Extracellular matrix during laminar pattern formation of neocortex in normal and reeler mutant mice

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

The Role of Chondroitin Sulfate Proteoglycans in Nervous System Development

The orderly development of the nervous system is characterized by phases of cell proliferation and differentiation, neural migration, axonal outgrowth and synapse formation, and stabilization. Each of these processes is a result of the modulation of genetic programs by extracellular cues. In particular, chondroitin sulfate proteoglycans (CSPGs) have been found to be involved in almost every aspect of this well-orchestrated yet delicate process. The evidence of their involvement is complex, often contradictory, and lacking in mechanistic clarity; however, it remains obvious that CSPGs are key cogs in building a functional brain. This review focuses on current knowledge of the role of CSPGs in each of the major stages of neural development with emphasis on areas requiring further investigation.

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

C-Terminal Region Truncation of RELN Disrupts an Interaction with VLDLR, Causing Abnormal Development of the Cerebral Cortex and Hippocampus

We discovered a hypomorphic reelin (Reln) mutant with abnormal cortical lamination and no cerebellar hypoplasia. This mutant, Reln^CTRdel, carries a chemically induced splice-site mutation that truncates the C-terminal region (CTR) domain of RELN protein and displays remarkably distinct phenotypes from reeler The mutant does not have an inverted cortex, but cortical neurons overmigrate and invade the marginal zone, which are characteristics similar to a phenotype seen in the cerebral cortex of Vldlr^null mice. The dentate gyrus shows a novel phenotype: the infrapyramidal blade is absent, while the suprapyramidal blade is present and laminated. Genetic epistasis analysis showed that Reln^CTRdel/Apoer2^null double homozygotes have phenotypes akin to those of reeler mutants, while Reln^CTRdel/Vldlr^null mice do not. Given that the receptor double knock-out mice resemble reeler mutants, we infer that Reln^CTRdel/Apoer2^null double homozygotes have both receptor pathways disrupted. This suggests that CTR-truncation disrupts an interaction with VLDLR (very low-density lipoprotein receptor), while the APOER2 signaling pathway remains active, which accounts for the hypomorphic phenotype in Reln^CTRdel mice. A RELN-binding assay confirms that CTR truncation significantly decreases RELN binding to VLDLR, but not to APOER2. Together, the in vitro and in vivo results demonstrate that the CTR domain confers receptor-binding specificity of RELN.

SIGNIFICANCE STATEMENT

Reelin signaling is important for brain development and is associated with human type II lissencephaly. Reln mutations in mice and humans are usually associated with cerebellar hypoplasia. A new Reln mutant with a truncation of the C-terminal region (CTR) domain shows that Reln mutation can cause abnormal phenotypes in the cortex and hippocampus without cerebellar hypoplasia. Genetic analysis suggested that CTR truncation disrupts an interaction with the RELN receptor VLDLR (very low-density lipoprotein receptor); this was confirmed by a RELN-binding assay. This result provides a mechanistic explanation for the hypomorphic phenotype of the CTR-deletion mutant, and further suggests that Reln mutations may cause more subtle forms of human brain malformation than classic lissencephalies.

Proteoglycans and neuronal migration in the cerebral cortex during development and disease

Chondroitin sulfate proteoglycans and heparan sulfate proteoglycans are major constituents of the extracellular matrix and the cell surface in the brain. Proteoglycans bind with many proteins including growth factors, chemokines, axon guidance molecules, and cell adhesion molecules through both the glycosaminoglycan and the core protein portions. The functions of proteoglycans are flexibly regulated due to the structural variability of glycosaminoglycans, which are generated by multiple glycosaminoglycan synthesis and modifying enzymes. Neuronal cell surface proteoglycans such as PTPζ, neuroglycan C and syndecan-3 function as direct receptors for heparin-binding growth factors that induce neuronal migration. The lectican family, secreted chondroitin sulfate proteoglycans, forms large aggregates with hyaluronic acid and tenascins, in which many signaling molecules and enzymes including matrix proteases are preserved. In the developing cerebrum, secreted chondroitin sulfate proteoglycans such as neurocan, versican and phosphacan are richly expressed in the areas that are strategically important for neuronal migration such as the striatum, marginal zone, subplate and subventricular zone in the neocortex. These proteoglycans may anchor various attractive and/or repulsive cues, regulating the migration routes of inhibitory neurons. Recent studies demonstrated that the genes encoding proteoglycan core proteins and glycosaminoglycan synthesis and modifying enzymes are associated with various psychiatric and intellectual disorders, which may be related to the defects of neuronal migration.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Hyaluronan-associated adhesive cues control fiber segregation in the hippocampus

In various brain regions, particularly in the hippocampus, afferent fiber projections terminate in specific layers. Little is known about the molecular cues governing this laminar specificity. To this end we have recently shown that the innervation pattern of entorhinal fibers to the hippocampus is mimicked by the lamina-specific adhesion of entorhinal cells on living hippocampal slices, suggesting a role of adhesion molecules in the positioning of entorhinal fibers. Here, we have analyzed the role of extracellular matrix components in mediating this lamina-specific adhesion. We show that hyaluronidase treatment of hippocampal slices abolishes lamina-specific adhesion as well as layer-specific growth of entorhinal fibers to the dentate outer molecular layer in organotypic slice cultures. We conclude that hyaluronan-associated molecules play a crucial role in the formation of the lamina-specific entorhinal projection to the hippocampus.

Specification of layer-specific connections in the developing cortex

One of the basic tasks of neurobiology is to understand how the precision and specificity of neuronal connections is achieved during development. In this paper we reviewed some recent in vitro studies on the developing mammalian cerebral cortex that have been made towards this end. The results of these experiments provided evidence that membrane-associated molecules are instrumental for the formation of specific afferent and efferent cortical projections. Substrate-bound molecules guide growing axons towards their target, regulate the timing of thalamocortical innervation and mediate target cell recognition. Moreover, a newly described glycoprotein, defined by a monoclonal antibody, revealed a molecular heterogeneity in the developing white matter. Since this molecule has opposite effects on thalamic and cortical axons, it might play a role in the segregation of axons running to and from the cortex. Substrate-bound cues are important during the formation of local cortical circuits. In vitro assays demonstrated that molecular components confined to individual cortical layers control the laminar specificity of cortical axon branching. This suggests that similar developmental strategies contribute to the laminar specification of extrinsic and intrinsic cortical circuits. Thus substrate-bound molecules might provide the framework for subsequent activity-dependent mechanisms that control the elaboration of precise connections between the cortical columns. A major challenge ahead is to identify the factors that mediate these processes and to determine their mode of action. Recently, two families of proteins, the netrins and the semaphorins/collapsins, have been identified as growth cone signals in the developing spinal cord (reviewed in Goodman, 1994; Colamarino and Tessier-Lavigne, 1995a; Dodd and Schuchardt, 1995; Kennedy and Tessier-Lavigne, 1995). Semaphorins/collapsins appear to regulate axonal guidance by repelling growth cones and by inhibiting axonal branching and synapse formation. Originally, netrins have been purified as diffusible chemoattractants for commissural axons of the dorsal spinal cord, but it is now well established that they can also function as chemorepellent factors for other classes of neurons. Since netrins are related to extracellular matrix components and since they can bind to the cell surface, they might also act as local guidance cues. A possible role of netrins and semaphorins/collapsins in the development of cortical connections is likely to be resolved in the near future. The identification of the factors that regulate specific branching patterns of cortical neurons might provide a better understanding of cortical development, but it might also be relevant to some aspects of plasticity and repair in the adult cortex.

