1 and G2 phases from very early cleavage. The very early maternal to zygotic transition also allows the embryo to establish its own polarity without relying heavily on maternal determinants. In many amniotes including avians and non-rodent mammals, this enables an ability of the embryo to "regulate": a single multicellular embryo can give rise to more than one individual-monozygotic twins. Here we discuss the embryological, cellular, molecular and evolutionary underpinnings of gastrulation in avian embryos as a model amniote embryo. Many of these properties are shared by human embryos.

Transcriptome analysis of chicken ES, blastodermal and germ cells reveals that chick ES cells are equivalent to mouse ES cells rather than EpiSC

Pluripotent Embryonic Stem cell (ESC) lines can be derived from a variety of sources. Mouse lines derived from the early blastocyst and from primordial germ cells (PGCs) can contribute to all somatic lineages and to the germ line, whereas cells from slightly later embryos (EpiSC) no longer contribute to the germ line. In chick, pluripotent ESCs can be obtained from PGCs and from early blastoderms. Established PGC lines and freshly isolated blastodermal cells (cBC) can contribute to both germinal and somatic lineages but established lines from the former (cESC) can only produce somatic cell types. For this reason, cESCs are often considered to be equivalent to mouse EpiSC. To define these cell types more rigorously, we have performed comparative microarray analysis to describe a transcriptomic profile specific for each cell type. This is validated by real time RT-PCR and in situ hybridisation. We find that both cES and cBC cells express classic pluripotency-related genes (including cPOUV/OCT4, NANOG, SOX2/3, KLF2 and SALL4), whereas expression of DAZL, DND1, DDX4 and PIWIL1 defines a molecular signature for germ cells. Surprisingly, contrary to the prevailing view, our results also suggest that cES cells resemble mouse ES cells more closely than mouse EpiSC.

Local cell interactions and self-amplifying individual cell ingression drive amniote gastrulation

Gastrulation generates three layers of cells (ectoderm, mesoderm, endoderm) from a single sheet, while large scale cell movements occur across the entire embryo. In amniote (reptiles, birds, mammals) embryos, the deep layers arise by epithelial-to-mesenchymal transition (EMT) at a morphologically stable midline structure, the primitive streak (PS). We know very little about how these events are controlled or how the PS is maintained despite its continuously changing cellular composition. Using the chick, we show that isolated EMT events and ingression of individual cells start well before gastrulation. A Nodal-dependent ‘community effect’ then concentrates and amplifies EMT by positive feedback to form the PS as a zone of massive cell ingression. Computer simulations show that a combination of local cell interactions (EMT and cell intercalation) is sufficient to explain PS formation and the associated complex movements globally across a large epithelial sheet, without the need to invoke long-range signalling.

DOI:http://dx.doi.org/10.7554/eLife.01817.001

A key process during the development of an embryo involves a single layer of cells reorganizing into three ‘germ layers’: the ectoderm, which becomes the skin and nervous system; the mesoderm, which gives rise to the skeleton, muscles and the circulatory and urinogenital systems, and the endoderm, which gives rise to the lining of the gut and associated organs. The process of forming these three layers is known as gastrulation.

To date most experiments on gastrulation in vertebrates have been performed on frog embryos. However, the embryos of amniotes, the group of ‘higher’ vertebrates that comprises reptiles, birds and mammals, differ from those of frogs in a number of ways. Now Voiculescu et al. have used a combination of experimental and computational techniques to shed new light on gastrulation in chick embryos.

Just prior to gastrulation, the cells of the amniote embryo are arranged in a flat disk, one cell thick, called the epiblast. The cells of the epiblast then move to form the mesoderm and endoderm (in a process called epithelial-to-mesenchymal transition). These cell movements also lead to the formation of a structure called the primitive streak that establishes the left-right symmetry of the organism, and also defines the midline of the body.

Now Voiculescu et al. have shown that the epithelial-to-mesenchymal transition starts before the primitive streak appears, and that two main processes drive gastrulation. One involves cells inserting themselves between other cells at the midline of the epiblast, which causes a double whorl-like movement within the plane of the epiblast. At the same time small numbers of cells leave the epiblast, and as these cells accumulate under the epiblast, they initiate a positive feedback effect by which they encourage more cells to leave the epiblast. Voiculescu et al. found that this ‘community effect’ involves signalling by a protein called Nodal. This protein effectively amplifies the epithelial-to-mesenchymal transition and leads to the appearance of the primitive streak at the midline.

