tion early in the first decade of life. In contrast, associative cortical areas show increased amounts of myelin staining only by the second decade, although some ...
VOL. 15, NO. 4, 1989
Myelination of CorticalHippocampal Relays During Late Adolescence
by Francine M. Benes
Abstract The normal developmental series of brains in the Yakovlev Collection has been examined to explore the possibility that various brain regions implicated in schizophrenia may show changes in myelination during late adolescence, a period coinciding with the appearance of early symptoms of this disorder. The prefrontal, cingulate, and parahippocampal (entorhinal) cortex, as well as the perforant pathway, cingulum bundle, and hippocampus, were closely examined because these regions have recently been found to show various neuropathological differences in schizophrenia. Observation of these specimens has confirmed earlier reports by Yakovlev and Lecours (1967) that primary motor and sensory cortices show robust myelination early in the first decade of life. In contrast, associative cortical areas show increased amounts of myelin staining only by the second decade, although some cortical areas, like the cingulate and basofrontal cortex, remain poorly myelinated throughout life. The most striking finding, however, was the appearance of increased myelination of the subicular and presubicular regions during the late adolescent period. Increased myelination in the subiculum was localized to a discrete region at the surface where fibers of the perforant pathway are known to aggregate as they course toward the area dentata. The comparable region in the adjacent presubicular area that also showed increased myelin staining probably contains distal portions of the cingulum bundle. Support for this latter possibility was obtained from a single case in which a stereotaxically placed lesion causing interruption of the cingulum bundle
585
showed less myelin in the presubicular area of the effectively lesioned side. These previously unreported findings are discussed in relation to normal brain circuitry mediating motivational, attentional, and affective behavior. It is suggested that myelination of key linkages in this circuitry may be "permissive" for the expression of a previously latent defect in schizophrenic brain. Recent anatomical investigations suggest that schizophrenic brain may contain structural abnormalities in several different areas that include dorsolateral prefrontal cortex (Benes et al. 1986), anterior cingulate cortex (Benes and Bird 1987), the entorhinal cortex (Jacob and Beckmann 1986), the parahippocampal gyrus (Bogerts et al. 1985; Brown et al. 1986), and the hippocampus (Kovelman and Scheibel 1984; Bogerts et al. 1985). While the etiological significance of these various anatomical defects is unknown, their presence nevertheless suggests the possibility that schizophrenia may involve a defect in the central corticolimbic circuitry of the brain (Benes and Bird 1987; Benes et al. 1987). The brain areas that make up this loop were believed by Papez (1937) to subsume a major role in the emotional component of cognitive behavior, and more recently Mesulam (1983) suggested that some of these areas might mediate volitionally driven attentional responses. Since motivation, attention, and affect are centrally disturbed in schizophrenia, neuroanatomical findings
Reprint requests should be sent to Dr. F.M. Benes, McLean Hospital, 115 Mill St., Belmont, MA 02178.
SCHIZOPHRENIA BULLETIN
586
in these brain regions may reflect altered integration among the elements of this circuit. One strategy for deciphering the pathophysiology of schizophrenia is to relate various neuroanatomical findings to clinical phenomena associated with the disorder. A question that is frequently asked is why schizophrenic symptomatology tends to become manifest between 16 and 20 years of age (Kraepelin 1919). One interpretation of this observation is that some normal developmental event in the brain may serve to trigger the disorder. It has long been known that development of the human brain continues well beyond childhood. For example, increases in the myelination of the prefrontal cortex are believed to occur up to the fourth decade and possibly beyond (Yakovlev and Lecours 1967). Since abnormalities in this cortical area have also been observed in schizophrenic brain (Benes et al. 1986), these late developmental changes may play some role in the pathophysiology of schizophrenia. Since recent investigations in schizophrenic brain also suggest that these patients may have abnormalities in the loop of Papez (Benes and Bird 1987; Benes et al. 1987), it seems reasonable to consider whether elements of this circuit may show changes in myelination during late adolescence. This question is relevant because this corticolimbic circuit, in conjunction with the dorsolateral prefrontal cortex, appears to mediate several functions that are centrally disturbed in schizophrenia (Benes and Bird 1987; Benes et al. 1987). Methods The specimens in the Yakovlev Collection at the Armed Forces Institute
