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Fig. 2. Postmitotic TTFs in Drosophila and mammals. (A) In
Drosophila, wild-type (wt) mushroom body (MB) neuroblasts (large
circles) generate MB neurons (small circles). Early-born MB neurons express
high levels of the transcription factor Chinmo (dark blue), whereas later-born
MB neurons express either low levels (light blue) or none (white). Postmitotic
levels of Chinmo specify the different temporal identities of 
,
'/β', p/β or /β neurons. A
decrease in Chinmo expression (Chinmo-) leads to fewer
neurons and to the precocious generation of supernumerary
p/β neurons. Chinmo does not appear to specify the temporal
identity of /β neurons. If high levels of Chinmo are maintained in
all postmitotic neurons (+ Chinmo), the early temporal identity
() is continuously generated at the expense of all later temporal
identities ('/β', p/β and /β).
(B) (a) In the mouse cerebral cortex, multipotent progenitors (mP)
generate two distinct pools of progenitors: deep-layer progenitors (dP) and
superficial-layer progenitors (sP). In turn, dP and sP sequentially generate
the different neuronal subtypes (coloured circles) that are associated with
deep (SP/VI/V) and superficial (IV/III/II) cortical layers, respectively.
Postmitotic projection neurons of the different layers express different
combinations of Sox5 (SRY-box 5), Ctip2 (Coup-TF-interacting protein 2) and
Satb2 (special AT-rich sequence binding protein 2). Sox5 is normally expressed
at different levels in the neurons of each of the layers SP, VI and V. (b)
Sox5 inactivation leads to a reduction in the sub-plate (SP) neuronal
layer. SP neurons appear to be replaced by ectopic Ctip2+ neurons
(yellow), characteristic of layer V. For clarity, ectopic Ctip2+
neurons located in superficial layers IV/III/II have been omitted as their
origin is unclear. Inactivation of Satb2, which is predominantly
expressed by layer IV/III/II neurons, leads to these late-born neurons
acquiring an earlier Ctip2+ identity. (c) Sox5 and Satb2 repress
Ctip2, and biochemical studies suggest that the Satb2 repression is
direct (Alcamo et al., 2008;
Britanova et al., 2008). As
Ctip2 and Satb2 are transiently coexpressed by some layer V neurons
(Alcamo et al., 2008), it might
be that a stable layer-specific cell identity is only acquired sometime after
neurons become postmitotic.
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