Differential distribution of ganglioside GM1 and sulfatide during the development of Xenopus embryos

A frozen section technique for frog oocytes was developed without using any organic solvent. It was applied to examine the distribution of acidic glycosphingolipids (ganglioside GM1 and sulfatide) in Xenopus oocytes, eggs and embryos by indirect immunofluorescence microscopy with specific monoclonal antibodies against the acidic glycolipids. Although glycolipids are generally present on the cell surface, GM1 and sulfatide were distributed in the cytoplasm of animal and vegetal hemispheres, respectively, of the fully grown oocytes and oviposited and fertilized eggs. In blastula, GM1 was present on the cell boundaries and in the Golgi of the blastomeres of animal hemisphere and marginal zone, whereas the staining of the outermost layer of animal blastomeres became faint or negligible at stage 9. Sulfatide in blastula was still observed in vegetal blastomeres. In gastrula, GM1 was distributed in the inner layer of ectoderm and the involuting mesoderm. In neurula, GM1 was concentrated in the dorsal midline including the closing neural tube, notochord and somites, while sulfatide was present in endoderm. The unique distribution of GM1 and sulfatide in oocytes, eggs and early embryos may help to elucidate one aspect of the biochemical bases laid on the animal–vegetal polarity.     

Topographic changes in nascent and early mesoderm in amphioxus embryos studied by DiI labeling and by in situ hybridization for a Brachyury gene

 In amphioxus embryos, the nascent and early mesoderm (including chorda-mesoderm) was visualized by expression of a Brachyury gene (AmBra-2). A band of mesoderm is first detected encircling the earliest (vegetal plate stage) gastrula sub-equatorially. Soon thereafter, the vegetal plate invaginates, resulting in a cap-shaped gastrula with the mesoderm localized at the blastoporal lip and completely encircling the blastopore. As the gastrula stage progresses, DiI (a vital dye) labeling demonstrates that the entire mesoderm is internalized by a slight involution of the epiblast into the hypoblast all around the perimeter of the blastopore. Subsequently, during the early neurula stage, the internalized mesoderm undergoes anterior extension mid-dorsally (as notochord) and dorsolaterally (in paraxial regions where segments will later form). By the late neurula stage, AmBra-2 is no longer transcribed throughout the mesoderm as a whole; instead, expression is detectable only in the posterior mesoderm and in the notochord, but not in paraxial mesoderm where definitive somites have formed. Received: 28 November 1996 / Accepted: 2 January 1997     

Localized synthesis of the Vg1 protein during early Xenopus development

The Xenopus Vg1 gene encodes a maternal mRNA that is localized to the vegetal hemisphere of both oocytes and embryos and encodes a protein related to the TGF-beta family of small secreted growth factors. We have raised antibodies to recombinant Vg1 protein and used them to show that Vg1 protein is first detected in stage IV oocytes and reaches maximal levels in stage VI oocytes and eggs. During embryogenesis, Vg1 protein is synthesized until the gastrula stage. The embryonically synthesized Vg1 protein is present only in vegetal cells of an early blastula. We find that Vg1 protein is glycosylated and associated with membranes in the early embryo. Our results also suggest that a small proportion of the full-length Vg1 protein is cleaved to give a small peptide of M(r) = approximately 17 x 10(3). These results support the proposal that the Vg1 protein is an endogenous growth-factor-like molecule involved in mesoderm induction within the amphibian embryo.     

Developmental expression of the Xenopus int-2 (FGF-3) gene: activation by mesodermal and neural induction.

We have used a probe specific for the Xenopus homologue of the mammalian proto-oncogene int-2 (FGF-3) to examine the temporal and spatial expression pattern of the gene during Xenopus development. int-2 is expressed from just before the onset of gastrulation through to prelarval stages. In the early gastrula, it is expressed around the blastopore lip. This is maintained in the posterior third of the prospective mesoderm and neuroectoderm in the neurula. A second expression domain in the anterior third of the neuroectoderm alone appears in the late gastrula, which later resolves into the optic vesicles, hypothalamus and midbrain-hindbrain junction region. Further domains of expression arise in tailbud to prelarval embryos, including the stomodeal mesenchyme, the endoderm of the pharyngeal pouches and the cranial ganglia flanking the otocyst. It is shown, by treatment of blastula ectoderm with bFGF and activin, that int-2 can be expressed in response to mesoderm induction. By heterotypic grafting of gastrula ectoderm into axolotl neural plate, we have also demonstrated that int-2 can be expressed in response to neural induction. These results suggest that int-2 has multiple functions in development, including an early role in patterning of the anteroposterior body axis and a later role in the development of the tail, brain-derived structures and other epithelia.     

Changes in states of commitment of single animal pole blastomeres of Xenopus laevis

The experiments described in this paper were designed to compare the normal fates of animal pole blastomeres of Xenopus laevis with their state of commitment. Single animal pole blastomeres were labeled with a lineage marker and transplanted into the blastocoels of host embryos of different stages. The distribution of labeled daughter cells in the tadpole reflects the state of commitment of the parent cell at the time of transplantation. It is known that cells from the animal pole of the early blastula normally contribute predominantly to ectoderm with a small, but significant, contribution to the mesoderm. We show that on transplantation to the blastocoels of late blastula host embryos these blastomeres are pluripotent, contributing to all three germ layers. At later stages the normal fate of these cells becomes restricted solely to ectoderm and concomitantly the proportion of pluripotent cells is reduced, although the results depend upon the stage of the host embryo. Blastomeres from late blastula donors transplanted to mid gastrulae contribute solely to ectoderm in 34% of cases; however, in earlier hosts, when the vegetal hemisphere cells have "mesoderm inducing" or "vegetalizing" activity, late blastula animal pole blastomeres contribute to mesoderm and endoderm rather than ectoderm. Thus during the blastula stage animal pole cells pass from pluripotency to a labile state of commitment to ectoderm.