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

14 NORMA L DEVELOPMENT
depends o n the synthesis and destruction o f cyclins as
well as on the function o f Cdk-activating kinases and
Cdk inhibitor s (LaBaer et al. , 1997 ; Morgan , 1997 ;
Cheng et al, 1999 ; Sherr and Roberts, 1999 ; Murray,
2004). Eac h Cd k ha s a preferre d cycli n partner(s) .
For example, Cdk 2 associate s wit h cyclin s A and E ,
whereas Cdk 4 an d Cdk o associat e wit h D-typ e cy -
clins. Association of Cdks wit h cyclin s results in th e
regulation of the cel l cycle at two principal transition
points: th e transition s fro m th e G l t o S phase, an d
from G 2 t o M (Murra y and Hunt , 1993 ) (Fig . 2-3) .
Cdk2, Cdk4 , an d Cdk 6 ar e principall y involved i n
the regulation of the transition from Gl t o S, whereas
Cdkl regulate s the G2- M transition . The transitio n
from G l t o S phas e i s associate d wit h additiona l
mechanisms o f regulatio n involvin g th e retinoblas -
toma protei n (Rb ) and th e transcriptio n factor E2F .
Association o f Cdk s wit h cyclin s inactivate s R b b y
phosphorylation, which in turn result s in the releas e
of E2F . E2 F promotes th e transitio n fro m G l t o S
and DNA synthesis (Dyson, 1998 ; Frolov et al, 2001 ).
Control ove r cell cycl e progressio n a t the Gl-to -
S-phase transitio n i s exercise d b y Cd k inhibitors ,
which inhibi t th e activit y of th e Cdk-cycli n com -
plexes (Ferguso n e t al., 2000) . Inhibitio n o f Cdk ac -
tivity b y specifi c Cd k inhibitor s suc h a s p27 Ki P ] o r
p21 Wafl suppresse s the entr y into the S phase (Zind y
et al, 1997 , 1999 ; Delall e e t al., 1999 ; Siegenthale r
and Miller, 2005) (Fig. 2-3). Muc h o f the regulation
of cytogenesi s i n th e developin g brai n i s thought t o
occur a t the Gl/S transitio n (Cavines s e t al, 1996 ;
Ross, 1996) . Transcriptio n factors , growt h factors ,
hormones, an d neurotransmitter s ar e amon g th e
most commo n regulator s o f cel l proliferatio n i n
the developin g brain . They are believed t o act upo n
the Gl/S transitio n poin t t o increase o r decrease th e
output of neurons or glia (Caviness et al., 1996 ; Ross ,
1996).
Much o f our understandin g of cell cycl e regula -
tion derives from i n vitro studies. That said, the PV E
and SP P cell s i n th e intac t mammalia n brai n diffe r
from th e precursor cells maintained i n a Petri dish i n
one importan t aspect. PV E an d SP P cell s in th e in -
tact brain serve a histogenetic agenda, which warrants
generation o f a specific number o f specific cell type s
in a spatiotemporal gradient over a restricted period of
time. Therefore, t o understand mechanisms that reg -
ulate CNS histogenesi s we need to analyze cell prolif -
eration i n population s o f PV E o r SP P cell s i n th e
intact brain . A challeng e face d b y developmenta l
neurobiologists i s to assimilat e mechanisti c insights ,
gained b y analysis of cell cycl e regulatory machinery
from studie s of dissociated cells , into analyses of cel l
cycle kinetic s an d cel l outpu t fro m population s o f
PVE or SPP cells i n th e intac t brain . Th e followin g
section describe s recent progress i n efforts along those
lines. Complementary insigh t into these issue s is provided
b y studyin g the proliferativ e response i n th e
developing brai n followin g ethano l exposur e (se e
Chapters 1 1 and 12) . Much of our knowledg e o f th e
kinetics of PVE and SPP cell cycle in the intact brain
is derive d fro m analyse s o f neocortica l PV E cells .
Therefore, th e followin g section focuse s mainl y o n
cell cycle kinetics of neocortical PVE .
CELL CYCLE KINETICS
AND DYNAMIC S OF
CELL PROLIFERATION I N
THE NEOCORTEX
In an y tissue or organ system, the precursor cell popu -
lation initiall y expands by producing large numbers of
new precurso r cells . Th e precurso r cel l populatio n
subsequently deplete s itsel f through (a ) the productio n
of large numbers o f daughter cells , which exi t the cel l
cycle, differentiate , an d acquir e matur e phenotypes ,
and (b ) th e deat h o f cyclin g cell s (se e Chapte r 5) .
This type o f biphasic growth pattern ensure s the pro -
duction o f a large numbe r o f cells withi n a specified
time interval . Th e neocortica l PV E follow s thi s
growth pattern. It undergoes two phases of growth: an
initial phase of expansion during which mos t daugh -
ter cell s retur n t o th e precurso r pool , an d a late r
phase o f depletion whe n mos t o f the daughte r cell s
differentiate int o neuron s o r glia (Smart, 1976 ; Cavi -
ness e t al, 1995 ; Mille r an d Kuhn , 1995 ; Takahash i
et al, 1996 , 1997) . Interestingly , during the phas e of
expansion, th e PV E grow s in th e tangentia l dimen -
sion, and thi s growth result s in expansion o f the PV E
such tha t i t cover s th e underlyin g subcortica l struc -
tures. There i s little expansio n o f the PV E i n th e ra -
dial dimension ; growt h i n th e radia l dimensio n i s
seemingly reserve d fo r late r developmenta l period s
when newbor n neuron s an d gli a are release d fro m a
depleting PV E (Miller, 1989) .
The us e o f th e S-phas e marke r [ 3 H]thymidine
(pH]dT) revolutionize d th e analysi s of neuroepithelial
cel l cycl e kinetics by permitting identification of
nuclei in S phase and tracking the nuclei labeled i n S