Dendritic Spines: Synaptogenesis and Synaptic Pruning for the Developmental Organization of Brain Circuits.

Synaptic overproduction and elimination is a regular developmental event in the mammalian brain. In the cerebral cortex, synaptic overproduction is almost exclusively correlated with glutamatergic synapses located on dendritic spines. Therefore, analysis of changes in spine density on different parts of the dendritic tree in identified classes of principal neurons could provide insight into developmental reorganization of specific microcircuits.The activity-dependent stabilization and selective elimination of the initially overproduced synapses is a major mechanism for generating diversity of neural connections beyond their genetic determination. The largest number of overproduced synapses was found in the monkey and human cerebral cortex. The highest (exceeding adult values by two- to threefold) and most protracted overproduction (up to third decade of life) was described for associative layer IIIC pyramidal neurons in the human dorsolateral prefrontal cortex.Therefore, the highest proportion and extraordinarily extended phase of synaptic spine overproduction is a hallmark of neural circuitry in human higher-order associative areas. This indicates that microcircuits processing the most complex human cognitive functions have the highest level of developmental plasticity. This finding is the backbone for understanding the effect of environmental impact on the development of the most complex, human-specific cognitive and emotional capacities, and on the late onset of human-specific neuropsychiatric disorders, such as autism and schizophrenia.

The Zagreb Skull Collection-The unique identified collection of human skulls from fetuses to centenarians.

Skull anatomy and development have been extensively studied due to their significance in evolutionary biology, forensic anthropology, and clinical medicine. Bone collections are an indispensable resource for conducting such anthropological and anatomical studies. However, worldwide there are only few skull collections containing specimens covering the entire fetal and postnatal period. Herein we describe the Zagreb Skull Collection, an identified collection comprising more than 1100 skulls and skull bone sets from the early fetal period to centenarians. The Zagreb Skull Collection consists of two main parts: the unique Collection of Skull Bones containing 386 sets of separated skull bones from the early fetal period to adulthood and the Collection of Skulls containing 742 skulls (age range 4-101 years). The collection was the core source for numerous anatomical studies on the development, postnatal changes, and anatomical variations of the skull. However, the Zagreb Skull Collection is still an underexploited resource for anthropological, forensic, and anatomical studies with translatability to contemporary clinical practice.

Cortical interneurons in schizophrenia - cause or effect?

GABAergic cortical interneurons are important components of cortical microcircuits. Their alterations are associated with a number of neurological and psychiatric disorders, and are thought to be especially important in the pathogenesis of schizophrenia. Here, we reviewed neuroanatomical and histological studies that analyzed different populations of cortical interneurons in postmortem human tissue from patients with schizophrenia and adequately matched controls. The data strongly suggests that in schizophrenia only selective interneuron populations are affected, with alterations of somatostatin and parvalbumin neurons being the most convincing. The most prominent changes are found in the prefrontal cortex, which is consistent with the impairment of higher cognitive functions characteristic of schizophrenia. In contrast, calretinin neurons, the most numerous interneuron population in primates, seem to be largely unaffected. The selective alterations of cortical interneurons are in line with the neurodevelopmental model and the multiple-hit hypothesis of schizophrenia. Nevertheless, a large number of data on interneurons in schizophrenia is still inconclusive, with different studies yielding opposing findings. Furthermore, no studies found a clear link between interneuron alterations and clinical outcomes. Future research should focus on the causes of changes in the cortical microcircuitry in order to identify potential therapeutic targets.

In the eye of the beholder - how course delivery affects anatomy education.

BACKGROUND: The COVID-19 pandemic caused major shifts in students' learning strategies as well as teaching environments that profoundly affected the delivery of anatomy courses in medical schools. The Department of Anatomy at the University of Zagreb School of Medicine had a unique experience where the anatomy course in 2019/2020 was first taught in-person before transferring to an online course delivery, while the inverse happened in 2020/2021. The core curriculum, course material and examination criteria were the same in both academic years. The aim of the study was to determine whether course delivery affected students' perceptions of the course and whether it impacted students' engagement and success. METHODS: The students' perceptions of the course were assessed via an anonymous course survey (student evaluation of teaching, SET). The questions in the SET assessed the usefulness of teaching modalities rather than students' satisfaction. Most questions were in the form of statements to which students responded with their level of agreement on a five-point Likert scale. Differences between responses in 2019/2020 and 2020/2021 were analyzed using the Mann-Whitney test. Effect size was estimated using Cliff's delta and association between responses was assessed using Spearman's r coefficient. RESULTS: Students' perceptions were significantly affected by changes in course delivery. Students' success and engagement were higher in 2019/2020 when in-person teaching preceded online teaching. Furthermore, students' views on course organization and the usefulness of continuous assessment were more positive in 2019/2020. Finally, students' perceptions of the usefulness of online materials and activities were more positive in 2019/2020. All comparisons between the two academic years were statistically significant (P ≤ 0.0001 for all comparisons, Mann-Whitney test). CONCLUSIONS: Students' perceptions of the anatomy course were dependent on the teaching environment they were exposed to at the beginning of the course. A transfer from in-person to online course delivery was more successful than vice-versa. This has important implications for structuring hybrid courses in medical education in the future.

