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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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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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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 ...