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

phase as they passed through G2 , M, and Gl phase s
of the cel l cycl e (Sidman e t al., 1959 ; Angevine and
Sidman, 1961 ; Fujit a an d Miyake , 1962 ; Sidman ,
1970). I t becam e possibl e t o estimat e cel l cycl e pa -
rameters an d th e tim e o f generation o f different cel l
types throug h cumulativ e o r puls e labelin g wit h
[ 3 H]dT. Recen t developmen t o f nonradioactiv e
S-phase marker s bromodeoxyuridin e (BrdU ) an d
iododeoxyuridine further enhance d th e versatilit y of
these method s (Mille r and Nowakowski , 1988, 1991 ;
Takahashi e t al , 1992 ; Haye s an d Nowakowski ,
2000; Tarui et al., 2005).
S-phase labeling methods hav e been exploited, often
combinin g th e radioactiv e an d nonradioactiv e
markers t o perfor m sophisticate d analyse s of cell cy -
cle kinetic s an d cel l outpu t function s o f PV E an d
SPP cells in the embryonic murine neocortex (Miller
and Nowakowski , 1991; Cavines s e t al, 1995 ; Haye s
and Nowakowski , 2000 ; Takahash i e t al , 2002 ;
Siegenthaler and Miller , 2005 ; inter alia) . This work
shows that the neocortical PVE cells in fetal mic e executed
1 1 cell cycles throughout the cours e of generation
of neocortical neuron s over the week from Gil
to G17. During the 1 1 cell cycles, the length of the cell
cycle increases (from abou t 8 to 1 8 hours) (Fig. 2-4) ,
the increas e bein g du e t o th e lengthenin g o f G l
(from abou t 3 t o 1 2 hours). This pattern i s also de -
tected i n ra t neocortica l proliferativ e zone s (Mille r
and Kuhn, 1995).
The probabilit y that a new daughter cel l exit s the
cell cycl e (calle d Q ) increase d durin g th e perio d of
cortical neuronogenesi s (Miller , 1993 , 1999 ; Mille r
and Kuhn , 1995 ; Takahashi e t al, 1999) . I t was near
zero fo r cel l cycl e 1 (o n Gil fo r the mouse ) an d
nearly 100 % for cell cycle 1 1 (G16-17). Eac h cell cycle
is associated with specific values of Q and cell cycle
length (i.e. , the lengt h o f the Gl-phase) , the two
parameters that change (increase) systematically over
the cours e o f the 1 1 cel l cycle s (Fig . 2-4) . Th e Q
value reache s th e halfwa y mar k during the sevent h
and eight h cel l cycles , o n approximatel y G14 (Fig .
2-4). Thi s probability value marks a significant milestone
i n the lif e history of neocortical PVE , a s it indicates
the switc h fro m th e expansiv e to the depletiv e
phase of growth and fro m tangentia l to radial expansion
o f the neocortex . Furthermore , i t mark s a significant
mileston e i n neocortica l neuronogenesis :
generation of neurons for the infragranular layers (layers
V an d VI ) cease s an d tha t fo r the granula r layer
(layer IV ) and supragranula r layers (layer s II and III)
CELL PROLIFERATION 1 5
begins (Fig . 2-4) . Thus , quantitative estimates of the
values o f Q a t each intege r cell cycl e hav e enable d
the linkag e of Q to laminar destination of the daugh -
ter cells. In other words, a novel concept emerges, i n
which th e mechanism s regulatin g Q ar e linke d t o
mechanisms regulatin g cel l fat e o r cel l clas s (Taru i
et al, 2005).
Neuronogenesis acros s th e cortica l mantl e doe s
not procee d simultaneously ; i t abides by various spatiotemporal
gradients (Smart, 1961; Smar t and McSh -
erry, 1982 ; Miller , 1988a ; Baye r and Altman , 1991) .
The transvers e gradients reflect th e gradient s in cel l
cycle kinetics, especially the valu e of Q, i n the PV E
(Caviness et al., 2000, 2003; Tarui et al, 2005). Thus,
PVE cell s are distribute d along a spatia l continuu m
of cell cycl e parameters and Q . PV E cell s located a t
the rostrolatera l locatio n ar e mor e advance d tha n
those a t the caudomedial location with respect to cell
cycle length and values of Q. That is, rostrolateral precursor
cell s hav e longe r cel l cycl e time s an d highe r
values of Q than those of caudomedial precurso r cell s
on any given day during the interval Gil t o G17. For
example, a given location i n the neocortical PV E contains
precursor cell s passin g through th e entir e range
of cell cycle times and Q values over the interval Gil
to G17. At any given time, th e neuroepithelium con -
tains a range of cell cycl e times and Q values . Cells
destined to multiple layer s in multiple neocortica l ar -
eas aris e contemporaneousl y a t multipl e location s
within the neuroepithelium.
Apart fro m th e tangentia l neurogeneti c gradien t
discussed above, neocortical neuronogenesis proceeds
along a radial dimension, a characteristic common t o
laminar structures throughout th e nervou s system. At
any give n locatio n o f the neocortica l anlagen , neu -
rons are generated i n an inside-out pattern (Angevin e
and Sidman , 1961 ; Berr y et al , 1964 ; Berr y and
Rogers, 1965 ; Caviness , 1982 ; Miller , 1985) . Deep -
layer neuron s ar e generate d earlie r than th e superficial
laye r neurons . Usin g a double S-phas e labelin g
method, Takahashi an d colleague s (1999 ) correlate d
the lamina r position of a neocortical neuron with its
"birth hour. " Throug h th e us e o f this method , the y
were able to correlate the time of generation and laminar
destination of neocortical neurons with considerably
higher spatiotempora l resolutio n tha n ha d bee n
possible previously.
A model correlatin g th e probabilit y of cell cycl e
exit (which is equivalent to Q) with the probabilit y of
the destinatio n o f a neuron i n a specifi c neocortica l