Upper cortical layer-driven network impairment in schizophrenia.

Schizophrenia is one of the most widespread and complex mental disorders. To characterize the impact of schizophrenia, we performed single-nucleus RNA sequencing (snRNA-seq) of >220,000 neurons from the dorsolateral prefrontal cortex of patients with schizophrenia and matched controls. In addition, >115,000 neurons were analyzed topographically by immunohistochemistry. Compositional analysis of snRNA-seq data revealed a reduction in abundance of GABAergic neurons and a concomitant increase in principal neurons, most pronounced for upper cortical layer subtypes, which was substantiated by histological analysis. Many neuronal subtypes showed extensive transcriptomic changes, the most marked in upper-layer GABAergic neurons, including down-regulation in energy metabolism and up-regulation in neurotransmission. Transcription factor network analysis demonstrated a developmental origin of transcriptomic changes. Last, Visium spatial transcriptomics further corroborated upper-layer neuron vulnerability in schizophrenia. Overall, our results point toward general network impairment within upper cortical layers as a core substrate associated with schizophrenia symptomatology.

Reorganization of the Brain Extracellular Matrix in Hippocampal Sclerosis.

The extracellular matrix (ECM) is an important regulator of excitability and synaptic plasticity, especially in its highly condensed form, the perineuronal nets (PNN). In patients with drug-resistant mesial temporal lobe epilepsy (MTLE), hippocampal sclerosis type 1 (HS1) is the most common histopathological finding. This study aimed to evaluate the ECM profile of HS1 in surgically treated drug-resistant patients with MTLE in correlation to clinical findings. Hippocampal sections were immunohistochemically stained for aggrecan, neurocan, versican, chondroitin-sulfate (CS56), fibronectin, Wisteria floribunda agglutinin (WFA), a nuclear neuronal marker (NeuN), parvalbumin (PV), and glial-fibrillary-acidic-protein (GFAP). In HS1, besides the reduced number of neurons and astrogliosis, we found a significantly changed expression pattern of versican, neurocan, aggrecan, WFA-specific glycosylation, and a reduced number of PNNs. Patients with a lower number of epileptic episodes had a less intense diffuse WFA staining in Cornu Ammonis (CA) fields. Our findings suggest that PNN reduction, changed ECM protein, and glycosylation expression pattern in HS1 might be involved in the pathogenesis and persistence of drug-resistant MTLE by contributing to the increase of CA pyramidal neurons' excitability. This research corroborates the validity of ECM molecules and their modulators as a potential target for the development of new therapeutic approaches to drug-resistant epilepsy.

The Distinct Characteristics of Somatostatin Neurons in the Human Brain.

Somatostatin cells are frequently described as a major population of GABAergic neurons in the cerebral cortex. In this study, we performed a comprehensive analysis of their molecular expression, morphological features, and laminar distribution. We provided a detailed description of somatostatin neurons in the human prefrontal cortex, including their proportion in the total neuron population, laminar distribution, neurotransmitter phenotype, as well as their molecular and morphological characteristics using immunofluorescence and RNAscope in situ hybridization. We found that somatostatin neurons comprise around 7% of neocortical neurons in the human Brodmann areas 9 and 14r, without significant difference between the two regions. Somatostatin cells were NeuN positive and synthesized vesicular GABA transporter and glutamate decarboxylase 1 and 2, confirming their neuronal nature and GABAergic phenotype. Somatostatin cells in the upper cortical layers were small, had a high expression of somatostatin mRNA, a relatively low expression of somatostatin peptide, and co-expressed calbindin. In the lower cortical layers, somatostatin cells were larger with complex somato-dendritic morphology, typically showed a lower expression of somatostatin mRNA and a high expression of somatostatin peptide, and co-expressed neuronal nitric oxide synthase (nNOS) and neuropeptide Y (NPY), but not calbindin. Somatostatin neurons in the white matter co-expressed MAP2. Based on their somato-dendritic morphology, cortical somatostatin neurons could be classified into at least five subtypes. The somatostatin neurons of the human prefrontal cortex show remarkable morphological and molecular complexity, which implies that they have equally complex and distinct functions in the human brain.