Anatomy, development and lesion-induced plasticity of rodent corticospinal tract

In this review the current knowledge of the anatomy, development and plasticity of the rodent corticospinal tract is summarised. Recent technical advancements, especially in neuronal tracing methods, have provided much new data concerning the anatomy of the corticospinal tract. The rodent corticospinal axons project to the subcortical nuclei via collateral branches. These collateral branches of corticospinal axons are formed by delayed interstitial budding during early postnatal periods. Corticospinal neurons are generated in the ventricular zone during a short time lag, migrate into the cortical plate, and settle in layer V of the cerebral cortex. The migration of corticospinal neurons is experimentally deranged by prenatal exposure to alcohol or genetically affected by the reeler genetic locus (rl), resulting in generation of ectopic corticospinal neurons. Such experimentally or genetically induced ectopic corticospinal neurons are a good model for examining whether target recognition and path finding are affected by the intracortical position of corticospinal neurons. Some chemical molecules (e.g. L1 and B-50/GAP43) are transiently expressed in the corticospinal tract during the perinatal period, while others (e.g. protein kinase C gamma subspecies and alpha CaM kinase II) are permanently expressed in the adult corticospinal tract. The only chemical marker specific for layer V corticofugal neurons is an antibody to a soluble protein, protein 35. Since the corticospinal tract in the rodent is an easily identified group of fibers situated in the most ventral portion of the dorsal funiculus of the spinal cord and exhibits considerable postnatal development, it has often been utilized in the neurological studies on plasticity and regenerative capacity of the lesioned central nervous system. Recently, it has been clarified that growing corticospinal fibers have the ability to penetrate and traverse across the lesion sites under certain special conditions.

Particulate forms of APP in the extracellular milieu of cultured cells

The principle externalized forms of amyloid precursor protein (APP) are soluble and well-characterized, but some evidence has suggested the additional presence of externalized APP in a nonsoluble form. To further assess this possibility, the current study has applied high resolution microscopy protocols in addition to immunoprecipitation to characterize externalized APP in three commonly used cell culture models (SH-SY5Y human neuroblastoma cells, fetal rat brain cells, and HEK 293 human embryonic kidney cells). Confocal immunofluorescence microscopy, using an antiserum against the c-terminal domain of APP, showed typical cell-associated APP, but hot spots of APP also were evident in cell-free areas, apparently associated with the culture substrata. These hot-spots were examined for evidence of cellular deterioration by whole mount transmission electron microscopy. Neither cell debris nor disrupted cells were present. Instead, the hot spots of substratum-bound APP comprised discrete microparticles, approximately 50-100 nm across. These microparticles also could be found near cells and in some cases were attached to cell surface fibrils. Substratum-bound APP also could be found clustered within the extracellular matrix made by primary cell cultures. Occurrence of APP in extracellular microparticles was verified by centrifugation-immunoprecipitation analysis of media conditioned by APP-transfected cells. Radiolabeling data showed that particulate APP was from metabolically active cells. Metabolic labeling of particle-associated APP, as well as the absence of cellular debris near the APP-containing particles, suggests that the occurrence of nonsoluble APP in the extracellular milieu derives from a physiologically active process.

Role of the basement membrane in neurogenesis and repair of injury in the central nervous system

Although remarkable progress has been made during the last two decades concerning the biosynthesis, expression and assembly of extracellular matrix (ECM) macromolecules in nonneural cells, we are still far from a complete understanding of the role and function of the ECM and the basement membrane (BM) in the central nervous system (CNS). With the aid of correlative light and electron microscopic, Golgi and immunohistochemical studies of the developing neural tube of both early human fetus and mouse, we have shown that the establishment of the pial-glial barrier (PGB) is one of the earliest histogenetic events in neurogenesis. This is accomplished by coordinated interaction among the processes of radial glia, various ECM components, and mesenchymal cells at the pial surface, with the formation of a BM that tightly abuts the glia limitans. The PGB and the BM appear to be critical to the migration and final positioning of neurons and to the differentiation of the laminar cortical pattern within the developing neopallium. This hypothesis is further supported by our study of the brain of a human newborn infant in whom multiple sites of disruption of the BM and PGB resulted in abnormal neuronal migration and massive ectopia of neurons within the subarachnoid space, with abnormal cortical lamination. Finally, studies of the experimental cryogenic injury to the neonatal rat cerebrum have shown that the final positioning of neurons within the developing cortical plate appears to depend largely on the reconstitution of the BM and PGB, which presumably provide crucial positional signals for migrating neurons. Also, one of the essential reparative features seen following cryogenic injury to the adult rat cerebrum is an orderly and dynamic interaction between various ECM components and neural cells, resulting in the formation of the BM.

Developmentally regulated expression of a brain specific species of chondroitin sulfate proteoglycan, neurocan, identified with a monoclonal antibody IG2 in the rat cerebrum

The mammalian brain contains many species of proteoglycan. To identify each proteoglycan species, we have raised monoclonal antibodies against soluble chondroitin sulfate proteoglycans purified from 10-day-old rat brains. One monoclonal antibody, named monoclonal antibody 1G2, recognized two proteoglycan species with 220,000 and 150,000 mol. wt core glycoproteins (chondroitin sulfate proteoglycan-220 and chondroitin sulfate proteoglycan-150). Partial amino acid sequences of N-termini of their core proteins coincided with those of neurocan, a brain-unique chondroitin sulfate proteoglycan species, whose complete coding sequence was recently reported [Rauch et al. (1992) J. biol. Chem. 269, 19,536-19,547]. Western blots revealed that chondroitin sulfate proteoglycan-220 became detectable in the rat cerebrum on embryonic day 14, and that it disappeared from the brain around postnatal day 30. In contrast, a fairly large amount of chondroitin sulfate proteoglycan-150 remained in the mature brain. Immunohistochemical studies revealed that 1G2 antigen was first localized in the preplate zone, then both in the marginal zone and in the subplate of the rat cerebrum on embryonic day 16, prior to arrival of the first thalamic afferents at the cortex. On embryonic day 20, immunolabeling with monoclonal antibody 1G2 began to spread from the subplate into the developing cortical plate. On postnatal day 10, the neuropil of the cerebrum, except for the barrel field, was diffusely stained with the antibody, intensely in the hippocampus and superficial layers (I-III) of the cerebral cortex and weakly elsewhere. The barrel hollows were stained very weakly compared with the barrel walls at this stage. The immunoreactivity in the hippocampus and superficial cortical layers was weakened in the mature brain, so that no particular staining pattern, but weak and diffuse staining was observed in the adult rat cerebrum. The 1G2 antigen was immunohistochemically associated largely with glial fibrillary acidic protein-positive cells in primary cultures of the neonatal rat cerebrum. Both chondroitin sulfate proteoglycan-220 and chondroitin sulfate proteoglycan-150 were detected in the conditioned media not only of highly enriched cultures of fetal rat cortical neurons but also of pure cultures of mature astrocytes; more (12- to 20-fold) in the astrocyte conditioned media. Astrocytes, in addition to neurons, may be a cellular source of neurocan in brain at least under certain physiological conditions. The spaciotemporal expression pattern of 1G2 epitope-bearing proteoglycan, or neurocan, suggests that this proteoglycan species plays some roles at least in forming the elongation pathway for early cortical afferent fibers as well as the functional barrel structure in the somatosensory cortex.

