Protogenin mediates cell adhesion for ingression and re-epithelialization of paraxial mesodermal cells

In the avian embryo, precursor cells of the paraxial mesoderm that reside in the epiblast ingress through the primitive streak and migrate bilaterally in an anterolateral direction. Herein, we report on the roles of Protogenin (PRTG), an immunoglobulin superfamily protein expressed on the surface of the ingressing and migrating cells that give rise to the paraxial mesoderm, in paraxial mesoderm development. An aggregation assay using L-cells showed that PRTG mediates homophilic cell adhesion. Overexpression of PRTG in the presumptive paraxial mesoderm delayed mesodermal cell migration due to augmented adhesiveness. In contrast, siRNA knockdown of PRTG impaired successive ingression of epiblast cells and disorganized the epithelial structure of the somites. These results suggest that PRTG mediates cell adhesion to regulate continuous ingression of cells giving rise to the paraxial mesodermal lineage, as well as tissue integrity.     

Fate maps of the primitive streak in chick and quail embryo: ingression timing of progenitor cells of each rostro-caudal axial level of somites

Developmental fates of cells emigrating from the primitive streak were traced by a fluorescent dye Dil both in chick and in quail embryos from the fully grown streak stage to 12-somite stage, focusing on the development of mesoderm and especially on the timing of ingression of each level of somitic mesoderm. The fate maps of the chick and quail streak were alike, although the chick streak was longer at all stages examined. The anterior part of the primitive streak predominantly produced somites. The thoracic and the lumbar somites were shown to begin to ingress at the 5 somite-stage and 10 somite-stage in a chick embryo, and 6 somite-stage and 9 somite-stage in a quail embryo, respectively. The posterior part of the streak served mainly as the origin of more lateral or extra embryonic mesoderm. As development proceeded, the fate of the posterior part of the streak changed from the lateral plate mesoderm to the tail bud mesoderm and then to extra embryonic, allantois mesoderm. The fate map of the primitive streak in chick and quail embryo presented here will serve as basic data for studies on mesoderm development with embryo manipulation, especially for transplantation experiments between chick and quail embryos.     

Non-canonical Wnt signaling through Wnt5a/b and a novel Wnt11 gene, Wnt11b, regulates cell migration during avian gastrulation

Knowledge of the molecular mechanisms regulating cell ingression, epithelial–mesenchymal transition and migration movements during amniote gastrulation is steadily improving. In the frog and fish embryo, Wnt5 and Wnt11 ligands are expressed around the blastopore and play an important role in regulating cell movements associated with gastrulation. In the chicken embryo, although Wnt5a and Wnt5b are expressed in the primitive streak, the known Wnt11 gene is expressed in paraxial and intermediate mesoderm, and in differentiated myocardial cells, but not in the streak. Here, we identify a previously uncharacterized chicken Wnt11 gene, Wnt11b, that is orthologous to the frog Wnt11 and zebrafish Wnt11 (silberblick) genes. Chicken Wnt11b is expressed in the primitive streak in a pattern similar to chicken Wnt5a and Wnt5b. When non-canonical Wnt signaling is blocked using a Dishevelled dominant-negative protein, gastrulation movements are inhibited and cells accumulate in the primitive streak. Furthermore, disruption of non-canonical Wnt signaling by overexpression of full-length or dominant-negative Wnt11b or Wnt5a constructions abrogates normal cell migration through the primitive streak. We conclude that non-canonical Wnt signaling, mediated in part by Wnt11b, is important for regulation of gastrulation cell movements in the avian embryo.     

