Myelination provides another parallel index of development and maturation of the nervous system and is apparently related to the functional activity of the fiber systems. The timing and precision of these connecting pathways are no less precise and time locked than is neuronal development (see Flechsig myelinogenic cycle as shown in Fig. 27-3). The acquisition of myelin sheaths by the spinal nerves and roots by the 10th week of fetal life is associated with the beginning of reflex motor activities. Segmental and intersegmental fiber systems in the spinal cord myelinate soon afterward, followed by ascending and descending fibers to and from the brainstem (reticulospinal, vestibulospinal). The acoustic and labyrinthine systems stand out with singular clarity in myelin-stained preparations by the 28th to 30th weeks, and the spinocerebellar and dentatorubral systems by the 37th week.
After birth, the brain continues to grow dramatically. From an average weight of 375 to 400 g at birth (40 weeks), it reaches about 1,000 g by the end of the first postnatal year. Glial cells (oligodendrocytes and astrocytes) derived from the matrix zones continue to divide and multiply during the first 6 months of postnatal life. The visual system begins to myelinate about the 40th gestational week; its myelination cycle proceeds rapidly, being nearly complete a few months after birth. The corticospinal tracts are not fully myelinated until halfway through the second postnatal year. Most of the principal tracts are myelinated by the end of this period. In the cerebrum, the first myelin is seen at 40 weeks in the posterior frontal and parietal lobes, and the occipital lobes (geniculocalcarine tracts) myelinate soon thereafter. Myelination of the anterior frontal and temporal lobes occurs later, during the first year of postnatal life. By the end of the second year, myelination of the cerebrum is largely complete (Fig. 27-3). These steps in myelination can be followed by MRI, where increasing myelination results in progressive T1 hyperintensity and T2 hypointensity of white matter. Despite these careful anatomic observations, their correlation with developmental clinical and electroencephalographic data has not been precise.
Childhood, Puberty, and Adolescence
Growth of the brain continues, at a much slower rate than before, until 12 to 15 years, when the average adult weight of 1,230 to 1,275 g in females and 1,350 to 1,410 g in males is attained. Myelination also continues slowly during this period. Yakovlev and Lecours, who reexamined Flechsig’s classic findings on the ontogeny of myelination (the term Flechsig myelinogenic cycle is still used), traced the progressive myelination of the middle cerebellar peduncle, acoustic radiation, and bundle of
Vicq d’Azyr (mammillothalamic tract) beyond the third postnatal year; the nonspecific thalamic radiations continued to myelinate beyond the 7th year and fibers of the reticular formation, great cerebral commissures, and intracortical association neurons to at least the 10th year and beyond (Fig. 27-3). These investigators noted that there was an increasing complexity of fiber systems through late childhood and adolescence, and perhaps even into middle adult life. Similarly, in the extensive studies of Conel and Rabinowicz, depicting the cortical architecture at each year from mid-fetal life to the 20th year, the dendritic arborizations and cortical interneuronal connections were observed to increase progressively in complexity; the “packing density” of neurons, that is, the number of neurons in any given volume of tissue increases through the age of approximately 15 months and then decreases (Fig. 27-2).