The cytological organization and the timetable of emergence and dissolution of the transient subplate zone subjacent to the developing visual and somatosensory cortex were studied in a series of human and monkey fetal brains. Cerebral walls processed with Nissl, Golgi, electron-microscopic, and histochemical methods show that this zone consists of migratory and postmigratory neurons, growth cones, loosely arranged axons, dendrites, synapses, and glial cells. In both species the subplate zone becomes visible at the beginning of the mid-third of gestation as a cell-poor/fiber-rich layer situated between the intermediate zone and the developing cortical plate. The subplate zone appears earlier in the somatosensory than in the visual area and reaches maximal width at the beginning of the last third of gestation in both regions. At the peak of its size the ratio between the width of the subplate zone and cortical plate in the somatosensory cortex is 2:1 in monkey and 4:1 in man while in the occipital lobe these structures have about equal width in both species. The dissolution of the subplate zone begins during the last third of gestation with degeneration of some subplate neurons and the relocation of fiber terminals into the cortex. The subplate zone disappears faster in the visual than in the somatosensory area. The present results together with our previous findings support the hypothesis that the subplate zone may serve as a "waiting" compartment for transient cellular interactions and a substrate for competition, segregation, and growth of afferents originated sequentially from the brain stem, basal forebrain, thalamus, and from the ipsi- and contralateral cerebral hemisphere. After a variable and partially overlapping time period, these fibers enter the cortical plate while the subplate zone disappears leaving only a vestige of cells scattered throughout the subcortical white matter. A comparison between species indicates that the size and duration of the subplate zone increases during mammalian evolution and culminates in human fetuses concomitantly with an enlargement of cortico-cortical fiber systems. The regional difference in the size, pattern, and resolution of the subplate zone correlates also with the pattern of cerebral convolutions. Our findings indicate that, contrary to prevailing notions, the subplate may not be a vestige of the phylogenetically old network but a transient embryonic structure that expanded during evolution to subserve the increasing number of its connections.

The major mechanism for generating diversity of neuronal connections beyond their genetic determination is the activity-dependent stabilization and selective elimination of the initially overproduced synapses [Changeux JP, Danchin A (1976) Nature 264:705-712]. The largest number of supranumerary synapses has been recorded in the cerebral cortex of human and nonhuman primates. It is generally accepted that synaptic pruning in the cerebral cortex, including prefrontal areas, occurs at puberty and is completed during early adolescence [Huttenlocher PR, et al. (1979) Brain Res 163:195-205]. In the present study we analyzed synaptic spine density on the dendrites of layer IIIC cortico-cortical and layer V cortico-subcortical projecting pyramidal neurons in a large sample of human prefrontal cortices in subjects ranging in age from newborn to 91 y. We confirm that dendritic spine density in childhood exceeds adult values by twoto threefold and begins to decrease during puberty. However, we also obtained evidence that overproduction and developmental remodeling, including substantial elimination of synaptic spines, continues beyond adolescence and throughout the third decade of life before stabilizing at the adult level. Such an extraordinarily long phase of developmental reorganization of cortical neuronal circuitry has implications for understanding the effect of environmental impact on the development of human cognitive and emotional capacities as well as the late onset of human-specific neuropsychiatric disorders.association cortex | critical period | schizophrenia | synaptogenesis

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The developing human cerebrum displays age-specific changes in its patterns of lamination. Among these, the subplate zone is the most prominent transient compartment because growing major afferent systems temporarily reside in this zone, establish synapses and take part in cellular interactions that are crucial for subsequent cortical development. We explored the potential of magnetic resonance imaging (MRI) for tracing the developmental history of the most prominent cortical layer (the subplate zone) and other laminar compartments of the fetal cerebral wall between 15 and 36 weeks post-ovulation. We found that changes in the MRI lamination pattern of the human fetal cerebral wall are predominantly caused by changes in the subplate zone. Histochemical staining of the extracellular matrix (ECM) enables selective visualization of the subplate zone and correlation with an increase in MRI signal intensity in the subplate zone and ingrowth and accumulation of thalamocortical and corticocortical afferents and their subsequent relocation to the cortical plate. Thus, dynamic changes in the MRI appearance of the subplate zone and histochemical staining of its ECM can be used as indirect parameters for an assessment of normal versus disturbed unfolding of crucial histogenetic events that are involved in prenatal shaping of the human cerebral cortex.

