Current problems of in vivo developmental neurotoxicity tests and a new in vivo approach focusing on each step of the developing central nervous system.

Developmental neurotoxicity (DNT) tests usually focus on postnatal indicators, such as behavior and neuropathology, for the detection of chemically induced neurodevelopmental defects in the central nervous system (CNS). However, low reliability, especially low reproducibility, of behavioral results often causes concern among scientists and the scientific community in general. Guidance of neurohistopathological examination in the DNT guideline also has some shortcomings, especially relating to the methodological aspects. Ongoing international trends in DNT tests have shifted from the use of original in vivo animal (mammalian) studies to in vitro experiments using cell cultures and/or non-mammalian species, such as fish. In vitro systems might initially be useful to screen test chemicals for their DNT potential. Although in vitro systems are employed as alternative approaches for DNT studies, the use of in vivo studies based on animal models remains an important factor when data are to be extrapolated to the human case. In this review, a new in vivo approach that focuses on histopathological observation of each developmental step of the CNS, such as proliferation of neural stem cells, migration of immature neurons, and formation of neural networks, using fetal and neonatal brains after chemical exposure is introduced, and some queries and arguments for current DNT experimental guidelines are discussed.