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The formation of the embryonic
midline fates
Embryonic
midline fates of the chordate embryo include the
notochord, the ventral part of the neural tube and the anterior
endoderm.
In
ascidians, two rounds of cell divisions of blastomeres called A5.1 and
A5.2 of the 16-cell embryo generate precursors of each of these midline
structures (Figure 1).
Figure 1. Cell lineage of A5.1 and
A5.2 blastomeres, progenitors
of the midline tissues. Embryos are of the vegetal pole view. Black
barconnecting blastomeres reprisents sister cell relationship.
Note that Ciona embryos are bilaterally symmetrical. En: endoderm,
No: notochord, Neu: neural, Mes: mesenchyme.
We have been
focusing on the cell fate
choice between notochord and neural which takes place when the
A6.2/A6.4 blastomeres divide to give rise to A7.3 and A7.7 notochord
precursors and A7.4
and A7.8 neural precursors.
Following the formation of the precursors of notochord and neural
tissue,
a distinct set of genes starts to be expressed in each precursor,
suggesting that
cell-fate specification begins soon after these cell lineages emerge
(Figure 2).
Figure 2. Differential gene activation
between notochord and neural
lineages. Gene expression
was revealed by in situ hybridisation. Zygotic
expression of Ci-Brachyury and Ci-FoxB, both of which
encodes a transcription
factor, is initiated in notochord and neural precursors, respectively.
We are
analysing the roles of cell-cell
interactions and the cell signalling pathways and transcriptional
controls of lineage-specific genes that govern these cell-fate
descisions.
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