Post-hatching development of the porcine and bovine embryo--defining criteria for expected development in vivo and in vitro

Particular attention has been paid to the pre-hatching period of embryonic development although blastocyst development is a poor indicator of embryo viability. Post-hatching embryonic development in vitro would allow for establishment of more accurate tools for evaluating developmental potential without the need for transfer to recipient animals. Such a system would require (1) definition of milestones of expected post-hatching embryonic development in vivo; and (2) development of adequate culture systems. We propose a stereomicroscopical staging system for post-hatching embryos defining the following stages: (1) Expanded hatched blastocyst stage where the embryo presents an inner cell mass (ICM) covered by trophoblast. (2) Pre-streak stage 1 where the embryonic disc is formed. (3) Pre-streak stage 2 where a crescent-shaped thickening of the caudal portion of the embryonic disk appears. (4) Primitive streak stage where the primitive streak has developed as an axis of cell ingression of cells for meso- and endoderm formation. (5) Neural groove stage where the neural groove is developing from the rostral pole of the embryo along with a proportional shortening of the primitive streak; and (6) Somite stage(s) where paraxial mesoderm gradually condensates to form somites. Post-hatching development of bovine embryos in vitro is compromised and although hatching occurs and elongation can be physically provoked by culture in agarose tunnels, the embryonic disk characterizing the pre-streak stage 1 is never established. Thus, particular focus should be placed on establishing culture conditions that support at least some of the above-mentioned critical phases of development that in vivo occur within the initial two (pig) to three (cattle) weeks.     

Formation of the avian primitive streak from spatially restricted blastoderm: evidence for polarized cell division in the elongating streak

Gastrulation in the amniote begins with the formation of a primitive streak through which precursors of definitive mesoderm and endoderm ingress and migrate to their embryonic destinations. This organizing center for amniote gastrulation is induced by signal(s) from the posterior margin of the blastodisc. The mode of action of these inductive signal(s) remains unresolved, since various origins and developmental pathways of the primitive streak have been proposed. In the present study, the fate of chicken blastodermal cells was traced for the first time in ovo from prestreak stages XI-XII through HH stage 3, when the primitive streak is initially established and prior to the migration of mesoderm. Using replication-defective retrovirus-mediated gene transfer and vital dye labeling, precursor cells of the stage 3 primitive streak were mapped predominantly to a specific region where the embryonic midline crosses the posterior margin of the epiblast. No significant contribution to the early primitive streak was seen from the anterolateral epiblast. Instead, the precursor cells generated daughter cells that underwent a polarized cell division oriented perpendicular to the anteroposterior embryonic axis. The resulting daughter cell population was arranged in a longitudinal array extending the complete length of the primitive streak. Furthermore, expression of cVg1, a posterior margin-derived signal, at the anterior marginal zone induced adjacent epiblast cells, but not those lateral to or distant from the signal, to form an ectopic primitive streak. The cVg1-induced epiblast cells also exhibited polarized cell divisions during ectopic primitive streak formation. These results suggest that blastoderm cells located immediately anterior to the posterior marginal zone, which secretes an inductive signal, undergo spatially directed cytokineses during early primitive streak formation.     

Rauber's sickle and not the caudal marginal zone induces a primitive streak, blood vessels, blood cell formation and coelomic vesicles in avian blastoderms

By using the quail-chicken chimera technique, we studied the reactivity and the eventual developmental or inducing capacities of the avian caudal marginal zone (in comparison with Rauber's sickle), when associated in vitro with different avian blastoderm components. If a fragment of quail sickle endoblast is placed on the caudal marginal zone of a whole unincubated chicken blastoderm, then a secondary miniature embryo will develop in this caudal marginal zone. The primitive streak and accompanying neural plate of the secondary embryo are directed peripherally into the caudal germ wall, away from Rauber's sickle. Thus, the 'mirror image development' indicates that the upper layer of the caudal marginal zone can react in the same way as the upper layer of the area centralis, because of the presence of sickle endoblast. A quail Rauber's sickle fragment placed on an isolated anti-sickle region always induces a primitive streak directed centrally. After prolonged culture, blood vessels and associated coelomic vesicles are formed. By contrast if a quail caudal marginal zone is placed on an isolated chicken anti-sickle region, the primitive streak, blood vessels and coelomic vesicles do not form. Thus, in contrast to the inducing effect of Rauber's sickle, the caudal marginal zone has no inducing effect by itself, even in the absence of the dominating effect of Rauber's sickle.     

