Myelination of each of the multiple connecting fiber systems of the CNS takes place at a different time in early development. Some fiber systems start to myeli-nate halfway through gestation or later and rapidly attain their maximal degree of myelination, whereas other systems attain their maximal degree of myeli-nation only slowly. It is, therefore, not correct to refer to myelination as a singular process. There is a marked, temporal diversity in topographic patterns of myelination throughout the last half of gestation and during the first 2 postnatal years. Thus, at any time in the early development of the human brain there are multiple separate or intermixed regions of unmyelinated, partly myelinated, or completely myelinated tracts.
Myelination of the nervous system follows a fixed pattern consisting of ordered sequences of myelinat-ing systems apparently governed by some rules:
1. The first rule, probably governing all other rules, is that tracts in the nervous system become myeli-nated at the time they become functional.
2. Most tracts become myelinated in the direction of the impulse conduction.
3. Myelination starts in the PNS before it starts in the CNS.
5. Myelination in the brain occurs earlier in areas of primary function than in association areas.
6. Roughly speaking, myelination progresses from caudal (spinal cord) to rostral parts (brain) and spreads from central (diencephalon, pre- and postcentral gyri) to peripheral parts of the brain. However, there are many exceptions to this rule.
It is important to note that the times mentioned below for myelination of the different tracts and structures of the brain are only generalizations and approximations. In the first place, there is a considerable degree of normal variation. Secondly, the onset of myelination is difficult to define. It can be defined as the first myelin tube found on light microscopic examination, as the appearance of the first myelin lamella on ultrastructural examination, or as the first evidence of the presence of myelin constituents in im-munological investigations.
In the 4th month of gestation myelin is first seen in the anterior motor roots and soon appears in the posterior roots.
In the 6th month of gestation myelination proceeds rapidly cephalad in the medial lemniscus and spinothalamic tracts in the brain stem tegmentum. Myelin begins to appear in the statoacoustic tectum and tegmentum and the lateral lemniscus for the conduction of acoustic stimuli. Myelin is seen in the inner, vestibulocerebellar part of the inferior cerebellar peduncle.
In the 7th month of gestation myelination is still largely confined to structures outside the dien-cephalon and cerebral hemispheres. Progress of myelination is seen in the optic nerve, optic chiasm and tracts, inferior cerebellar peduncle, the parasagit-tal part of the cerebellum, the descending trigeminal tract, superior cerebellar peduncle, capsule of the red nucleus, capsule of the inferior olivary nucleus, vestibulospinal, reticulospinal and tectospinal descending tracts to the spinal cord and posterior limb of the internal capsule.
In the eighth month of gestation, myelination starts in the corpus striatum (in particular globus pallidus), anterior limb of the internal capsule, subcortical white matter of the post- and precentral gyri, rostral part of the optic radiation as well as corti-cospinal tracts in midbrain and pons, transpontine fibers, middle cerebellar peduncles and cerebellar hemispheres.
In the ninth month of gestation, myelination continues in the thalamus (in particular ventrolateral nucleus), putamen, central part of the corona radiata, distal part of the optic radiation, acoustic radiation, anterior commissure, midportion of the corpus callo-sum and fornix.
However, in a child born at term, most of the structures and tracts mentioned are not fully myelinated and, in fact, in some myelination has just started. Apart from some myelin in the central tracts of the corona radiata connected with the pre- and postcentral gyri, and the primary optic and acoustic radiations, the cerebral hemispheres are still largely un-myelinated. During the first postnatal year, myelin spreads throughout the entire brain. By the postnatal age of 12 weeks myelination is well advanced in the corona radiata, the optic radiation and the corpus cal-losum, but the frontal and temporal white matter are still largely unmyelinated. By the age of about 8 months, the adult state is foreshadowed in that none of the fiber systems is still completely devoid of myelin sheaths. Myelin sheaths are still sparse in the temporal and frontal areas. It is not until the end of the second postnatal year that an advanced state of myelination is seen in all subcortical areas. Histologi-cally, myelination reaches completion in early adulthood.
1.10 Compositional Changes in the Developing Brain 13
Was this article helpful?
Are You Sick And Tired Of All The Fat-Burning Tricks And Trends That Just Don’t Deliver? Well, Get Set To Discover The Easy, Safe, Fast, And Permanent Way To Mega-Charge Your Metabolism And Lose Excess Fat Once And For All! This Weight Blasting Method Is Easy AND Natural… And Will Give You The Hot Body And Killer Energy Levels You’ve Been Dreaming Of.