BEHAB, a new member of the proteoglycan tandem repeat family of hyaluronan-binding proteins that is restricted to the brain

Hyaluronan (HA) is a ubiquitous component of the extracellular matrix of all tissues. In the mammalian central nervous system (CNS) HA is present throughout development and into adulthood. While the functions of HA are likely to be mediated by HA-binding proteins, no cell or tissue specific HA-binding proteins have been reported. In an effort to characterize the composition of the extracellular matrix of the CNS, we sought to identify neural HA-binding proteins. We report here the isolation and characterization of a cDNA with a high degree of sequence homology to members of the proteoglycan tandem repeat (PTR) family of HA-binding proteins. Unlike other HA-binding proteins, the expression of this cDNA is restricted to the CNS. We propose the name BEHAB, Brain Enriched HyAluronan Binding protein, for this gene. The expression of BEHAB mRNA is developmentally regulated; expression is first detected in the late embryonic period and peaks during the first two postnatal weeks. In the embryo, BEHAB is expressed at highest levels in mitotically active cells. The sequence of BEHAB has long stretches of identity between rat and cat, suggesting that the encoded protein is functionally important. The size and sequence of BEHAB are consistent with the possibility that it could serve a function like link protein, stabilizing interactions between HA and brain proteoglycans. These observations suggest that existence of other tissue specific HA-binding proteins.

Development of geniculocortical projections to visual cortex in rat: evidence early ingrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Large chondroitin sulfate proteoglycans of developing chick CNS are expressed in cerebral hemisphere neuronal cultures

Chondroitin sulfate proteoglycans (CSPG) of the extracellular matrix may play regulatory roles in central nervous system (CNS) development. We have examined the expression of two large CSPGs of the embryonic chick brain, which can be differentiated using the monoclonal antibodies HNK-1 and S103L, in cultures of embryonic day 6 chick cerebral hemisphere neurons. Western blot analysis following immunoprecipitation and endoglycosidase treatment revealed that these cultures produce S103L- and HNK-1-reactive proteoglycans which are biochemically indistinguishable from the CSPGs (previously) identified in homogenized chick embryo brain extracts. The HNK-1-reactive CSPG accumulated in the medium throughout the course of cultures. In contrast, the S103L-reactive CSPG was found in a neuron-associated form during the period of aggregate establishment in culture, as well as in a soluble form secreted into the medium. Immunocytochemical staining of cultures with the S103L antibody localized reactivity to most neurons during the period of aggregate formation, while neuronal processes and the few flat cells present (presumably neuroblasts and early glia) were negative. Cell selection experiments confirmed that neurofilament-positive cells were the source of the S103L-reactive CSPG. The use of differential fixation techniques suggested that the cell-associated S103L reactivity may be intracellular. Because of this pattern of expression and localization, we propose that the developmentally regulated S103L-reactive CSPG may play a role in neuronal migration arrest and organization of neurons into functional aggregates.

Biochemical comparison of brain glycosaminoglycans between normal and reeler mutant mice

Glycosaminoglycans (GAGs) were isolated from the brains of reeler and normal mice on postnatal days 13 and 20. The GAG content of the reeler mouse brain, based upon the amount of DNA, was about 150% that of the normal mouse brain on both days. The GAGs consisted of chondroitin sulfate (CS), heparan sulfate (HS), hyaluronic acid (HA) and polysialosyl glycopeptides. There was no significant difference in the composition of GAGs isolated from either reeler or normal brain. Repeating disaccharide compositions of CS and HS were also similar in reeler and normal brains. Core proteins of brain chondroitin sulfate proteoglycans (CSPGs), solubilized with phosphate buffered saline, were prepared by digesting purified CSPGs with chondroitinase ABC, and were analyzed by SDS-polyacrylamide slab gel electrophoresis. There was no difference in the composition of core proteins from either reeler or normal brain. These results indicate that, although the GAG content of the reeler mouse brain is higher than the normal, all structural parameters of GAGs/CSPGs so far examined were normal. The rate of synthesis and/or degradation of brain GAGs may be affected in the mutant mouse brain.

A chondroitin sulfate proteoglycan that is developmentally regulated in the cerebellar mossy fiber system

It is known that the mammalian brain contains many kinds of proteoglycans, but almost all of them remain to be characterized. In this study, we prepared a monoclonal antibody against a phosphate-buffered saline-soluble brain proteoglycan (MAb 6B4). MAb 6B4 recognized a 600- to 1000-kDa chondroitin sulfate proteoglycan with a 250-kDa core protein (6B4 proteoglycan). The core protein of 6B4 proteoglycan carried the HNK-1 epitope. Immunohistochemical analysis of the adult rat brain indicated that this proteoglycan was expressed on the cell surfaces of a subset of neurons. In the hindbrain, 6B4 proteoglycan was highly expressed on the cerebellar Purkinje cells and Golgi cells, and at particular nuclei including the pontine nuclei and lateral reticular nucleus. Almost all of these nuclei were connected to the cerebellum through the mossy fiber system. A developmental study indicated that the expression of this proteoglycan changed dramatically during the formation of the cerebellar mossy fiber system. The mossy fibers from the pontine nuclei expressed 6B4 proteoglycan transiently from Embryonic Day 20 (E20) to Postnatal Day 30 (P30), during which time the axonal outgrowth and glomerular synapse formation occurred. The Purkinje cells, glomeruli, and Golgi cells began to be stained with MAb 6B4 from P10, P16, and P20, respectively. These expression stages correspond with the onset of their synapse formation. These results suggest that 6B4 proteoglycan is closely involved in the development of the cerebellar mossy fiber system.

