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[The theory of germ layers: the debatable aspects].

Data on early embryonic development of animals provided the basis for the following conclusions. In most (if not in all) species, germinal layers are not primitive structures recapitulating the initial organs of ancestral forms. The initial cell differentiation in the embryo is not directly associated with gastrulation, and the common concept of a fundamental relationship between germinal layer formation and cell differentiation is erroneous. Specification of cell types during early development and morphogenetic movements during gastrulation are relatively autonomous processes controlled by different genetic systems. The formation of germinal layers during individual development of different multicellular animals can be understood taking into account that segregation of cell layers is an essential element of epithelial morphogenesis. Movement of cells and cell layers during gastrulation is a specific manifestation of a general morphogenetic principle.

Animals↗
PubMed2002

Cell lineage, axis formation, and the origin of germ layers in the amphipod crustacean Orchestia cavimana.

Embryos of the amphipod crustacean Orchestia cavimana are examined during cleavage, gastrulation, and segmentation by using in vivo labelling. Single blastomeres of the 8- and 16-cell stages were labelled with DiI to trace cell lineages. Early cleavage follows a distinct pattern and the a/p and d/v body axes are already determined at the 4- and 8-cell stages, respectively. In these stages, the germinal rudiment and the naupliar mesoderm can be traced back to a single blastomere each. In addition, the ectoderm and the postnaupliar mesoderm are separated into right and left components. At the16-cell stage, naupliar ectoderm is divided from the postnaupliar ectoderm, and extraembryonic lineages are separated from postnaupliar mesoderm and endoderm. From our investigation, it is evident that the cleavage pattern and cell lineage of Orchestia cavimana are not of the spiral type. Furthermore, the results of the labelling show many differences to cleavage patterns and cell lineages in other crustaceans, in particular, other Malacostraca. The cleavage and cell lineage patterns of the amphipod Orchestia are certainly derived within Malacostraca, whose ancestral cleavage mode was most likely of the superficial type. On the other hand, Orchestia exhibits a stereotyped cell division pattern during formation and differentiation of the germ band that is typical for malacostracans. Hence, a derived (apomorphic) early cleavage pattern is the ontogenetic basis for an evolutionarily older cell division pattern of advanced developmental stages. O. cavimana offers the possibility to trace the lineages and the fates of cells from early developmental stages up to the formation of segmental structures, including neurogenesis at a level of resolution that is not matched by any other arthropod system.

Animals↗
PubMed2003

Degenerative changes and cell death in long-living homo- and heterotopic transplants from embryonic germ layers of rat neocortex.

Morphological study of allotransplants of rat embryonic neocortex 14-18 months after transplantation into the neocortex, lateral cerebral ventricle, and sciatic nerve of adult animals revealed death of nerve and glial cells in the delayed postoperation period independently on the site of transplantation. After heterotopic transplantation the count of degenerated neurons was 2 times higher that after homotopic transplantation. In heterotopic transplants a considerable number of grafted neurons underwent reversible and irreversible degenerative changes accompanied by their premature aging. Neuronal death is probably determined by insufficiency of trophic influence from afferent structures and target tissues. We hypothesized that antiapoptotic preparations can be used for prevention of transplanted cell death. It was also found that degeneration of neurons was associated with impaired vascularization of transplants and pronounced immune reaction of the recipient in late posttransplantation period. Transplantation of embryonic brain structures can serve as a model system in studies concerning involutive and pathological processes in the central nervous system and in the search for factors improving survival of neurons.

Animals↗
PubMed2003

Introduction of cell markers into germ layer tissues of the mouse gastrula by whole embryo electroporation.

We have optimized the technique of electroporation for introducing genetic markers into cells of the gastrulating mouse embryo to follow cell fates, tissue movement, and lineage differentiation. Using a plate-needle electrode combination and specific route of plasmid delivery, labeling could be targeted to discrete regions of the epiblast or the endoderm of the late gastrula. Among the various types of fluorescent and chromogenic reporter constructs tested, those driven by CMV promoter are efficient and strong expression can be detected as soon as 2-3 h after electroporation. The efficacy of marking cell lineages by CRE-mediated activation of reporters proved to be inefficient for tracking cell lineages due to an obligatory 8-9-h lag from the electroporation of constructs to the expression of reporter. This significant time lag also raises concern of the temporal precision at which tissue- or stage-specific knock-out or activation of genetic activity may be achieved by the Cre-loxP mechanism.

Alkaline Phosphatase↗
PubMed2005

Germ-layer specification and control of cell growth by Ectodermin, a Smad4 ubiquitin ligase.

