Vulnerable processes of nervous system development: a review of markers and methods.

The susceptibility of the developing nervous system to damage following exposure to environmental contaminants has been well recognized. More recently, from a regulatory perspective, an increased emphasis has been placed on the vulnerability of the developing nervous system to damage following pesticide exposure. The publication of the National Academy of Sciences (NAS) report on Pesticides in the Diets of Infants and Children (1995) and the passage of the Food Quality Protection Act (FQPA) and Safe Drinking Water Act (SDWA) amendments have significantly escalated the scientific debate regarding age-related susceptibility. Key concerns raised in the NAS report include the qualitative and quantitative differences that distinguish the developing nervous system from that of the adult. It was suggested that neurotoxicity testing on adult animals alone may not be predictive of these differences in susceptibility. The age-related susceptibility of the nervous system is compounded by the protracted period of time over which this complex organ system develops. This temporal vulnerability spans the embryonic, fetal, infant, and adolescent periods. Normal development of the nervous system requires the concomitant and coordinated ontogeny of proliferation, migration, differentiation, synaptogenesis, gliogenesis, myelination and apoptosis to occur in a temporally- and regionally-dependent manner. Perturbations of these processes during development can result in long-term irreversible consequences that affect the structure and function of the nervous system and could account for qualitative differences in age-related susceptibility of the developing nervous system as compared to the adult nervous system. A discussion of developmental milestones and the relevance of transient effects on developmental endpoints are presented. Transient effects following developmental perturbations can be missed or dismissed depending on the experimental design or screening strategy employed. This subject is discussed in light of scientific uncertainties regarding perturbation-induced compensation in the developing nervous system. Thus, utilization of age-appropriate tests of these developmental processes may improve the detection and reduce uncertainty about the nature of adverse effects following developmental exposure to environmental neurotoxicants.