The chicken blastoderm: current views on cell biological events guiding intercellular communication

Molecular anatomy of the pre-primitive-streak chick embryo

The early stages of development of the chick embryo, leading to primitive streak formation (the start of gastrulation), have received renewed attention recently, especially for studies of the mechanisms of large-scale cell movements and those that position the primitive streak in the radial blastodisc. Over the long history of chick embryology, the terminology used to define different regions has been changing, making it difficult to relate studies to each other. To resolve this objectively requires precise definitions of the regions based on anatomical and functional criteria, along with a systematic molecular map that can be compared directly to the functional anatomy. Here, we undertake these tasks. We describe the characteristic cell morphologies (using scanning electron microscopy and immunocytochemistry for cell polarity markers) in different regions and at successive stages. RNAseq was performed for 12 regions of the blastodisc, from which a set of putative regional markers was selected. These were studied in detail by in situ hybridization. Together this provides a comprehensive resource allowing the community to define the regions unambiguously and objectively. In addition to helping with future experimental design and interpretation, this resource will also be useful for evolutionary comparisons between different vertebrate species.

The behavior of chick gastrula mesodermal cells under the unidirectional tractive force parallel to the substrata

Advancement of leading lamellae of a migratory cell inevitably causes a strain inside the cell body. We investigated the effect of the tension arisen inside a mesodermal cell on its behavior by pulling the cell body unidirectionally along the substratum. Chick gastrula mesodermal cells, known as highly migratory, were dissociated into single cells in sodium citrate buffer, conjugated with paramagnetic beads activated by tosyl-residue (4.5 microns in diameter) and seeded onto coverglasses coated with fibronectin. After the cells spread on the substratum and protruded cellular processes in all directions, they were exposed to a non-uniform magnetic field by a magnet. Thus the cells bearing the beads were pulled with a force in the order of 10(−10) N. The behavior of such cells was recorded with a time-lapse video taperecorder and assessed quantitatively. Shortly after the magnetic force was applied, the beads stuck to the cells were aligned in tandem along the line of magnetic force at the site for the magnet. Subsequently, they frequently came to extend their leading lamella precisely counter to the traction on the line of the beads. Observation with scanning electron microscope revealed that a large part of the beads attached to the cells were wrapped in the cell membrane. In this condition, the cells were stretched locally between the attachment site of the beads and adhesion plaques beneath the leading edge, which was formed in a direction away from the traction. It was proved statistically that such cells tended to locomote away from the magnet at the 0.1% significance level with Hotelling's T2-test. In contrast, the mesodermal cells free of the artificial traction in three kinds of control experiments did not show such a preference in the direction of locomotion. These results proved that migratory cells tended to move in the direction away from the tractive force parallel to the substratum, suggesting that advancement of a leading lamella is accelerated when it is stretched along the direction of projection by a mechanical force of sufficient strength. Implication of this finding to the mechanism of cell locomotion will be discussed.

A homeobox gene involved in node, notochord and neural plate formation of chick embryos

We have isolated a chicken cDNA clone, Cnot, resembling in sequence and expression pattern the Xenopus homeobox gene Xnot. The major, early transcription domains of Cnot are the node, the notochord and prenodal and postnodal neural plate caudal from the prospective hindbrain level. All these cell populations appear to be descendants of the Cnot-expressing cells of the node, suggesting a cell lineage relationship. After the onset of somitogenesis, a second, independent expression domain appears in the neural folds at the prospective mid- and forebrain levels, and further transcripts are found in the epiphysis, the ventral diencephalon, the preoral gut and the limb buds. Transplantation of nodes from extended streak embryos leads to the formation of ectopic notochords, which express Cnot in the typical, cranially decreasing gradient. Transplantation of young nodes to young hosts has previously been described to induce secondary embryos. We observed that secondary chick embryos express Cnot in node derived, notochord-like structures and in the anterior neural plate, similar to the domains seen in primary embryos. However, expression was absent from the posterior neural plate, which in the induction experiments is excluded from the node lineage. This finding corroborates our initial conclusion about a cell lineage relationship between node, notochord, and neural plate defined by Cnot expression. The midline mesoderm of vertebrate embryos consists of two tissues, the prechordal mesoderm and the notochord. The anterior notochord, the head process, may represent an intermediate form. The transition from prechordal to chordal mesoderm can be followed by the expression of the two marker homeobox genes goosecoid and Cnot, first in the primitive streak, and then in the head process. We suggest that expression of goosecoid or Cnot is involved in the specification of a prechordal or notochordal identity, respectively. A transition from goosecoid to Cnot expression may proceed, while cells are still in the epiblast, but not after becoming mesodermal. A molecular coding of axial positions in the midline mesoderm may occur by specific homeobox genes, similar to the situation in the neural tube and the somitic mesoderm.

