The surface of the developing cerebral cortex: still special cells one century later

Parental Education, Household Income, and Cortical Surface Area among 9-10 Years Old Children: Minorities' Diminished Returns

Introduction: Although the effects of parental education and household income on children's brain development are well established, less is known about possible variation in these effects across diverse racial and ethnic groups. According to the Minorities' Diminished Returns (MDRs) phenomenon, due to structural racism, social stratification, and residential segregation, parental educational attainment and household income show weaker effects for non-White than White children. Purpose: Built on the MDRs framework and conceptualizing race as a social rather than a biological factor, this study explored racial and ethnic variation in the magnitude of the effects of parental education and household income on children's whole-brain cortical surface area. Methods: For this cross-sectional study, we used baseline socioeconomic and structural magnetic resonance imaging (sMRI) data of the Adolescent Brain Cognitive Development (ABCD) study. Our analytical sample was 10,262 American children between ages 9 and 10. The independent variables were parental education and household income. The primary outcome was the children's whole-brain cortical surface area. Age, sex, and family marital status were covariates. Race and ethnicity were the moderators. We used mixed-effects regression models for data analysis as participants were nested within families and study sites. Results: High parental education and household income were associated with larger children's whole-brain cortical surface area. The effects of high parental education and high household income on children's whole-brain cortical surface area were modified by race. Compared to White children, Black children showed a diminished return of high parental education on the whole-brain cortical surface area when compared to White children. Asian American children showed weaker effects of household income on the whole-brain cortical surface area when compared to White children. We could not find differential associations between parental education and household income with the whole-brain cortical surface area, when compared to White children, for non-Hispanic and Hispanic children. Conclusions: The effects of parental educational attainment and household income on children's whole-brain cortical surface area are weaker in non-White than White families. Although parental education and income contribute to children's brain development, these effects are unequal across racial groups.

Morphological and Physiological Characteristics of Ebf2-EGFP-Expressing Cajal-Retzius Cells in Developing Mouse Neocortex

Cajal-Retzius (CR) cells are one of the earliest populations of neurons in the cerebral cortex of rodents and primates, and they play a critical role in corticogenesis and cortical lamination during neocortical development. However, a comprehensive morphological and physiological profile of CR cells in the mouse neocortex has not yet been established. Here, we systematically investigated the dynamic development of CR cells in Tg(Ebf2-EGFP)58Gsat/Mmcd mice. The morphological complexity, membrane activities and presynaptic inputs of CR cells coordinately increase and reach a plateau at P5–P9 before regressing. Using 3D reconstruction, we delineated a parallel-stratification pattern of the axonal extension of CR cells. Furthermore, we found that the morphological structure and presynaptic inputs of CR cells were disturbed in Reelin-deficient mice. These findings confirm that CR cells undergo a transient maturation process in layer 1 before disappearing. Importantly, Reelin deficiency impairs the formation of synaptic connections onto CR cells. In conclusion, our results provide insights into the rapid maturation and axonal stratification of CR cells in layer 1. These findings suggest that both the electrophysiological activities and the morphology of CR cells provide vital guidance for the modulation of early circuits, in a Reelin-dependent manner.

Time-Lapse Imaging and Cell Tracking of Migrating Cells in Slices and Flattened Telencephalic Vesicles

Neuronal migration is a vital process needed for subsequent assembly and function of neural circuitry during embryonic development. The vast majority of neural progenitors are generated far from their final destination and need to migrate considerable distances to reach their specific cortical layer. Innovations in cell culture techniques and fluorescence microscopy now facilitate the direct visualization of cell movements during cortical development. Here, a detailed protocol to record and analyze a particular type of early migrating neurons, the Cajal Retzius Cells, during the development of the telencephalic vesicles in mammals is described. This method applied to other reporter mouse lines or to electroporated mouse embryos can be also used to analyze the migration of different types of moving neurons during cortical development. © 2017 by John Wiley & Sons, Inc.

