Key events: germ band elongation
to 70% egg length, early neuroblast delamination
Stage
9 lasts for approximately 40 min (3:40-4:20 h) and is initiated by a transient
parasegmentation of the mesodermal layer.
By
the form of bulges that protrude slightly into the yolk sac, the mesoderm
shows a metameric pattern. In the ectoderm only a slight and also transient
parasegmentation becomes visible at this time.
The
proctodeal opening reaches 70% egg length (0% egg length = posterior pole).
The end of stage 9 is defined by the onset of foregut invagination
the stomodeal invagination.
In
stage 9 the first neuroblasts delaminate from the ectoderm; therefore,
by the end of stage 9 the germ band exhibits a clearly three-layered architecture:
mesoderm - neuroblasts - outer ectoderm.
At the beginning of stage 9, mitotic divisions in the mesoderm cease;
the mesodermal cells then reorganize, changing from the loose arrangement
into a monolayer.
The
ectoderm of the germ band in stage 9 is divided in two clearly defined
regions. One is ventromedial and consists of large cells; the neuroblasts
and the ventrolateral epidermis derive from the cells of this region (also
called neurogenic region). The other ectodermal region is located laterally,
and consists of a palisade of narrow, tightly packed cells; the dorsal
epidermis and the tracheal placodes will develop from this lateral region.
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Media list
Stage 9, in vivo (1.8 MB)
Stage
9, Parasegmentation of mesoderm
Stage 9, anti-Crumbs..........additional
information
Stage 9 (0.4 MB)
Stage 9, interactive (4 MB)
Stage 9, Mesoderm (0.3 MB)..........additional
information
Genes discussed
Gene
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Gene product - Domains
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Function
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Links
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crumbs (crb)
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transmembrane -EGF repeats - laminin A homolog
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involved in epithelial polarity, expressed in the apical
membrane of ectodermal cells
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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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