Neuronal Migration in CNS Development

After neurons are born, they migrate to their final destinations:

Radial migrationTangential migration

Radial migration along radial glia cells in the developing CNS:

Serial section electron microscopy:Migrating neurons in the intermediate zone are intimately apposed to radial glial fibers (striped vertical shafts, RF1-6), which extend short lamellate expansions (LE) at a right angle to their main axis. Nuclei (N) of migrating neurons are elongated, and their leading processes (LP) are thicker and richer in organelles than their trailing processes (TP). Each leading process extends several pseudopodial endings (PS). Several cross sections through a migrating neuron are shown (a–d); migrating cell partially encircles the shaft of the radial glial fiber and these intimate contacts are continuous throughout the length of the cell. OR, optic radiation, From Sidman and Rakic (1973).

Radial migration along radial glia in the developing Cerebellum, Hippocampus and Cortex

In vitro migration of hippocampal neuronsalong the process of astroglia cells fromthe cerebellum. Neurons can migrate along a variety of radial glia fibers.

Migrating neurons are apposed to gliaCells, which guide them from the ventri-cular zone to their final destination.

Cortical neurons are migrating over long distances

Diagram of various trajectories taken by migrating cortical cells co-generated at the same embryonic day, but destined to settle in various areas of the cortical plate.

Some cells generated in the neocortical neuroepithelium migrate in the lateral cortical stream for four or more days before reaching their target destination.

The role of the Reelin protein in cortical

development.

A) Reelin is expressed by Cajal Retzius cells in the outer layer of the developing cortex. As neurons migrate out along the glial fibers, Reelin is proposed to organize the cortical plate. B) Reelin binds to a receptor, VLDLR or ApoER2 in the surface membrane, which leads to downstream signaling via Dab1, resulting in alterations in gene expression. In addition, Cdk5 phosphorylates cytoskeletal components such as tau and neurofilaments, which may affect organization of the cytoskeleton and properties of migrating neurons. Procadherins act as another class of reelin receptors.

Players in the formation of the neuronal layers of cerebral cortex.

Layer 1 (Cajal-Retzius cells, blue), secrete Reelin. Cells migrate along the radial glia (green) using genes that provide components of the cytoskeleton (Lis1, Dcx, Filamin1, and Cdk5/p35) or neuron-glia binding (Astn1, and Integrin 3). Mutations in any of these genes results in brain malformations. ME Hatten, 2002

• A cell’s position in the embryo is important in development because its differentiation is often dictated by location.

• A cell’s final location is important because neural function depends on precise connections between neurons and their targets; presynaptic and postysynaptic elements must be in the right place at the right time.

• The final position in vertebrates requires active migration.

• In the CNS (neural tube origin), development builds from a basic columnar organization of cells.

• Many of the structures vary in the degree of layering: cerebral cortex (6), hippocampus, cerebellar cortex (simpler 2 layer arrangement), retina, spinal cord (2-3) are layered in their final development.

• Show several brain areas including subdivisions of vesicles.

• This laminar structure is essential for the formation of complex circuits.

• Structures that are not layered: brainstem, midbrain, diencephalon.

• As a result, one important aspect of the migrating movements = radial migration:

• A special class of cells, radial glial cells, are differentiated for this purpose.

General Movement Pattern and Building of Basic Embryonic Zones

• Cells becoming post-mitotic earliest end up in the deepest cortical layers (3H-thymidine autorad) .

• The wall of the neural tube and its various vessicles begins 1 cell deep.

• [the ventricular zone – replication and migration will be outward]

• One of the 1st cells to differentiate is the radial glial cell, with processes reaching outward to span the thickness of the wall of the growing brain vessicles. These provide a scaffold for the dividing cells to migrate away from the ventricular zone.

• As cell division progresses, the cortical structure progresses from a simple neuroepithelial sheet to a multi-laminar structure.

• Illustrate here.• Preplate: 1st step for early dividing cells to

migrate to (single layer) (post-mitotic neuronal precursors).

• Intermediate Zone: between ventricular zone and preplate containing the axons of these neurons.

