Alteration of cell adhesion system in amphibian ectoderm cells during primary embryonic induction: changes in reaggregation pattern of induced neurectoderm cells and ultrastructural features of the reaggregate

Summary Cell adhesion was studied during primary embryonic induction. The disaggregation rate and reaggregation patterns were analysed in the ectoderm cells of various developing Cynopus gastrulae and neurulae. The neurectoderm cells disaggregated more slowly with gastrulation, and the neural plate cells of early neurula showed a lesser capacity for disaggregation. Although no differences in reaggregation were found between dorsal and ventral ectoderm at the early gastrula stage, there were significant differences between the induced neurectoderm and the non-induced ventral epidermal cells at the late gastrula stage. Neural plate cells of the early neurula stage were seen to form a chain-like reaggregate, but the ventral epidermal cells of the same embryo formed a cluster-like spherical reaggregate. Scanning electron microscope observations of reaggregates also showed significant differences in adhesive properties between induced neurectoderm and non-induced epidermal cells. The adhesion field of the induced neurectoderm cells was smooth, differing from the distinct ridges of the non-induced epidermal cells. These results suggest that changes in the cell adhesion system, resulting in the formation of a columnar cell shape, may occur immediately after a neural-inducing action.     

ANALYSIS OF CELL PROLIFERATION DURING EARLY EMBRYOGENESIS

Cell proliferation was examined during early embryogenesis of the newt ( Triturus pyrrhogaster ) by various methods. After the two-cell stage, at 23°C, the blastomere (cell) number per whole embryo increased logarithmically until the mid-blastula stage (for about 19 hr) and the rate of increase slowed down in and after the late blastula stage. On the other hand, the synchronous cleavage of the blastomeres at the animal pole continued for 18 hr until the twelfth cleavage (mid-blastula) and the transition from synchronous to asynchronous division occurred abruptly at and after the thirteenth cell division (late blastula). The study also showed that the presumptive neuro-ectoderm consisted mainly of cells of the fifteenth generation (G-15) at the onset of gastrulation (pigment stage).
The present study suggested that the number of ectodermal cells of the early gastrula (stage 12a) nearly doubled during gastrulation at the presumptive neuro-ectoderm. This means that most of the ectodermal cells are in G-16 at the end of gastrulation. On the other hand, both mitotic activity and the rate of cell increase gradually diminished during gastrulation in the ectoderms of both the presumptive neural and epidermal regions, and there are evidently significant differences in both activities between the neuro-ectoderm and the epidermal ectoderm after stage 13b: the epidermal ectoderm showed greater decrease in the rate of both mitotic activity and cell proliferation than the neuro-ectoderm.
These facts suggested that, whether the ectodermal cells will differentiate into neural cells or epidermal cells is determined during G-15 or G-16 in normal primary induction.     

Allocation of epiblast cells to germ layer derivatives during mouse gastrulation as studied with a retroviral vector

The embryonic ectoderm, or epiblast, is the source of the three primary germ layers that form during gastrulation in the mouse embryo. Previous studies have investigated the fate of epiblast cells in early gastrulation stages using clonal analysis of cell lineage and in late gastrulation stages using transplantation of labeled grafts. In this study, we studied the fate of late gastrulation stage epiblast using a clonal analysis based on a retroviral vector encoding the Escherichia coli lacZ gene. We found that by reducing the volume of viral suspension injected into each embryo, it was possible to achieve single infectious events. Our analysis of 20 embryos singly infected at the late streak stage and 21 at the head fold stage revealed clonal descendants in only a single germ layer in each embryo. These results indicate that allocation of epiblast progenitors to a single germ layer fate has occurred by late gastrulation in mouse embryos. © 1995 Wiley-Liss, Inc.     

Cell surface proteins in the early embryogenesis of Pleurodeles waltlii

Surface proteins in the first embryonic stages (8–32 cells, morula, blastula, early and late gastrula) of Pleurodeles waltlii were selectively labelled by ¹²⁵I using lactoperoxidase and glucose/glucose oxidase. Iodination was effected either on non-dissociated embryos or after their dissociation with EDTA. On the outer surface of non-dissociated embryos the two-dimensional electrophoresis revealed only three groups of ¹²⁵I-labelled proteins which did not change during all studied stages. Quite different results were obtained with the cells of dissociated embryos. In addition to the iodinated proteins of the embryonic outer surface seven major iodinated proteins were identified. These proteins originate from the regions of cell-cell contacts in intact embryo. Their two-dimensional pattern in dissociated cells changes between stages 8–32 cells and morula. The next important difference was observed during gastrulation, which corresponds in Pleurodeles waltlii to the first morphogenetic movements. Therefore the outside and inside cell surfaces of embryo are different already at stage 8–32 cells (and probably earlier), before the first step of morphogenesis. The changes of cell surface proteins at early embryonal development take place inside the embryo, in the regions of cell-cell interactions.     