Using computational modelling, Voiculescu et al. argue that the movements of gastrulation can be explained entirely based on local interactions between cells, without the need for cells to send signals over long distances to guide cell movements, as had been generally believed.

DOI:http://dx.doi.org/10.7554/eLife.01817.002

Regulated expression of the HNK-1 carbohydrate is essential for medaka (Oryzias latipes) embryogenesis

Carbohydrates are known to play essential roles in various biological processes including development. However, it remains largely unknown which carbohydrate structure takes part in each biological event. Here, we examined the roles of the human natural killer-1 (HNK-1) carbohydrate in medaka embryogenesis. We first cloned two medaka glucuronyltransferases, GlcAT-P and GlcAT-S, key enzymes for HNK-1 biosynthesis. Overexpression of these glucuronyltransferases affected morphogenetic processes. In addition, loss-of-function experiments revealed that GlcAT-P is physiologically indispensable for head morphogenesis and GlcAT-P depletion also led to markedly increased apoptosis. However, even when the apoptosis was blocked, abnormal head morphogenesis caused by GlcAT-P depletion was still observed, indicating that apoptosis was not the main cause of the abnormality. Moreover, in situ hybridization analyses indicated that GlcAT-P depletion resulted in the abnormal formation of the nervous system but not in cell specification. These results suggest that tight regulation of HNK-1 expression is essential for proper morphogenesis of medaka embryos.

The mechanisms underlying primitive streak formation in the chick embryo

Formation of the primitive streak is one of the key events in the early development of amniote embryos. The streak is the site where during gastrulation the mesendoderm cells ingress to take up their correct topographical positions in the embryo. The process of streak formation can be conveniently observed in the chick embryo, where the streak forms as an accumulation of cells in the epiblast in the posterior pole of the embryo and extends subsequently in anterior direction until it covers 80% of the epiblast. A prerequisite for streak formation is the differentiation of mesoderm, which is induced in the epiblast at the interface between the posterior Area Opaca and Area Pellucida in a sickle shaped domain overlying Koller's sickle. Current views on the molecular mechanisms of mesoderm induction by inducing signals from the Area Opaca and inhibitory signals from the hypoblast are briefly discussed. During streak formation the sickle of mesoderm cells transforms into an elongated structure in the central midline of the embryo. We discuss possible cellular mechanisms underlying this process, such as oriented cell division, cell-cell intercalation, chemotactic cell movement in response to attractive and repulsive signals and a combination of chemotaxis and contact following. We review current experimental evidence in favor and against these different hypotheses and outline some the outstanding questions. Since many of the interactions between cells signaling and moving are dynamic and nonlinear in nature they will require detailed modeling and computer simulations to be understood in detail.

Endoderm development in vertebrates: fate mapping, induction and regional specification

The formation of the vertebrate body plan begins with the differentiation of cells into three germ layers: ectoderm, mesoderm and endoderm. Cells in the endoderm give rise to the epithelial lining of the digestive tract, associated glands and respiratory system. One of the fundamental problems in developmental biology is to elucidate how these three primary germ layers are established from the homologous population of cells in the early blastomere. To address this question, ectoderm and mesoderm development have been extensively analyzed, but study of endoderm development has only begun relatively recently. In this review, we focus on the 'where', 'when' and 'how' of endoderm development in four vertebrate model organisms: the zebrafish, Xenopus, chick and mouse. We discuss the classical fate mapping of the endoderm and the more recent progress in characterizing its induction, segregation and regional specification.

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.