PDGF signalling controls the migration of mesoderm cells during chick gastrulation by regulating N-cadherin expression

In the early chick embryo, Pdgfa is expressed in the epiblast, outlining the migration route that mesoderm cells expressing the receptor, Pdgfralpha, follow to form somites. Both expression of a dominant-negative PDGFRalpha and depletion of endogenous PDGFRalpha ligands through injection of PDGFRalpha-Fc fragments, inhibit the migration of mesoderm cells after their ingression through the primitive streak. siRNA-mediated downregulation of Pdgfa expression in the epiblast on one side of the streak strongly blocks the migration of mesoderm cells into that side. Beads soaked in PDGFA elicit a directional attractive movement response in mesoderm cells, showing that PDGFA can provide directional information. Surprisingly, however, PDGF signalling is also required for directional movement towards other attractants, such as FGF4. PDGF signalling controls N-cadherin expression on mesoderm cells, which is required for efficient migration. PDGF signalling activates the PI3 kinase signalling pathway in vivo and activation of this pathway is required for proper N-cadherin expression.     

Epithelial type, ingression, blastopore architecture and the evolution of chordate mesoderm morphogenesis

Chordate embryos show an evolutionary trend in the mechanisms they use to internalize presumptive mesoderm, relying predominantly on invagination in the basal chordates, varying combinations of involution and ingression in the anamniote vertebrates and reptiles, and predominantly on ingression in birds and mammals. This trend is associated with variations in epithelial type and changes in embryonic architecture as well as variations in the type of blastopore formed by an embryo. We also note the surprising conservation of the involution, during gastrulation, of at least a subset of the notochordal cells throughout the chordates, and suggest that this indicates a constraint on morphogenic evolution based on a functional linkage between architecture and patterning. Finally, we propose a model for the evolutionary transitions from gastrulation through a urodele amphibian-type blastopore to gastrulation through a primitive streak, as in chick or mouse.     

Regionalisation of cell fate and morphogenetic movement of the mesoderm during mouse gastrulation

The developmental fate of cells in the epiblast of early-primitive-streak-stage mouse embryos was assessed by studying the pattern of tissue colonisation displayed by lac Z-expressing cells grafted orthotopically to nontransgenic embryos. Results of these fate-mapping experiments revealed that the lateral and posterior epiblast contain cells that will give rise predominantly to mesodermal derivatives. The various mesodermal populations are distributed in overlapping domains in the lateral and posterior epiblast, with the embryonic mesoderm such as heart, lateral, and paraxial mesoderm occupying a more distal position than the extraembryonic mesoderm. Heterotopic grafting of presumptive mesodermal cells results in the grafted cells adopting the fate appropriate to the new site, reflecting a plasticity of cell fate determination before ingression. The first wave of epiblast cells that ingress through the primitive streak are those giving rise to extraembryonic mesoderm. Cells that will form the mesoderm of the yolk sac and the amnion make up a major part of the mesodermal layer of the midprimitive-streak-stage embryo. Cells that are destined for embryonic mesoderm are still found within the epiblast, but some have been recruited to the distal portion of the mesoderm. By the late-primitive-streak-stage, the mesodermal layer contains only the precursors of embryonic mesoderm. This suggests that there has been a progressive displacement of the midstreak mesoderm to extraembryonic sites, which is reminiscent of that occurring in the overlying endodermal tissue. The regionalisation of cell fate in the late-primitive-streak mesoderm bears the same spatial relationship as their ancestors in the epiblast prior to cell ingression. This implies that both the position of the cells in the proximal-distal axis and their proximity to the primitive streak are major determinants for the patterning of the embryonic mesoderm. © 1995 Wiley-Liss, Inc.     