TGF-beta signaling is essential for development and proliferative homeostasis. During embryogenesis, maternal determinants act in concert with TGF-beta signals to form mesoderm and endoderm. In contrast, ectoderm specification requires the TGF-beta response to be attenuated, although the mechanisms by which this is achieved remain unknown. In a functional screen for ectoderm determinants, we have identified Ectodermin (Ecto). In Xenopus embryos, Ecto is essential for the specification of the ectoderm and acts by restricting the mesoderm-inducing activity of TGF-beta signals to the mesoderm and favoring neural induction. Ecto is a RING-type ubiquitin ligase for Smad4, a TGF-beta signal transducer. Depletion of Ecto in human cells enforces TGF-beta-induced cytostasis and, moreover, plays a causal role in limiting the antimitogenic effects of Smad4 in tumor cells. We propose that Ectodermin is a key switch in the control of TGF-beta gene responses during early embryonic development and cell proliferation.

Animals↗
PubMed2005

Germ layers to organs: using Xenopus to study "later" development.

The amphibian embryo is a highly successful model system with great promise for organogenesis research. Since the late 1800s, amphibians have been employed to understand vertebrate development and since the 1950s, the African clawed frog Xenopus laevis has been the amphibian of choice. In the past two decades, Xenopus has led the way forward in, among other things, identifying transcription factors, gene regulatory networks and inter- and intracellular signaling pathways that control early development (from fertilization through gastrulation and neurulation). Perhaps the best measure of how successful Xenopus has been as a model for early mammalian development is the observation that much of the knowledge gleaned from Xenopus studies has subsequently directly translated to discoveries of similar mechanisms operating in mouse development. Despite this great success in early development, research on organogenesis in Xenopus has lagged behind the mouse. However, recent technical advances now make Xenopus amenable for studies on later development, including organogenesis. Here, we discuss why Xenopus is well suited for such research and, we believe, permits addressing questions that have been difficult to approach using other model systems. We also highlight how Xenopus researchers have already begun studying a number of major organs, pancreas, liver, kidney and heart, and suggest how Xenopus might contribute more to these areas in the near future.

Animals↗
PubMed2004

Epithelial and mesenchymal hamartomatous changes in a mature port-wine stain: Morphologic evidence for a multiple germ layer field defect.

The port-wine stain (PWS) is a congenital cutaneous venulocapillary malformation of unknown pathogenesis. Many patients with facial PWS develop thickening with cobblestoning and nodularity during adult life. The histologic correlates of this maturational change are poorly documented and its mechanisms remain unclear. In this case study we present new histologic observations that may elucidate this phenomenon. An extensive PWS on the face of a 75-year-old man exhibited gross thickening with cobblestoning and nodularity. Histologic examination revealed not only the expected vascular abnormalities, but also a number of widely distributed epithelial, neural, and mesenchymal hamartomatous changes. Epithelial changes included epidermal nevus, sebaceous trichofolliculoma, and basaloid follicular hamartoma. Changes of connective tissue nevus, smooth-muscle hamartoma, neural hamartoma, and subcuticular hamartoma were also noted. The complex hamartomatous changes observed in the PWS of this patient involved multiple germ lines and were distributed in a widespread pattern. These changes not only offer an explanation for the skin thickening and nodularity of this patient, but also suggest a genetically determined, multilineage developmental field defect in the pathogenesis of this lesion. Further studies of other patients are necessary to understand the full implications of these findings in the late stage of PWS.

Aged↗
PubMed2002

A consensus RNA signal that directs germ layer determinants to the vegetal cortex of Xenopus oocytes.

RNA localization is an important mechanism for generating cellular diversity and polarity in the early embryo. In Xenopus, the correct localization of the RNA encoding the T-box transcription factor VegT is essential for the correct spatial organization and identity of endoderm and mesoderm. Although localization signals in the 3' UTR have been identified for many localized RNAs, insight into what constitutes an RNA localization signal remains elusive. To investigate possible common features between signals that direct different RNAs to the same subcellular region, we carried out a detailed analysis of the uncharacterized VegT RNA localization signal and compared it with the well-studied Vg1 localization signal. Both RNAs localize to the vegetal cortex during the same period of oogenesis. Our results suggest a common RNA localization signal at the level of clustered redundant protein-binding motifs and trans-acting factors. We propose that what characterizes RNA localization signals in general is not the nucleotide sequence or secondary structure per se, but the critical clustering of specific redundant protein-binding motifs.

3' Untranslated Regions↗
PubMed1985

Transfer of extracellular matrix components between germ layers in chimaeric chicken-quail blastoderms.

