MYELINATION OF A KEY RELAY ZONE IN THE HIPPOCAMPAL-FORMATION OCCURS IN THE HUMAN BRAIN DURING CHILDHOOD, ADOLESCENCE, AND ADULTHOOD

Background: A previous study demonstrated that myelination of the supe rior medullary lamina along the surface of the parahippocampal gyrus i s occurring in human brain during adolescence. To further investigate whether postnatal increases of myelination may continue during the sec ond decade and possibly even longer, the extent of myelination in this region has been analyzed in 164 psychiatrically normal individuals ag ed newborn to 76 years. Methods: Cross sections of the hippocampal for mation with adjoining hippocampal gyrus were analyzed on a blinded bas is using either a global rating scale or measurements of the area of m yelin staining. Results: A curvilinear increase in the extent of myeli nation between the first and sixth decades of life (r=.71 and r=.67, r espectively) was observed. When the area of myelination was expressed relative to brain weight, there was a twofold increase between the fir st and second decades and an additional increase of 60% between the fo urth and sixth decades. Female subjects showed a significantly greater degree of myelin staining than did male subjects during the interval of ages 6 to 29 years; however, after the third decade, there were no gender differences in the area of myelin staining. Conclusions: The in creased staining of myelin during the first and second decades princip ally occurred in the subicular region and adjacent portions of the pre subiculum. During the fourth through sixth decades, however, it extend ed to progressively more lateral locations along the surface of the pr esubiculum. The precise origin(s) of the axons showing progressive mye lination is unknown; however, the axons in the subiculum may include s ome perforant path fibers, while those found in the presubiculum may i nclude cingulum bundle projections. Overall, our data are consistent w ith the idea that both early and late postnatal increases of myelinati on occur in a key corticolimbic relay area of the human brain and unde rscore the importance of applying a neurodevelopmental perspective to the study of psychopathology during childhood, adolescence, and even a dulthood.