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

112 NORMA L DEVELOPMENT
Even i f the numbe r i s much lower , the recognitio n
of developmental disorde r and terminatio n o f development,
particularl y i n earl y stages , ar e a s muc h
an importan t face t o f developmen t a s constructiv e
mechanisms.
If a commitment t o the offsprin g i s unavoidable —
for example , if it is the onl y chance a t reproduction ,
or a life-threatenin g metaboli c commitmen t ha s al -
ready bee n made—the n ever y subsequent choic e t o
preserve the offsprin g should be made a s is physically
possible. Particularl y in shortag e situations , clearly
strategic choices ar e made—for example , t o preserve
the brai n of the infant , which i s generated onl y early
in development, ove r and against the musculoskeletal
system, whos e growt h ca n b e delayed, o r to favor th e
infant's nutritiona l need s ove r th e calori c require -
ments o f the mothe r (Marty n et al., 1996 ; Luca s an d
Campbell, 2000) . One "difficulty " i n research on developmental
disorders, which may not be entirely obvious
to those working in well-established paradigms,
is to be able to find just those regime s that lie on th e
cusp betwee n th e plateau s o f embryoni c rejectio n
and death, an d full normality .
Analogous strategi c similaritie s can be seen in various
developmenta l mechanism s i n th e nervou s sys -
tem, fo r example , a certai n numbe r o f neurons , a
certain convergenc e rati o between neurons , or a feature
of physiology. Central mechanism s ca n gir d th e
system against environmental stressors , but a t a critical
value, this defense will suddenly and catastrophi -
cally collapse. For example, if the number o f neurons
in th e corte x is progressively depleted b y the mitoti c
inhibitor methylazoxymethanol during early develop -
ment, the thalamus projecting to that cortex will show
no los s of neurons unti l mor e tha n 70%-80 % o f its
target i s gone, an d the n collaps e itsel f (Wo o e t al. ,
1996). Simila r event s ma y b e a t pla y with th e anti -
mitogen ethanol. Prenatal exposur e to ethanol causes
a