Neural development

Neural Development Study Notes

Early Neural Development Learning Objectives

• A. Neural Induction

  • The process of initiating the formation of neural tissue from precursor cells.

• B. Patterning

  • The spatial arrangement and structure formation of neural tissue.

• C. Mitosis in Early Nervous System

  • The process of cell division specifically in the developing nervous system.

• D. Homeotic/Hox Genes

  • Important genes responsible for determining the body plan and identity of segments.

• E. Notch Signaling and Differentiation

  • A signaling mechanism that regulates cellular differentiation and maintenance of progenitors.

• F. Migration and Layer Formation

  • How precursor cells migrate to their destined locations to form layers in the developing brain.

• G. Defects in Layer Formation

  • Pathologies arising from improper migration and layering of neural cells.

Developmental History of a Frog (Xenopus laevis)

• Stages of Development

  • Fertilization

  • Cleavage: Early cell divisions that lead to the morula and blastula stages.

  • Gastrulation: Formation of the three germ layers (ectoderm, mesoderm, endoderm).

    • Blastula: Formation of the blastocoel, leading to the next developmental stage.

    • Morula: Solid ball of cells prior to forming a blastocoel.

    • Metamorphosis: Developmental process in certain species.

  • Organogenesis: Formation of organs from germ layers.

Neurulation in the Mammalian Embryo

• Primitive Streak: Formation during gastrulation, allowing inward migration of cells.

• Germ Layers:

  • Ectoderm: Forms the outermost layer, leading to the development of the nervous system.

  • Mesoderm: Diverges at the midline to form the notochord, critical for embryonic symmetry.

  • Endoderm: Forms inner structures and organs.

Neural Plate and Tube Formation

• Neural Plate Formation: The ectoderm above the notochord develops into a neural plate post-gastrulation.

• Neurulation: The process where the neural plate forms a cylindrical neural tube.

• Neural Tube:

  • Fuses at the peripheral edges, giving rise to the central nervous system (CNS).

  • Neural Crest: Structure at the dorsal edge, contributes to peripheral nervous system and other tissues.

Neural Crest Development

• Migrations: Neural crest cells migrate to form different structures.

  • Four Routes of Migration:

    1. Dorsal root ganglia.

    2. Autonomic ganglia.

    3. Adrenal medulla cells.

    4. Melanocytes and non-neural cells.

  • The routes influence the microenvironments encountered, thus affecting differentiation outcomes.

  • Patterning: Anterior/Posterior and dorso-ventral patterning initiated around day 20.

Regional Specification of the Developing Brain

• Neural Tube Subdivision:

  • Prosencephalon: Anterior

  • Mesencephalon: Midbrain

  • Rhombencephalon: Posterior

  • Spinal Cord: Precursor section.

• Further Specifications:

  • Prosencephalon splits into Telencephalon and Diencephalon.

  • Rhombencephalon splits into Metencephalon and Myelencephalon.

Detailed Specification by Region

• Telencephalon: Becomes the cerebral hemispheres.

• Diencephalon: Becomes thalamus, hypothalamus, pineal gland, retinas.

• Mesencephalon: Becomes midbrain.

• Metencephalon: Becomes pons and cerebellum.

• Myelencephalon: Becomes medulla.

Gene Expression in Development

• Homeotic Selector Genes (HOX genes):

  • Act as transcription factors, determining the identity of body segments.

  • Common across species, with clusters on chromosomes.

• Segmentation Process: Originates from studies on Drosophila, leading to the specification of neural tube regions.

Inductive Signaling Events

• Cell Identity: Generated from spatial and temporal regulation by signaling molecules.

• Major Signaling Molecules:

  • Retinoic Acid: Derived from vitamin A, influences posteriorization, acts via Retinoic Acid Receptors (RAR α, β, γ; RXR α, β, γ).

  • FGFs (Fibroblast Growth Factors): Peptide hormones, critically involved in neural induction.

  • Sonic Hedgehog (Shh): Signaling through patched and smoothened receptors.

  • BMPs (Bone Morphogenetic Proteins): Alters neural differentiation and maintains dorsal-ventral polarity, can be inhibited by Noggin and Chordin.

  • WNT Pathways: Non-canonical and canonical pathways, involved in establishing cell fate and polarity.

Cell Differentiation and Migration

• Neural Precursor Cells: Undergo mitosis primarily in the ventricular zone, producing both neuroblasts and progenitor cells.

• Migration Patterns: Radial migration through the use of radial glia as scaffolds.

• Cell Maturation: Shapes neural layer with defined structures in the cortex.

Notch Signaling Mechanism

• Role: Prevents precursors from differentiating prematurely.

• Mechanism:

  • Notch receptor activation leads to the Notch Intracellular Domain (NICD) translocating to the nucleus, triggering transcriptional changes that inhibit differentiation.

• Proneural Factors: Basic helix-loop-helix (bHLH) factors, necessary for neuronal differentiation, inhibited by Notch signaling.

Neuronal and Glial Differentiation

• Types of Cell Division:

  • Symmetrical Division: Produces two daughter cells that remain active in the cell cycle.

  • Asymmetrical Division: Results in one active cell and one differentiated neuron or glial cell, based on Notch signaling levels.

Radial Glia and Cell Migration

• Migration Mechanism: Radial glial cells facilitate movement of neuroblasts.

• Stop Signals: Molecules like Reelin provide signals for neurons to cease migration and settle into their layers.

• Clinical Relevance: Conditions such as Reelin deficiencies lead to improper cortical layer formation, influencing brain morphology.

Diseases Related to Neural Migration

• Reelin and Doublecortin (DCX) Mutations: Cause significant disruptions in cortical structure, potentially leading to conditions like lissencephaly (smooth brain).

• MRI Findings: Demonstrate enlarged lateral ventricles and altered cortical structures in affected individuals.