Repair and reconstruction of the cortical plate following closed cryogenic injury to the neonatal rat cerebrum

A cryogenic lesion was induced in the parietal cortex of neonatal rats at postnatal day 2, and the chronological sequence of cellular events during repair and reconstruction of the cortical plate examined. Serial sections of cerebra obtained at varying intervals ranging from 1 to 60 days postinjury were studied by light and electron microscopy and by immunocytochemistry for fibronectin, laminin, type IV collagen, vimentin and glial fibrillary acidic protein. In addition, localization of heavily labeled neurons (generated on embryonic day 20) in the cerebral cortical plate was examined by [3H]thymidine radioautography. Repair of a well-defined coagulative lesion was accomplished with little or no mesenchymal cell proliferation in either the necrotic zone or the leptomeninges. Eventually, fusion of the adjacent cortical plates took place with the formation of a microsulcus. Migration of neurons continued to take place along the outer margins of the lesion, and postmigratory neurons accumulated within the upper cortical layers. Around the microsulcus, heavily labeled neurons aligned themselves with layers II-III of the adjacent normal cortical plate. Irregular clusters of neurons closely abutting the leptomeningeal surface were frequently noted when repair took place without an intervening molecular layer and/or a well-defined pial-glial barrier. Supplementing intrinsic information inherent in migrating neurons, local environmental signals provided by the radial glia, glia limitans, basal lamina and pial-glial barrier appear to influence the polarity and final positioning of postmigratory neurons within the cortical plate. The necrotic zone within the deeper layers of the cortex eventually healed with a cell-sparse gliotic layer. The end result was a histological pattern that, in many respects, resembled that of human micropolygyria.(ABSTRACT TRUNCATED AT 250 WORDS)

Laminin and s-laminin are produced and released by astrocytes, Schwann cells, and schwannomas in culture

Components of the extracellular matrix (ECM) have been implicated in the regulation of neuronal migration, axonal growth, and synaptogenesis. We have examined cultures of glial cells, Schwann cells, and schwannomas for the expression of two components of the ECM, laminin and s-laminin, using immunohistochemical and Western blot techniques. Laminin is a potent promotor of neurite outgrowth in cultures of both central and peripheral neurons, and is present in all ECMs. In contrast, s-laminin (for synaptic laminin), a recently described homolog of laminin, is highly localized at the neuromuscular synaptic cleft (Sanes and Chiu, Cold Spring Harbor Symp. Quant. Biol. 1983;48:667-678; Chiu and Sanes, Dev. Biol. 1984;103:456-467) and shows selective adhesivity for motor neurons (Hunter et al. Cell 1989;59:905-913). While the distribution of these ECM components have been well documented in situ, the sources of these extracellular molecules are unclear. We report that astrocytes cultured in serum-free medium maintain an organized ECM that only bears laminin immunoreactivity; s-laminin appears to be sequestered intracellularly. However, both molecules are found in the astrocyte conditioned medium. Thus, under these growth conditions, astrocytes produce and release laminin and s-laminin, but only incorporate the former into an ECM. In contrast, neither molecule is present in comparable cultures of oligodendrocytes. Although no established ECM is seen in cultures of Schwann cells or schwannomas, laminin and s-laminin immunoreactivity are present within cells and in the conditioned media. These results indicate that certain populations of non-neuronal support cells and cell lines can produce and release both synaptic and extrasynaptic components of the ECM. The assembly of these different molecules into an organized basal lamina may require the presence of additional factors or interaction with neurons.

Changes in glycosaminoglycans during the neuritogenesis in PC12 pheochromocytoma cells induced by nerve growth factor

Previously, we had suggested that heparan sulfate (HS) makes some contribution to a flat-shaped morphology of PC12D cells. Therefore, we carried out quantitative and qualitative analyses of glycosaminoglycans (GAGs), the polysaccharide moiety of proteoglycans, during neuritogenesis in PC12 cells that is induced by nerve growth factor (NGF). (a) In PC12 cells, NGF induced a flat-shaped morphology with a few short processes after 3 days of culture, and then it elicited short and long neurites after 6 (in approximately 30% of cells) and 9 (in 60-70%) days of culture, respectively. (b) HS and chondroitin sulfate (CS) were detected in the cell layer at all times. Only CS was found in the medium at 3 and 6 days, whereas a low level of HS, in addition to CS, was detectable on day 9. (c) In the NGF-treated cultures, the amounts of cell-associated HS per cell were two to three times as high as those in the respective nontreated cultures at all times, whereas the amount based on phospholipid was about twofold higher after 3 days of culture. (d) The levels of HS labeled with [35S]sulfate during the last 48 h of the culture were 1.5- to twofold higher in the NGF-treated cultures than in the respective controls at any time. (e) The amount of cell-associated CS per cell (or per unit of phospholipid), but not of labeled CS per cell, was transiently enhanced at 3 days in culture with or without NGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Sulfated proteoglycans in astroglial barriers inhibit neurite outgrowth in vitro

In vivo studies of the roof plate of the spinal cord and midline optic tectum in rodent and the developing subplate in the telencephalon of the chick showed that two glycosaminoglycans, keratin sulfate and chondroitin sulfate, possibly in the proteoglycan form (KS-PG, CS-PG, or KS/CS-PG), were present at times when axons approach closely but do not invade these territories. To address the question of whether KS/CS-PG actively inhibits growth cone elongation and to determine which component(s) of the proteoglycan may be critical to this phenomenon, we used a technique employing nitrocellulose-coated petri dishes onto which stripes of various purified macromolecules were attached. Isolated E9 chick dorsal root ganglia were grown on lanes of KS/CS-PG in alteration with lanes of the growth-promoting molecule laminin (LN). Neurite outgrowth was abundant along stripes of LN. In contrast, upon encountering a stripe containing KS/CS-PG, neurites either stopped abruptly or turned and traveled along the KS/CS-PG stripe border. The effect was dependent upon the concentration of the proteoglycan with intermediate concentrations producing intermittent patterns of crossing. We mixed LN with the KS/CS-PG, where the LN was in concentrations which alone support outgrowth, and observed that the KS/CS-PG was still inhibitory when such a growth-promoting molecule was present. A 10-fold higher concentration of LN was able to overcome the inhibitory effect of the KS/CS-PG. These results suggest that the interaction of inhibitory and growth-promoting molecules can interact to produce a wide spectrum of neurite patterns ranging from complete inhibition to totally unimpeded outgrowth. Selective enzymatic removal of the KS or CS from the KS/CS-PG permitted various degrees of neurite outgrowth to occur across the previously inhibitory lanes, and digestion of both glycoaminoglycan moieties, leaving only the protein core of the molecule, resulted in a complete lack of inhibition. These assays demonstrated that KS/CS-PG is inhibitory to embryonic dorsal root ganglia neurites in vitro and that complete inhibition requires contributions from both KS and CS moieties.