Expression of HNK1 epitope by the cardiomyocytes of the early embryonic chick: in situ and in vitro studies

Monoclonal antibody HNK1 reacts with a carbohydrate epitope in cell surface glycoproteins and glycolipids. During development, in various species the HNK1 epitopes are expressed in migrating neural crest cells and in the developing conduction cardiomyocytes. The conduction system is generally thought to be developed from cardiomyocytes, but some investigators have hypothesized that it is derived from the neural crest because conduction myocytes express neural antigens, including HNK1. Using immunohistochemistry, we examined the spatiotemporal expression of HNK1 in early chick cardiogenesis (stages 4 to 18) and whether cultured precardiac mesoderm does or does not express HNK1 as well as sarcomeric myosin (MF20). HNK1 was first expressed in the premyocardium at stage 8. At stage 10, HNK1-positive cardiomyocytes were scattered along the straight heart tube. By stage 18, HNK1-positive cardiomyocytes had become restricted to the atrium and sinus venosus. Atrioventricular cushion mesenchyme also expressed an HNK1 epitope. Immunostaining of HNK1 and MF20 in cultured precardiac mesoderm showed that there are at least three types of cells: 1) cardiomyocytes without HNK1 expression, 2) cells possessing both HNK1- and MF20-immunoreactivity, and 3) mesenchymal cells with HNK1. Immunogold electron microscopy showed that cardiomyocytes containing sparsely distributed myofibrils associated with the Z-band react with anti-HNK1 antibody. Our observations showed a direct evidence for the first time that the precardiac mesoderm generates HNK1-positive cardiomyocytes with morphological features similar to those of conduction cardiomyocytes.

Intrahippocampal administration of an antibody against the HNK-1 carbohydrate impairs memory consolidation in an inhibitory learning task in mice

Many cell adhesion molecules express the HNK-1 carbohydrate involved in formation and functioning of synapses. To assess its role in learning, we injected the monoclonal HNK-1 antibody or nonimmune IgG into the hippocampus of C57BL/6J mice 1 h after training in a step-down avoidance task. In animals treated with the HNK-1 antibody, latencies of step down in a recall session 48 h after injection did not change compared to training values and were significantly shorter versus IgG-treated controls, which acquired the task normally. Similar differences between the two treatments were also observed after a stronger training protocol in a step-down avoidance paradigm. The HNK-1 antibody was effective only when injected 1 h, but not 48 h after training, thus affecting memory consolidation but not memory recall itself. The HNK-1 antibody impaired memory also in tenascin-R knock-out mice, indicating that extracellular matrix molecule tenascin-R, one of the carriers of the HNK-1epitope in the hippocampus, does not mediate the function of the HNK-1 carbohydrate in this task. Our observations show that the HNK-1 carbohydrate is critically involved in memory consolidation in hippocampus-dependent learning in mammals.

Characterization of N-glycans from mouse brain neural cell adhesion molecule

The N-glycosylation pattern of the neural cell adhesion molecule (NCAM), isolated from brains of newborn mice, has been analyzed. Following digestion with trypsin, generated glycopeptides were fractionated by serial immunoaffinity chromatography using immobilized monoclonal antibodies specifically recognizing polysialic acid (PSA) units or the HNK1-carbohydrate epitope. Subsequent analyses of the resulting (glyco)peptides by Edman degradation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed polysialylated glycans to be exclusively linked to glycosylation sites 5 (Asn(431)) and 6 (Asn(460)), whereas glycans carrying the HNK1-epitope could be assigned to sites 2 (Asn(297)), 5, 6, and, to a lesser extent, site 3 (Asn(329)). PSA-, HNK1-, and non-PSA/HNK1-glycan fractions were characterized by carbohydrate constituent and methylation analyses as well as MALDI-TOF-MS in conjunction with chromatographic fractionation techniques. The results revealed that the core structures of PSA-glycans represented predominantly fucosylated, partially sulfated 2,6-branched isomers of triantennary as well as tetraantennary complex-type glycans, whereas carbohydrate chains bearing the HNK1-epitope were dominated by diantennary species carrying in part bisecting GlcNAc residues. Non-PSA/HNK1-glycans exhibited a highly heterogeneous pattern of partially truncated, mostly diantennary structures being characterized by the presence of additional fucose, bisecting GlcNAc and/or sulfate residues. In conclusion, our results revealed that the glycosylation pattern of murine NCAM displays high structural and regional selectivity, which might play an important role in controlling the biological activities of this molecule.

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.