of Pathology, Washington, DC, are under the curatorship of Mohammed Haleem. Normal specimens in the collection cover a wide range from the earliest stages of the embryonic period through the 10th decade of life. The specimens used for this study were from the perinatal period, and 3.5, 4, 6.5, 11, 19, 20, 32, 36, 40, 54, and 64 years of age. An additional specimen from a healthy 17-year-old that had been prepared in a manner identical to those of the Yakovlev Collection has also been included. All specimens had been imbedded in celloidin and cut serially at a thickness of 50 nm in the coronal, sagittal, or horizontal plane. For this report, only coronal sections were used for the photomicrography. In some cases, however, specimens cut in other orientations were also examined to give different perspectives of fiber orientation in a given brain region. Appropriate levels of cut that included dorsolateral prefrontal, basofrontal, anterior cingulate, primary motor, primary sensory, primary visual, superior temporal, parahippocampal, and hippocampal cortices were photographed at the same magnification. In most cases, associative areas of cortex adjacent to the primary areas were also examined. To control for variability of the intensity of myelin staining, specimens were photographed at a magnification of approximately 5x. This permitted comparisons of the degree of staining of various structures in relation to each other. When the staining for a given specimen was more or less intense than for other specimens, it occurred proportionately throughout. Assessments of the degree of myelination for a particular region were made in relation to other structures within the same section, as well as similar structures in other specimens. An
additional control for interspecimen variability involved printing of the photomicrographs at different exposures so that similar densities could be obtained across the various specimens. Results In general, many of the observations that have been made here were consistent with concepts described earlier by Yakovlev and Lecours (1967). For example, in the first decade of life, primary motor and sensory areas show a robust development of myelination that is generally complete by approximately 6 years of age. Associative cortical regions, on the other hand, show very poor myelination until the second decade, while some remain poorly myelinated throughout life. The following areas were examined in more detail: Prefrontal Cortex. The prefrontal
cortex could be divided into basofrontal cortex and dorsolateral cortex. Basofrontal or orbital cortex showed poor myelination throughout all specimens regardless of age. The dorsolateral prefrontal area, however, showed sparse myelination confined to the subcortical zone beneath layer 4 during the first decade (figure 1/4), but by 11 years of age, (figure IB) there was possibly a slight increase in the degree of myelination of fibers radiating through layers 4 and 6 and forming the so-called faciculi radiata. If present, this increase did not constitute a striking difference. At 20 and 32 years of age (figures 1C and ID), little difference in the amount of myelin could be discerned. Cingulate Cortex. The anterior and posterior cingulate cortices showed a
VOL. 15, NO. 4, 1989
587
Figure 1. Photomicrographs of Weigert-stained specimens of dorsolateral prefontal cortex
light degree of myelination that appeared in the second decade and remained relatively constant throughout subsequent decades (not shown). Parahippocampal Gyrus. The parahippocampal gyrus (figure 2) showed a relatively heavy degree of myelination in the subcortical zone beneath layer VI that was similar throughout all the specimens (figure 3). Lighter staining of myelin in the cortical mantle itself was also seen in the second decade and beyond.
-f
,
A. A 65-year-old. B. An 11-year-old. C. A 20-year-old. D. A 32-year-old. Bar in A = 05 mm and applies to other micrographs shown. Cortical laminae 1-6 are indicated as numbers in C.
Subiculum and Presubiculum. At 3.5 (figure 3A), 4 (not shown), 6.5 (figure 3B), and 11 years of age (figure 3C), the subiculum and presubiculum (figure 2) showed light staining of myelinated fibers immediately beneath the surface. At 17 (not shown), 19 (figure 3D), and 20 years of age (figure 3E), there was an increase in the amount of myelinstained material in this same region, and this did not appear to change appreciably at 32, 40, or 54 years of age (figures 3F, G, and H). The differences between the 11-year-old (figure 4A) and the 17-year-old (figure 4B) are seen more strikingly at higher magnification. One of the cases in the Yakovlev Collection had been subjected to a cingulotomy procedure for the control of chronic pain. Examination of this specimen revealed that only the lesion on the left side affected this structure (figure 5A). The presubicular region on the left side (figure 5B) of this specimen showed less myelinated material at the surface than the right side (figure 5Q. Hippocampal Formation. The alveus and fornix showed heavy myelination even at birth (not shown), and
58fi
SCHIZOPHRENIA BULLETIN
Figure 2. Photomicrograph of medial portion of the temporal lobe
The location of the hippocampus (hipp), area dentata (ad), aiveus (open arrowheads), fornix (fx), subiculum (sub), presubiculum (ps), and parahippocampal gyrus (phg) are shown. The section shown is at the level of the lateral geniculate nucleus (Ign), which lies above the subiculum. Bar = 4 mm and applies to all other micrographs shown.
