Brain Development: Normal Processes and the Effects of Alcohol ...

78 NORMA L DEVELOPMENT
FIGURE 5- 3 Examinatio n of caspase-9 and caspase-3
knockout mice. Capase-9 +/ ~ (A) and caspas e 9~ /- (B)
fetuses a t G16.5 , showin g th e large r an d mor -
phologically abnorma l brai n i n th e knockout . Scal e
bar= 1.0mm. C-F . Caspase- 3 mutant s hav e a n expanded
proliferative zone. C, D. BrdU immunolabeling
i n a sectio n throug h th e developin g cerebra l
cortex of a hétérozygote an d mutan t embryo , respec -
tively. E, E DAP I staining of the adjacent section i n a
series fro m C an d D , respectively . I n th e caspase- 3
knockout fetus , th e corte x i s distorte d int o fold s
and th e latera l ventricle i s identifiable only as a thin
slit in the center of the cortical mass. The thicknes s of
the proliferativ e laye r relative to the corte x looks very
much alike in the two embryos. Statistics showed that
the percentage o f BrdU-labeled cells was only slightly
increased i n th e -/ - fetuse s compare d t o th e +/ -
fetuses, v , latera l ventricle . Scal e ba r = 500 Jim.
(Source: A, B: adapted from Kuid a e t al., 1998 . C-F :
adapted from Pompeian o e t al., 2000)
Glial Death within
the Developing Brain
The rol e o f astrocytes or other gli a i n neurona l cel l
death i s poorly understood. Interestingly, phagocytes
of macrophage lineages such as microglia contribut e
to the eliminatio n o f dead cell s i n the brain . Phagocytes,
from mic e lackin g the gen e encodin g PS R are
defective i n removin g apoptoti c cells . Thes e PCR -
deficient mic e hav e developmenta l abnormalities ,
such as in the accumulation of dead cells in the lun g
and brain . Som e o f these animal s also present a hyperplasic
brai n phenotyp e resemblin g tha t o f mic e
deficient i n the cel l death-associated gene s APAF-1,
caspase-3, an d caspase- 9 (Fig . 5-3 ) suggestin g tha t
phagocytes ma y also be involved in promoting apop -
tosis (L i e t al. , 2003) . Actually , in th e developin g
cerebellum, th e deat h o f Purkinje neuron s ca n b e
promoted b y microglia. This findin g suggest s that a
form o f engulfment-related cel l death ma y link PC D
to th e clearanc e o f dea d cell s (Marin-Tev a e t al. ,
2004).
DIFFERENT PHASES OF
DEVELOPMENT, DIFFERENT
MECHANISMS OF PROGRAMMED
CELL DEATH
Using an early molecular marker for post-mitotic neurons,
Weine r an d Chu n (1997) , foun d tha t PC D i n
neuroproliferative zones does not become pronounce d
until NPC s begi n t o differentiat e int o postmitoti c
cells. This correlation suggest s that the initial generation
o f postmitotic neuron s ma y trigger the onse t of
PCD i n a given proliferative zone. The natur e of this
link; however, is currently unclear.
Similar to the developing cerebral cortex, the retina
of newborn mice and rat s is composed o f two strata
containing cell s i n variou s stage s o f development .
One laye r of differentiated cell s i s the ganglio n cel l
layer an d th e othe r i s the neuroblasti c laye r (NBL).
Like the cortica l VZ, the NB L is comprised o f NPCs
(Alexiades and Cepko , 1997) . During differentiation,
NPCs that are exiting the cell cycle, migrate from th e
NBL to form other layers of postmitotic neurons (Linden
et al, 1999) .
PCD ca n be induced i n the NBL following inhibition
o f protein synthesis . Interestingly, as the retin a
matures, retinal cells become less sensitive to protei n
synthesis inhibitors, and i n adult retinal tissue , thes e
inhibitors have n o effec t o n cel l death (Rehe n et al,
1996). Whe n BrdU , a marke r o f proliferatin g cells
(Miller an d Nowakowski , 1988), i s used i n conjunction
wit h protein synthesi s inhibitors, the vas t majority
of cells undergoing PCD ar e not labeled with BrdU
(Fig. 5.4) . This result suggests that the populatio n of