Mechanisms and in vivo functions of contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a process whereby a cell ceases motility or changes its trajectory upon collision with another cell. CIL was initially characterized more than half a century ago and became a widely studied model system to understand how cells migrate and dynamically interact. Although CIL fell from interest for several decades, the scientific community has recently rediscovered this process. We are now beginning to understand the precise steps of this complex behaviour and to elucidate its regulatory components, including receptors, polarity proteins and cytoskeletal elements. Furthermore, this process is no longer just in vitro phenomenology; we now know from several different in vivo models that CIL is essential for embryogenesis and in governing behaviours such as cell dispersion, boundary formation and collective cell migration. In addition, changes in CIL responses have been associated with other physiological processes, such as cancer cell dissemination during metastasis.

"Subpial Fan Cell" - A Class of Calretinin Neuron in Layer 1 of Adult Monkey Prefrontal Cortex

Layer 1 of the cortex contains populations of neurochemically distinct neurons and afferent fibers which markedly affect neural activity in the apical dendritic tufts of pyramidal cells. Understanding the causal mechanisms requires knowledge of the cellular architecture and synaptic organization of layer 1. This study has identified eight morphological classes of calretinin immunopositive (CRet+) neurons (including Cajal-Retzius cells) in layer 1 of the prefrontal cortex (PFC) in adult monkey (Macaca fasicularis), with a distinct class — termed “subpial fan (SPF) cell” — described in detail. SPF cells were rare horizontal unipolar CRet+ cells located directly beneath the pia with a single thick primary dendrite that branched into a characteristic fan-like dendritic tree tangential to the pial surface. Dendrites had spines, filamentous processes and thorny branchlets. SPF cells lay millimeters apart with intralaminar axons that ramified widely in upper layer 1. Such cells were GABA immunonegative (-) and occurred in areas beyond PFC. Interspersed amidst SPF cells displaying normal structural integrity were degenerating CRet+ neurons (including SPF cells) and clumps of lipofuscin-rich cellular debris. The number of degenerating SPF cells increased during adulthood. Ultrastructural analyses indicated SPF cell somata received asymmetric (A — presumed excitatory) and symmetric (S — presumed inhibitory) synaptic contacts. Proximal dendritic shafts received mainly S-type and distal shafts mostly A-type input. All dendritic thorns and most dendritic spines received both synapse types. The tangential areal density of SPF cell axonal varicosities varied radially from parent somata — with dense clusters in more distal zones. All boutons formed A-type contacts with CRet- structures. The main post-synaptic targets were dendritic shafts (67%; mostly spine-bearing) and dendritic spines (24%). SPF-SPF cell innervation was not observed. Morphometry of SPF cells indicated a unique class of CRet+/GABA- neuron in adult monkey PFC — possibly a subtype of persisting Cajal-Retzius cell. The distribution and connectivity of SPF cells suggest they act as integrative hubs in upper layer 1 during postnatal maturation. The main synaptic output of SPF cells likely provides a transminicolumnar excitatory influence across swathes of apical dendritic tufts — thus affecting information processing in discrete patches of layer 1 in adult monkey PFC.

Cajal-Retzius cells: update on structural and functional properties of these mystic neurons that bridged the 20th century

Cajal-Retzius cells (CRc) represent a mostly transient neuronal cell type localized in the uppermost layer of the developing neocortex. The observation that CRc are a major source of the extracellular matrix protein reelin, which is essential for the laminar development of the cerebral cortex, attracted the interest in this unique cell type. In this review we will (i) describe the morphological and molecular properties of neocortical CRc, with a special emphasize on the question which markers can be used to identify CRc, (ii) summarize reports that identified the different developmental origins of CRc, (iii) discuss the fate of CRc, including recent evidence for apoptotic cell death and a possible persistence of some CRc, (iv) provide a detailed description of the electrical membrane properties and transmitter receptors of CRc, and (v) address the role of CRc in early neuronal circuits and cortical development. Finally, we speculate whether CRc may provide a link between early network activity and the structural maturation of neocortical circuits.

Historical first descriptions of Cajal-Retzius cells: from pioneer studies to current knowledge

Santiago Ramón y Cajal developed a great body of scientific research during the last decade of 19th century, mainly between 1888 and 1892, when he published more than 30 manuscripts. The neuronal theory, the structure of dendrites and spines, and fine microscopic descriptions of numerous neural circuits are among these studies. In addition, numerous cell types (neuronal and glial) were described by Ramón y Cajal during this time using this “reazione nera” or Golgi method. Among these neurons were the special cells of the molecular layer of the neocortex. These cells were also termed Cajal cells or Retzius cells by other colleagues. Today these cells are known as Cajal–Retzius cells. From the earliest description, several biological aspects of these fascinating cells have been analyzed (e.g., cell morphology, physiological properties, origin and cellular fate, putative function during cortical development, etc). In this review we will summarize in a temporal basis the emerging knowledge concerning this cell population with specific attention the pioneer studies of Santiago Ramón y Cajal.

