Embryology Chapter: Nervous System Development

Embryonic Development of the Nervous System

• Neural Plate: A thickened region of the ectoderm, precursor to the neural tube which forms the CNS.
• Neuroectoderm: The embryonic tissue giving rise to the nervous system (both CNS and PNS)
• Induction: A process where one group of cells influences another's development through signaling molecules.
• Neural Tube: The structure arising from the neural plate which forms the brain and spinal cord (CNS).
• Neurulation: The transformation of the neural plate into a neural tube; starts CNS development and indirectly impacts Peripheral Nervous System (PNS) formation. Initiated around week 3.

Primitive Streak Regression

• The primitive streak, a structure involved in gastrulation, gradually regresses during neurulation.
• As the streak moves cranially, it leaves a trail of notochordal precursor cells behind.
• The regression process creates the neural plate from the ectoderm.
• The ectoderm now differentiates into surface ectoderm and neural ectoderm.

Neural Induction

• Chordamesoderm, a mesodermal precursor of the notochord, is key to neural induction.
• Signaling molecules from the chordamesoderm influence the ectoderm, causing it to become a neural ectoderm and form the neural plate.
• Critical aspects include influencing the ectoderm to adopt a "neural fate" and inhibiting epidermal development.
• Signaling molecules like Noggin and Chordin regulate this process by inhibiting BMP-4, crucial for development pathways.

Neurulation Events

• Neural Groove and Folds: The neural plate begins to develop a groove, which then folds to create neural folds, important steps in the neurulation process.
• Tube Formation: Neural folds meet together and fuse to generate the neural tube; which overlies the ectoderm.
• Neural Crest Cells: During neural tube formation, neural crest cells, which contribute to the PNS, develop.
• Neuropore Closure: The neural tube closes in two places- Anterior and Posterior neuropores.
• Molecular Mechanisms: Molecular signals from the notochord guide the formation and closure of the neural tube.

Impact of Cell Structure

• Bottle Cells: These cells exhibit specific cytoskeletal characteristics (microfilaments and microtubules) essential for the tube formation process; particularly during the neurulation stage
  • disruption of either microfilaments or microtubules will interrupt neurulation.
• Inhibitors (e.g., Colchicine and Cytochalasin D): Disrupting cell cytoskeletons, for example via specific drugs, can affect neurulation.

Neural Tube Defects

• Errors in Closure: Defects in the closure of the neural tube lead to a variety of severe birth defects, including anencephaly and spina bifida.
• Anencephaly: Failure of the anterior neuropore to close, resulting in severe brain malformation. Usually leading to death prior to birth or shortly after.
• Spina Bifida: Failure of the posterior neuropore to close; varying in severity, from occulta to more serious forms like myelomeningocele, affecting the spinal cord.
• Risk Factors: Folate deficiency, genetics, and other environmental factors can increase the risk of neural tube defects.
• Treatment & Prevention: Treatment options (surgical) exist and prevention through sufficient folic acid intake is crucial.

Zika Virus and Congenital Defects

• Zika Virus: The Zika virus, spread by mosquitoes, can be passed from pregnant women to their fetuses.
• Microcephaly: A significant association between Zika virus infection and microcephaly– a birth defect characterized by a smaller than normal head size and possible brain damage in babies.