Microdissection by ultrasonication: application to early chick embryos

A technique utilizing microdissection by ultrasonication was applied to scanning electron microscopy of chick embryos during the first three days of incubation. Using a tank cleaner operating at 80 kHz, whole embryos immersed in pure acetone were sonicated until fragmentation became evident. At 12 hr incubation disintegration occurred by one second or less. At 18 hr, three sonic bursts of one second each produced only partial fragmentation. All three germ layers retained their original relationships to each other. During the second day of incubation, large pieces of integument were removed and somites began to microdissect after 10-20 seconds of sonication. Late in the third day of incubation, sonication for 1 min or more was required to produce significant microdissection. Living embryos exposed to 0.1% collagenase for 10 min prior to standard fixation fragmented in a different manner. Lamellipodia and filopodia were most sensitive and were largely destroyed. The three major germ layers (ectoderm, endoderm, mesoderm), however, retained their structural integrity and original relationships to each other. Factors contributing to the results reported here include: 1) extracellular fibrils of varying chemical composition, 2) primitive cell junctions, 3) biomechanical stability in the nonfibrillar portions of the extracellular matrix, and 4) effects of technical procedures performed prior to sonication. Sonicated tissues of early embryos reveal features that are difficult to demonstrate in other ways and may be unrecognized in conventional preparations.

Gastrulation in the mouse embryo: ultrastructural and molecular aspects of germ layer morphogenesis

Ultrastructural studies and lineage analyses of gastrulating mouse embryos have revealed that different morphogenetic tissue movements are involved in the formation of the three definitive germ layers. Definitive ectoderm is formed by epibolic expansion of the pre-existing progenitor population in the embryonic ectoderm. Formation of the mesoderm and the endoderm is initiated by cellular ingression at the primitive streak. The mesodermal layer is established by cell migration and cell sheet spreading, but the endoderm is formed by replacing the original primitive endodermal population. To this date, genes that are expressed during mouse gastrulation mostly encode cell surface adhesion or signalling molecules, growth factors and their receptors, and putative transcriptional factors. Their precise role during gastrulation remains to be investigated.

Microinjection of antifibronectin antibodies in the chicken blastoderm: inhibition of mesoblast cell migration but not of cell ingression at the primitive streak

