Файл: The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O\'Rahilly-1.pdf

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C H A P T E R 26

SUPPLEMENT: EARLY

POSTNATAL LIFE

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C h a p t e r 2 6 : EARLY POSTNATAL LIFE

Figure 26–14. (Continued ) (C) At 3 years, most of the adult features are already clearly visible. Structures seen more readily in this image include the septum pellucidum, the aqueduct, and the spheno-occipital joint. In the cerebellum the fissura prima, the nodule, and the tonsil are distinguishable. Many of the white areas represent either fat or bone marrow, whereas solid bone does not produce signals.

Postnatal Prolongation of Fetal growth

Human development shows a general retardation relative to other primates, resulting in retention of juvenile features (neoteny). The human infant is relatively undeveloped and helpless at birth, and the completion of growth and maturation is postponed. It has been proposed (particularly by Adolf Portmann) that because human birth is

accelerated, growth rates during the first postnatal year follow the fetal trends shown by other primates. Indeed it has been estimated that were birth to be delayed in proportion to retarded development in general, then existence in utero would be increased to 21 months. The maintenance of fetal growth rates postnatally allows the brain to increase considerably in mass. By plotting brain weight versus body weight it has been shown that the high fetal slope of the graph is, in the human, continued well into postnatal life.

Myelin(iz)ation

Myelination in the central nervous system begins after its onset in the peripheral system in trimester 2. It continues for a number of years after birth at a gradually decreasing rate. In the CNS, myelination is undertaken by oligodendrocytes and is very slow, and one axon can be enveloped in several sheaths. The determination of the onset and end of myelination depends on whether light or electron microscopy is used, and the appearances on magnetic resonance images lag about a month behind histological data.

Myelination expresses the functional maturity of the brain and is correlated with psychomotor development, although transmission of impulses and functional activity begin before myelin sheaths develop. Large afferent tracts become myelinated early, and tracts that appear early in development generally undergo early myelination, e.g., the medial longitudinal fasciculus. Although myelination is found in the pyramidal decussation by the middle of

prenatal life (Wozniak´ and O’Rahilly, 1982), in the pyramidal tracts it begins late in trimester 3 and is not completed until about two years. Cortical association fibers are among the last to become myelinated.

At birth the human brain is only moderately myelinated (Fig. 26–14A,B) and the cerebral hemispheres contain little myelin. Myelin can be shown histologically in the brain stem, the cerebellar white matter, and the posterior limb of the internal capsule, with extensions to the thalamus and the basal nuclei. Myelination is greatest during the first two postnatal years and is practically complete in early adulthood, although it continues throughout life. Tables showing the progress of myelination are available (e.g., Larroche, 1966; Gilles et al., 1983).

A few examples of magnetic resonance imaging serve to illustrate the increasing degree of myelination. Postnatal myelination in the central nervous system has been studied extensively by Brody and Kinney and their colleagues (Brody et al., 1987; Kinney et al., 1988).

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