Richard L. Sidman

Neuronal migration, with special reference to developing human brain: a review

A general rule in the developing central nervous system is that cells are generated in sites different from those in which they will later reside. The intervening migrations, particularly in the human nervous system, form the subject of this review. The basic columnar organization in the early stages of development favors radial migration of cells. During later stages in primates, when young neurons migrate to the distant cerebral cortex, they follow radial glial guides across the widening intermediate zone as they pass from the juxtaventricular site of genesis to the cortical plate. Somas of later-generated cells take positions external to somas of their predecessors. The final position along the radial vector may be influenced by afferent axons. Cell relationships in the developing cerebellar cortex are essentially similar, though the key migration of granule cell neurons is in the reverse direction, from the external surface inward past Purkinje dendrites and somas. Bergmann glial fibers provide the radial guidance in this instance. The degree of dependence of developing neurons upon other cells and cell processes in their immediate environment has been clarified by study of mutant mice in which cerebral or cerebellar cortices are malformed. Other special migrations in the fetal human brain are reviewed, particularly the passage of neurons from the rhombic lip through the transient corpus pontobulbare to mainly the inferior olives and pontine gray nuclei, and from the ganglionic eminence of the cerebrum through the corpus gangliothalamicum into the pulvinar region of the thalamus. It was suggested that the special relationships involved in these various migrations are probably mediated by cell surface properties, and that such surface properties will come to be defined through analysis of reaggregation tissue cultures, experimental and natural chimeras, and by immunological definition of antigens on CNS cells at different stages of development.

Directed migration of neural stem cells to sites of CNS injury by the stromal cell-derived factor 1α/CXC chemokine receptor 4 pathway

Migration toward pathology is the first critical step in stem cell engagement during regeneration. Neural stem cells (NSCs) migrate through the parenchyma along nonstereotypical routes in a precise directed manner across great distances to injury sites in the CNS, where they might engage niches harboring local transiently expressed reparative signals. The molecular mechanisms for NSC mobilization have not been identified. Because NSCs seem to home similarly to pathologic sites derived from disparate etiologies, we hypothesized that the inflammatory response itself, a characteristic common to all, guides the behavior of potentially reparative cells. As proof of concept, we show that human NSCs migrate in vivo (including from the contralateral hemisphere) toward an infarcted area (a representative CNS injury), where local astrocytes and endothelium up-regulate the inflammatory chemoattractant stromal cell-derived factor 1α (SDF-1α). NSCs express CXC chemokine receptor 4 (CXCR4), the cognate receptor for SDF-1α. Exposure of SDF-1α to quiescent NSCs enhances proliferation, promotes chain migration and transmigration, and activates intracellular molecular pathways mediating engagement. CXCR4 blockade abrogates their pathology-directed chain migration, a developmentally relevant mode of tangential migration that, if recapitulated, could explain homing along nonstereotypical paths. Our data implicate SDF-1α/CXCR4, representative of the inflammatory milieu characterizing many pathologies, as a pathway that activates NSC molecular programs during injury and suggest that inflammation may be viewed not simply as playing an adverse role but also as providing stimuli that recruit cells with a regenerative homeostasis-promoting capacity. CXCR4 expression within germinal zones suggests that NSC homing after injury and migration during development may invoke similar mechanisms.

An autoradiographic analysis of histogenesis in the mouse cerebellum.

Embryonic and young postnatal mice were exposed once to thymidine-H3, to label cells preparing for division. Histogenesis of cerebellum was studied in autoradiograms. Purkinje cells and neurons of roof nuclei form simultaneously in the primitive ependyma of the young embryo and migrate outward to reach their final positions. A transient external granular layer arises by proliferation of cells on the lateral caudal cerebellar surface lining the fourth ventricle. These cells migrate over the external cerebellar surface, and continue to proliferate abundantly until a few weeks after birth. The layer disappears in the third postnatal week. Most external granule cells divide a few times and then migrate inward past the Purkinje cells to become granule cell neurons. A smaller population of cells arises in the granule layer itself and is characterized by an unusually long interval between deoxyribonucleic acid (DNA) synthesis and mitosis. These cells migrate short distances and may assume positions around the perikarya of Purkinje cells; cells of this class are probably neuroglia. An hypothesis is presented which accounts for the extensive cell migrations during histogenesis as a means for attaining particular synaptic contacts.

Engraftable human neural stem cells respond to development cues, replaceneurons, and express foreign genes

Stable clones of neural stem cells (NSCs) have been isolated from the human fetal telencephalon. These self-renewing clones give rise to all fundamental neural lineages in vitro. Following transplantation into germinal zones of the newborn mouse brain they participate in aspects of normal development, including migration along established migratory pathways to disseminated central nervous system regions, differentiation into multiple developmentally and regionally appropriate cell types, and nondisruptive interspersion with host progenitors and their progeny. These human NSCs can be genetically engineered and are capable of expressing foreign transgenes in vivo. Supporting their gene therapy potential, secretory products from NSCs can correct a prototypical genetic metabolic defect in neurons and glia in vitro. The human NSCs can also replace specific deficient neuronal populations. Cryopreservable human NSCs may be propagated by both epigenetic and genetic means that are comparably safe and effective. By analogy to rodent NSCs, these observations may allow the development of NSC transplantation for a range of disorders.

Mutant mice (quaking and jimpy) with deficient myelination in the central nervous system

Two mutant mice with deficient myelination are described. Quaking is a new autosomal recessive mutant mouse with marked tremor of the hindquarters. The mice eat, swim, breed, and nurse well even though the entire central nervous system is very deficient in myelin by histological and chemical criteria. Myelin formation is impaired; no destruction is seen. Peripheral nerves are myelinated. Jimpy, a known sex-linked mutation, has similar but more severe symptoms and similar pathology, with the additional feature of sudanophilic (nonpolar) lipid distributed in some white-matter tracts. Both mutants offer new opportunities for study of the formation and functions of myelin.