Obstructive sleep apnea and multiple facets of a neuroinflammatory response: a narrative review.

Background: Obstructive sleep apnea (OSA) is a chronic, highly prevalent, multi-system and sleep disorder, which may contribute to cognitive impairment and a variety of structural and neurophysiologic changes. The focus on OSA is warranted given its recognized links with major psychiatric and neurologic disorders, including Alzheimer's disease. Some preliminary studies suggest a dual effect of the inflammatory response in OSA. Neuroinflammation may present with initial, potentially adaptive and homeostatic, and later, a more distinctly maladaptive, precipitating and perpetuating role. Objective: We here propose and argue in favour of the inflammatory process in the brain as a likely binding mechanism behind at least some effects that OSA may have on the brain and its function. Several OSA-triggered molecular and cellular events, that could lead to a neurodegenerative cascade, are similarly discussed. Methods: This perspective reviews the body of literature that investigates potential links between the inflammatory processes in the brain and the OSA. A special emphasis is placed on a potential role for neuroplastin, a novel transmembrane synaptic protein involved in the neuroplasticity and known to be differentially regulated in the OSA. Conclusions: The intricate interplay between neuroinflammation and other mechanistic correlates of OSA add to the evidence that neuroinflammation may be a key target for future therapeutic strategies in a number of comorbid disorders. The future studies will need to answer whether it is sleep fragmentation (SF) or intermittent hypoxia (IH) which may drive any such neuroinflammation.

The Signature of Moderate Perinatal Hypoxia on Cortical Organization and Behavior: Altered PNN-Parvalbumin Interneuron Connectivity of the Cingulate Circuitries.

This study was designed in a rat model to determine the hallmarks of possible permanent behavioral and structural brain alterations after a single moderate hypoxic insult. Eighty-two Wistar Han (RccHan: WIST) rats were randomly subjected to hypoxia (pO2 73 mmHg/2 h) or normoxia at the first postnatal day. The substantially increased blood lactate, a significantly decreased cytochrome-C-oxygenase expression in the brain, and depleted subventricular zone suggested a high vulnerability of subset of cell populations to oxidative stress and consequent tissue response even after a single, moderate, hypoxic event. The results of behavioral tests (open-field, hole-board, social-choice, and T-maze) applied at the 30-45th and 70-85th postnatal days revealed significant hyperactivity and a slower pace of learning in rats subjected to perinatal hypoxia. At 3.5 months after hypoxic insult, the histochemical examination demonstrated a significantly increased number of specific extracellular matrix-perineuronal nets and increased parvalbumin expression in a subpopulation of interneurons in the medial and retrosplenial cingulate cortex of these animals. Conclusively, moderate perinatal hypoxia in rats causes a long-lasting reorganization of the connectivity in the cingulate cortex and consequent alterations of related behavioral and cognitive abilities. This non-invasive hypoxia model in the rat successfully and complementarily models the moderate perinatal hypoxic injury in fetuses and prematurely born human babies and may enhance future research into new diagnostic and therapeutic strategies for perinatal medicine.

Von Economo Neurons - Primate-Specific or Commonplace in the Mammalian Brain?