Preplate

Marginal zone (future Layer I)

Subplate = a transient popof neurons that largelyby apoptosis in early in post-natal life.

• What mechanism halts the migration of post-mitotic neurons and induces them to form layers?

• Studies of a mutant mouse, reeler, have provided some clues. Mouse gets its name because of disruption of the formation of embryonic layers leads to cerebellar disfiguration (recall earlier slides about reeler).

• The mutated gene encodes an ECM-related protein, called reelin.

• This suggests that ECM molecules may function in the arrest of radial migration and formation of neuronal lamina.

Primary Radial Migrations Along Glial Fibers

• The close apposition of post-mitotic neurons to the glial processes and the timing of their appearance (revealed by detailed 3-D reconstruction studies) led to proposal that they are a substitute for primary migration of neurons in the cortical structure in which they appear.

• This can be a very long journey (3,000 μm in primates.

• Illustration: neuron takes on a very extended bipolar shape.

• EM studies have revealed specialized “migration junctions” between the neuron soma and glial fibers (interstitial junctions) – widening of intercellular space with filamentous material, which is contiguous with submembrane cytoskeletal elements.

• A receptor system highly expressed in this stage of development: astrostatin (neuronal glycoprotein).

• Earlier studies: the function of radial glia have stemmed from another neurological mouse mutant, weaver, whose granule cells never migrate out to their final location and was found to involve a defect in cerebellar radial glia.

Secondary Migration

• In many brain areas, more complex secondary migrations occur after the initial establishment of a cortical scaffold.

• The example we will examine is the 2° migration of granule cells in the developing cerebellar cortex.

• As alluded to earlier, granule neurons do undergo a 1° migration from ventricular zone with assistance of radial glia (known to be disordered in _____________ mutant mouse.

• Cortex example follow-up: In general, the 2° germinal zone in a subventricular zone overlaying the ventricular zone (illustrate here).

• This has developed evolutionarily to supply the large #s of neurons later in development (in the human, 2° neurogenesis occurs through the 2nd yr of life.

• What is the significance of this in terms of trauma or pathology (e.g., stroke)?

In Cerebellum• Dividing precursor cells stream across a

structure, known as the rhombic lip, then onto the surface of the developing cerebellum, establishing a 2° (displaced) germinal zone, known as the external germinal layer (EGL).

• During prenatal development, this zone develops over the entire surface of the cerebellum 2 cells thick. After birth, rapid proliferation expands the EGL 8 cells.

• Next, the downward migration of cells to the final position (internal granular layer) by 15 days after birth.

• Recent molecular biology studies have demonstrated that the separation of this superficial zone of dividing cells and internal zone of differentiating cells is accompanied by expression of genes that are differentially regulated at the transcriptional level:

• Different genes are transcribed during:1. Neurogenesis2. Initiation of neuronal differentiation3. Axon outgrowth4. Forming synaptic connections

Cell Migration Patterns Reflect Neuronal Fate Specification

• As noted earlier, 3H-thymidine labeling studies showed that layers of neurons are generated in an “inside out” pattern.

• [Recall: 1st post-mitotic neurons settle in the deepest layers to differentiate; later, they migrate through these to more superficial layers]

How do Cells Know when to Start Differentiating?

1. Cells might possess an “internal clock” – measure time or # of divisions.

2. Cells might be signaled by their local environment (“community” to start differentiating.

The latter is most supported in modern tissue culture studies:

e.g., progenitor cells would “switch” their fate when transplanted (i.e., an “internal timing” mechanism would not be able to explain this).

A Few Notes Regarding Cell Migration in Nonlayered Structures

• In brainstem and diencephalon, the final adult structure is not layered.

• In these structures, cells still migrate outward from a ventricular zone, but they aren’t directed radially by glial fibers.

• Rather, they aggregate into nuclei or condensed group of cells, all geared towards a particular function (e.g., thalamus, brainstem, hypothalamus).

• One clue to the way this can occur is with cadherins, expressed by these cells; e.g., R-cadherins.

• 2° migrations are used here to allow great cell proliferation later in development.