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.Edited by D. Tautz     

Limited left-right cell migration across the midline of the gastrulating avian embryo

During avian development the earliest phase in which the avian embryo expresses axial features of a left-right axis is at the primitive streak stage. Until the stage of definitive primitive streak (streak 4 H&H), the axis seems to possess morphological bilateral symmetry. Morphological asymmetry begins only during the next few hours of incubation, with development of overt morphological and molecular asymmetry within Hensen's node (stage 5 H&H). In this report, we present an experimental study aimed at following the pattern of cell movements during primitive streak formation and gastrulation of specific left-right regions from earlier stages of the avian embryo. To determine the origin of cells contributing to each side of the primitive streak, we applied the dye Lysinated-Rodamine-Dextran (LRD) to one half, either left or right, of the pre-streak blastoderm (stages X–XIII, EG&K). We tried to estimate the relative cell contribution to primitive streak formation, and to the three germ layers evolving during gastrulation in the context of the left-right axis. Moreover, we asked whether the midline serves as a border, that is, as a physiological barrier preventing cell passing during gastrulation. Our results demonstrate that on each side of the axis, either the right or the left, most of the cells originate from the same half of a pre-streak blastoderm, populate the same half of the PS and contribute to tissues largely confined to that particular side. However, along the primitive streak, a few cells were detected on the opposite side of the midline. Moreover, variation in the number of cells crossing the midline at specific regions along the primitive streak was found. Most crossing cells were located near the mid rostrocaudal extent of the primitive streak, from 25–85% of its length. At the posterior end of the primitive streak, fewer crossing cells were detected. At the anterior region of the PS, that is, within Hensen's node, cells do not cross the midline. These results suggest that differences occur in the process of ingression along the rostrocaudal extent of the PS. Dev. Genet. 23:175–184, 1998. © 1998 Wiley-Liss, Inc.     

The enigmatic primitive streak: prevailing notions and challenges concerning the body axis of mammals

The primitive streak establishes the antero‐posterior body axis in all amniote species. It is thought to be the conduit through which mesoderm and endoderm progenitors ingress and migrate to their ultimate destinations. Despite its importance, the streak remains poorly defined and one of the most enigmatic structures of the animal kingdom. In particular, the posterior end of the primitive streak has not been satisfactorily identified in any species. Unexpectedly, and contrary to prevailing notions, recent evidence suggests that the murine posterior primitive streak extends beyond the embryo proper. In its extraembryonic site, the streak creates a node‐like cell reservoir from which the allantois, a universal caudal appendage of all amniotes and the future umbilical cord of placental mammals, emerges. This new insight into the fetal/umbilical relationship may explain the etiology of a large number of umbilical‐associated birth defects, many of which are correlated with abnormalities of the embryonic midline.     

Analysis of extraembryonic mesodermal structure formation in the absence of morphological primitive streak

During mouse gastrulation, the primitive streak is formed on the posterior side of the embryo. Cells migrate out of the primitive streak to form the future mesoderm and endoderm. Fate mapping studies revealed a group of cell migrate through the proximal end of the primitive streak and give rise to the extraembryonic mesoderm tissues such as the yolk sac blood islands and allantois. However, it is not clear whether the formation of a morphological primitive streak is required for the development of these extraembryonic mesodermal tissues. Loss of the Cripto gene in mice dramatically reduces, but does not completely abolish, Nodal activity leading to the absence of a morphological primitive streak. However, embryonic erythrocytes are still formed and assembled into the blood islands. In addition, Cripto mutant embryos form allantoic buds. However, Drap1 mutant embryos have excessive Nodal activity in the epiblast cells before gastrulation and form an expanded primitive streak, but no yolk sac blood islands or allantoic bud formation. Lefty2 embryos also have elevated levels of Nodal activity in the primitive streak during gastrulation, and undergo normal blood island and allantois formation. We therefore speculate that low level of Nodal activity disrupts the formation of morphological primitive streak on the posterior side, but still allows the formation of primitive streak cells on the proximal side, which give rise to the extraembryonic mesodermal tissues formation. Excessive Nodal activity in the epiblast at pre‐gastrulation stage, but not in the primitive streak cells during gastrulation, disrupts extraembryonic mesoderm development.     