The pioneering work by von Economo in 1925 on the cytoarchitectonics of the cerebral cortex revealed a specialized and unique cell type in the adult human fronto-insular (FI) and anterior cingulate cortex (ACC). In modern studies, these neurons are termed von Economo neurons (VENs). In his work, von Economo described them as stick, rod or corkscrew cells because of their extremely elongated and relatively thin cell body clearly distinguishable from common oval or spindle-shaped infragranular principal neurons. Before von Economo, in 1899 Cajal depicted the unique somato-dendritic morphology of such cells with extremely elongated soma in the FI. However, although VENs are increasingly investigated, Cajal's observation is still mainly being neglected. On Golgi staining in humans, VENs have a thick and long basal trunk with horizontally oriented terminal branching (basilar skirt) from where the axon arises. They are clearly distinguishable from a spectrum of modified pyramidal neurons found in infragranular layers, including oval or spindle-shaped principal neurons. Spindle-shaped cells with highly elongated cell body were also observed in the ACC of great apes, but despite similarities in soma shape, their dendritic and axonal morphology has still not been described in sufficient detail. Studies identifying VENs in non-human species are predominantly done on Nissl or anti-NeuN staining. In most of these studies, the dendritic and axonal morphology of the analyzed cells was not demonstrated and many of the cells found on Nissl or anti-NeuN staining had a cell body shape characteristic for common oval or spindle-shaped cells. Here we present an extensive literature overview on VENs, which demonstrates that human VENs are specialized elongated principal cells with unique somato-dendritic morphology found abundantly in the FI and ACC of the human brain. More research is needed to properly evaluate the presence of such specialized cells in other primates and non-primate species.

The anatomy lesson of the SARS-CoV-2 pandemic: irreplaceable tradition (cadaver work) and new didactics of digital technology.

AIM: To compare the efficacy of different components of online and contact anatomy classes as perceived by medical students. METHODS: An anonymous course evaluation survey was conducted at the end of the academic year 2019/2020. The organization of classes due to the SARS-CoV-2 pandemic provided our students with a unique opportunity to compare online and contact classes. Students' responses were analyzed according to the type of obtained data (ratio, ordinal, and categorical). RESULTS: The response rate was 95.58%. Approximately 90% of students found anatomical dissection and practical work in general to be the most important aspect of teaching, which could not be replaced by online learning. During online classes, students missed the most the interaction with other students, followed by the interaction with student teaching assistants and teaching staff. Very few students found contact lectures useful, with most students reporting that they could be replaced with recorded video lectures. In contrast, recorded video lectures were perceived as extremely helpful for studying. Regular weekly quizzes were essential during online classes as they gave students adequate feedback and guided their learning process. Students greatly benefitted from additional course materials and interactive lessons, which were made easily available via e-learning platform. CONCLUSIONS: Anatomical dissection and interaction during contact classes remain the most important aspects of teaching anatomy. However, online teaching increases learning efficiency by allowing alternative learning strategies and by substituting certain components of contact classes, thus freeing up more time for practical work.

Axon morphology of rapid Golgi-stained pyramidal neurons in the prefrontal cortex in schizophrenia.

AIM: To analyze axon morphology on rapid Golgi impregnated pyramidal neurons in the dorsolateral prefrontal cortex in schizophrenia. METHODS: Postmortem brain tissue from five subjects diagnosed with schizophrenia and five control subjects without neuropathological findings was processed with the rapid Golgi method. Layer III and layer V pyramidal neurons from Brodmann area 9 were chosen in each brain for reconstruction with Neurolucida software. The axons and cell bodies of 136 neurons from subjects with schizophrenia and of 165 neurons from control subjects were traced. The data obtained by quantitative analysis were compared between the schizophrenia and control group with the t test. RESULTS: Axon impregnation length was consistently greater in the schizophrenia group. The axon main trunk length was significantly greater in the schizophrenia than in the control group (93.7 ± 36.6 µm vs 49.8 ± 9.9 µm, P = 0.032). Furthermore, in the schizophrenia group more axons had visibly stained collaterals (14.7% vs 5.5%). CONCLUSION: Axon rapid Golgi impregnation stops at the beginning of the myelin sheath. The increased axonal staining in the schizophrenia group could, therefore, be explained by reduced axon myelination. Such a decrease in axon myelination is in line with both the disconnection hypothesis and the two-hit model of schizophrenia as a neurodevelopmental disease. Our results support that the cortical circuitry disorganization in schizophrenia might be caused by functional alterations of two major classes of principal neurons due to altered oligodendrocyte development.

THE ANATOMY OF OROFACIAL INNERVATION.

The whole human body receives rich sensory innervation with only one exception and that is the brain tissue. The orofacial region is hence no exception. The head region consequently receives a rich network of sensory nerves making it special because the two types of sensory fibres, visceral and somatic overlap, especially in the pharynx. Also, different pain syndromes that affect this region are rather specific in comparison to their presentation in other body regions. With this review article we wanted to show the detailed anatomy of the peripheral sensory pathways, because of its importance in everyday body functions (eating, drinking, speech) as well as the importance it has in pathological conditions (pain syndromes), in diagnostics and regional analgesia and anaesthesia.