this persisted throughout life (figures 2>A to 3H). A faint, but distinct band of myelination was noted in the stratum moleculare at 3.5 years of age. A slight increase may occur early in the second decade (see specimen from the 11-year-old in figure 3C).
Discussion The data reported in this study must be considered quite preliminary due to the small numbers of specimens, the variability of myelin staining, and the inability to perform quantitative analyses. Given these qualifications, however, if the current findings prove to be accurate,
they may suggest that normal brain ontogeny could involve late adolescent increases in the myelination of certain portions of the central corticolimbic circuitry of the brain. The interval during which this occurs is notable for its overlap with the time of onset of schizophrenia (Kraepelin 1919). Earlier work by Yakovlev and Lecours (1967) had suggested that prefrontal cortex undergoes continued myelination through possibly the fourth decade. The current study, however, has failed to corroborate this finding. The reason for this discrepancy is unclear, but it is possible that quantitative methods might reveal differences that are otherwise too subtle to appreciate by
visual inspection. It is conceivable that the earlier study employed higher magnifications and may have thereby emphasized more subtle differences than those reported here. The changes noted in the subiculum and presubiculum during development were far more striking than those seen in other areas during the adolescent period. It is noteworthy, however, that they seemed to occur most extensively during the late adolescent period. The subiculum and presubiculum are notable for their strategic location within the corticolimbic circuitry of the brain. Although little is known about the function of the subiculum and presubiculum, anatomical investigations have demonstrated that these two brain areas show different patterns of afferents and efferents. The subiculum receives its principal input from the CA1 sector of the hippocampus (Hjorth-Simonsen 1973). It sends efferent fibers, via the postcommissural portion of the fornix, to the hypothalamus (Raisman et al. 1966) and, via the precommissural fornix, to lateral portions of the medial septal nuclei and the nucleus accumbens (Chronister and White 1973). The subiculum also projects to the frontal pole, particularly medial prefrontal cortex (Irle and Markowitsch 1982). The presubicular region, on the other hand, receives its primary inputs from widespread areas of the cortex including the dorsolateral prefrontal (Goldman-Rakic et al. 1984) and inferior parietal (Seltzer and Van Hoesen 1979) areas. The presubiculum sends efferents to many sites, including the dorsal and medial entorhinal area (Shipley 1974),and dorsolateral prefrontal (Irle and Markowitsch 1982) cortex. The perforant pathway carries fibers from all
VOL. 15, NO. 4, 1989
Figure 3. Set of photomicrographs of the hippocampal formation, subiculum, and presubiculum
589
regions of the entorhinal area (Hjorth-Simonsen and Jeune 1972; Van Hoesen and Pandya 1975) that pass through the subicular region en route to the stratum moleculare of the area dentata where they terminate on granule cell dendrites (Lorente de No 1934; Ramon Y Cajal 1911, 1955; Blackstad 1956; Nafstad 1967; Fifkova 1975). This so-called perforant pathway represents one of
the most extensive inputs to the hippocampal formation. Interestingly, fibers of this pathway aggregate at the surface of the subiculum where the significant increases in myelination were noted during late adolescence, although they are not actually considered to be part of the region. The cingulum bundle, a vital link of the cingulate cortex and to some extent the dorsolateral prefrontal area (Goldman-Rakic et al. 1984), with the hippocampal formation, terminates in a similar location within the presubiculum (Ramon Y Cajal 1911, 1955; Domesick 1969; Pandya et al. 1981; Vogt and Miller 1983; Goldman-Rakic et al. 1984). The fact that differences in the structure of the entorhinal cortex, cingulate cortex, and hippocampus have been recently reported in schizophrenic brain suggests the possibility that changes in myelination of the perforant pathway and the cingulum bundle could somehow be related to the appearance of certain early symptoms of schizophrenia.