Boundaries during normal and abnormal brain development: in vivo and in vitro studies of glia and glycoconjugates

This paper focuses on transient boundaries of glia and glycoconjugates during development of the mouse central nervous system (CNS). Lectin-bound glycoconjugates, glial fibrillary acidic protein, and the J1/tenascin glycoprotein are distributed coextensively within boundaries around developing substructural arrangements (e.g., developing nuclei, and at a finer level, somatosensory cortical "barrels" related to individual facial vibrissae) throughout the CNS during pattern formation events. Electron microscopy has shown that the J1/tenascin glycoprotein, for example, is present in immature astrocytes, on glial and neuronal plasma membranes, and within the pericellular space that could be extracellular matrix (ECM). The findings presented on the expression of this well-characterized ECM molecule suggest that previously described glial and glycoconjugate boundaries reported by our group are in part composed of specific recognition molecules. The J1/tenascin glycoprotein, a chondroitin sulfate-containing antigen termed the 473 proteoglycan, and the adhesion molecule on glia are expressed within discrete boundary regions and associated axonal pathways. There, they may sculpture fine aspects of functional cytoarchitectonic arrangements and help guide axons to specific targets. The expression and developmental regulation of glycoproteins such as J1/tenascin may thus be integral events during pattern formation and synaptogenesis in the CNS. The presence of abnormal glial arrangements and glycoconjugate boundaries in the cortices of the genetic mutant mouse reeler, and findings on plasticity of boundaries following various perturbations, suggest that boundary expression is controlled by both genetic and epigenetic factors. Some future directions for studying developmental boundaries, including use of cultured explants for in vitro "bioassays," are also discussed.

Molecular and cellular characterization of the glial roof plate of the spinal cord and optic tectum: a possible role for a proteoglycan in the development of an axon barrier

Certain types of glial structures, located at strategic positions along axon pathways, may provide the mechanical and/or chemical elements for the construction of barriers which can grossly direct the elongation of axons during development. The roof plate, a putative axon barrier, is located along the dorsal midline of the developing spinal cord and may be important for the guidance of the commissural and dorsal column axons. We examined the roof plate to determine the developmental morphology of the region and to determine which molecules were correlated with the barrier function when axons were growing nearby. Light and electron microscopic observations of the roof plate revealed that this glial domain undergoes a dramatic change in shape from a "wedge" with large extracellular spaces between the cell apices at E12.5 to a thin, dense septum with reduced extracellular space at E15.5. Immunocytochemical techniques demonstrated that highly sialylated neural cell adhesion molecule (N-CAM), the carbohydrate recognized by L2 monoclonal antibody, cholinesterase, stage-specific embryonic antigen 1, and a ligand that binds tetragonolobus purpureas agglutinin are expressed by the roof plate. These molecules, however, were also found in other regions of the spinal cord which are permissive or attractive to axon growth. A molecule which is unique to the roof plate when axons grow close to, but do not cross, the dorsal midline is a glycosaminoglycan (GAG), keratan sulfate. Keratan sulfate is also present in the tectal midline and in other noninnervated regions such as the outer epidermis and developing cartilage. Our data suggest that keratan sulfate, alone or in combination with other molecules expressed by the roof plate, may be responsible, in part, for the inhibition of axon elongation through the roof plate in the embryonic spinal cord.

Granule cell behavior on laminin in cerebellar microexplant cultures

In order to study roles of the extracellular matrix (ECM) in the cerebellar granule cell migration, cerebellar microexplants of neonatal to postnatal 11-day-old mice were cultured on 3 kinds of substrata, poly-L-lysine (PL), PL/fibronectin and PL/laminin. A prominent outgrowth of small granule cells, which did not uptake GABA, was observed only on the PL/laminin substratum. The granule cells showed the following sequence of events: (1) Many polygonal undifferentiated cells migrated out from the microexplants. These blast cells differentiated into small bipolar neurons with long fine neurites which extended radially from the explants. (2) These cells then changed their orientation perpendicular to their radial neurites, by protruding a short process from the cell body at right angles. (3) Finally, cell bodies of these granule cells adhered to each other to form cell aggregates. Quantitative labelings by bromodeoxyuridine revealed that there were less mitotic cells in explants from the later postnatal cerebellar compared to the earlier postnatal ones. Anti-MAP2 immunoreactivity was localized in short perpendicular processes of the aggregated granule cells. Thus, this unique cell behavior exhibited on the PL/laminin substratum provides the first defined experimental system for studying the granule cell differentiation in vitro.

Extracellular matrix of the superior olivary nuclei in the dog

The extracellular matrix around nerve cell bodies in canine lateral and medial superior olivary nuclei was examined by conventional electron microscopy, Golgi impregnation and histochemical techniques. Each neuron is surrounded by a region of myelin-free neuropil embedded amongst the myelinated fibres of the trapezoid body. In the myelin-free neuropil there are astrocytes, axons, synaptic boutons and extracellular matrix. The extracellular matrix fills the spaces between slender axons near the terminals, synaptic boutons and glial processes, but not the synaptic cleft. Golgi impregnation selectively stains the perineuronal nets which cover some of all of the nerve cell bodies and dendrites. The Golgi-EM method revealed that the impregnated profiles of the nets are restricted to the extracellular matrix. Synaptic boutons are situated in the holes of the perineuronal nets. Peanut (PNA) and soybean (SBA) agglutinins bound the extracellular matrix but not the synaptic boutons, glial processes, nerve cell bodies or basal lamina of blood capillaries. Light microscopic immunohistochemistry of the glial fibrillary acidic protein (GFAP) and S-100 protein did not stain a layer corresponding to the extracellular matrix and synapses but showed an intensely positive reaction immediately outside this layer. These data suggest the existence of a unique microenvironments associated with glycoconjugates around nerve cell bodies in canine superior olivary nuclei.

Extracellular space in the developing retina assessed by electron microscopy: laminar and topographic distribution

The extent of extracellular space (ECS) in the developing retina of the cat has been measured by electron microscopy in material fixed using techniques developed by others to preserve ECS. ECS is generally greater in foetal than in adult material. It is particularly marked in the plexiform layers of retina at the time of synaptogenesis and in the axon layer at the time of axon growth. The changes in ECS occur first in the central retina, and spread to the periphery. These observations suggest that the high volumes of ECS found in the foetus are not artefactual, but accompany and may play a role in developmental processes.

Astroglia-released neurite growth-inducing activity for embryonic hippocampal neurons is associated with laminin bound in a sulfated complex and free fibronectin

Neurons from embryonic (E18) rat hippocampus were chosen to identify and characterize neurite growth-stimulating proteins accumulating in serum-free conditioned media (CM) obtained from primary or secondary cultures of cerebral astrocytes (less than 5% nonglial cells) using a quantitative cell culture bioassay. CM were fractionated by FPLC on an anion exchange column (Mono Q) and by gel filtration (Superose 6). Column fractions were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting and enzyme-linked immunosorbent assay (ELISA) using antibodies to laminin (LN) and fibronectin (FN). The neurite-promoting activity (NPA) was tested by incubating aliquots of the eluted fractions with poly-L-lysine precoated glass coverslips prior to addition of neurons suspended in chemically defined medium. We provide evidence that the NPA in astroglial CM could be assigned mainly to a negatively charged, highly sulfated LN complex consisting predominantly of the B-chains of LN and presumably a sulfated proteoglycan that was sensitive for chondroitinase and to a lower degree to heparinase degradation. In addition, a smaller proportion of the NPA was associated with uncomplexed LN and free FN. FN reached approximately 10 times the concentration of LN in astroglial CM. As revealed by immunofluorescence microscopy, both LN and FN are simultaneously expressed by cultured astrocytes; however, only the production of FN, measured by ELISA, increased during the time astrocytes were in culture, whereas the release of LN remained unchanged. We conclude that, besides the most active LN complex, FN bound to a polycationic matrix is able to induce neurite growth in hippocampal neurons in vitro.

