Hemicentrotus pulcherrimus Genes in Development: Brachyury (T) homologue
HpTa
Function
HpTa is the sea urchin Brachyury (T) gene homologue, member of the T-box family of genes.
In vertebrates Brachyury (T) gene is essential for notochord formation.
During sea urchin development,
the T gene is utilized to specify the developmental
fate of embryonic cells to the secondary mesenchyme.
Southern blot analysis indicated that that HpTa is present as a
single copy gene, and that the sea urchin genome contains T domain genes other
than HpTa (Harada et al., 1995).
HpTa mRNA is 3.7 kb in length.
Northern blotting of the HpTa gene during embryogenesis of
H. pulcherrimus revealed that the HpTa transcripts are
transient.
Hybridization signals were undetectable in unfertilized eggs
and early embryos at the cleavage and early blastula stages.
This result was confirmed by longer exposure. A weak
but distinct hybridization signal was first detected in hatching
blastulae. A distinct band was found at the mesenchymal
blastula and gastrula stages, at which time the intensity of the
band was maximal. The intensity of the band decreased after
the gastrula stage. Although a band was detected in prism
larvae, it was barely detectable at the pluteus larva stage.
This pattern of transient HpTa expression coincides with
that of the chordate T genes.
The first stage at which a clear result was obtained by in situ hybridization
was the mesenchyme blastula, when the distribution of
hybridization signals was not even but restricted to cells of the
vegetal plate. At the early gastrula stage, the signals
were found in cells of the invaginating tip of the vegetal plate
or newly formed archenteron. Neither aboral and oral
ectoderm nor primary mesenchyme cells that have delaminated
from the vegetal wall into the blastocoel showed signals above
the background.
At the mid-gastrula stage, the archenteron has extended to
nearly a half of the blastocoel and two islands of skeletogenic
sites are formed by the primary mesenchyme cells.
Hybridization signals were found in cells of extending
tip of the archenteron and secondary mesenchyme cells
migrating from there. In contrast, signals were not
so evident in cells of the middle and posterior parts of the
archenteron.
At the late gastrula stage, the archenteron has almost reached
the wall of the most anterior part of the oral ectoderm
and some secondary mesenchyme cells have separated
from the archenteron wall into blastocoel. Intense hybridization
signals were detected in the secondary mesenchyme cells
and cells of the archenteron tip, whereas the other cells of the archenteron
showed barely detectable signals.
At the prism larval stage, the secondary mesenchyme cells
are scattered over the inner wall of blastocoel. At this stage,
hybridization signals were not detected above the background
level. The distinct signal above the background
level could not be detected at the late prism and the pluteus larval stages.
Thus, HpTa is expressed in the secondary mesenchyme
cells, but not in the endoderm cells (Harada et al., 1995)
mRNA level
Temporal accumulation
Method: RNase protection assay
Reference: Harada et al., 1995
Stage
Unfertilized egg
16 cells
Early blastula
Hatching blastula
Gastrula
Prism larvae
Pluteus larvae
Level
-
-
-
+ -
+ +
+
+ -
Spatial localization
Method: Whole mount in situ hybridization
Reference: Harada et al., 1995
Stage
Mesenchyme blastula
Early gastrula
Mid-gastrula
Late gasrula
Prism larvae
Pluteus larvae
Tissue
Cells of the vegetal plate
Intense signals in cells at the tip of the
invaginating archenteron
Secondary mesenchyme precursor cells
around the tip of the archenteron, and secondary mesenchyme cells
migrating from there
Secondary mesenchyme precursor cells, cells of the tip of archenteron
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