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nding Neurons become more widely separated as differentiation proceeds, owing to an increase in the size and complexity of dendrites and axons and enlargement of synaptic surfaces (Fig 28-2) The cytoarchitectural patterns that demarcate one part of the cerebral cortex from another are already in evidence by the 30th week of fetal life and become de nitive at 40 weeks and in succeeding months As the maturational process of cortical neurons proceeds, the patterns of neuronal organization in different regions of the brain (motor, premotor, sensory and striate cortices, Broca and Wernicke areas) continue to change Myelination provides another index of the development and maturation of the nervous system and is believed to be related to the functional activity of the ber systems The acquisition of myelin sheaths by the spinal nerves and roots by the 10th week of fetal life is associated with the beginning of re ex motor activities Segmental and intersegmental ber systems in the spinal cord myelinate soon afterward, followed by ascending and descending bers 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 (Fig 28-3) Neonatal Period and Infancy After birth, the brain continues to grow dramatically From an average weight of 375 to 400 g at birth (40 weeks), it reaches about 1000 g by the end of the rst postnatal year Glial cells (oligodendrocytes and astrocytes) derived from the matrix zones continue to divide and multiply during the rst 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 rst 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 rst year of postnatal life By the end of the second year, myelination of the cerebrum is largely complete (Fig 28-3) These steps in myelination can be followed by magnetic resonance im-

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4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 11 12

MOTOR ROOTS SENSORY ROOTS STATO ACOUSTIC TECTUM 8 TEGM MEDIAL LEMNISCUS INNER DIV INF CEREB PED AAK OUTER DIV INF CEREB PED

SUP CEREBELLAR PED MID CEREBELLAR PED RETICULAR FORMATION BR INF COL BR SUP COL 8 OPTIC NERVE 8 TRACT HI and VICQ d AZYR ANSA and PI HZ and PE OPTIC RAD SOMESTHETIC RAD ACOUSTIC RAD NON-SPECIFIC THALAMIC RAD STRIATUM PYRAMIDAL TRACTS FRONTO-PONTINE TRACT FORNIX CINDULUM GREAT CEREBRAL COMMISSURES INTRA-CORTIC, NEUROPIL ASSOC AREAS

Figure 28-3 The myelogenetic chronology (Reproduced by permission from Yakovlev and Lecours)

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aging (MRI) Despite these careful anatomic observations, their correlation with developmental clinical and electroencephalographic (EEG) 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 1230 to 1275 g in females and 1350 to 1410 g in males is attained Myelination also continues slowly during this period Yakovlev and Lecours, who re-examined Flechsig s classic ndings on the ontogeny of myelination (the term Flechsig s 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 nonspeci c thalamic radiations continued to myelinate beyond the seventh year and bers of the reticular formation, great cerebral commissures, and intracortical association neurons to at least the 10th year and beyond (Fig 28-3) These investigators noted that there was an increasing complexity of ber 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, ie, the number of neurons in any given volume of tissue increases through the age of approximately 15 months and then decreases (Fig 28-2) Interesting questions are (1) whether neurons begin to function

only when their axons have acquired a myelin sheath; (2) whether myelination is under the control of the cell body, the axon, or both; and (3) whether the classic myelin stains yield suf cient information as to the time of onset and degree of the myelination process At best these correlations can be only approximate It seems likely that systems of neurons begin to function before the rst appearance of myelin, as shown in conventional myelin stains These correlations need to be restudied, using more delicate measures of function and ner staining techniques, as well as the techniques of quantitative biochemistry and phase and electron microscopy