A. A 35-year-old. B. A 65-year-old. C. An 11-year-old. D. A 19-year-old. E. A 20-year-old. F. A 32-yearold. G. A 44-year-old. H. A. 54-year-old. Arrow heads indicate the location of myelin-stained material that is sparsely represented until 19-20 years of age. The amount of myelin-stained material appears to increase up to 32 years of age, but not beyond. Bar in A = 2 mm and applies to other micrographs shown.
Core features of the schizophrenic syndrome include disturbances in motivation, attention, and affective experience (Bleuler 1911/1950). Mesulam (1983) has theorized that attentional mechanisms mediated through the cingulate cortex are linked to motivational drives, motor and sensory representations, and
SCHIZOPHRENIA BULLETIN
590
Figure 4. Comparison of myelin staining of the perforant pathway during early and late adolescence
A. An 11-year-old. B. A 17-year-old. Both specimens shown are at the level of the lateral geniculate nucleus (LGN). The pathway (arrows) is located at the surface above the subiculum (SUB) and perforant presubiculum (PS). The region containing the perforant pathway is lightly stained for myelin in the 11-year-old, but is heavily stained in the 17-year-old. The fornix (F) and other regions containing myelinated fibers in the 11-year-old are darkly stained. Both micrographs have been carefully matched for the degree of myelin staining and photographic exposure. Bar in B = 2 mm and also applies to A.
ocular responses. Attentional behavior can be conceptualized as involving activity that not only arises in the cingulate cortex, but also motivational responses mediated through the dorsolateral prefrontal cortex, affective responses mediated through the hippocampus, and finally, ocular saccades initiated through frontal eye field 8 (figure 6). The reciprocal linkages among dorsolateral prefrontal, anterior cingulate, frontal eye field 8, and hippocampal cortices could represent a level of integration that is
critical to normal motivational, attentional, and affective behaviors. When these behaviors are disturbed, as in schizophrenia, there might be altered activity in these brain areas due to independent defects occurring in each, or alternatively, to a primary defect in one causing a cascade effect in the others. The question that the current study raises is whether myelination of one or perhaps two linkages of this circuitry could in some way permit a preexisting latent defect to become manifest. Some retrospec-
tive analyses have suggested that many schizophrenic patients have sustained hypoxic insults at birth (Parnas et al. 1982). Studies in monkey have shown that early lesions of the prefrontal area do not produce any immediate impairment of the delayed response task, a performance skill specifically mediated through this area, until puberty (Goldman 1974). These data have suggested that developmental changes during pubescence, such as hormonal ones, may produce alterations that allow the latent defect to appear (Goldman 1974; Nonneman et al. 1984). Similar mechanisms in schizophrenia that involve normal developmental events during adolescence have been postulated by others (Murray and Lewis 1987; Weinberger 1987). Such changes could involve increased myelination of certain pathways that mediate performance of the delayed response task. In the case of schizophrenia, a preexisting defect, whether arising from a genetic predisposition, an acquired perinatal insult, or perhaps some combination of the two, could remain dormant until some new developmental epoch begins. The changes in myelination of inputs to the hippocampus during the late adolescent period could provide a "permissive" environment for the expression of faulty corticolimbic integration that may occur in relation to schizophrenia. At present, too little is known about the neuroanatomical substrates of schizophrenia to speculate any further on how this might occur. It does seem evident, however, that a consideration of integration among widespread aspects of corticolimbic circuitry will be necessary to understand the complex and diverse symptoms that arise in this disorder.
VOL. 15, NO. 4, 1989
591
Figure 5. Set of photomicrographs of Weigert-stained sections from a patient with chronic pain subjected to a cingulotomy procedure
A. A stereotaxically placed lesion is in evidence in a position inferior to the cingulate gyms on the left side. The right side shows faint evidence of an electrolytic lesion, but it spares the cingulum bundle. B. The hippocampus and parahippocampal gyms on the left side of the brain. C. Same as in B, but on the right side of the brain. There is myelin-stained material located at the surface of the subiculum on both sides of the brain. In the presubicular area (arrowheads), the myelin-stained material is reduced on the left side of the brain. Bar in A = 4 mm and also applies to the other micrographs.