Origin of primordial germ cells in the prestreak chick embryo

The temporal and spatial pattern of segregation of the avian germline from the formation of the area pellucida to the beginning of primitive streak formation (stages VII-XIV, EG&K) was investigated using the culture of whole embryos and central and peripheral embryo fragments on vitelline membranes at stages VII-IX, immunohistological analysis of whole mount embryos and sections with monoclonal antibodies MC-480 against stage-specific embryonic antigen-1 (SSEA-1) and EMA-1, and with the culture of dispersed blastoderms at stages IX-XIV with and without on STO feeder layer. Whole embryos at intrauterine stages developed up to the formation of the primitive streak despite the absence of area pellucida expansion. Primordial germ cells (PGCs) appeared in the cultures of whole embryos and only in central fragments containing a partially formed area pellucida at stages VII-IX. When individual stage IX-XIV embryos were dispersed and cultured without a feeder layer, 25-45 PGCs/embryo were detected only with stage X-XIV, but not with stage IX blastoderms. However, the culture of dispersed cells from the area pellucida of stages IX-XIII on STO feeder layers yielded about 150 PGCs/embryo. The carbohydrate epitopes recognized by anti-SSEA-1 and EMA-1 first appeared at stage X on cells in association with polyingressing cells on the ventral surface of the epiblast and later on the dorsal surface of the hypoblast. The SSEA-1-positive hypoblast cells gave rise to chicken PGCs when cultured on a feeder layer of quail blastodermal cells. From these observations, we propose that the segregation and development of avian germline is a gradual, epigenetic process associated with the translocation of SSEA-1/EMA-1-positive cells from the ventral surface of the area pellucida at stage X to the dorsal side of the hypoblast at stages XI-XIV.

Getting organized: new insights into the organizer of higher vertebrates

The Organizer of higher vertebrates (e.g., Hensen's node in birds and the node in mammals) functions much like the Organizer of lower vertebrates (e.g., embryonic shield in fish and dorsal lip of the blastopore in amphibians). In all classes of vertebrates, the Organizer displays a number of unique properties including the fate, migratory patterns, morphogenetic movements, and the level of commitment of its cells; its pattern of gene expression; its ability to induce neural differentiation; and its ability to organize and regionalize a secondary embryo when grafted ectopically. The importance of Organizer activity to formation of the neuraxis is highlighted by results from studies in which the Organizer is eliminated experimentally. Such studies demonstrate that an auxiliary system is present that can generate a reconstituted Organizer, which completely mimics the activity of the original Organizer. For almost 50 years after the discovery of Spemann's Organizer, the molecular nature of Organizer activity was virtually unknown. However, recent progress in identifying the morphoregulatory molecules underlying Organizer activity has been substantial, and a full understanding of the molecular basis of this activity is imminent. Thus, the intriguing question of how the Organizer organizes, raised by the seminal experiments of Spemann and Mangold, is finally being answered in this exciting renaissance of developmental biology driven by new molecular and genetic approaches.

Cardiac neural crest is essential for the persistence rather than the formation of an arch artery

Double-label immunohistochemistry was used to compare early aortic arch artery development in cardiac neural crest-ablated and sham-operated quail embryos ranging from stage 13 to stage 18. The monoclonal antibody QH-1 labeled endothelial cells and their precursors, and HNK-1 labeled migrating neural crest cells. In the sham-operated embryos, the third aortic arch artery developed from a lumenizing strand of endothelial precursors that became separated from the pharyngeal endoderm by migrating cardiac neural crest cells as they ensheathed the artery. The arch artery of the neural crest-ablated embryos lumenized but failed to become separated from the pharyngeal endoderm, indicating that neural crest is unnecessary for the early formation of the aortic arch artery. However, once blood flow was initiated through the third arch artery of crest-ablated embryos at stage 16, the artery became misshapen and sinusoidal. By embryonic day 3, abnormal connections to the dorsal aorta occurred and bilateral symmetry was lost, suggesting that the loss of neural crest-derived ectomesenchyme destabilizes the nascent artery. Although here we show no loss of the third arch artery, past studies have reported hypoplasia or missing carotids in older neural crest-ablated embryos (Bockman et al. [1987] Am. J. Anat. 180:332-341; Bockman et al. [1989] Anat. Rec. 225:209-217; Nishibatake et al. [1987] Circulation 75:255-264; Tomita et al. [1991] Circulation 84:1289-1295). We suggest that the cardiac neural crest is essential for the persistence of an arch artery, but not its formation. Furthermore, since changes in the development of the arch artery are seen prior to the formation of the tunica media, it is suggested that a critical period is reached in the development of the arch artery, after lumenization, but prior to the formation of the tunica media, which necessitates the presence of the cardiac neural crest.

