Gray and White Matter of the Developing Brain

Marked changes occur in the relative proportions of the gray and white matter of the brain during development. During early developmental stages, the proportion of gray matter increases, and then, when myelination begins, the proportion of the white matter increases. For example, biochemical studies showed that at 2 weeks of age (i.e., at the time when active myelination has just begun), the rat brain is composed mainly of gray matter (85%). During the following 4 weeks of growth, the gray matter proportion decreases rapidly to approximately 70%, whereas the proportion of the white matter increases from the initial value of 15% to approximately 30%. Later, this change in percentage proportions is gradual, and at 52 weeks of age the two matters share the total mass of the brain almost equally (Fig. 5).

However, when the absolute amounts of gray and white matter in the developing brain are considered, the amount of gray matter is found to increase (by approximately 12%) only up to 3 weeks of age and remain almost constant thereafter (up to the age of 9 weeks), whereas the absolute amount of white matter increases steadily throughout this growth period (Fig. 5).

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