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		In the extracellular matrix (ECM) the NC cells adhere to both fibronectin and laminin, which are widely distributed. However, the pattern of migration must also be delineated by other ECM components that either do not support or inhibit crest cell migration. It is believed that the interaction between crest cells and the ECM may also be important for events following migration (Marusich, 1991). Research has been done to show that there are a number of soluble growth factors found in the ECM that play a role in NC cell development (proliferation and differentiation). Fibroblast growth factor 1 and 2 (FGF1 and FGF2)in vitro are known to cause NC cell proliferation (Murphy, 1994). It is known that retinoic acid (RA) selectively increases neurogenesis by promoting the survival and stimulating the proliferation of NC cells (Henion, 1994). Leukemia inhibitory factor (LIF) is known to play a potent role in the differentiation of sensory neurons (Murphy, 1994). Many studies have been done on the toxicity, specifically neurotoxicity, of lead. Lead is most detrimental when an organism is still developing and directly after birth. As an organism ages, the effects of lead are still dangerous if in high doses. Lead has been connected to defects in the brain, specifically the hippocampus and cortical regions. Individuals with high lead levels may suffer from problems with memory, emotional disorders, and ADD. Recent studies done by Reams-Brown et al.(2000), has shown that the neurotoxicity of lead may be a result of increased neurite outgrowth. They were able to show that lead, in the presence of nerve growth factor (NGF), enhances neurite outgrowth in PC12 cells.