Onset of the segmentation clock in the chick embryo: evidence for oscillations in the somite precursors in the primitive streak

Vertebrate somitogenesis is associated with a molecular oscillator, the segmentation clock, which is defined by the periodic expression of genes related to the Notch pathway such as hairy1 and hairy2 or lunatic fringe (referred to as the cyclic genes) in the presomitic mesoderm (PSM). Whereas earlier studies describing the periodic expression of these genes have essentially focussed on later stages of somitogenesis, we have analysed the onset of the dynamic expression of these genes during chick gastrulation until formation of the first somite. We observed that the onset of the dynamic expression of the cyclic genes in chick correlated with ingression of the paraxial mesoderm territory from the epiblast into the primitive streak. Production of the paraxial mesoderm from the primitive streak is a continuous process starting with head mesoderm formation, while the streak is still extending rostrally, followed by somitic mesoderm production when the streak begins its regression. We show that head mesoderm formation is associated with only two pulses of cyclic gene expression. Because such pulses are associated with segment production at the body level, it suggests the existence of, at most, two segments in the head mesoderm. This is in marked contrast to classical models of head segmentation that propose the existence of more than five segments. Furthermore, oscillations of the cyclic genes are seen in the rostral primitive streak, which contains stem cells from which the entire paraxial mesoderm originates. This indicates that the number of oscillations experienced by somitic cells is correlated with their position along the AP axis.     

Morphologic characteristics of definitive and provisional structures of normal 17-day-old human embryos]

The structure of the presomite human embryo was investigated at embryogenesis. The embryonic shield is a three-layer gastrula 810 mkm long in the anteroposterior direction and 855 mkm wide (at the level of the primitive nodule). The primitive streak is 200 mkm long; the primitive nodule is well pronounced. All three germ layers are separately followed only in the cranial end of the embryo. The chordo-mesodermal process, 80 mkm long, is seen and is situated anterior to the primitive nodule, between ecto- and endoderm; in its zone, as well as in the area of the primary nodule and the primary streak, along the middle line, the germ layers are in close contact with each other. In the caudal end the mesoderm grows thin, and the external and internal layers come into contact forming the cloacal membrane. Extraembryonic formations are described: amniotic vesicle, yolk sac, amniotic peduncle, allantois and chorionic membrane wall. Together with the extraembryonic ecto- and endoderm, exocoelomic mesoderm participates in the formation of walls of the primitive germ vesicles. The yolk sac wall contains blood islets. Primary blood vessels are detected in the connective tissue matrix of the chorionic layer and in the amniotic peduncle. According to the anamnesis, morphological data and comparing to the data of the literature on presomitic human embryos, the age of the embryo "Krym" is determined as old as 17 days.     

Fate Mapping Identifies the Origin of SHF/AHF Progenitors in the Chick Primitive Streak

