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A
C
P1
P1
Adult Cortex
Adult Cortex
Layer II, III
Layer II, III
E36
CP
CP
CP
E29
CP
IZ
IZ
IZ
IZ
VZ
VZ
VZ
VZ
Young to old transplants
Intermediate to old transplants
P1
B
D
Adult Cortex
Adult Cortex
Layer II, III
E36
CP
CP
E29
E29
Layer VI
CP
CP
IZ
IZ
IZ
IZ
VZ
VZ
VZ
VZ
Old to young transplants
Intermediate to young transplants
FIGURE 4.19 Progressive restrictions in fate determination in the cerebral cortex. A. Transplantation of
cells from the VZ of an E29 ferret to a P1 ferret leads these cells to change from a deep layer (early) to a super-
ficial layer (late) fate. B. But when P1 cells are transplanted to E29 hosts, they retain their superficial layer
fates. C. Intermediate E36 generated cells normally destined for Layer IV can assume later fates when trans-
planted into an older host. D. But they cannot assume younger fates when transplanted into younger hosts.
neurons of the retina, and their axons form the optic
nerve that relays the visual image to the brain. Thus,
there are just six basic types of neuron in the retina
and one type of intrinsic glial cell, called the Müller
cell. Vertebrate retinal cells develop from a population
of neuroepithelial progenitors, which produce this
diversity of neurons and glia. Injection and infection
of single retinoblasts with heritable markers pro-
duces clones of mixed, and seemingly random, cellu-
lar composition (Turner and Cepko, 1987; Holt et al.,
1988; Wetts and Fraser, 1988), indicating that, as in
the Drosophila retina, lineage is not the dominant
mechanism of fate determination in these cells
(Figure 4.20).
As in the cortex, it seems that the competence of
retinoblasts becomes more restricted as development
proceeds (Adler and Hatlee, 1989; Reh and Kljavin,
1989; Livesey and Cepko, 2001). During normal devel-
opment, RGCs are born and differentiate first, next are
amacrines, horizontal cells, and cones, and the later
born neurons are rods and bipolar cells (Sidman, 1961;
Cepko et al., 1996). The final cell type to be born is the
Müller glial cell (Figure 4.21). This pattern is largely,
though not entirely, preserved among vertebrates. To
test the idea of changing competence, progenitors at
various stages of retinal development are forced to dif-
ferentiate in culture. If progenitor cells are isolated at
a time when RGCs are normally born, they tend to turn
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