Human brain development, like central nervous system development in all vertebrates, takes place by an overproduction and then selective elimination of cells. Most of the dynamic activity of brain development takes place in utero. However, as indicated in Fig. 1, competitive elimination, myelination, and dendritic and axonal arborization continue throughout childhood and adolescence.
Brain cells are of two general types—neurons and glial cells. Although neuronal number peaks during gestation, neuronal size changes with age as axonal thickness, dendritic number, and the number of synaptic connections undergo cyclic changes throughout development. Environmental factors influence which synaptic connections and neurons thrive and remain viable. For instance, children with cataracts who do not receive treatment prior to age 1 suffer irreversible cortical blindness.
The other main class of central nervous system cells is the glial cells, which unlike neurons continue to
Encyclopedia of the Human Brain Volume 1
actively proliferate and die postnatally. Glial cells outnumber neurons by ratios ranging from 1.7 to 10. Myelination by a subclass of glial cells, oligodendrocytes, is an important determinant of increases in structure size during childhood and adolescence. Ultimate structure size is determined by this dynamic interplay between glial cells and decreasing numbers but increasing size of neurons. Synaptic pruning is an important aspect in the functional development of the brain but may have little impact on overall structure size. Estimates from research on the primary visual cortex of the macaque monkey indicate that a total loss of all boutons would result in only a 1-2% decrease in volume. However, synaptic pruning may have an effect on the thickness of the parent axon or dendritic branches. Another parameter to consider in structure size is packing density, which is influenced by degree of vascularity, extracellular volume, and hydration.
Genetics, hormones, growth factors, nutrients in the developing nervous system, diet, infections, toxins, trauma, stress, and degree of enriched environment all play a role in determining structure size, and the complexity of these factors and their interactions should be considered in any interpretation of the significance of gross structural volume.
total size of the cerebral hemispheres changes little from childhood to adolescence, reaching 95% of its adult size by the age of 5 years. This is perhaps surprising to anyone who has watched an adult's hat falling down over the eyes of a child. The seeming discrepancy is due to the fact that head circumference does indeed increase from ages 4 to 18 (approximately 2.0 in. in boys and 1.9 in. in girls), but the increase is accounted for by an increase in skull thickness and less so by an increase in ventricular volume. Many factors, including intelligence, handedness, psychiatric illness, body size, and gender, have been related to total brain size in teens as well as adults.
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