It was Flechsig (1920) who originally put forward the view that the degree of myelination of the CNS might be correlated with functional capacity. In his theory he stated that myelination started in projection pathways before association pathways, in peripheral nerves before central pathways, and in sensory areas before motor ones. Although he modified his theory slightly in response to his critics, he continued to maintain that fibers always myelinated in the same order: first the afferent (sensory), then the efferent (motor), then the association fibers.

The histological study of fetal development has confirmed that myelination proceeds systematically and, in nerve pathways with several neurons, in the order of conduction of the impulse. The first signs of myelination appear in the column of Burdach at the gestational age (GA) of 16 weeks, becoming stronger from the 24th week onward. The column of Goll starts to myelinate at 23 weeks of gestation. Cerebellar tracts start to myelinate at about 20 weeks of gestation, and the amount of myelin at birth is considerable. Pyramidal tracts start to myelinate at 36 weeks at the level of the pons, but at birth the amount of myelin is still small. In other tracts, for example, the rubrospinal tracts, the pattern of the pyramidal tract is followed. In a term neonate at a GA of 40 weeks myelin stains reveal myelin in the medulla oblongata, in the central parts of the cerebellar white matter, in the cerebellar peduncles and the vermis, in the medial lemniscus and fasciculus medialis longitudinalis in the pons and mesencephalon, in the posterior limb of the internal capsule, spreading into the globus pallidus and thalamus and, in the thalamocortical connections in the centrum semiovale, upwards to the parasagittal parts of the postcentral gyrus and backwards into the optic radiation. Paul Flechsig's lithographs (1920) demonstrate this myelination pattern beautifully (Fig. 4.1). Several authors, including Keene and Hewer (1931) and Yakovlev and Lecours (1967),have published diagrams of the progress of myelination (Fig. 4.2).

MRI is unique in making it possible to visualize the progress of myelination in vivo in astonishing detail. It is now possible to describe the state of myelination in preterm and term neonates in detail and to follow this process through up to full maturation.

In the preterm child with a GA of less than 30 weeks the following structures show myelination:

cerebellar vermis, inferior cerebellar peduncles, vestibular nuclei, superior cerebellar peduncles and their decussation, dentate nucleus, medial longitudinal fasciculus, medial geniculate bodies, subthalamic nuclei, inferior olivary nuclei and ventrolateral nuclei of thalamus. Myelin can also be seen in the fasciculus gracilis and cuneatus and in their nuclei (Counsell et al. 2002; Sie et al. 1997).

In the period between 30 and 36 weeks of gestation the quantity of myelin in the aforementioned structures increases, but from weeks 30-36 of gestation no myelin is seen in any new sites on MRI. This is not in agreement with histological descriptions of the myelin process. Reasons for this are the lower sensitivity of MRI to small quantities of myelin, also the reason for a time-lag in myelination timetables between histology and MRI, and the higher spatial resolution of histological methods. At a GA of 36 weeks evidence of the presence of myelin appears on T1-weighted images in the posterior limb of the internal capsules (with higher intensity in the area of the cor-ticospinal tracts) and in the tracts from and to the precentral and postcentral gyri in the corona radiata. In the period between 37 and 42 weeks of gestation myelination of these tracts also becomes visible on T2-weighted images. At this time myelination is visible in the tegmentum pontis but not in the basis pon-tis. Myelin now also appears in the lateral geniculate bodies and in the optic tracts, chiasm, and nerves. The myelin density in the basal ganglia and corticospinal tracts increases, and myelin shows up in the optic radiation.

During the 1st month after birth, myelination progresses rapidly. It becomes more prominent in the areas mentioned above. The pattern of myelin presence in the cerebellum changes (see below). On T1-weight-ed MR images myelin becomes visible in the rest of the striatum and caudate nucleus. Myelin in the optic pathways becomes more prominent, and myelin is also present in cortical layers of the primary motor and sensory cortex and in the hippocampus and parahip-pocampal gyrus. With increasing myelination in the occipital and parietal lobes, the splenium of the corpus callosum starts to myelinate. From the 3rd or 4th month onward myelination proceeds in the frontal direction, and from the 4th to 5th month onward, also in the temporal direction. The anterior limb of the internal capsule shows myelination from the 3rd to 4th month onward, proceeding in the 5th month to-

Myelin The Brain

Fig. 4.1. Lithograph in left upper row is reproduced from work of Paul Flechsig (1920), who used refined histological techniques to depict ongoing myelination in the brain. Progress of myelination of a young infant is presented here. Note that myelin (dark in the image) is already circling around the temporal horn to reach the hippocampus and parahip-pocampal gyrus. Also note myelination of the auditory pathway in the superior temporal gyrus.The two T2-weighted coronal MR images show the same features in vivo.Myelination in this case is somewhat further advanced than on the lithograph, already spreading towards the parietal U fibers wards the genu of the corpus callosum. At 6 months myelination starts to spread in the frontal lobes, and on the Ti-weighted images the pattern of myelination is seen to be more or less complete at about 8 months. The corpus callosum reflects the myelination of the parts it connects. T1-weighted images show myelin in the splenium at 3 months and in the genu at about 6 months of age; T2-weighted images show this 4-6 weeks later.

It should be clear that we are describing 'apparent' myelination, i.e. myelination as it appears on T1- or T2-weighted images, which is dependent on pulse sequences and field strength. The apparent progress in myelination on T2-weighted images lags behind that seen on T1-weighted images. Because of this,myelina-

tion can be followed for much longer on T2-weighted than on Trweighted images. On T2-weighted images myelination does not reach the arcuate fibers in the frontal and temporal areas before the 12th-14th and 14th-18th months, respectively.

Myelination is not an all-or-none process. Myelinated white matter gradually replaces the unmyelinated white matter. In T2-weighted series unmyelinat-ed white matter has higher signal intensity than gray matter. With ongoing myelination, white matter becomes darker, and eventually it can no longer be differentiated from gray matter. This transition or 'cross-over' period is reached in the parietal and occipital areas between 8 and 10 months after birth. After this period myelinated white matter has lower

Fig. 4.2. Classic diagram of progression of myelination as conceived by Yakovlev and Lecours. Most of the structures mentioned can also be made visible on MRI. Appearance of myelination on MR images is 1 or 2 weeks behind this schedule with conventional MR techniques. From Yakovlev and Lecours (1967), with permission

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