The aim of this review is to present clinically relevant data on prenatal development of thalamocortical connections in the human brain. The analysis is based on extensive Zagreb Neuroembryological Collection, including more than 500 prenatal human brains stained with various classical neurohistological, as well as modern histochemical and immunohistochemical methods. The connection of thalamocortical axons during the 'waiting' period with transient cortical subplate zone and subsequent synaptic engagement in the cortical plate is the main connectivity event in the late foetus and preterm infant. This connectivity is the structural substrate for the endogeneous subplate and sensory-driven circuitry generating transient electrical phenomena and may represent a transient network in the developmental history of consciousness.

The fine structure, synaptic relationships, distribution and time of origin of interstitial neurons situated within the white matter subjacent to the visual, somatosensory and motor cortices were studied in the human and monkey telencephalon. The analysis was carried out on Nissl-stained serial sections, rapid Golgi impregnations, by acetylcholinesterase (AChE) histochemistry, electron microscopy and [3H]thymidine ([3H]TdR) autoradiography. Interstitial neurons have a similar distribution, morphology and histochemistry in both human and monkey telencephalon. Their highest density and the most extensive distribution is found in the neonatal period in both species. The number of interstitial neurons decreases during infancy, but numerous cells remain in the adult. Two types of interstitial neuron can be recognized in Golgi preparations: polymorphic cells, usually situated close to the cortex and fusiform cells, located predominantly in the depths of the white matter. The polymorphic cell type is prevalent during neonatal and infant stages, while fusiform cells are relatively more numerous in the adult. Interstitial cells have ultrastructural features and organelles typical of neurons of the central nervous system with well-defined axosomatic and axodendritic synapses of both symmetrical and asymmetrical types. About 20% fo the interstitial cells show strong specific AChE activity. Autoradiographic analysis of postnatal monkeys exposed to [3H]TdR at various embryonic (E) and early postnatal days indicates that interstitial neurons which lie beneath the visual and somatosensory-motor cortices are generated between E38 and E48. Contrary to the prevailing notion that interstitial neurons are the latest generated cells arreste during migration across the maturing white matter, they prove to be produced at the end of the first third of the 165-day gestation in the rhesus monkey concomitantly with the generation of neurons destined for the deep neocortical layers. These findings raise the possibility that interstitial cells represent a vestige of the transient embryonic subplate layer.

The postnatal development and lifespan alterations in basal dendrites of large layer IIIC and layer V pyramidal neurons were quantitatively studied. Both classes of neurons were characterized by rapid dendritic growth during the first postnatal months. At birth, layer V pyramidal neurons had larger and more complex dendritic trees than those of layer IIIC; however, at 1 postnatal month both classes of neurons displayed a similar extent of dendritic outgrowth. In addition, after a more than year-long "dormant" period of only fine dendritic rearrangement, layer IIIC pyramidal neurons displayed a second period of dendritic growth, starting at the end of the second year and continuing in the third year. During that period, the dendritic tree of layer IIIC pyramidal neurons became more extensive than that of layer V pyramidal neurons. Thus, layer IIIC pyramidal neurons appear to show a biphasic pattern of postnatal dendritic development. Furthermore, the childhood period was characterized by transient increase in size of pyramidal cell somata, which was more pronounced for neurons in layer IIIC. These structural changes occurred during both the period of rapid cognitive development in preschool children and the period of protracted cognitive maturation during the childhood, puberty, and adolescence.

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