SCHIZOPHRENIA BULLETIN
592
Figure 6. Schematic diagram representing reciprocal connections among several brain regions involved with motivationaliy driven attentional behavior
Archives of General Psychiatry,
42:784-791, 1985. Brown, R.; Colter, N.; Corsellis, J.; Crow, T.J.; Frith, CD.; Jagoe, R.; Johnstone, E.C.; and Marsh, L. Postmortem evidence of structural brain changes in schizophrenia: Differences in brain weight temporal horn area, and parahippocampal gyrus compared with affective disorder.
Frontal Eye Field (Saccadic Eye Movement)
Cinyulate Cortex (Attention)
Prefrontal Cortex (Motivation)
schizophrenia: A morphometric study of brain volume and shrinkage.
Archives of General Psychiatry,
43:36-42, 1986. Chronister, R.B., and White, L.E. The fiber connections of allocortex: The subiculum hippocampi. Anatomical Record, 175:292, 1973.
Hippocampus (Affective Drive)
The prefrontal and cingulate cortices, as well as frontal eye field 8 and the hippocampus, are the principal components of this circuit.
Future studies will seek to replicate and extend these preliminary findings by obtaining a larger cohort with both male and female subjects and employing quantitative techniques to consider more systematically how individual variations and/or gender differences may influence the data reported from this exploratory project. References Benes, F.M.; Davidson, ].; and Bird, E.D. Quantitative cytoarchitectural studies of the cerebral cortex of schizophrenics. Archives of General Psychiatry, 43:31-35, 1986. Benes, F.M.; and Bird, E.D. An analysis of the arrangement of
neurons in the cingulate cortex of schizophrenic patients. Archives of General Psychiatry, 44:608-616, 1987.
Benes, F.M.; Majocha, R.; Bird, E.D.; and Marotta, C.A. Increased vertical axon counts in cingulate cortex of schizophrenics. Archives of General Psychiatry, 44:1011-1021, 1987. Blackstad, T.W. On the termination of some afferents to the hippocampus and fascia dentata. Ada Anatomica, 35:202-214, 1956. Bleuler, E. Dementia Praecox or the Group of Schizophrenias. (1911) New
York: International Universities Press, 1950. pp. 40-52. Bogerts, B.; Meertz, E.; and Schonfeldt-Bausch, R. Basal ganglia and limbic system pathology in
Domesick, V.B. Projections from the cingulate cortex in the rat. Brain Research, 12:296-230, 1969. Fifkova, E. Two types of terminal degeneration in the molecular layer of the dentate fascia following lesions of the entorhinal cortex. Brain Research, 96:169-175, 1975. Goldman, P.S. An alternative to developmental plasticity: Heterology of CNS structures in infants and adults. In: Stein, D.; Rosen, ].; and Butters, M., eds. Plasticity and Recovery of Function in the CNS.
New York: Academic Press, 1974. pp. 149-174. Goldman-Rakic, P.S.; Selemon, L.D.; and Schwartz, M.L. Dual pathways connecting the dorsolateral prefrontal cortex with the hippocampal formation and parahippocampal cortex in the rhesus monkey. Neuroscience, 12:719-743, 1984. Hjorth-Simonsen, A. Some intrinsic connections of the hippocampus in the rat: An experimental analysis. Journal of Comparative Neurology,
147:145-162, 1973. Hjorth-Simonsen, A., and Jeune, B.
