Abstract
Differences in response to pharmacologic or other exogenous agents occur when they are applied to individuals of different ages. The present report presents a discussion of the relative vulnerabilities of various early stages in the life cycle. Various published results, using several systems, including lung, kidney, gastrointestinal tract, and immune system, have clearly established that unique developmental events take place not just in embryonic development but also during fetal development and that the latter period may be highly vulnerable to perturbation by a teratogenic agent. Defects may be manifest in subtle ways and result in neonates that, although they may appear to be normal, in fact lack physiologic reserves or capabilities beyond those necessary for simply surviving. In the clinical world, it is possible that a prenatal insult often undetected by routine types of clinical evaluation could put an individual at jeopardy or increase its postnatal vulnerability to an insult tolerated by unaltered off spring.