Cell proliferation and migration in the primitive ependymal zone; An autoradiographic study of histogenesis in the nervous system

Thymidine was incorporated into deoxyribonucleic acid (DNA) of cells preparing for division. Autoradiography with tritium-labeled thymidine (thymidine-H3) provided a method of marking such cells in the reletively inaccessible mammalian embryo. Pregnant mice were injected intravenously with thymidine-H3 and killed at various intervals. Autoradiograms were prepared of sections through the embryonic brains. Eleven-day embryos fixed 1 hour after exposure to thymidine-H3 showed heavy labeling of most cell nuclei in the external half of the primitive ependymal layer in the wall of the cerebral vesicle, and almost no labeling in the inner half. Thus, the external half of the primitive ependymal layer is the site of DNA synthesis. Six hours after exposure to thymidine-H3, the labeled nuclei occupied the inner (ventricular) half of the primitive ependymal layer, and most mitotic figures at the ventricular surface contained labeled chromosomes. Forty-eight hours after injection, labeled nuclei had migrated laterally; some had entered the developing mantle layer, but many remained in the primitive ependyma and had repeated the cycle of DNA synthesis, migration, and division. Development of the primitive ependyma was similar throughout the embryonic nervous system. The data show that the cells of the primitive ependymal layer behave synchronously, and that the sites of DNA synthesis and mitosis are different. The views of Schaper and Sauer are confirmed: the primitive ependymal layer is a pseudostratified columnar epithelium within which nuclei of undifferentiated cells migrate to and fro in relation to the mitotic cycle.

Increased rate of peripheral nerve regeneration using bioresorbable nerve guides and a laminin-containing gel

The sciatic nerve of adult mice was transected and proximal and distal nerve stumps were sutured into a nontoxic bioresorbable nerve guide. Nerve guide lumens were either empty or filled with a gel containing 80% laminin and additional extracellular matrix components. Two weeks later cells in the L3 through L5 dorsal root ganglia and the ventral horn of the spinal cord were retrogradely filled with horseradish peroxidase. All animals with the laminin-containing gel but none with empty nerve guides displayed labeled cells. This suggests that the laminin-containing gel significantly hastened axonal regeneration in vivo.

Autoradiographic Methods and Principles for Study of the Nervous System with Thymidine-H3

The history of autoradiography as a method is closely interwoven with the idea of radioactivity itself. Niepce de St. Victor (1867) and Henri Becquerel (1896) made the first autoradiograms when they showed that uranium salts altered the emulsion in an adjacent photographic plate, though they had formulated the experimental design on the premise that light mediated the response. In 1898 Marie Curie reinterpreted Becquerel’s observations and added the measurements that established the concept of radioactivity (Romer, 1955). Subsequent applications to biological problems followed basically the same technical approach, but histological (microscopic) autoradiography became practical only after several more decades, with the development of methods for maintaining accurate alignment of a thin slice of tissue and a photographic film through the many steps of staining and development, so that a radioactive source could be localized in the tissue by direct microscopy (e.g., Belanger and Leblond, 1946). The subsequent development of tritium as a tracer, of a range of fine-grained precision emulsions, and of refinements in the methods for applying them to tissue specimens has been reviewed by Caro (1964, 1966), Belanger (1965), Salpeter (1966), Stevens (1966), and Rogers (1967).

Behavioral improvement in a primate Parkinson's model is associated with multiple homeostatic effects of human neural stem cells

Stem cells have been widely assumed to be capable of replacing lost or damaged cells in a number of diseases, including Parkinsons disease (PD), in which neurons of the substantia nigra (SN) die and fail to provide the neurotransmitter, dopamine (DA), to the striatum. We report that undifferentiated human neural stem cells (hNSCs) implanted into 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated Parkinsonian primates survived, migrated, and had a functional impact as assessed quantitatively by behavioral improvement in this DA-deficit model, in which Parkinsonian signs directly correlate to reduced DA levels. A small number of hNSC progeny differentiated into tyrosine hydroxylase (TH) and/or dopamine transporter (DAT) immunopositive cells, suggesting that the microenvironment within and around the lesioned adult host SN still permits development of a DA phenotype by responsive progenitor cells. A much larger number of hNSC-derived cells that did not express neuronal or DA markers was found arrayed along the persisting nigrostriatal path, juxtaposed with host cells. These hNSCs, which express DA-protective factors, were therefore well positioned to influence host TH+ cells and mediate other homeostatic adjustments, as reflected in a return to baseline endogenous neuronal number-to-size ratios, preservation of extant host nigrostriatal circuitry, and a normalizing effect on α-synuclein aggregation. We propose that multiple modes of reciprocal interaction between exogenous hNSCs and the pathological host milieu underlie the functional improvement observed in this model of PD.

Toll-like receptor 8 functions as a negative regulator of neurite outgrowth and inducer of neuronal apoptosis

Toll receptors in Drosophila melanogaster function in morphogenesis and host defense. Mammalian orthologues of Toll, the Toll-like receptors (TLRs), have been studied extensively for their essential functions in controlling innate and adaptive immune responses. We report that TLR8 is dynamically expressed during mouse brain development and localizes to neurons and axons. Agonist stimulation of TLR8 in cultured cortical neurons causes inhibition of neurite outgrowth and induces apoptosis in a dissociable manner. Our evidence indicates that such TLR8-mediated neuronal responses do not involve the canonical TLR–NF-κB signaling pathway. These findings reveal novel functions for TLR8 in the mammalian nervous system that are distinct from the classical role of TLRs in immunity.