Somato-dendritic morphology and axon origin site specify von Economo neurons as a subclass of modified pyramidal neurons in the human anterior cingulate cortex.

Von Economo neurons (VENs) are modified pyramidal neurons characterized by an extremely elongated rod-shaped soma. They are abundant in layer V of the anterior cingulate cortex (ACC) and fronto-insular cortex (FI) of the human brain, and have long been described as a human-specific neuron type. Recently, VENs have been reported in the ACC of apes and the FI of macaque monkeys. The first description of the somato-dendritic morphology of VENs in the FI by Cajal in 1899 (Textura del Sistema Nervioso del Hombre y de los Vertebrados, Tomo II. Madrid: Nicolas Moya) strongly suggested that they were a unique neuron subtype with specific morphological features. It is surprising that a clarification of this extremely important observation has not yet been attempted, especially as possible misidentification of other oval or fusiform cells as VENs has become relevant in many recently published studies. Here, we analyzed sections of Brodmann area 24 (ACC) stained with rapid Golgi and Golgi-Cox in five adult human specimens, and confirmed Cajal's observations. In addition, we established a comprehensive morphological description of VENs. VENs have a distinct somato-dendritic morphology that allows their clear distinction from other modified pyramidal neurons. We established that VENs have a perpendicularly oriented, stick-shaped core part consisting of the cell body and two thick extensions - an apical and basal stem. The perpendicular length of the core part was 150-250 µm and the thickness was 10-21 µm. The core part was characterized by a lack of clear demarcation between the cell body and the two extensions. Numerous thin, spiny and horizontally oriented side dendrites arose from the cell body. The basal extension of the core part typically ended by giving numerous smaller dendrites with a brush-like branching pattern. The apical extension had a topology typical for apical dendrites of pyramidal neurons. The dendrites arising from the core part had a high dendritic spine density. The most distinct feature of VENs was the distant origin site of the axon, which arose from the ending of the basal extension, often having a common origin with a dendrite. Quantitative analysis found that VENs could be divided into two groups based on total dendritic length - small VENs with a peak total dendritic length of 1500-2500 µm and large VENs with a peak total dendritic length of 5000-6000 µm. Comparative morphological analysis of VENs and other oval and fusiform modified pyramidal neurons showed that on Nissl sections small VENs might be difficult to identify, and that oval and fusiform neurons could be misidentified as VENs. Our analysis of Golgi slides of Brodmann area 9 from a total of 32 adult human subjects revealed only one cell resembling VEN morphology. Thus, our data show that the numerous recent reports on the presence of VENs in non-primates in other layers and regions of the cortex need further confirmation by showing the dendritic and axonal morphology of these cells. In conclusion, our study provides a foundation for further comprehensive morphological and functional studies on VENs between different species.

Integration of complementary biomarkers in patients with first episode psychosis: research protocol of a prospective follow up study.

In this project, we recruited a sample of 150 patients with first episode of psychosis with schizophrenia features (FEP) and 100 healthy controls. We assessed the differences between these two groups, as well as the changes between the acute phase of illness and subsequent remission among patients over 18-month longitudinal follow-up. The assessments were divided into four work packages (WP): WP1- psychopathological status, neurocognitive functioning and emotional recognition; WP2- stress response measured by saliva cortisol during a stress paradigm; cerebral blood perfusion in the resting state (with single photon emission computed tomography (SPECT) and during activation paradigm (with Transcranial Ultrasonography Doppler (TCD); WP3-post mortem analysis in histologically prepared human cortical tissue of post mortem samples of subjects with schizophrenia in the region that synaptic alteration was suggested by WP1 and WP2; WP4- pharmacogenetic analysis (single gene polymorphisms and genome wide association study (GWAS). We expect that the analysis of these data will identify a set of markers that differentiate healthy controls from patients with FEP, and serve as an additional diagnostic tool in the first episode of psychosis, and prediction tool which can be then used to help tailoring individualized treatment options. In this paper, we describe the project protocol including aims and methods and provide a brief description of planned post mortem studies and pharmacogenetic analysis.

The Protracted Maturation of Associative Layer IIIC Pyramidal Neurons in the Human Prefrontal Cortex During Childhood: A Major Role in Cognitive Development and Selective Alteration in Autism.

