10. Neural Development

Introduction to Neural Development

  • Neural development is a complex process that begins with the induction of the neural plate from the ectoderm.
  • It involves various cell types, signaling mechanisms, and tissue organization.

Learning Outcomes

  • At the end of this lecture, you should be able to:
    • Describe the cell types generated in the central nervous system (CNS).
    • Explain the events required for the development of the neural tube.
    • Identify sources and types of signals that control neural tissue development.
    • Describe types of proliferation in neural development and their regulation.

Organization of Nervous System

  • Central Nervous System (CNS):
    • Brain
    • Spinal Cord
  • Peripheral Nervous System (PNS):
    • Ganglia (collections of neuronal cell bodies)
    • Peripheral Nerves

Cell Types in the CNS

  • Neurons:
    • Many sizes and shapes: motor, sensory, and interneurons.
  • Glia:
    • Several types including:
    • Astrocytes: Support neurons, nutrient supply, clean up debris.
    • Oligodendrocytes: Myelinate axons, insulating support for multiple axons.
    • Ependymal Cells: Line fluid-filled cavities.
    • Microglia: Immune cells of the CNS.

Structure of Neurons

  • Parts of a Neuron:
    • Dendrites: Receive signals.
    • Cell Body (Soma): Contains the nucleus.
    • Axon: Transmits signals.
    • Axon Terminals: Send signals to the next cell.
    • Synapse: Junction between neurons for signaling.

Signaling in the CNS

  • Neurons communicate via synapses using chemical signals (neurotransmitters).
  • Process:
    1. Pre-synaptic neuron releases neurotransmitter.
    2. Neurotransmitter binds to receptors on the post-synaptic neuron.
    3. This binding triggers an electrical signal in the post-synaptic neuron.

Neural Tissue Development

  • The nervous system originates from the ectoderm layer.
  • Neural Induction: Signals induce the ectoderm to form the neural plate.
  • Neurulation: This process involves folding the neural plate to form the neural tube.
    • Apical constriction and differential cadherin expression drive morphogenesis.

Neurulation Stages

  • The neural tube closes from the middle and zips up toward both ends (neuropores).
  • Failure to close can result in conditions like anencephaly or spina bifida.

Regions of the CNS Post-Neurulation

  • Differentiation into:
    • Forebrain (telencephalon and diencephalon)
    • Midbrain
    • Hindbrain
    • Spinal Cord
  • Signaling is essential for regional specification, with growth factors (BMPs, Fgfs, Wnts, Shh) guiding development.

Stem Cells and Neurogenesis

  • Proliferation in the neuroepithelium occurs via:
    • Symmetrical Division: Producing more progenitor cells.
    • Asymmetrical Division: Producing neurons and maintaining progenitors.
Types of Asymmetrical Cell Division
  1. Type I: Producing one apical progenitor and one neuroblast.
  2. Type II: Producing basal/intermediate progenitors which divide before differentiation.
  • Contact-Dependent Signaling: Involves mechanisms like the Notch pathway to maintain progenitor populations.

CNS Organization and Function

  • Neurons migrate away from the ventricular layer to form distinct layers:
    • Ventricular Layer: Ependymal cells lining the central canal.
    • Mantle Layer: Contains neuronal cell bodies (grey matter).
    • Marginal Layer: Contains axon tracts (white matter).
  • Increased basal progenitors correlate with greater brain complexity and the presence of gyri.

Summary of Key Points

  • The neural plate derives from ectoderm, forming the neural tube leading to the development of the CNS.
  • Major cell types in the CNS are neurons and glial cells (astrocytes and oligodendrocytes).
  • Neural tube closure is crucial for normal CNS development; defects can lead to severe outcomes.
  • Signaling pathways are pivotal in determining the architecture and cellular composition of the developing brain and spinal cord.