Development of intersecting CNS fiber tracts: the corpus callosum and its perforating fiber pathway

What are the mechanisms acting during development at points of intersection of central nervous system fiber tracts which influence the direction taken by a population of growing axons? In order to address this question, the ontogeny of the intersecting rostral corpus callosum and its perforating fiber pathway (PF), and the microenvironment through which these fiber systems grow, were examined in a series of mouse embryos and early postnates. Our results show that the perforating fibers are identifiable in silver-stained sections between embryonic days (E) 15 and 16, at least 1 day prior to the initial appearance of the callosal projection. Soon after the PF can be identified, a dense accumulation of subventricular cells surrounds the PF at a point just ventral to the location where the callosum and PF will intersect (i.e., at the corticoseptal boundary). Callosal axons, which are present at the point of intersection beginning on E17, do not joint the perforating fibers, nor do they appear to penetrate the underlying population of subventricular cells. Instead, the callosal fibers turn across the PF and enter the contralateral cerebral hemisphere. Thus, the intersection of the callosal and perforating fiber systems during development may be related both to the sequential development of each pathway and to the altered nonneuronal environment at the point of intersection.

A fibronectin-like molecule is present in the developing cat cerebral cortex and is correlated with subplate neurons

The subplate is a transient zone of the developing cerebral cortex through which postmitotic neurons migrate and growing axons elongate en route to their adult positions within the cortical plate. To learn more about the cellular interactions that occur in this zone, we have examined whether fibronectins (FNs), a family of molecules known to promote migration and elongation in other systems, are present during the fetal and postnatal development of the cat's cerebral cortex. Three different anti-FN antisera recognized a single broad band with an apparent molecular mass of 200-250 kD in antigen-transfer analyses (reducing conditions) of plasma-depleted (perfused) whole fetal brain or synaptosome preparations, indicating that FNs are present at these ages. This band can be detected as early as 1 mo before birth at embryonic day 39. Immunohistochemical examination of the developing cerebral cortex from animals between embryonic day 46 and postnatal day 7 using any of the three antisera revealed that FN-like immunoreactivity is restricted to the subplate and the marginal zones, and is not found in the cortical plate. As these zones mature into their adult counterparts (the white matter and layer 1 of the cerebral cortex), immunostaining gradually disappears and is not detectable by postnatal day 70. Previous studies have shown that the subplate and marginal zones contain a special, transient population of neurons (Chun, J. J. M., M. J. Nakamura, and C. J. Shatz. 1987. Nature (Lond.). 325:617-620). The FN-like immunostaining in the subplate and marginal zone is closely associated with these neurons, and some of the immunostaining delineates them. Moreover, the postnatal disappearance of FN-like immunostaining from the subplate is correlated spatially and temporally with the disappearance of the subplate neurons. When subplate neurons are killed by neurotoxins, FN-like immunostaining is depleted in the lesioned area. These observations show that an FN-like molecule is present transiently in the subplate of the developing cerebral cortex and, further, is spatially and temporally correlated with the transient subplate neurons. The presence of FNs within this zone, but not in the cortical plate, suggests that the extracellular milieu of the subplate mediates a unique set of interactions required for the development of the cerebral cortex.

Distribution in cesium chloride gradients of proteoglycans of chick embryo brain and characterization of a large aggregating proteoglycan

Proteoglycans were extracted from 14-day chick embryo brains, which had been labelled in vitro with [35S]sulfate or 3H-labelled amino acids. 4.0 M guanidinium chloride (containing proteinase inhibitors) extracted 94% of the 35S-labelled glycoconjugates. Following cesium chloride equilibrium centrifugation, the proteoglycans in each fraction were characterized by chromatography on Sepharose CL-2B. The most dense fraction (D1), which contained no detectable non-proteoglycan proteins, contained a large, aggregating chondroitin sulfate proteoglycan in addition to small chondroitin sulfate and heparan sulfate proteoglycans. The less dense fractions (D2-D6) contained both small chondroitin sulfate and heparan sulfate proteoglycans. Removal of hyaluronate from the D1 sample by digestion with Streptomyces hyaluronidase in the presence of proteinase inhibitors showed that aggregation of the large chondroitin sulfate proteoglycan is hyaluronate-dependent. Aggregation was restored by re-addition of hyaluronate. Reduction and alkylation, which blocked aggregation of a cartilage A1 proteoglycan, did not interfere with aggregation of the large brain proteoglycan.

Immunoreactivity for laminin in the developing ventral longitudinal pathway of the brain

The first long tract to form in the brain of a vertebrate embryo is the ventral longitudinal pathway. In order to investigate what chemical cues may guide nerve growth cones along this pathway, affinity-purified antibodies to laminin and collagen type IV were used to stain sections of mouse embryos from Embryonic Days 8 through 17. A monoclonal anti-neurofilament antibody was used to show the development of the ventral longitudinal pathway in relationship to immunoreactivity for laminin and collagen type IV. At Day 8 fluorescent immunoreactivity for laminin is bright in the external limiting membrane of the neural tube, but the neuroepithelium does not show bright laminin or neurofilament immunoreactivity. At E9 the ventral longitudinal pathway is forming and punctate immunoreactivity for laminin is present on the surfaces of neuroepithelial cells in the marginal zone, through which axons of the ventral pathway extend. Punctate immunofluorescence for laminin remains concentrated in the marginal zone on Days E10 through E14, but on E16 punctate immunofluorescence was much reduced, although immunoreactivity for laminin remained bright in the maturing pial and arachnoid membranes and on blood vessels in the brain. Immunoreactivity for collagen type IV was strong in the external limiting membrane and on blood vessels, but never showed concentrated punctate immunofluorescence in the marginal zone. These results indicate that laminin may be available on cell surfaces and in extracellular spaces as an adhesive ligand for growth cones during the formation of the ventral longitudinal pathway.

Glial and glycoconjugate boundaries during postnatal development of the central nervous system

The localization of glycosylated molecules and glia has been studied during early postnatal development in the mouse central nervous system (CNS) using autoradiographic detection of radiolabeled fucose incorporation, and in sections processed either for histochemistry or immunocytochemistry following binding of labeled lectins or an antibody to glial fibrillary acidic protein. Radiolabeled sugar incorporation, lectin binding of glycoconjugates, and glial labeling all reveal borders between nuclei within the diencephalon, midbrain, and brainstem through the first postnatal week. Glycoconjugate and glial boundaries exist throughout the CNS during pattern formation events, and they also are seen in relation to fine aspects of developing functional organization within individual structures (e.g. segmentation associated with the representation of mystacial vibrissae within the brainstem trigeminal complex). The observation that each of the probes employed in this study fails to label boundary organization during later postnatal times suggests that the distribution and chemistry of the glial/glycoconjugate network are dynamic, and they change in accordance with distinct maturational states of the nervous system.