A chemical basis for the transmission of signals during gastrulation has been investigated by using chimaeric embryos resulting from the combination of 3H-glucosamine-labelled and unlabelled hypoblast with epiblast taken from chicken and quail embryos at stage 3 of Vakaet (1970). The ability to distinguish chicken from quail cells on the basis of their different nuclear distribution of heterochromatin after Feulgen staining made it possible to determine the origin of the cells in the chimaerae. Tritiated quail hypoblast (after incubation of the embryo in the presence of 3H-glucosamine) was transplanted onto unlabelled chicken blastoderm deprived of its hypoblast. After culture of the chimaera for 5 h, the autoradiographic pattern shows silver grains not only over the graft, but also at the ventral surface of the epiblast of the host. Transfer of label may occur to mesoblast cells, but not between chicken and quail hypoblast cells. Chase experiments exclude the possibility that unprocessed, tritiated glucosamine is transferred. Chemical fixation of the host before transplantation of a labelled quail hypoblast also allows visualization of a transfer of macromolecules from hypoblast to the basement membrane of the epiblast, suggesting that an intervention of the epiblast cells in this process is not necessary. The morphology of the chimaeric embryos, as studied by scanning electron microscopy, suggests a direct deposition of these macromolecules by filopodia of the dorsal surface of the hypoblast. The possibility of diffusion of free macromolecules has been considered and can reasonably be discarded on the basis of several observations.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Ontogenetic expression of cell adhesion molecules: L-CAM is found in epithelia derived from the three primary germ layers.

Immunofluorescence techniques using specific antibodies against the liver cell adhesion molecule, L-CAM, were used to explore the appearance of L-CAM during early embryogenesis and organogenesis, as well as in adult tissue. Immunoblots of L-CAM from embryonic and adult organs indicated that molecules detected in each tissue were L-CAM, and that the antibodies were not simply detecting cross-reacting molecules. L-CAM was found in low levels on pregastrulation embryos. During gastrulation, the molecule remained present on ectoderm but was not detected on mesodermal and definitive endodermal cells. During neurulation, L-CAM disappeared from the neural ectoderm, in which staining for the neural cell adhesion molecule, N-CAM, had previously been shown to increase markedly. During organogenesis, L-CAM appeared in all endodermal structures, in ectoderm other than neural derivatives, in placodes, in extraembryonic ectoderm and endoderm, and in some mesodermal structures such as Wolffian ducts, oviduct, and kidney epithelium. Other mesodermal derivatives were not stained and the molecule was not detected in hemangioblastic areas of the lateral plate mesoderm nor in splanchnopleural derivatives such as spleen, adrenal glands, and gonads. During embryonic induction, for example, neurulation and in early kidney development, changes in L-CAM distribution were correlated with both locations and times of induction events. Analysis of distribution in the adult revealed that L-CAM was present in the stratum germinativum of the skin, in endodermally derived epithelia, in the female reproductive tract, and in the kidneys. In several fully differentiated glandular organs, L-CAM staining was restricted to basal or apical parts of the cell surface. When correlated with previous results obtained for N-CAM, these findings support the idea that local cell surface modulation of a small number of cell adhesion molecules may regulate other primary processes of development to yield specific patterns, both in early development and in organogenesis. Reflections of these patterns remain in adult life.

Animals↗
PubMed1988

Differential regulation of Ultrabithorax in two germ layers of Drosophila.

The homeotic gene Ultrabithorax (Ubx) is expressed in specific parts of Drosophila embryos: in a single metamer in the visceral mesoderm and forming a complex pattern limited to a broad domain in the ectoderm and in the somatic mesoderm. Here we use a linked beta-galactosidase gene to identify cis-acting regulatory sequences. In the visceral mesoderm, correct expression of Ubx depends on localized upstream sequences. In the ectoderm, all galactosidase-positive transformants show the same characteristic pattern. The repeated elements of this basal pattern appear to be a sub-pattern of engrailed (en) expression; they depend on en function as well as on sequences in the Ubx RNA leader. We use a mutant (Haltere-mimic) to show that sequences that normally restrict segmental expression of Ubx in the ectoderm are located downstream from the RNA leader.

Animals↗
PubMed2000

dtrap-1 encodes a novel member of the heat shock super family of proteins and is expressed in derivatives of all three germ layers during Drosophila embryogenesis.

Heat shock proteins (Hsps) comprise a highly conserved superfamily of proteins that are required for stress tolerance in living cells. At physiological conditions, these proteins act as chaperones during protein folding and protein assembly processes (Parsell and Lindquist, 1993. Annu. Rev. Genet. 27, 427-496). Members of the subfamily of Hsp90 proteins were shown to be additionally involved in the presentation and structural modification of components of diverse cellular signal transduction pathways including steroid hormone reception and regulatory kinase activities (Pratt, 1993. J. Biol. Chem. 268, 21455-21458); Cutforth et al., 1994. Cell 77, 1027-1036; van der Straten et al., 1997. EMBO J. 16, 1961-1997; Hunter and Poon, 1997. Trends Cell Biol. 7, 157-161). We have identified a Drosophila gene, called dtrap-1, which encodes a Hsp of a novel subfamily that is related to the Hsp90 family of proteins. During oogenesis dtrap-1 is expressed in nurse cells and its transcripts accumulate in the oocyte. The maternal transcripts remain in the egg but rapidly degrade during early embryogenesis, except in the posterior pole region. Zygotic expression is initiated after the onset of gastrulation showing dynamic patterns of transcripts in the developing mid- and hindgut as well as a subset of mesoderm derivatives.

Animals↗