Activity-dependent endogenous taurine release facilitates excitatory neurotransmission in the neocortical marginal zone of neonatal rats

In the developing cerebral cortex, the marginal zone (MZ), consisting of early-generated neurons such as Cajal-Retzius cells, plays an important role in cell migration and lamination. There is accumulating evidence of widespread excitatory neurotransmission mediated by γ-aminobutyric acid (GABA) in the MZ. Cajal-Retzius cells express not only GABA_A receptors but also α2/β subunits of glycine receptors, and exhibit glycine receptor-mediated depolarization due to high [Cl⁻]_i. However, the physiological roles of glycine receptors and their endogenous agonists during neurotransmission in the MZ are yet to be elucidated. To address this question, we performed optical imaging from the MZ using the voltage-sensitive dye JPW1114 on tangential neocortical slices of neonatal rats. A single electrical stimulus evoked an action-potential-dependent optical signal that spread radially over the MZ. The amplitude of the signal was not affected by glutamate receptor blockers, but was suppressed by either GABA_A or glycine receptor antagonists. Combined application of both antagonists nearly abolished the signal. Inhibition of Na⁺, K⁺-2Cl⁻ cotransporter by 20 µM bumetanide reduced the signal, indicating that this transporter contributes to excitation. Analysis of the interstitial fluid obtained by microdialysis from tangential neocortical slices with high-performance liquid chromatography revealed that GABA and taurine, but not glycine or glutamate, were released in the MZ in response to the electrical stimulation. The ambient release of taurine was reduced by the addition of a voltage-sensitive Na⁺ channel blocker. Immunohistochemistry and immunoelectron microscopy indicated that taurine was stored both in Cajal-Retzius and non-Cajal-Retzius cells in the MZ, but was not localized in presynaptic structures. Our results suggest that activity-dependent non-synaptic release of endogenous taurine facilitates excitatory neurotransmission through activation of glycine receptors in the MZ.

Contact repulsion controls the dispersion and final distribution of Cajal-Retzius cells

Cajal-Retzius (CR) cells play a fundamental role in the development of the mammalian cerebral cortex. They control the formation of cortical layers by regulating the migration of pyramidal cells through the release of Reelin. The function of CR cells critically depends on their regular distribution throughout the surface of the cortex, but little is known about the events controlling this phenomenon. Using time-lapse video microscopy in vivo and in vitro, we found that movement of CR cells is regulated by repulsive interactions, which leads to their random dispersion throughout the cortical surface. Mathematical modeling reveals that contact repulsion is both necessary and sufficient for this process, which demonstrates that complex neuronal assemblies may emerge during development through stochastic events. At the molecular level, we found that contact repulsion is mediated by Eph/ephrin interactions. Our observations reveal a mechanism that controls the even distribution of neurons in the developing brain.

Meninges control tangential migration of hem-derived Cajal-Retzius cells via CXCL12/CXCR4 signaling

Cajal-Retzius cells are critical in the development of the cerebral cortex, but little is known about the mechanisms controlling their development. Three focal sources of Cajal-Retzius cells have been identified in mice-the cortical hem, the ventral pallium and the septum-from where they migrate tangentially to populate the cortical surface. Using a variety of tissue culture assays and in vivo manipulations, we demonstrate that the tangential migration of cortical hem-derived Cajal-Retzius cells is controlled by the meninges. We show that the meningeal membranes are a necessary and sufficient substrate for the tangential migration of Cajal-Retzius cells. We also show that the chemokine CXCL12 secreted by the meninges enhances the dispersion of Cajal-Retzius cells along the cortical surface, while retaining them within the marginal zone in a CXCR4-dependent manner. Thus, the meningeal membranes are fundamental in the development of Cajal-Retzius cells and, hence, in the normal development of the cerebral cortex.