The involvement of fibronectin in adhesion and migration of individual mesoblast cells during chicken gastrulation was examined after microinjection of functional and nonfunctional antifibronectin antibodies in the blastoderm during the period of rapid migration of mesoblast cells. The injection of affinity-purified polyclonal antihuman fibronectin antibody (total IgG or Fab fragment) or of monoclonal antichicken cellular fibronectin caused a thickening of the primitive streak, which was composed of loosely connected cells. This effect was most evident at the level of Hensen's node, and very few mesoblast cells were observed migrating in the space between upper layer and deep layer. The obvious explanation of this effect was that the de-epithelialization of upper layer cells persisted in the presence of antibodies, but ingressed cells failed to emigrate from the primitive streak. Immunostaining of microinjected antibodies showed binding to the basement membrane, to the cell surface of mesoblast cells that had migrated before microinjection occurred, and to the cell surface of deep layer cells. Cells that ingressed and detached in the course of reincubation of the embryo possessed little immunolabelling along their cell surface. The results suggest that the failure of ingressed cells to emigrate from the primitive streak and to form mesoblast was due (1) to alterations in adhesion between newly ingressed primitive streak cells, which had the ability to detach but possessed relatively little fibronectin along their cell surfaces and a small number of cell protrusions, and (2) probably to a lack of adhesion of detached cells to the basement membrane, which was blocked by the presence of antifibronectin antibodies. We conclude that the presence of fibronectin in the basement membrane is required for emigration of ingressed cells and migration of mesoblast cells to occur. Once migration has commenced, fibronectin is also deposited along the cell surface of migrating cells, a factor that may increase their mutual adhesion.

Scanning electron microscopy of developing primary endoderm during first 6 hours of incubation in the chick

Development of primary endoderm in the domestic fowl (Gallus domesticus) is described in scanning electron microscopy (SEM) supplemented by transmission electron microscopy (TEM). Although complicated by great variability, the ventral surface of the blastoderm reveals this process during the first 6 hours of incubation. Primary endoderm arises (1) from the hypoblast, (2) from the margin of the area pellucida, and (3) from intervening portions of the area pellucida. The early hypoblast becomes several cells thick while individual cells are still spherical. TEM reveals a variety of immature cell junctions. During subsequent flattening of these cells into primary endodermal epithelium, numerous filopodia arise from their surfaces. These are 0.20-0.25 microns in diameter. They become long and branched, attaching to each other and to other cell bodies. Similar filopodial processes are present less conspicuously among cells in the margin of the area pellucida. Here, there is pseudopodial evidence that cells or cell sheets creep along the ventral surface of the epiblast. The filopodia disappear as cell flattening proceeds. The ventral surface of the exposed epiblast delaminates cells that become free after their exploratory filopodia and lamellipodia are put forth. Lateral contacts among cell bodies from the above three sources increase until a continuous epithelium is formed. The primary endoderm of the embryo, a simple squamous epithelium that separates the connective tissue space above from the gastrocoele below, is generated by these developmental events.

Immunohistochemical localisation of monoclonal antibody R 24-recognized ganglioside Glac2 in early chick embryos

The spatio-temporal cellular expression and biosynthesis of ganglioside Glac2 was investigated in early chick embryogenesis. For demonstration of embryonic Glac2-biosynthesis, chick embryos of stage 0 and of stages 4-5 were incubated in vitro in the presence of radioactive sugar precursors. It was found that chick embryos synthesize Glac2 as early as at the blastula stage as well as at the gastrula stage, both within the area pellucida and the area opaca. In contrast to the biosynthetical findings immunohistochemical staining of the chick embryos at various stages by aid of the mouse monoclonal antibody (mAb) R 24, specific for the immunoepitope NeuAc alpha, 8NeuAc alpha, 3Gal beta less than, as present on the ganglioside Glac2, revealed a spatio-temporal cellular pattern of expression of this ganglioside in early chick embryos. Immunohistochemical staining of the chick embryo at stage 0 shows that all cells of the embryo, the extraembryonic epiblast and the yolk endoderm included, are mAb R 24-positive. At the intermediate streak stage (stage 3), the cranial part of the deep layer, the so-called endophyll, is strongly mAb R 24-positive, whereas at the end of gastrulation (stage 5), mAb R 24-recognized epitopes appear to be restricted to a narrow band of deep-layer cells in the endophyllic crescent and to the yolk endoderm of the area opaca. At this stage, no labelling by the antibody is observed in cell layers of the future embryo. The beginning of neurulation (stage 7) is characterized by the expression of the mAb R 24-recognized epitope in the notochord, whilst the deep layer in the cranial part of the neural fold still expresses this epitope. No ecto- or mesodermal structures are stained by the antibody at this developmental stage. During further development (stage 12 and 13), mAb R 24-reactivity is restricted to the cranial part of the embryo with a preferential staining of cells of endodermal origin. At these stages, the notochord expresses mAb R 24 binding sites only in its cranial region. The spatial and temporal correlation between the presence of mAb R 24-recognized epitopes and the morphogenetic positioning of tissues may be indicative for a possible role of the ganglioside Glac2 in corresponding cellular interactions.