The human specific cognitive shift starts around the age of 2 years with the onset of self-awareness, and continues with extraordinary increase in cognitive capacities during early childhood. Diffuse changes in functional connectivity in children aged 2-6 years indicate an increase in the capacity of cortical network. Interestingly, structural network complexity does not increase during this time and, thus, it is likely to be induced by selective maturation of a specific neuronal subclass. Here, we provide an overview of a subclass of cortico-cortical neurons, the associative layer IIIC pyramids of the human prefrontal cortex. Their local axonal collaterals are in control of the prefrontal cortico-cortical output, while their long projections modulate inter-areal processing. In this way, layer IIIC pyramids are the major integrative element of cortical processing, and changes in their connectivity patterns will affect global cortical functioning. Layer IIIC neurons have a unique pattern of dendritic maturation. In contrast to other classes of principal neurons, they undergo an additional phase of extensive dendritic growth during early childhood, and show characteristic molecular changes. Taken together, circuits associated with layer IIIC neurons have the most protracted period of developmental plasticity. This unique feature is advanced but also provides a window of opportunity for pathological events to disrupt normal formation of cognitive circuits involving layer IIIC neurons. In this manuscript, we discuss how disrupted dendritic and axonal maturation of layer IIIC neurons may lead into global cortical disconnectivity, affecting development of complex communication and social abilities. We also propose a model that developmentally dictated incorporation of layer IIIC neurons into maturing cortico-cortical circuits between 2 to 6 years will reveal a previous (perinatal) lesion affecting other classes of principal neurons. This "disclosure" of pre-existing functionally silent lesions of other neuronal classes induced by development of layer IIIC associative neurons, or their direct alteration, could be found in different forms of autism spectrum disorders. Understanding the gene-environment interaction in shaping cognitive microcircuitries may be fundamental for developing rehabilitation and prevention strategies in autism spectrum and other cognitive disorders.

Histological and MRI Study of the Development of the Human Indusium Griseum.

To uncover the ontogenesis of the human indusium griseum (IG), 28 post-mortem fetal human brains, 12-40 postconceptional weeks (PCW) of age, and 4 adult brains were analyzed immunohistochemically and compared with post-mortem magnetic resonance imaging (MRI) of 28 fetal brains (14-41 PCW). The morphogenesis of the IG occurred between 12 and 15 PCW, transforming the bilateral IG primordia into a ribbon-like cortical lamina. The histogenetic transition of sub-laminated zones into the three-layered cortical organization occurred between 15 and 35 PCW, concomitantly with rapid cell differentiation that occurred from 18 to 28 PCW and the elaboration of neuronal connectivity during the entire second half of gestation. The increasing number of total cells and neurons in the IG at 25 and 35 PCW confirmed its continued differentiation throughout this period. High-field 3.0 T post-mortem MRI enabled visualization of the IG at the mid-fetal stage using T2-weighted sequences. In conclusion, the IG had a distinct histogenetic differentiation pattern than that of the neighboring intralimbic areas of the same ontogenetic origin, and did not show any signs of regression during the fetal period or postnatally, implying a functional role of the IG in the adult brain, which is yet to be disclosed.

Biphasic dendritic growth of dorsolateral prefrontal cortex associative neurons and early cognitive development.

AIM: To analyze postnatal development and life-span changes of apical dendrite side branches (oblique dendrites) from associative layer IIIC magnopyramidal neurons in the human dorsolateral prefrontal cortex and to compare the findings with the previously established pattern of basal dendrite development. METHODS: We analyzed dendritic morphology from 352 rapid-Golgi impregnated neurons (10-18 neurons per subject) in Brodmann area 9 from the post-mortem tissue of 25 subjects ranging in age from 1 week to 91 years. Data were collected in the period between 1994 and 1996, and the analysis was performed between September 2017 and February 2018. Quantitative dendritic parameters were statistically analyzed using one-way analysis of variance and two-tailed t tests. RESULTS: Oblique dendrites grew rapidly during the first postnatal months, and the increase in the dendrite length was accompanied by the outgrowth of new dendritic segments. After a more than one-year-long "dormant" period of only fine dendritic rearrangements (2.5-16 months), oblique dendrites displayed a second period of marked growth, continuing through the third postnatal year. Basal and oblique dendrites displayed roughly the same growth pattern, but had considerably different topological organization in adulthood. CONCLUSION: Our analysis confirmed that a biphasic pattern of postnatal dendritic development, together with a second growth spurt at the age of 2-3 years, represents a unique feature of the associative layer IIIC magnopyramidal neurons in the human dorsolateral prefrontal cortex. We propose that these structural changes relate to rapid cognitive development during early childhood.