Abnormal glial and glycoconjugate dispositions in the somatosensory cortical barrel field of the early postnatal reeler mutant mouse

During early postnatal development in reeler mutant mice, lectin binding delineates prospective abnormal barrels as they will appear in the adult mutant somatosensory cortex. Glial fibers also may be more condensed within fascicles in developing reeler barrels. These fibers also appear to be misaligned, coursing predominantly in the tangential plane within the abnormal reeler barrel sides as opposed to having a radial orientation as seen in normal mouse barrels. The thalamic barreloid complex, however, reveals a disposition of glycoconjugates that is completely normal in reeler. Thus, there are anomalies in glia and associated glycoconjugates during mainly cortical development in the reeler mutant mouse that might be related to the primary action of the abnormal gene.

Fibronectin and collagens modulate the proliferation and morphology of astroglial cells in culture

The proliferation and morphology of astroglia derived from neonatal rat cortex and cultured in serum-free medium on either untreated, or fibronectin-, or collagen I-, or collagen IV-treated substrates were investigated using tritiated thymidine autoradiography and immunocytochemical staining of glial fibrillary acidic protein (GFAP) and actin. Modification of culture substratum with fibronectin enhanced the rate of proliferation of astroglial cells and increased the proportion of process-bearing astroglial cells. The distribution of actin and patterns of adhesion observed were typical for motile cells. Both types of collagen decreased the proportion of astroglial cells undergoing mitosis. Many of the astroglial cells exhibited a flat morphology and displayed prominent stress fibres in the cell body and processes. The data suggest that specific interactions with the substratum modulate the proliferation and morphological behaviour of astroglial cells.

Developmental change in the glycosaminoglycan composition of the rat brain

Glycosaminoglycans (GAGs) were isolated from the brains of pre-and postnatal rats. The GAG content of the brain, based on the amount of DNA, was constant during the period from day 13 to day 15 of gestation. After day 15, the GAG content began to increase and reached a plateau by 10 days after birth. Hyaluronate (HA) was the main GAG (greater than 60% of the total) in the fetal rat brain, and the relative amount of HA decreased after birth. Conversely, the relative amount of chondroitin sulfate increased with development and reached the adult level by 20 days after birth. Heparan sulfate (HS) was the major sulfated GAG in the fetal rat brain at early developmental stages, but HS accounted for approximately 10% of the total GAG in the postnatal brains. In addition to these GAGs, a polysialosyl glycoconjugate was isolated from rapidly growing brains of the rat. These three GAGs could be isolated either from the cerebellum, cerebrum, or brainstem of the newborn rat. A closely similar age-related change in the GAG composition was observed in each of these different regions of the brain. The developmental change could be implicated in morphogenesis or maturation of the brain.

Lectins demarcate the barrel subfield in the somatosensory cortex of the early postnatal mouse

Plant lectins were used to examine the disposition of glycosylated molecules in vibratome sections through the barrel subfield of mouse somatosensory cortex at selected times during postnatal development. The peroxidase conjugates of peanut agglutinin (PNA, specific for N-acetylgalactosamine), concanavalin A (specific for mannose), and wheat germ agglutinin (specific for N-acetylglucosamine and N-acetylneuraminic acid) were used to study lectin binding in aldehyde-fixed tissue sections of cortex. Following peroxidase cytochemistry and light microscopy, it was found that all three lectins bound in the region of the barrel subfield as early as postnatal day 3 (day of birth = postnatal day 1). The lectins bound to the prospective sides and/or septae of individual barrels in preference to the prospective hollows. This lectin demarcation of the barrel field occurred prior to the detection of this region with cresyl violet staining and was still demonstrable on postnatal day 6, when the individual barrels became discernible with cresyl violet. This suggests that the lectin binding material is present before the barrel field becomes a fully formed and organized region. A decrease in lectin affinity for binding sites in these tissue sections occurs during postnatal development (Cooper and Steindler: Soc. Neurosci. (Abstr.) 10: 43a, '84) and this study demonstrates that lectins do not delineate the barrel field of more mature animals (2-3 months old), whereas barrels can be detected with cresyl violet at this time. A preliminary electron microscope analysis of the postnatal day 6 somatosensory cortex demonstrates that the lectin PNA binds to elements of the forming neuropil and also to Golgi apparatus intermediate saccules in neuronal cells. The prospective barrel field can be detected with lectins during a critical period in development in which alterations can occur in the barrel field in response to peripheral deprivation (Jeanmonod et al: Neuroscience 6:1503-35, '81) and therefore we suggest that the glycans visualized with lectin-peroxidase conjugates denote possible candidates for molecules involved in shaping barrel structure.

Immunocytological localization of cell adhesion molecules L1 and N-CAM and the shared carbohydrate epitope L2 during development of the mouse neocortex

The expression of the two adhesion molecules L1 and N-CAM and their shared carbohydrate epitope recognized by monoclonal antibody L2, was studied during development of the embryonic mouse neocortex by immunohistology at light- and electron-microscopic levels between embryonic days 9 and 18. Throughout this time period N-CAM is expressed in all layers of the telencephalic anlage. L1 antigen shows a more restricted expression than N-CAM. It is not detectable at day 9. From day 10 onward it is expressed on young neurons in the marginal zone, but not in the ventricular layer. At embryonic day 13 L1 antigen appears also in the intermediate zone on afferent fibers from subcortical structures and on migrating neurons. Neuronal cell bodies in the cortical plate and subplate express L1 antigen only transiently on embryonic days 13-16. These observations suggest that L1 antigen does not play a prominent role in the initiation of neuronal migration in the ventricular zone, but could be functional during later stages of migration and in the aggregation of neuronal cell bodies at their final position in the cortical plate. The L2 epitope also shows a more restricted expression than N-CAM during the time period studied. Similar to L1 antigen, it first appears at embryonic day 10 in the marginal zone and remains undetectable in the ventricular layer also at later stages. In the marginal zone the L2 epitope is strongly expressed on neuroepithelial endfeet at the basal lamina. The basal lamina itself is L2 epitope-negative. From embryonic day 10 onward the L2 epitope is most strongly expressed in the marginal zone and subplate and more weakly in the cortical plate and intermediate zone. In the subplate it is not only associated with the surface membrane, but also with the extracellular matrix. These observations support previous biochemical data which show that the L2 epitope is not present on all N-CAM molecules of the embryonic or adult forms and suggest that the independent regulation or L2 epitope expression may have functional implications during development.

