Recommendation 1. To improve the understanding of the mechanisms of action of toxicants, the committee recommends that critical molecular targets of toxicants be identified among the components of developmental processes.
1.1.1. Conserved signaling pathways and genetic regulatory circuits as potential targets of toxicants.
Approximately 10 kinds of signaling pathways are repeatedly used at all times and places of development before cytodifferentiation (i.e., during cleavage, gastrulation, neurulation, and organogenesis). Development is built on coupled signaling and genetic regulation. As differentiated cell function begins during cytodifferentiation, seven other pathways are added. Thus, in the mammalian fetus, all 17 pathways are in use. Almost all activities of cells of multicellular organisms, including all embryonic stages, are contingent on extracellular signals. These activities include secretion, motility, adhesion, proliferation, differ-