Prenatal exposure to environmental factors and congenital limb defects.

Limb congenital defects afflict approximately 0.6:1000 live births. In addition to genetic factors, prenatal exposure to drugs and environmental toxicants, represents a major contributing factor to limb defects. Examples of well-recognized limb teratogenic agents include thalidomide, warfarin, valproic acid, misoprostol, and phenytoin. While the mechanism by which these agents cause dymorphogenesis is increasingly clear, prediction of the limb teratogenicity of many thousands of as yet uncharacterized environmental factors (pollutants) remains inexact. This is limited by the insufficiencies of currently available models. Specifically, in vivo approaches using guideline animal models have inherently deficient predictive power due to genomic and anatomic differences that complicate mechanistic comparisons. On the other hand, in vitro two-dimensional (2D) cell cultures, while accessible for cellular and molecular experimentation, do not reflect the three-dimensional (3D) morphogenetic events in vivo nor systemic influences. More robust and accessible models based on human cells that accurately replicate specific processes of embryonic limb development are needed to enhance limb teratogenesis prediction and to permit mechanistic analysis of the adverse outcome pathways. Recent advances in elucidating mechanisms of normal development will aid in the development of process-specific 3D cell cultures within specialized bioreactors to support multicellular microtissues or organoid constructs that will lead to increased understanding of cell functions, cell-to-cell signaling, pathway networks, and mechanisms of toxicity. The promise is prompting researchers to look to such 3D microphysiological systems to help sort out complex and often subtle interactions relevant to developmental malformations that would not be evident by standard 2D cell culture testing. Birth Defects Research (Part C) 108:243-273, 2016.