The chronology of lesion repair in the developing rat brain: biological significance of the pre-existing extracellular space

We observed the histological peculiarities of the repair process in a destructive lesion of the developing rat brain during neurogenesis. Degeneration was induced selectively in certain cells of the proliferating phase in the rat fetal neopallium on embryonic day 16 by transplacental administration of ethylnitrosourea. Successive elimination of necrotic cells and the restoration process were observed. The repair process was divided into the following steps: elimination of individually affected cells by phagocytes in the pre-existing extracellular space; successive restoration of the disintegrated area by cells which differentiated from remaining matrix cells. No reactive gliosis, fibrosis, abnormal vascularization or infiltration of granulocytes and lymphocytes was observed at any time. The thinned neopallium on postnatal day 21 revealed only a small number and abnormal distribution of the cortical neurons. It may be assumed that the fetal brain owes its unique repair features to the presence of a vast extracellular space under normal conditions. In this pre-existing extracellular space, every kind of cell seems to exist separately without the intracellular adhesions characteristic of the adult brain. When degeneration occurs in certain cells the phagocytes would be able to eliminate the degenerate cells completely in this space without having to break intercellular adhesions. As a result, after the completion of cell elimination, the injured brain is restored to its original state with no cell reaction, giving the appearance of a small brain with normal-looking histological architecture, save only for the sparseness of cells.

Responses of cultured neural retinal cells to substratum-bound laminin and other extracellular matrix molecules

The responses of cultured chick embryo retinal neurons to several extracellular matrix molecules are described. Retinal cell suspensions in serum-free medium containing the "N1" supplement (J. E. Bottenstein, S. D. Skaper, S. Varon, and J. Sato, 1980, Exp. Cell Res. 125, 183-190) were seeded on tissue culture plastic surfaces pretreated with polyornithine (PORN) and with one of the factors to be tested. Substantial cell survival could be observed after 72 hr in vitro on PORN pretreated with serum or laminin, whereas most cells appeared to be degenerating on untreated PORN, PORN-fibronectin, and PORN-chondronectin. Cell attachment, although quantitatively similar for all these substrata, was temperature-dependent on serum and laminin but not on fibronectin or untreated PORN. In a short-term bioassay, neurite development was abundant on laminin, scarce on serum and fibronectin, and absent on PORN. No positive correlation between cell spreading and neurite production could be seen: cell spreading was more extensive on PORN and fibronectin than on laminin or serum, while on laminin-treated dishes, spreading was similar for neurite-bearing and non-neurite-bearing cells. Laminin effects on retinal neurons were clearly substratum dependent. When bound to tissue culture plastic, laminin showed a dose-dependent inhibitory effect on cell attachment and did not stimulate neurite development. PORN-bound laminin, on the other hand, did not affect cell attachment but caused marked stimulation of neurite development, suggesting that laminin conformation and/or the spatial distribution of active sites play an important role in the neurite-promoting function of this extracellular matrix molecule. Investigation of the embryonic retina with ELISA and immunocytochemical methods showed that laminin is present in this organ during development. Therefore, in vivo and in vitro observations are consistent with the possibility that laminin might influence neuronal development in the retina.

Immunocytochemical localization of a chondroitin sulfate proteoglycan in nervous tissue. II. Studies in developing brain

In contrast to the intracellular (cytoplasmic) localization of chondroitin sulfate proteoglycans in adult brain (Aquino, D. A., R. U. Margolis, and R. K. Margolis, 1984, J. Cell Biol. 99:940-952), immunoelectron microscopic studies in immature (7 d postnatal) rat cerebellum demonstrated almost exclusively extracellular staining in the granule cell and molecular layers. Staining was also extracellular and/or associated with plasma membranes in the region of the presumptive white matter. Axons, which are unmyelinated at this age, generally did not stain, although faint intracellular staining was present in some astrocytes. At 10 and 14 d postnatal there was a significant decrease in extracellular space and staining, and by 21 d distinct cytoplasmic staining of neurons and astrocytes appeared. This intracellular staining further increased by 33 d so as to closely resemble the pattern seen in adult brain. Analyses of the proteoglycans isolated from 7-d-old and adult brain demonstrated that they have essentially identical biochemical compositions, immunochemical reactivity, size, charge, and density. These findings indicate that the antibodies used in this study recognize the same macromolecule in both early postnatal and adult brain, and that the localization of this proteoglycan changes progressively from an extracellular to an intracellular location during brain development.

Studies on the factors that govern directionality of axonal growth in the embryonic optic nerve and at the chiasm of mice

What are the forces residing at the presumptive chiasm of embryonic mice that control the directionality (i.e., side specificity) of the optic axons? In an attempt to answer this question, the overall trajectories of individual fascicles of early growing axons and the various environments that they encounter along their pathway have been charted from the eye through the nerve and into the base of the diencephalon. Serial sections and reconstructive computer graphic techniques were used for the analysis. The early optic axons (embryonic (E) day 13.5) arrive at the chiasm in a stereotyped topographic arrangement. However, the fiber array at the primitive chiasm is not retinotopically organized nor is it maintained with the same level of spatial precision as it is at the disc. Thus, the annular, inverted retinotopic contingent of "pioneering" axons that exists in the primitive nerve becomes reorganized at the chiasm into a crescent-shaped configuration, with fascicles from ventrotemporal and ventronasal retina at either side of the crescent and with fascicles from dorsal retina interposed. Because of their gross locations in the crescent, particular clusters of fibers, each largely originating from different retinal sectors, but "contaminated" with fibers from other regions, come in contact with different types of nonneuronal structures at the chiasm. One, a dense, knotlike glial formation that lies along the margin of the diencephalic-telencephalic junction, directs all adjacent (ventronasal) fibers contralaterally. The other, a discrete pathway of lengthy marginal glial processes, separated by an anastomotic system of large extracellular spaces, guides all nearby fibers from ventrotemporal retina ipsilaterally. The results suggest that fiber topography as well as local environmental factors may play important roles in guiding axons at the chiasm.

An SEM examination of granule cell migration in the mouse cerebellum

The inward migration of external granule cells (EGC) from the pial surface of the developing cerebellum to form the (internal) granule cell layer was examined using SEM. Cerebella from male mice ranging in age from days 1-20 were fixed, then fractured through the developing pyramid region. EGC were initially unspecialized cells, forming 2-3 layers at the pial surface. EGC layers increased to 6-8, granule cells in the deeper regions elongated, and a prominent space formed between superficial and deep (premigratory) strata. During peak migration (days 8-12), nests of 4-6 EGC were associated with Bergmann glial fibers (BF) of the Golgi epithelial cells, which crossed molecular and EGC layers to terminate as spiny endfeet at the pial surface. Fibrils of extracellular material (ECM) often linked both premigratory and migrating EGC with a nearby BF. The molecular layer thickened considerably and the parallel fibers were traversed by an increasing number of Bergmann fibers and Purkinje cell processes during this period. As active migration slowed (days 13-20) and EGC reached their destination below the Purkinje cell layer, they lost their polarity and were enmeshed in ECM. The role of the Bergmann fibers and extracellular material in granule cell migration is considered.