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Studies of morphogenesis in early Xenopus embryos have focused primarily on gastrulation and neurulation. Immediately following these stages is another period of intense morphogenetic activity, the neurula-to-tailbud transition. During this period the embryo is transformed from the spherical shape of the early stages into the long, thin shape of the tailbud stages. While gastrulation and neurulation depend largely on active cell rearrangement and cell shape changes in dorsal tissues, we find that the neurula-to-tailbud transition depends in part on activities of ventral cells. Ventral explants of neurula lengthen autonomously as much as the ventral sides of intact embryos, while dorsal explants lengthen less than the dorsal sides of intact embryos. Analyses of cell division, cell shapes, and cell rearrangement by transplantation of labeled cells and by time lapse recordings in live intact embryos concur that cell rearrangements in ventral mesoderm and ectoderm contribute to the autonomous anterior-posterior axis lengthening of ventral explants between neurula and tailbud stages.
FIG. 1. Dissections were made through the body wall of stage 16 embryos leaving dorsal and ventral explants lined on both surfaces with an
epithelial layer. Confocal cross (A) and sagittal sections (B) of stage 16 embryos with arrows indicating where cuts were made to separate embryos
into dorsal and ventral explants. Video micrographs of a dorsal explant (C), a ventral explant, with the archenteron floor visible as the lightly
pigmented upper surface (D), and stage 16 embryo (E). Video micrographs of the same dorsal explant (C9), ventral explant (D9), and embryo (E9)
at stage 27. Arrowed, superimposed lines indicate where measurements were made. In BâE anterior is to the left and dorsal is up. Scale bar is 500 mm.
FIG. 2. Ventral explants lengthen autonomously. Photomicrographs of the same dorsal explants (A and A9), ventral explants (B and B9), and
intact embryos (C and C9) at stages 16 (A, B, and C) and 27 (A9, B9, and C9). Anterior is to the left and dorsal is up. Scale bar is 500 mm.
FIG. 3. Ventral explants lengthen as much as the ventral sides of
intact embryos, while dorsal explants lengthen less than dorsal
sides of intact embryos. Dorsal and ventral sides of 28 intact
embryos, 78 ventral explants, and 77 dorsal explants were measured
at stages 16 and 27. Bars represent one standard deviation.
FIG. 4. Few mitotic cells were found at any stage and any region, and total cell numbers did not increase significantly with age. Regions
of ventral ectoderm and mesoderm of identical dimensions and locations were surveyed for mitotic figures at stages 21 through 24 (A). Each
row of data refers to the embryonic stage indicated in the diagrams at left. Low percentages of division were observed in any region at any
stage; many regions had no dividing cells. The labels âparallelâ and âperpendicularâ refer to the orientation of the division plane relative
to the A-P axis (B). Cell counts averaged for six or seven embryos per stage indicate no significant change in number with age (C). A total
of 31,506 cells were scored, with 7,076, 7,400, 9,260, and 7,770 cells at stages 21 through 24, respectively.
FIG. 5. Cell shapes can be indicative of morphogenetic mechanisms. If a cell sheet lengthens by cell shape change, then the cells must
get longer in the dimension of sheet lengthening, whereas if lengthening occurs by cell rearrangement, intercalating cells become longer
perpendicular to the direction of sheet lengthening (A). SEM of ventral mesoderm of stage 24 embryo reveals that cells have the long axis
perpendicular to the A-P axis. Note the rosette arrangement at the upper right (B, arrow). Traced cell outlines of cells in B to illustrate
length:width characteristics (B9). Bisected embryo with the mesodermal sheet extending beyond the ectoderm due to the fracture.
Wedge-shaped cells (arrowhead) are underlying a rosette (arrow) in the ventral mesoderm of a stage 24 embryo (C). Traced cell outlines of
wedge-shaped cells in C (C9). Rosettes appear to be correlated with wedge-shaped cells undergoing radial intercalation. In B and B9, anterior
is to the left and dorsal is up. In C and C9, dorsal is up. Scale bar is 10 mm.
FIG. 6. Length-width ratios indicate that ventral mesoderm cells
are longer mediolaterally than anterioposteriorly. Length-width
ratios increase posterior to anterior and in both regions with
increasing age. Anterior and posterior regions at each stage are from
the same embryo; numbers of cells per region ranged between 8 and
13. Error bars indicate one standard deviation. The anteriorposterior
difference is likely due to the delay in development seen
between those regions, similar to that seen dorsally during somite
formation.
FIG. 7. A strip of ventral meso-ectoderm from TRITC-dex-labeled embryos was transplanted into unlabeled hosts to see if cells rearrange.
Donor embryos were injected with TRITC-dex in all cells of the 4-cell stage, at stage 16 strips of ventral meso-ectoderm were transplanted
from donors to hosts, and at stage 27 hosts were fixed and labeled cells examined on a confocal microscope (A). Parasagittal (B) and cross
section (C) of a neurula fixed shortly after transplantation to show the size, shape, and position of the labeled cells. Higher magnification
view of the embryo in B and C showing the rectangular shape and coherence of the initial graft (D). Confocal images of parasagittal (E) and
cross section (F) of a tailbud embryo showing the more ventral location of the labeled mesodermal cells. Higher magnification view of the
tailbud embryo in E and F showing labeled mesodermal cells separated from the bulk of the transplant in A-P dimension (G, arrows) and
some ectoderm cells separated from the bulk of the transplant in the D-V dimension (G, arrowhead). Autofluorescence of yolk platelets
makes unlabeled cells of the host visible. Anterior is to the left and dorsal is up in B, D, E, and G. Dorsal is up in C and F. Scale bar in B
applies to B, C, E, and F and is 500 mm. Scale bar in D applies to D and G and is 150 mm.
FIG. 8. Active cell rearrangement of Nile blue-stained ventral
mesoderm cells was recorded in time-lapse on a confocal microscope.
Cells moving into alignment along the A-P axis are indicated
with an * and a 1. Note that the cells are longer in the mediolateral
dimension. Anterior is to the lower left and dorsal is toward the
top. The numbers indicate minutes elapsed after the recording
began. The outlines of the marked cells were traced and placed to
the side of the confocal images to illustrate cell shapes and
positions. Scale bar is 10 mm.
