Vulnerable periods and processes during central nervous system development.

The developing central nervous system (CNS) is the organ system most frequently observed to exhibit congenital abnormalities. While the developing CNS lacks a blood brain barrier, the characteristics of known teratogens indicate that differential doses to the developing vs mature brain are not the major factor in differential sensitivity. Instead, most agents seem to act on processes that occur only during development. Thus, it appears that the susceptibility of the developing brain compared to the mature one depends to a great extent on the presence of processes sensitive to disruption. Yet cell proliferation, migration, and differentiation characterize many other developing organs, so the difference between CNS and other organs must depend on other properties of the developing CNS. The most important of these is probably the fact that nervous system development takes much longer than development of other organs, making it subject to injury over a longer period.