Healthcare and Medicine Reference
In-Depth Information
A
B
C
msh
ind
vnd
A
A
A
Neuroblasts
D
V
D
V
D
V
P
Drosophila embryo
neurogenic region
P
AP genes
P
DV genes dividing
up the neurogenic region
along the DV axis
D
1
2
3
4
HB
KR
PDM
CAS
1
2
1
2
1
3
NB
NB
NB
NB
NB
HB
KR
PDM
CAS
E
1
1
1
1
1
1
2
2
2
2
1
2
3
4
3
4
NB
NB
NB
NB
hb -
kr -
HB
KR
FIGURE 4.5 Positional and temporal coordinates of neuroblasts in Drosophila . A. The neurogenic region
of a Drosophila embryo showing the rows and columns of neuroblasts. B. The embryo and the neurogenic
region is divided segmentally into AP domains as described in Chapter 2. C. The neurogenic region is further
divided into DV domains by the expression of transcription factors such as msh, ind, and vnd, creating a
grid whereby each neuroblast has its own specific spatial coordinates. D. Neuroblasts transiently express tem-
porally coordinated transcription factors Hb, then Kr, then Pdm, then Cas, but their progeny the GMCs main-
tain the transcription factor profile that was present at their birth. E. In Hb and Kr mutants or in animals that
misexpress Hb or Kr, certain GMCs are missing or take on fates associated with the factors that the mother
neuroblast expressed at the time when they were born.
express a transcription factor called hunchback ( hb)
and the GMCs generated at this time inherit this hb.
Later, the same neuroblasts turn off hb and express a
different transcription factor, Krueppel (Kr) instead.
Now, all the GMCs generated at this stage inherit Kr
expression, but not hb expression. If hb is eliminated
from the neuroblasts when they are making GMCs, the
neuroblasts generate early GMCs that cannot make
early neuron fates. Similarly, if Kr is eliminated, then
later neural fates are missing. If instead, Hb is experi-
mentally maintained in the neuroblasts at the stages
when they would normally start expressing Kr, the
neuroblasts keep making early GMCs (Figure 4.5)
(Isshiki et al., 2001). The expression of the successive
transcription factors is linked to the cell cycle, which
functions as a kind of clock, since blocking the cell
cycle blocks the succession and reactivating it reacti-
vates the succession.
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