Vertical Signalling Involves Transmission of Hox Information from Gastrula Mesoderm to Neurectoderm

Development and patterning of neural tissue in the vertebrate embryo involves a set of molecules and processes whose relationships are not fully understood. Classical embryology revealed a remarkable phenomenon known as vertical signalling, a gastrulation stage mechanism that copies anterior-posterior positional information from mesoderm to prospective neural tissue. Vertical signalling mediates unambiguous copying of complex information from one tissue layer to another. In this study, we report an investigation of this process in recombinates of mesoderm and ectoderm from gastrulae of Xenopus laevis. Our results show that copying of positional information involves non cell autonomous autoregulation of particular Hox genes whose expression is copied from mesoderm to neurectoderm in the gastrula. Furthermore, this information sharing mechanism involves unconventional translocation of the homeoproteins themselves. This conserved primitive mechanism has been known for three decades but has only recently been put into any developmental context. It provides a simple, robust way to pattern the neurectoderm using the Hox pattern already present in the mesoderm during gastrulation. We suggest that this mechanism was selected during evolution to enable unambiguous copying of rather complex information from cell to cell and that it is a key part of the original ancestral mechanism mediating axial patterning by the highly conserved Hox genes.     

Correlation between the cell cycle in the neurectoderm and differentiation during the early development of Xenopus laevis. 2 Inhibition of DNA synthesis and mitosis during gastrulation

The hypothesis that cell differentiation during primary induction may be coupled with a 'quantal' cell cycle in the presumptive neurectoderm is considered. Embryos of Xenopus laevis were incubated with fluorodeoxyuridine or mitomycin C during gastrulation or neurulation. Embryos treated during the gastrula stage developed abnormalities of the brain but not of other tissues. This is seen as lending support to the idea that the wave of DNA synthesis and mitosis in the presumptive neurectoderm during gastrulation is an important event in the process of primary induction.     

Cell fate and cell lineage in the endoderm of the presomite mouse embryo, studied with an intracellular tracer

The fate of the embryonic endoderm (generally called visceral embryonic endoderm) of midstreak to neural plate stages of the mouse embryo was studied by microinjecting horseradish peroxidase (HRP) into single axial endoderm cells in situ, and tracing the labeled descendants to early somite stages in vitro. Axial endoderm cells along the anterior fifth of the late streak/neural plate stage embryo contributed descendants either to the yolk sac endoderm or to the anterior intestinal portal. Cells of the exposed head process contributed to the trunk endoderm and notochord; neighboring endoderm cells contributed to the dorsal foregut. Contributions to the ventral foregut came from endoderm at, and anterior to, the distal tip of the younger, midstreak embryo (in which the head process was not yet exposed). Endoderm over the primitive streak contributed to the postsomite endoderm. We argue from these results and those in the literature that during gastrulation the axial embryonic endoderm is of mixed lineage: (1) an anterior population of cells is derived from primitive endoderm and contributes to the yolk sac endoderm; (2) a population at, and anterior to, the distal tip of the midstreak embryo, extending more anteriorly at late streak/neural plate stages, is presumed to emerge from primitive ectoderm at the beginning of gastrulation and contributes to the foregut and anterior intestinal portal; (3) the axial portion of the head process that begins to incorporate into the ventral surface at the late streak stage contributes to notochord and trunk endoderm. Cells or their descendants that were destined to die within 24 hr were evident at the midstreak stage. There was a linear trend in the incidence of cell death among labeled cells at the late streak/neural plate stages, ranging from 27% caudal to the node to 57% in the anterior fifth of the embryo. The surviving axial endoderm cells divided sufficiently fast to double the population in 24 hr.     