593
VOL. 15, NO. 4, 1989
Origin and termination of the hippocampal perforant path in the rat studied by silver impregnation. Journal of Comparative Neurology,
144:215-232, 1972. Irle, E., and Markowitsch, H.J. Widespread cortical projection of the hippocampal formation in the cat. Neuroscience, 7:2637-2647, 1982. Jakob, H., and Beckmann, H. Prenatal developmental disturbances in limbic allocortex in schizophrenics. Journal of Neural Transmission, 65:303-326, 1986. Kovelman, J.A., and Scheibel, A.B. A neurohistological correlate of schizophrenia. Biological Psychiatry, 19:1601-1621, 1984. Kraepelin, E. Dementia Praecox and Paraphrenia. Edinburgh: E. & S. Livingston, 1919. p. 224. Lorente de No, R. Studies on the structure of the cerebral cortex: II. Continuation of the study of the
Vol. II. New York: Academic Press, Vogt, B.A., and Miller, M. Cortical connections between rat cingulate 1984. pp. 139-153. cortex and visual, motor and postPandya, D.N.; Van Hoesen, G.W., subicular cortices. Journal of Comand Mesulam, M.M. Efferent conparative Neurology, 216:192-210, 1983. nections of the cingulate gyrus in Weinberger, D.R. Implications of the rhesus monkey. Experimental normal brain development for the Brain Research, 42:319-330, 1981. pathogenesis of schizophrenia. Papez, J.W. A proposed mechanism Archives of General Psychiatry, of emotion. Archives of Neurology and 44:660-669, 1987. Psychiatry, 38:725-743, 1937. Yakovlev, P.I., and Lecours, A. The Parnas, J.; Schulsinger, F.; Teasdale, myelogenetic cycles of regional T.W.; Schulsinger, H.; Feldman, maturation of the brain. In: P.M.; and Mednick, S.A. Perinatal Minkowski, A., ed. Regional Developcomplications and clinical outcome ment of the Brain in Early Life. Oxwithin the schizophrenic spectrum. ford: Blackwell, 1967. pp. 3-70. British Journal of Psychiatry,
140:416-420, 1982. Raisman, G.; Cowan, M.W.; and Powell, T.P.S. An experimental analysis of the efferent projection of the hippocampus. Brain, 89:83-108, 1966. Ramon Y Cajal, S. Histologi du Systeme Nerveux del Homme et du ammonic system. Journal of PsychoVertebrates. Tome II. Madrid, Spain: logical Neurology, 46:113-177, 1934. Consejo Superior de Investigaciones Cientificos Instituto Ramon Y Cajal, Mesulam, M.M. The functional 1911. pp. 772-779. anatomy and hemispheric specialization for directed attention: The Ramon Y Cajal, S. Studies on the role of the parietal lobe and its conCerebral Cortex. Chicago: The Year nectivity. Trends in Neuroscience, Book Publishers, Inc., 1955. 6:384-387, 1983. pp. 80-99. Murray, R.M., and Lewis, S.W. Is Seltzer, B., and Van Hoesen, G.W. A schizophrenia a neurodevelopmental direct inferior parietal lobule projecdisorder? British Medical Journal, tion to the presubiculum in the 95:681-682, 1987. rhesus monkey. Brain Research, 179:157-161, 1979. Nafstad, P.H.J. An electron microscope study on the termination of Shipley, M.T. Presubiculum afferents the perforant pathy fibers in the hipto the entorhinal area and the Papez pocampus and the fascia dentata. circuit. Brain Research, 67:162-168, Zeitschrift Zellforschung Mikroskopie 1974. Anatomie, 57:532-542, 1967. Van Hoesen, G., and Pandya, D.M. Nonneman, A.J.; Corwin, J.V.; Some connections of the entorhinal Sahlley, C; and Vicedomini, J.P. (area 28) and perirhinal (area 35) Functional development of the cortices of the rhesus monkey: III. prefrontal system. In: Finger, S., and Efferent connections. Brain Research, Almli, C.R., eds. Early Brain Damage. 95:39-59, 1975.
Acknowledgments This work has been supported by USPHS grant MH-42261, a Level II Research Scientist Development Award MH-00423, and the Schizophrenia Collaborative Program MH-31154 from the National Institute of Mental Health; and an award from the Schizophrenia Research Program of the Scottish Rite Foundation. The author thanks Mohamed Haleem, Curator, and other staff at the Armed Forces Institute of Pathology for their help in examining and photographing the specimens in the Yakovlev Collection. The author is also grateful to Dr. Alfred Pope for his comments and criticisms concerning this article.
The Author Francine M. Benes, M.D., Ph.D., is Associate Professor of Psychiatry (Neuroscience), Harvard Medical School, Boston, MA, and is Associate Neurobiologist, Mailman Research Center, McLean Hospital, Belmont, MA.
