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This chapter provides an overview of brain development and the role of experience in sculpting the developing brain that provides a construct for understanding the neural basis of cognitive development.

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The brain develops over a prolonged period of time; the most rapid period of development occurs prenatally and during the first few postnatal years. The major events that occur prenatally include the construction of the neural tube, cell proliferation and migration, and cellular differentiation. Although myelination and synaptogenesis both begin during the last trimester of pregnancy, these events extend well into postnatal life.

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Neurulation

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Between the second and third weeks of gestation, the dorsal region of the ectodermal layer of the embryo begins to thicken and form a pear-shaped plate. As cell proliferation continues, this plate becomes a groove and then a tube. Toward the end of the third week of gestation, the anterior end of the neural tube forms a set of swollen enlargements that give rise to 3 primary vesicles: the forebrain (which will become the cerebral hemispheres), the midbrain (which will contain important pathways to and from the forebrain), and the hindbrain (which will consist of the brainstem and cerebellum). The remainder of the neural tube gives rise to the spinal cord, peripheral nerves, and certain endocrine glands in the body. The neural tube completes its closure by the end of the third prenatal week.

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This phase of development may be compromised, leading to a class of disorders called neural tube defects, which are further discussed in Chapter 549.2

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Neurogenesis

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After the neural tube has closed, a new phase of brain development commences. Within the neural tube, the innermost cells divide rapidly and repeatedly, giving rise first to the cells that primarily become neurons and later to precursors of both neurons and the supportive tissue components called glia (which will include elements such as astrocytes, oligodendrocytes, etc). In most areas of the brain, the process of neurogenesis is completed by the third trimester of pregnancy, with 2 known (and accepted) exceptions. First, cells that line the olfactory bulb turn over on a near-weekly basis for the entire life span. Second, postnatal neurogenesis is known to occur in a region of the hippocampus known as the dentate gyrus.3-5 In this region, new neurons are born through at least middle age. Interestingly, these new neurons possess all the functional properties of prenatally derived neurons (eg, developmental processes); in addition, they appear to be influenced by experience. For example, when learning and memory functions are challenged in the rodent, the number of new cells produced in the dentate increases; by contrast, if the rodent is reared in a stressful environment, there is a reduction in the number of new cells produced.6

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Although there have been reports of postnatal neurogenesis in other regions of the cortex, there is controversy as ...

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