Developmental abnormalities of NT mouse embryos appear early after implantation

In mammals, cloning by nuclear transfer (NT) into an enucleated oocyte is a very inefficient process, even if it can generate healthy adults. We show that blastocysts derived from embryonic stem (ES) donor cells develop at a high rate, correctly express the pluripotential marker gene Oct4 in ICM cells and display normal growth in vitro. Moreover, the majority of them implant in the uterus of recipient females. We combine embryological studies, gene expression analysis during gastrulation and generation of chimaeric embryos to identify the developmental origin (stage and tissue affected) of NT embryo mortality. The majority died before mid-gestation from defects arising early, either at peri-implantation stages or during the gastrulation period. The first type of defect is a non-cell autonomous defect of the epiblast cells and is rescued by complementation of NT blastocysts with normal ES or ICM cells. The second type of defect affects growth regulation and the shape of the embryo but does not directly impair the initial establishment of the patterning of the embryo. Only chimaeras formed by the aggregation of NT and tetraploid embryos reveal no growth abnormalities at gastrulation. These studies indicate that the trophoblast cell lineage is the primary source of these defects. These embryological studies provide a solid basis for understanding reprogramming errors in NT embryos. In addition, they unveil new aspects of growth regulation while increasing our knowledge on the role of crosstalk between the extra-embryonic and the embryonic regions of the conceptus in the control of growth and morphogenesis.     

The program of protein synthesis during the development of the micromere-primary mesenchyme cell line in the sea urchin embryo

Changes in the pattern of protein synthesis were analyzed during the in vitro development of the micromere-primary mesenchyme cell line of the sea urchin embryo. Micromeres were isolated and cultured from 16-cell stage embryos, and primary mesenchyme cells were isolated and cultured from early gastrulae. Both cell isolates developed normally in culture with about the same timing as their in situ counterparts in control embryos. Newly synthesized proteins were labeled with [³H]valine at several stages of development and were analyzed by two-dimensional polyacrylamide gel electrophoresis and fluorgraphy. The electrophoretic pattern of labeled proteins changed dramatically during development. More than half of the analyzed proteins underwent qualitative or quantitative changes in their relative rates of valine incorporation and these changes were highly specific to this cell line. Almost all of the changes were initiated prior to gastrulation and many prior to hatching. The highest frequency of changes in the micromere pattern of protein synthesis occurred between hatching and the start of gastrulation. This peak of activity coincided with the normal time of ingression of the primary mesenchyme and preceded the differentiation of spicules by more than 30 hr. Most of the observed changes were characterized as either decreases in the synthesis of proteins that showed maximum incorporation at the 16-cell stage or increases in the synthesis of proteins that showed maxima in the fully differentiated cells. Very few proteins exhibited transient synthetic maxima at intermediate stages. Thus, the program of protein synthesis associated with the development of micromeres consists largely of a switch in emphasis from early to late proteins, with the primary time of switching being between hatching and the onset of gastrulation.     

Retinoic acid can mimic endogenous signals involved in transformation of the Xenopus nervous system.

In the frog Xenopus laevis, signals from the mesoderm divert part of the ectoderm from an epidermal to a neural fate. In the course of neural induction, the neurectoderm also acquires anterior-posterior polarity. In this report, the early expression of two genes, XlHbox6 and the neurofilament gene XIF6, is examined. The pattern of expression of the two genes seen in the tailbud embryo develops progressively over a 4 hr period following gastrulation. Physiological concentrations of retinoic acid can mimic this effect in isolated embryonic explants, consistent with the involvement of retinoic acid, or a closely related molecule, in localizing gene expression along the anterior-posterior axis of the neural tube.     

Localized axis determinant in the early cleavage embryo of the goldfish, Carassius auratus

 The teleost dorsoventral axis cannot be distinguished morphologically before gastrulation. In order to examine whether the yolk cell affects axis determination, we bisect early cleavage embryos of the goldfish, Carassius auratus. When the vegetal yolk hemisphere is removed by bisection along the equatorial plane at the 2-cell stage, the embryos develop abnormally and exhibit a symmetrical morphology. No dorsal structures, such as notochord, somites and neural tube, differentiate and no embryonic shield is formed during gastrulation. In addition, no goosecoid mRNA is expressed before gastrulation. The frequency of abnormality decreases as the age at which the vegetal yolk hemisphere is removed increases. Most embryos removed at the 32-cell stage develop normally. Their morphological phenotype is similar to that of a Xenopus ventralized embryo generated by ultraviolet irradiation on the vegetal hemisphere soon after fertilization. We also observed that, when the embryos were bisected along the first cleavage plane at the 2-cell stage, the proportion of pairs of embryos of which one embryo developed normally was 44.8%. These results indicate that the vegetal yolk hemisphere of the early cleavage embryo of the goldfish contains axis determination factor(s), which are necessary for generation of dorsal structures. Furthermore, it is suggested that these determinant(s) are distributed asymmetrically within the vegetal yolk hemisphere. Received: 25 May 1996 / Accepted: 19 September 1996     

