Stage 6
Key events: onset
of gastrulation, formation of ventral and cephalic furrow, dorsal shift
of pole cells
Stage 6 lasts approximately 10 min only (2:50-3 h). It is the stage of
early gastrulation, in which the major morphogenetic movements
start:
- The invagination of the mesoderm and the anterior endodermal primordium is initiated.
- The blastoderm cells at the posterior pole shift their position to form a dorsal plate underneath the pole cells. This is the onset of the germ band elongation.
- The cephalic furrow becomes visible at a position corresponding to about 65% egg length (0% egg length = posterior).
Stage
6 begins with the formation of the ventral furrow from which mesoderm and
anterior endoderm originate. The ventral furrow is initiated as a median
longitudinal cleft that extends between 20% and 70% egg length, along the
ventral embryonic midline; over a period of approximately 10 min the ventral
furrow will extend further by incorporating additional cells at its anterior
and posterior tips, until it extends between 6% and 85% egg length. The
ventral furrow forms as a result of cell shape changes which affect an area
about 12 cells in width centered on the ventral midline. A total of about
800 cells will become internalized through the ventral furrow: 730 cells
represent the mesoderm primordium and 70 cells represent the anterior endodermal
midgut primordium.
At
the posterior pole of the embryo, a cell plate that carries the pole cells
shifts dorsally. This cell plate is flanked anteriorly by a prominent ridge
of blastoderm cells, which becomes more and more striking as germ band elongation
proceeds. At the same time, other shape changes become evident within the dorsal half of the embryo; these preceed the formation of the dorsal folds in stage 7.
The
cephalic furrow forms at the same time as the ventral furrow. It first becomes
visible as a lateroventral slit at about 65% egg length. Later, it extends
transversely from the dorsal midline, at about 60% egg length, to the ventral
midline at about 75% egg length. 
Media list
Stage 6, invagination (0.5 MB)..........additional
information
Stage 6, sections..........additional
information
Stage 6, in vivo (0.9 MB)Genes discussed
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Gene
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Gene product - Domains
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Function
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Links
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twist (twi)
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transcription factor - bHLH
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required for mesoderm development, high Twi levels
block formation of visceral mesoderm and heart and induce somatic
myogenesis
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