Subpallial origin of a population of projecting pioneer neurons during corticogenesis

Pyramidal neurons of the mammalian cerebral cortex are generated in the ventricular zone of the pallium whereas the subpallium provides the cortex with inhibitory interneurons. The marginal zone contains a subpial stream of migratory interneurons and two different classes of transient neurons, the pioneer neurons provided with corticofugal axons, and the reelin-expressing Cajal-Retzius cells. We found in cultured slices that the medial ganglionic eminence provides the reelin-negative pioneer neurons of the marginal zone. Pioneer neurons sent long projection axons that went through the cortical plate and reached the subplate and the lateral border of the lateral ganglionic eminence. In the cultured slices, pioneer neurons were functionally mature: they displayed a voltage-gated sodium current, expressed functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and showed gamma-aminobutyric acid type A (GABAA) postsynaptic events that were modulated by presynaptic AMPA receptors. Pioneer neurons expressed the adhesion molecules L1 and TAG-1; the latter has been reported to control tangential migrations to the neocortex [Denaxa, M., Chan, C.-H., Schachner, M., Parnavelas, J. & Karagogeos, D. (2001) Development (Cambridge, U.K.) 128, 4635-4644], and we show here that the pioneer neurons of the marginal zone are the cellular substrate of such a function. Finally, we show that, in early corticogenesis, reelin controls both the tangential migration of cortical interneurons toward the cortical plate and the tangential migration of pioneer neurons toward the marginal zone.

DNA microarray profiling of developing PS1-deficient mouse brain reveals complex and coregulated expression changes

Presenilin 1 (PS1) plays a critical role in the nervous system development and PS1 mutations have been associated with familial Alzheimer's disease. PS1-deficient mice exhibit alterations in neural and vascular development and die in late embryogenesis. The present study was aimed at uncovering transcript networks that depend on intact PS1 function in the developing brain. To achieve this, we analyzed the brains of PS1-deficient and control animals at embryonic ages E12.5 and E14.5 using MG_U74Av2 oligonucleotide microarrays by Affymetrix. Based on the microarray data, overall molecular brain development appeared to be comparable between the E12.5 and E14.5 PS1-deficient and control embryos. However, in brains of PS1-deficient mice, we observed significant differences in the expression of genes encoding molecules that are associated with neural differentiation, extracellular matrix, vascular development, Notch-related signaling and lipid metabolism. Many of the expression differences between wild-type and PS1-deficient animals were present at both E12.5 and E14.5, whereas other transcript alterations were characteristic of only one developmental stage. The results suggest that the role of PS1 in development includes influences on a highly co-regulated transcript network; some of the genes participating in this expression network may contribute to the pathophysiology of Alzheimer's disease.

Loss of Emx2 function leads to ectopic expression of Wnt1 in the developing telencephalon and cortical dysplasia

Leptomeningeal glioneuronal heterotopias are a focal type of cortical dysplasia in which neural cells migrate aberrantly into superficial layers of the cerebral cortex and meninges. These heterotopias are frequently observed as microscopic abnormalities in the brains of individuals with central nervous system (CNS) malformations and epilepsy. Previous work has demonstrated that the function of Emx2, which encodes a homeodomain transcription factor, is essential for development of the cortical preplate, which gives rise to the marginal zone and subplate. However, transcriptional targets of EMX2 during CNS development are unknown. We report that leptomeningeal glioneuronal heterotopias form in Emx2(-/-) mice that are equivalent to human lesions. Additionally, we observed ectopic expression of Wnt1 in the embryonic roofplate organizer region and dorsal telencephalon. To determine the phenotypic consequences of such Wnt1 misexpression, we deleted a putative EMX2 DNA-binding site from the Wnt1 enhancer and used this to misexpress Wnt1 in the developing murine CNS. Heterotopias were detected in transgenic mice as early as 13.5 days postcoitum, consistent with a defect of preplate development during early phases of radial neuronal migration. Furthermore, we observed diffuse abnormalities of reelin- and calretinin-positive cell populations in the marginal zone and subplate similar to those observed in Emx2-null animals. Taken together, these findings indicate that EMX2 is a direct repressor of Wnt1 expression in the developing mammalian telencephalon. They further suggest that EMX2-Wnt1 interactions are essential for normal development of preplate derivatives in the mammalian cerebral cortex.