Inhibition of mitogen activated protein kinase signaling affects gastrulation and spiculogenesis in the sea urchin embryo

The mitogen activated protein (MAP) kinase signaling cascade has been implicated in a wide variety of events during early embryonic development. We investigated the profile of MAP kinase activity during early development in the sea urchin, Strongylocentrotus purpuratus, and tested if disruption of the MAP kinase signaling cascade has any effect on developmental events. MAP kinase undergoes a rapid, transient activation at the early blastula stage. After returning to basal levels, the activity again peaks at early gastrula stage and remains high through the pluteus stage. Immunostaining of early blastula stage embryos using antibodies revealed that a small subset of cells forming a ring at the vegetal plate exhibited active MAP kinase. In gastrula stage embryos, no specific subset of cells expressed enhanced levels of active enzyme. If the signaling cascade was inhibited at any time between the one cell and early blastula stage, gastrulation was delayed, and a significant percentage of embryos underwent exogastrulation. In embryos treated with MAP kinase signaling inhibitors after the blastula stage, gastrulation was normal but spiculogenesis was affected. The data suggest that MAP kinase signaling plays a role in gastrulation and spiculogenesis in sea urchin embryos.     

MITOTIC ACTIVITY AND CELL PROLIFERATION IN PRIMARY INDUCTION OF NEWT EMBRYO

Mitotic activity and cell proliferation of newt ( Triturus pyrrhogaster ) embryo were examined with special reference to primary induction.
Mitotic activity of gastrula ectoderm gradually decreases during gastrulation. The ectoderm, which is isolated from mid-gastrula (stage 12b) and cultured in vitro , also shows gradual decrease in mitotic activity during cultivation and the mitotic activity steeply decreases after 48 hr.
The ectoderm cultured with heterologous inductor (GPL-extract) shows a temporal suppression in mitotic activity. The ectoderm of the whole gastrula also shows a regional suppression where it is in contact with the chorda-mesoderm.
The number of the ectodermal cells increases about 2 times after 24 hr culture and to more than 3 times after 48 hr culture. Accordingly it is certain that the majority of the ectodermal cells divides at least one time in the course of 48 hr.
Histological examination of the ectoderm cultured together with the inductor reveals that differentiation of undifferentiated ectoderm to neural tissues is accomplished at least within 48 hr after cultivation with the inductor.
The present examination shows the possibility that the mitotic activity of the ectoderm may be temporarily suppressed by the inductor and that it then decreases along with neural cell differentiation after recovery of the activity.
The results also suggest that the determination of undifferentiated ectoderm to neural tissues occurs before the second cell division after the contact with the inductor and the events occurring during the first cell cycle after activating by the inducing stimulus are critical for the primary induction.     

Developmental toxicity of methanol: Pathogenesis in CD-1 and C57BL/6J mice exposed in whole embryo culture

BACKGROUND: Methanol causes axial skeleton and craniofacial defects in both CD-1 and C57BL/6J mice during gastrulation, but C57BL/6J embryos are more severely affected. We evaluated methanol-induced pathogenesis in CD-1 and C57BL/6J embryos exposed during gastrulation in whole embryo culture. METHODS: Conceptuses with five to seven somites were exposed to 0, 1, 2, 3, 4, or 6 mg methanol/ml culture medium for 24 hr and embryonic morphology was assessed. Cell death was evaluated by histology and LysoTracker red staining, and cell-cycle distribution was evaluated by flow cytometry. RESULTS: In C57BL/6J embryos, craniofacial defects were observed at 3 mg methanol/ml and greater. The response for CD-1 embryos was different, with increased dysmorphology only at 6 mg/ml. However, protein content in CD-1 embryos was reduced at 3 mg methanol/ml and above, indicating growth retardation. Yolk sac toxicity occurred only at 6 mg methanol/ml in both strains. Methanol caused only small changes in cell-cycle distribution, while cell death was induced at 4 and 6 mg methanol/ml in both strains after 8 hr. The extent of cell death after 8 hr was greater in C57BL/6J embryos, and increased over time through 18 hr; in contrast, CD-1 embryos showed less cell death at 18 than at 8 hr, suggesting recovery. CONCLUSIONS: Cell death plays a prominent role in methanol-induced dysmorphogenesis, while cell-cycle perturbation may not. Differences in the extent of cell death between CD-1 and C57BL/6J embryos correlated with differences in the severity of dysmorphogenesis.     

