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Patterning
of the ascidian neural tube
As well as the neural cells which
come from
the vegetal A-line cells from a cell fate choice between neural and
notochord (see
project 1), CNS precursors are also specified in the animal cells
(a-line
and b-line) following cell fate decisions between neural and epidermal
fates.
Figure 1. Cell lineages of the
ascidian larval CNS (from Hudson and Yasuo, 2005). Cell lineages are indicated as follows: the a-line is coloured red
(anterior sensory vesicle precursors) or pink (anterior epidermis and
pharynx/neurohypothesis precursors); b-line is green and A-line yellow at the
8-cell stage and light yellow
(medial cells) or tan (lateral cells) from the
32-cell stage. Bars connecting two blastomeres on the right-hand side of the
drawings indicate sister cell relationship.
Small circles in the lateral-A-line
precursors on the left hand side indicate that these blastomeres give rise to
the motoneurones; the final position of the motoneurones in the visceral
ganglion is also indicated on the drawing of the larvae. At the neural plate
stage, only the neural plate is shown and the dark
blue ovals represent the
secondary muscle lineage.
In order to form a fully
developed CNS, the
neural cells, once generated, obtain positional information which
govens
patterning along the dorsal-ventral and anterior-posterior axis. This
leads to
the generation of specific cell-types in the CNS, such as the
motoneurones
which form in the ventral-lateral part of the visceral ganglion (see
Fig.1).
Patterning of the neural cells begins as early as the early gastrula
stage,
when we first see a difference in the genes expressed between the
medial and
the lateral A-line neural precursors (Fig.2).
Figure 2. Differential expression of Ci-Snail and Ci-Delta2 in
lateral but not medial A-line neural
precursors. On the right is a picture of
an in situ hybridization
with a probe for Ci-ETR,a general
neural marker,
which is expressed in all 8 A-line neural precursors at the early
gastrula
stage.
Expression of Ci-ETR in the
lateral-most A-line neural precursor (A8.16) is weaker than the other
A-line
neural blastomeres, consistent with the fact that A8.16 is not yet fate
restricted to neural fate,
but still contains muscle fate. In contrast to Ci-ETR, Ci-Snail and Ci-Delta2 are specifically expressed
in the lateral neural precursors (A8.15 and A8.16)
and not the medial precursors (A8.7 and A8.8).
Expression of these two genes is
the earliest known molecular difference between medial and lateral
neural
precursors. This suggests that, at least by the early gastrula stage,
medial
and lateral A-line
neural precursors start to undergo a different molecular
program. Below each picture is a schematic
representation of an embryo at the
early gastrula stage. Different lineages are shown with the same
colour code as
in the Cell lineages of the ascidian
larval CNS figure. A spot
marks the
cells expressing
the gene analysed in the panel above. For more details see Hudson
and Yasuo, 2005.
By neural plate stages,
expression of genes
can be found in specific anterior-posterior as well as dorsal-ventral
domains.
Figure 3. A collection of neural
plate markers. At the right of the panels
is a schematic drawing of
the
neural
plate when it consists of 6 rows of cells (mid-gastrula stage), with
each
square representing
a neural plate cell. Names of each cell are indicated,
which should be prefixed with an 'a9.' for a-line
cells, 'A9.' for
A-line cells
and 'b9.' for b-line cells. I-VI indicate the row number with Row I the
closest
to
the blastopore, which is at the most caudal position of the neural
plate.
The colour scheme used is the
same as in the Cell lineages of the
ascidian
larval CNS figure. The gene expression pattern analysed
is indicated above
the photo of the embryo. In these in situ hybridizations the nuclei
have been
labelled
to allow easier identification of individual cells. A schematic
drawing of the neural plate with the
blastomeres expressing each marker
coloured in blue is presented below each embryo. Some of
these expression
patterns are published in Hudson
and
Yasuo, 2005; Ci-Otx expression
pattern is
published in Hudson
and Lemaire,
2001.
We are analysing cell-cell
interactions,
cell signalling pathways and transcriptional gene regulation to try to
understand how the patterning of the neural plate is generated.
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