Mesoderm induction and development of the embryonic axis in amniotes

The mechanisms controlling the formation of the embryonic axis, and specifically those that give rise to the mesoderm, have received renewed attention recently. In the frog, some of these mechanisms have begun to be elucidated, and several factors have been found to cause uncommitted ectoderm cells to differentiate into mesoderm. All of the factors identified to date are related either to fibroblast growth factor (FGF) or to transforming growth factor beta (TGF-beta). Do the mechanisms that generate the embryonic axis of amphibians also operate in chick and mouse embryos? Here I address how amphibian and amniote embryos might provide complementary pieces of a puzzle.

Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation

The expression and distribution of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during early avian embryogenesis has been analysed in detail. Three FGF-2 protein isoforms of 18.5, 20.0 and 21.5 kDa are expressed during gastrulation of chicken embryos. Using whole mount immunohistochemistry, these proteins were found to be predominantly nuclear in prestreak blastodiscs during mesoderm induction. Distribution of positive cells in the epiblast was mosaic, whereas all cells of the forming hypoblast expressed the FGF-2 proteins. During primitive streak formation, the proteins started to translocate to the cytoplasm in epiblast cells but remained nuclear in the hypoblast. The FGF-2 proteins became predominantly cytoplasmic in all cells during the subsequent developmental stages. Their highest levels were detected in endodermal cells underlying Hensen's node and the newly formed notochord, the dorsal apex of all epiblast cells and, most interestingly, in the extra-cellular basal lamina separating the epiblast from newly formed mesoderm. Heparin and suramin treatment of these advanced embryos (stage 4) revealed a dose-dependent inhibition on the regression of Hensen's node and formation of mesodermal derivatives such as somites. The results are discussed with respect to current models on FGF-mediated functions during vertebrate mesoderm induction and regionalization.

c-otx2 is expressed in two different phases of gastrulation and is sensitive to retinoic acid treatment in chick embryo

We cloned the chick homologue of the mouse Otx2 gene, c-otx2, and analyzed its expression pattern during gastrulation. During mouse embryogenesis, Otx2 expression is first detected in the entire epiblast and after the formation of the primitive streak becomes confined to the most anterior region of the embryo corresponding to presumptive fore- and mid-brain. Similarly, two distinct phases of c-otx2 expression were observed in the chick. c-otx2 transcripts were first detected in the unincubated egg and up to stage XIII, in all epiblast, and forming hypoblast and mesoblast cells. During primitive streak progression, c-otx2 expression becomes progressively restricted to anterior regions and is mainly associated with Hensen's node. When the extension of the streak is maximal, transcripts are only found in Hensen's node. A second phase of c-otx2 expression starts during streak regression. c-otx2 transcripts are lost from the node and present in higher abundance in anterior neuroectoderm and mesendoderm, with the exception of forming notochord and floor plate. The first phase of expression bears strong similarity with that of c-gsc, a gene shown to be a marker for cells that have organizer activity in the chick. Therefore, we compared the expression of the two genes by double staining on the same embryo. This analysis demonstrated that c-otx2 is transcribed first and its expression in the hypoblast precedes that of c-gsc. On the other hand, c-gsc is an earlier marker of primitive streak cells. The expression domains of the two genes transiently overlap in Hensen's node and anterior mesendoderm, whereas only c-otx2 is expressed in neuroectodermal areas. The second phase of c-otx2 expression is sensitive to an early treatment with retinoic acid. This treatment abolishes c-otx2 expression in mesendoderm and restricts it to most anterior regions in the forming neural plate. In conclusion, our results suggest that c-otx2 expression is first associated with cells with an anterior mesendoderm fate and subsequently extends to anterior neuroectoderm.