Heart development depends on the spatio-temporally regulated contribution of progenitor cells from the primary, secondary and anterior heart fields. Primary heart field (PHF) cells are first recruited to form a linear heart tube; later, they contribute to the inflow myocardium of the four-chambered heart. Subsequently cells from the secondary (SHF) and anterior heart fields (AHF) are added to the heart tube and contribute to both the inflow and outflow myocardium. In amniotes, progenitors of the linear heart tube have been mapped to the anterior-middle region of the early primitive streak. After ingression, these cells are located within bilateral heart fields in the lateral plate mesoderm. On the other hand SHF/AHF field progenitors are situated anterior to the linear heart tube, however, the origin and location of these progenitors prior to the development of the heart tube remains elusive. Thus, an unresolved question in the process of cardiac development is where SHF/AHF progenitors originate from during gastrulation and whether they come from a region in the primitive streak distinct from that which generates the PHF. To determine the origin and location of SHF/AHF progenitors we used vital dye injection and tissue grafting experiments to map the location and ingression site of outflow myocardium progenitors in early primitive streak stage chicken embryos. Cells giving rise to the AHF ingressed from a rostral region of the primitive streak, termed region ‘A’. During development these cells were located in the cranial paraxial mesoderm and in the pharyngeal mesoderm. Furthermore we identified region ‘B’, located posterior to ‘A’, which gave rise to progenitors that contributed to the primary heart tube and the outflow tract. Our studies identify two regions in the early primitive streak, one which generates cells of the AHF and a second from which cardiac progenitors of the PHF and SHF emerge.     

Developmental patterning of the carbohydrate antigen FC10.2 during early embryogenesis in the chick

An oligosaccharide antigen (FC10.2), formerly described only in mammalian cells and secreted glycoproteins, has been detected and found to display striking temporal and spatial patterning in the chick during early embryonic development. This antigen is expressed on type 1 chains, which are isomers of oligosaccharides of the poly-N-acetyllactosamine series (type 2 chains). Immunoreactivities before and after neuraminidase treatment of serial sections of chick embryos during the first 17 stages of development indicate that the FC10.2 structure occurs predominantly in the sialylated form (S-FC10.2). The FC10.2 and S-FC10.2 antigens are prominent markers of the primordial germ cells, being strongly expressed by these cells from the pre-primitive streak stage onwards. S-FC10.2 is also a clear marker of the pronephric duct from its first appearance. Initially present over the entire apical surface of the ectoderm, antigenicity diminishes in an antero-posterior direction as neurulation proceeds. A unique pattern for a carbohydrate antigen is displayed by cells of the primitive streak; antigenicity is lost with de-epithelialisation and ingression, but is regained in a pericellular distribution on the mesoderm cells that emerge from the primitive streak. Thereafter, successive changes in expression and distribution of FC10.2 and S-FC10.2 are features of mesodermal tissues, particularly during somitogenesis. These antigens are prominent components of the extracellular matrix around the notochord and sclerotome cells. They are also prominent posteriorly in the subectodermal region, ceasing abruptly at the lateral limits of the embryo proper. Although no absolute correlations can yet be made, several features of the distribution of these antigens suggest that they may be integral components of, or ligands for, cell adhesion molecules.     

Ingression of primary mesenchyme cells of the sea urchin embryo: a precisely timed epithelial mesenchymal transition

Epithelial-mesenchyme transitions (EMTs) are familiar to all scholars of development. Each animal system utilizes an EMT to produce mesenchyme cells. In vertebrates, for example, there are a number of EMTs that shape the embryo. Early, entry of epiblast cells into the primitive streak is followed by the emergence of mesoderm via an EMT process. The departure of neural crest cells from the margin of the neural folds is an EMT process, and the delamination of cells from the endomesoderm to form the supporting mesenchyme of the lung, liver, and pancreas are EMTs. EMTs are observed in Drosophila following invagination of the ventral furrow, and even in Cnidarians, which have only two germ layers, yet mesoglial and stem cells delaminate from the epithelia and occupy the matrix between the ectoderm and endoderm. This review will focus on a classic example of an EMT, which occurs in the sea urchin embryo. The primary mesenchyme cells (PMCs) ingress from the vegetal plate of this embryo precociously and in advance of archenteron invagination. Because ingression is precisely timed, the PMC lineage precisely known, and the embryo easily observed and manipulated, much has been learned about how the ingression of PMCs works in the sea urchin. Though the focus of this review is the sea urchin PMCs, there is evidence that all EMTs share many common features at both cellular and molecular levels, and many of these mechanisms are also shown to be involved in tumor progression, especially metastasizing carcinomas.