The behavior and cytoskeletal system of chick gastrula mesodermal cells on substrata coated with lines of fibronectin

In order to investigate the mechanism of the formation of the mesodermal layer during chick gastrulation, we observed the behavior of fragments of mesodermal cells explanted and cultured on substrata coated with parallel lines of fibronectin (FN). We also examined the distribution of F-actin, alpha-actinin, and vinculin in explanted fragments by immunocytochemical methods noting particularly their distribution with respect to FN lines. Explants of mesodermal cells flattened on FN-coated substrata and then became elliptical with the major axis of the ellipse oriented along the FN lines and migrated along them. The peripheral cells of explants extended filopodia and lamellipodia which attached preferentially to FN lines and then contracted, pulling other mesodermal cells in explants along passively. Vinculin and alpha-actinin in peripheral anchoring filopodia and lamellipodia co-localized with the terminations of F-actin bundles and with FN lines, suggesting that the peripheral cells were the moving force for explant translocation. We propose based on these results that in vivo, peripheral cells of invaginated cell mass are guided by the known FN-rich fibrous extracellular matrix on the basal surface of epiblast to move outwards; the rest linked to the peripheral cells are pulled away from the primitive streak to spread in tandem to form the mesodermal layer.

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)

Distribution and functional role of laminin during induction of the embryonic axis in the chick embryo

Laminin is a major glycoprotein of basement membranes and has been shown to promote cell adhesion, and movement of various nonepithelial cells and tumour cells. Using antibodies to laminin in paraffin sections and cultured embryos, we have studied the distribution of laminin and its involvement in the first morphogenetic events, beginning with the first extensive cellular migrations and interactions that result in the induction of the primitive streak (PS) and of the neural plate in the early chick embryo. Laminin immunogold labeling was not detected in the blastoderm at stage X. At stage XIII, laminin immunoreactivity was detected at the ventral surface of the epiblast and in the entire hypoblast. The intense labeling of the hypoblast indicated that these cells are active in laminin synthesis. Extracellular matrix (ECM) started accumulating as the first embryonic spaces were forming, before the morphogenetic movements of gastrulation were initiated. Immunogold labeling revealed a punctate pattern of laminin distribution in the ECM in the blastocoele, and in the space below the neural plate. Laminin, which is a multidomain molecule known to interact with other molecules of the ECM and with the cell surface, could serve as the scaffold for highly specific contact points of migrating cells and for the folding of epithelial sheets during this time in the developing embryo. We incubated blastoderms at stages X and XIII with laminin antibodies (1:30 dilution) for 4 h, then cultured the blastoderms further in plain egg albumin. The laminin antibodies did not interfere with triggering of PS cell movements, but perturbed the normal migration pattern of these cells. A normal PS did not form and, as a consequence, the embryonic axis was not induced.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of cell junctions in gametogenesis and in early embryonic development

Recent reviews of the role of cell junctions in development have focused primarily upon functions related to the relatively subtle physiological modulation of their subunits in relation to fundamental developmental processes in a wide variety of organisms. There is, however, considerable support from numerous laboratories that the more radical modulation of the presence and number of junctional subunits in many diverse tissues may play a pivotal role in a wide spectrum of developmental phenomena ranging from gametogenesis to organogenesis. Since a great deal of recent interest in this latter subject has concentrated upon vertebrate systems including mammals, this review will examine the functional significance of the modulation of gap junctions, tight junctions and desmosomes in a developing idealized mammalian system from gamete formation to tissue and organ differentiation during embryo-genesis.