Ectopic induction and reorganization of Wnt-1 expression in quail/chick chimeras

When grafted ectopically into the diencephalon of a chick host embryo, a portion of met-mesencephalon straddling the met-mesencephalic constriction has the capacity to induce En-2 expression in the surrounding host tissue. Subsequently, tectal and cerebellar structures, composed of both host and grafted cells, are reconstructed in this ectopic location at the expense of the host diencephalon. Previous experiments indicated that the induction of En-2 was correlated with Wnt-1 expression within the graft. The aim of the present study was: (i) to determine whether Wnt-1 expression was spatially regulated within the graft, (ii) to investigate whether host Wnt-1-expressing cells were also involved in the ectopic met-mesencephalic development and, if so, (iii) to localize these Wnt-1-positive domains in relation to the patterning of the ectopically developing met-mesencephalic territory. We studied the expression profile of Wnt-1, in relation with that of other positional markers, in quail/chick chimeras where various portions of met-mesencephalon had been grafted into the diencephalon. We found that Wnt-1 expression was reorganized within the graft, and that it was also induced in the host in contact with the graft. Moreover, these ectopic expressions of Wnt-1, in both the grafted and the surrounding host tissues, were organized in concert to form a continuous positive line at the host/graft junction, the location of which depended on the precise origin of the graft. Finally, we found that this line was frequently located at the limit between territories expressing different positional markers. We propose that Wnt-1 expression is turned on at the junction between domains of different phenotypes, and may be used as a border to stabilize these adjacent differently committed territories.

A fate map of the epiblast of the early chick embryo

We have used carbocyanine dyes (DiI and DiO) to generate fate maps for the epiblast layer of the chick embryo between stage X and the early primitive streak stage (stages 2–3). The overall distribution of presumptive cell types in these maps is similar to that described for other laboratory species (zebrafish, frog, mouse). Our maps also reveal certain patterns of movement for these presumptive areas. Most areas converge towards the midline and then move anteriorly along it. Interestingly, however, some presumptive tissue types do not take part in these predominant movements, but behave in a different way, even if enclosed within an area that does undergo medial convergence and anterior movement. The apparently independent behaviour of certain cell populations suggests that at least some presumptive cell types within the epiblast are already specified at preprimitive streak stages.

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

Small grafts from transfected mammalian cell lines that secrete activin or express Wnt-1 RNA were made to the marginal zone of entire chick blastoderms in culture. Grafts from appropriate control cell lines produced no effects on development. The activin-secreting grafts, implanted before streak formation, could cause the streak to form opposite their marginal position even when this was 180 degrees distant around the blastoderm from the original presumptive streak site. Alternatively, opposed twin streaks were observed, one at the original presumptive site and one in relation to the graft. Wnt-expressing grafts implanted early could also reposition axis formation, but only to graft sites within approximately 100 degrees of angular distance from the host's presumptive streak origin. No Wnt-induced twinning was observed. Grafts of both experimental cell types intermixed were the most effective in reorientating, twinning, or globally disturbing the axial pattern and led to second axes with the least delay, relative to normal development, in reaching headfold stages. The incidence and distribution of cells positive for the epitope HNK-1 was investigated during early stages of normal and of experimentally twinned development. Only two nonhypoblast regions of HNK-1 expression were consistently observed in normal early development; a sector in the germ wall area opaca, behind the site of streak formation, and then a localised region of intensely, newly expressing cells arising in epiblast and in anteriormost parts of the (epiblast-derived) streak at the half-length streak stage. Both "activin only" and "activin/Wnt" mixed grafts, although not control grafts, became surrounded by new sectors of "germ wall" HNK-1 positivity. Such positivity may therefore mark a cell group with a signaling role (but no anatomical participation) in streak initiation. However, there was no change of the local background incidence of epiblastic HNK-1 positivity in the structure of streaks induced by "activin only" grafts. This indicates that most cells of the streak are specified by relatively local induction, rather than deriving from selective aggregation. Only grafts including the Wnt-expressing cells gave rise to obvious new HNK-1 expression within epiblast-derived cells anteriorly, as does the complete normal streak. This suggests that the Wnt class of response pathway can complement the activin one in producing rostrocaudally complete axial pattern, as has been suggested for amphibian development.