Expression of cell adhesion molecule E-cadherin in Xenopus embryos begins at gastrulation and predominates in the ectoderm

The expression of the Ca2+-dependent epithelial cell adhesion molecule E-cadherin (also known as uvomorulin and L-CAM) in the early stages of embryonic development of Xenopus laevis was examined. E-Cadherin was identified in the Xenopus A6 epithelial cell line by antibody cross-reactivity and several biochemical characteristics. Four independent mAbs were generated against purified Xenopus E-cadherin. All four mAbs recognized the same polypeptides in A6 cells, adult epithelial tissues, and embryos. These mAbs inhibited the formation of cell contacts between A6 cells and stained the basolateral plasma membranes of A6 cells, hepatocytes, and alveolar epithelial cells. The time of E-cadherin expression in early Xenopus embryos was determined by immunoblotting. Unlike its expression in early mouse embryos, E-cadherin was not present in the eggs or early blastula of Xenopus laevis. These findings indicate that a different Ca2+-dependent cell adhesion molecule, perhaps another member of the cadherin gene family, is responsible for the Ca2+-dependent adhesion between cleavage stage Xenopus blastomeres. Detectable accumulation of E-cadherin started just before gastrulation at stage 9 1/2 and increased rapidly up to the end of gastrulation at stage 15. In stage 15 embryos, specific immunofluorescence staining of E-cadherin was discernible only in ectoderm, but not in mesoderm and endoderm. The ectoderm at this stage consists of two cell layers. The outer cell layer of ectoderm was stained intensely, and staining was localized to the basolateral plasma membrane of these cells. Lower levels of staining were observed in the inner cell layer of ectoderm. The coincidence of E-cadherin expression with the process of gastrulation and its restriction to the ectoderm indicate that it may play a role in the morphogenetic movements of gastrulation and resulting segregation of embryonic germ layers.     

Self-organization in the determination of the size of the axial structures in the embryogenesis of the clawed toad

Experiments were performed using X. laevis embryos during gastrulation and neurulation (stages 10, 11 1/2, 12 1/2, 13 1/2, 15 and 18). Part of presumptive epidermis and lateral plate mesoderm was removed, and embryos raised until stage 25. The size of axial structures (notochord, somite mesoderm, central nervous system) was determined using serial histological sections and compared with that of control embryos. In experimental embryos, the size of axial structures was decreased. Until a specific stage of development, close correlation was found between the volume of embryonic compartment corresponding to a particular, structure and the volume of presumptive epidermis and lateral plate mesoderm. This stage is individual for each axial organ: middle gastrula (stage 11 1/2) for notochord, late gastrula (stage 12 1/2) for somite mesoderm, and late neurula (stage 18) for central nervous system. This data suggest that differentiation pattern of ecto-mesodermal rudiment is subject to regulation during gastrulation-neurulation, and subdivision of ectoderm and mesoderm into axial and non-axial tissues is a self-organizing process.     

Changing rates of histone mRNA synthesis and turnover in Drosophila embryos

The rates of synthesis and turnover of histone mRNA in Drosophila embryos were determined by hybridization of in vivo and in vitro labeled embryonic RNA to Drosophila histone DNA of the recombinant plasmid cDm500. There is a large store of maternal histone mRNA, equivalent to at least 7 X 10(7) copies of each of the five classes of histone mRNA per embryo. Embryonic synthesis of histone mRNA begins at 90 min after oviposition, making the histone genes among the first to be transcribed by embryonic nuclei. Embryonic histone mRNA accumulates rapidly during the blastoderm and gastrula stages. The peak in the rate of histone mRNA synthesis per embryo coincides with the peak in the rate of DNA synthesis per embryo, which occurs at 6 hr after oviposition. After 6 hr, as the rate of DNA synthesis per embryo decreases, the rate of histone mRNA synthesis and the total mass of histone mRNA per embryo both drop sharply. The rate of histone mRNA synthesis per gene falls more than 60 fold in the first 13 hr after oviposition, from 1.3 -2.5 copies per gene-min at 2 hr to 0.02-0.03 copies per gene-min at 13 hr. From measurements of the mass of histone mRNA per embryo and of the rate of accumulation of newly synthesized histone mRNA at a number of stages of early embryogenesis we determined that the cytoplasmic half-life of histone mRNA decreases approximately 7 fold during early Drosophila development, from 2.3 hr at blastoderm to 20 min by the end of gastrulation. Thus the level of expression of histone genes in Drosophila development is controlled not only by the size of the maternal mRNA pool and changes in the rate of histone mRNA synthesis, but also by changes in the rate of histone mRNA turnover.