through a combination of genetic influences, experience and other external influences, and the interaction of genes and experience.

Setting Up the Nervous System

The nervous system begins to develop in the human fetus 2 to 3 weeks after conception in a process called neurulation, starting as a layer of undifferentiated precursor cells called the neural plate. These cells eventually give rise to all components of the nervous system. As the initial cells divide to create more cells, the neural plate expands, folds, and fuses to form the neural tube (Detrait, George, et al., 2005; Kibar, Capra, and Gros, 2007). The neural tube continues to enlarge while cells in different parts of the tube become specialized, following a spatial pattern established by predetermined molecular mechanisms. From front to back, the neural tube becomes the forebrain (the cerebral cortices), the midbrain (containing neural pathways to and from the forebrain), the hindbrain (the brainstem and cerebellum), and the spinal cord and peripheral nervous system (Rhinn, Picker, and Brand, 2006).

Various physiological and environmental factors can affect prenatal brain development in ways that are either lethal or seriously debilitating (Detrait, George, et al., 2005; Kibar, Capra, and Gros, 2007). Low levels of the vitamin folic acid, for example, produce anencephaly and spina bifida, disorders of formation of the neural tube. Other prenatal environmental exposures can predispose a developing fetus to the development of MEB disorders later in life. For example, common prenatal infections, such as influenza, and less common ones, such as rubella, toxoplasmosis, and cytomegalovirus, can increase the risk of developing mental retardation, schizophrenia, and autism (Fruntes and Limosin, 2008; Jones, Lopez, and Wilson, 2003; Meyer, Yee, and Feldon, 2007; Pearce, 2001; Penner and Brown, 2007). Prenatal exposure to various environmental toxins, including certain insecticides used in homes and for agricultural purposes (Rauh, Garfinkel, et al., 2006), tobacco smoke (Herrmann, King, and Weitzman, 2008), and alcohol (Alcohol Research and Health, 2000), can impair behavior and cognition later in childhood (Williams and Ross, 2007). Premature birth and low birth weight can also predispose to a wide variety of disorders (Peterson, 2003a), including schizophrenia (Kunugi, Nanko, and Murray, 2001), autism (Kolevzon, Gross, and Reichenberg, 2007), and learning disabilities and educational difficulties (Peterson, 2003a).

The Right Cells in the Right Place

Between weeks 5 and 25 of human fetal gestation, undifferentiated precursor cells divide repeatedly, rapidly giving rise to large numbers of

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