Transgenesis in chickens

The application of transgenic technology to domestic poultry offers an alternative means to conventional practice for improvement of this highly productive agricultural species. The hen's reproductive system has unique characteristics which have imposed limitations on the use of established methods for artificial gene transfer. In this article, we review the various strategies that have been adopted to overcome the problem. Target sites for gene insertion include the fertilized ovum, the blastodermal embryo in the unincubated egg, and the primordial germ cells. Notable success in obtaining somatic and germline transformation has been achieved with the use of retroviral vectors to infect the blastodermal embryo. Current attempts to introduce DNA directly into the genome, without resort to pathogen-derived vectors, are discussed.

Monoclonal antibodies identifying subsets of ectodermal, mesodermal, and endodermal cells in gastrulating and neurulating avian embryos

The goal of our laboratory research is to elucidate the mechanisms underlying gastrulation and neurulation, using the avian embryo as a model system. In previous studies, we used two approaches to map the morphogenetic movements involved in these processes: (1) we constructed quail/chick transplantation chimeras in which grafted quail cells could be identified within chick host embryos by the presence of nucleolar-associated heterochromatin, and (2) we microinjected exogenous cell markers. However, it would be advantageous to be able to detect endogenous markers to demarcate various subsets of cells within the unmanipulated embryo. To elucidate such a series of natural markers, we have used monoclonal antibodies to identify epitopes found on subsets of ectodermal, mesodermal, and endodermal cells. Antibodies were made by immunizing mice against either homogenized ectoderm (i.e., prospective neural plate and surface ectoderm) or primitive streak, which had been microdissected from stage 3 chick embryos. Additionally, we screened a panel of antibodies made against soluble protein obtained from isolates of cell nuclei from late embryonic chick brain. Here, we describe the labeling patterns of three monoclonal antibodies, called MAb-GL1, GL2, and GL3 (GL, germ layer), during avian gastrulation and neurulation. Our results show that labeling early avian embryos with monoclonal antibodies can reveal previously undetected distributions of cells bearing shared epitopes, providing new labels for subsets of cells in each of the three primary germ layers.

Early development of the chick embryo

The ultrastructure of the early chick embryo was investigated, using scanning (SEM) and transmission electron microscopy (TEM). Eggs were obtained from the shell gland by injecting hens intravenously with a synthetic prostaglandin or arginine vasopressin. Embryos were examined during late cleavage (stages IV-VI, Eyal-Giladi and Kochav, '76), formation of the area pellucida (stages VII-XI), and formation of the hypoblast (stages X-XIV). SEM highlighted the reduction in cell number at the underside of the embryo during formation of the area pellucida although it became apparent that the thickness of the embryo is not reduced to a single layer of cells at stage X. In addition, blastomeres at the perimeter of embryos (stages V-VI) project filopodial extensions onto a smooth membrane that separates the sub-embryonic cavity from the yolk. During hypoblast formation, epiblast cells generate stellate projections at their basal aspect, thus providing a meshwork for the advancing secondary hypoblast cells. By stage XII the epiblast was one cell thick and reminiscent of a columnar epithelium when viewed transversely. Cells of the deep portion of the posterior marginal zone were distinguished morphologically in the stage XII embryo by their many cell surface projections and ruffled appearance. Blastomeres at the perimeter of stage V-VI embryos projected filopodial extensions onto a smooth membrane which separates the sub-embryonic cavity from the yolk. This membrane is presumed to be confluent with the cytolemma. Evidence is presented demonstrating the presence of intracellular membrane-bound droplets which are hypothesised to contain sub-embryonic fluid.

Fibrillin delineates the primary axis of the early avian embryo

Chicken embryos were immunolabeled with antibodies to fibrillin, an extracellular matrix molecule implicated in the Marfan syndrome. The pattern of labeling, established using both whole-mounted and sectioned embryos, showed a striking localization at the primary axis of the early embryo, including Hensen's node. After stage 8, the labeling of Hensen's node diminished while other structures in the embryo exhibited increased immunolabeling. These structures include the region of the presumptive dorsal aorta, the ventral surface of the notochord, and the mesocardium. Injections of the antibody between the vitelline membrane and the blastoderm in vivo failed to perturb the development of embryos, thus precluding any conclusions regarding the function of fibrillin.

Cranial nerve growth in birds is preceded by cholinesterase expression during neural crest cell migration and the formation of an HNK-1 scaffold

The expression of the neural crest cell (NCC) markers acetylcholinesterase (AChE) and the HNK-1-epitope is compared from the emigration of cephalic NCC until the formation of the cranial nerves V-X in chicken and quail hindbrain. We show that NCC transiently express acetylcholinesterase (AChE) activity during their emigration; NCC migrate into butyrylcholinesterase (BChE)-positive areas of the cranial mesenchyme. Along these migratory tracks that foreshadow the course of later projecting cranial nerves, BChE increases strongly in cells that may represent immature Schwann cells. Both AChE and BChE, but not HNK-1, are expressed in the ectodermal placodes. In NCC, HNK-1 is expressed strongly only when they approach their destination sites. Their intense expression of HNK-1 then leads to the establishment of tunnel-shaped HNK-1 matrices, within which G4-positive cranial neurites begin to extend. We conclude that AChE and HNK-1 expression in cephalic NCC serve different functions, since AChE is related to their migration, and HNK-1 to their aggregation and the formation of an extracellular neurite scaffold.

Features of polyingression and primitive streak ingression through the basal lamina in the chicken blastoderm

The de-epithelialization of cells of the upper layer during the phenomena of polyingression and primitive streak ingression was studied by analyzing, from the time of laying to the end of gastrulation, the ultrastructure of the basal lamina underlying the upper layer. The electron density of the basal lamina and associated extracellular materials was enhanced by addition of tannic acid to the fixative. Special attention was also paid to the spatial and temporal distribution of blebs at the basal surface of the upper layer, and to the contribution of the de-epithelialized cells to the formation of the deep layer. The results indicate that a nascent basal lamina is already present at the time of laying, especially beneath regions of the area pellucida where polyingression is not apparent. From the onset of incubation, the basal lamina rapidly develops, and it is interrupted by a large number of blebs. However, during the first 6-8 h of incubation, i.e., stages 1-2 of Vakaet (Arch. Biol. (Liège) 81:387-426, 1970), a downward movement of de-epithelialized cells that insert into the deep layer and form the endophyll persists cranially. This phenomenon of polyingression, which starts during the intrauterine period, probably extends from caudal to cranial and comes to an end by stage 3. During these first three stages, the number of blebs progressively decreases, especially in the cranial part of the area pellucida, and a thicker, continuous basal lamina associated with numerous interstitial bodies is laid down. The caudal part of the upper layer is still actively blebbing at that time. Due to the convergence of this area toward the axis of the blastoderm, which leads to ingression at and elongation of the primitive streak up to and including stage 6, the number of blebs at the basal surface of the upper layer progressively decreases. From stage 7 on, blebs are virtually absent; shortening of the primitive streak and formation of the head process begin. At the level of the head process, primitive streak ingression has ceased and a novel basal lamina is progressively deposited beneath the upper layer. By stage 9, a thick, smooth basal lamina physically separates the upper layer from the head mesenchyme. Summarizing, at the time of gastrulation, the presence of blebs that perforate the basal lamina is correlated with the de-epithelialization of cells. Before incubation, however, de-epithelialization of upper-layer cells occurs before the assembly of the basal lamina.(ABSTRACT TRUNCATED AT 400 WORDS)

Origin of cells giving rise to mesoderm and endoderm in chick embryo

In amniotes, all of the tissues of the adult arise from the epiblast, one of the two layers of cells present in the early embryo; the mesoderm and gut endoderm arise from an epiblast-derived structure known as the primitive streak. The monoclonal antibody HNK-1 recognizes the cells of the primitive streak in the chick embryo. Before streak formation, HNK-1 identifies cells that are randomly distributed within the epiblast. We have now used two novel ways to study cell lineage and commitment to show that the epiblast of the early chick embryo contains two distinct populations of cells with different developmental fates at a stage during which 'mesodermal induction' is believed to occur. One cell population, recognized by monoclonal antibody HNK-1, is destined to form mesoderm and endoderm; the rest of the epiblast is unable to give rise to mesoderm if this population of cells is removed.