Repair and Regeneration of Nervous Tissue and Associated Diseases

Repair and Regeneration of Nervous Tissue

  • Nervous Tissue Characteristics:

    • Postmitotic nature: Neurons do not divide and are generally not replaced.
    • Neurons maintain connections for communication, with information stored in these connections.

  • Types of Neuronal Repair:

    1. Axon Regrowth:
    • After an injury, axons from surviving neurons can regrow.
    1. Limited Repair:
    • Damage causes loss of axon and neurons, leading to proliferation of glial cells, forming a glial scar where neurons once existed.
    1. Neurogenesis:
    • Damaged neurons can be replaced by new neurons generated from stem cells.
    • Example: Neuro stem cells can proliferate and give rise to new neurons after damage.

  • Peripheral Nervous System (PNS) Repair:

    • The PNS can regrow axons and achieve synaptic reinnervation after injury:
      • When a peripheral nerve is damaged, the distal part degenerates while axons from surviving neurons form growth cones towards their original targets.
      • Crushed nerves generally recover better due to availability of supportive structures.

  • Cellular Elements in PNS Repair:

    • Key Cells: Schwann cells, fibroblasts, macrophages, endothelial cells.
    • Process of repair involves macrophages cleaning up myelin debris, gene expression promoting axon growth, and Schwann cells guiding growth cones with signals and scaffolding structures.

  • Regeneration Signals:

    • Different cellular signals are involved in various stages of repair (e.g., Myelin proteins during demyelination and various growth factors during repair).

  • Artificial Biomaterials:

    • Engineered to enhance or replace the support provided by peripheral nerve bridges.

CNS Injury and Repair Mechanisms

  • Types of CNS Injury:

    1. Traumatic brain injury
    2. Oxygen deprivation (hypoxia or stroke)
    3. Neurodegenerative diseases

  • Challenges in CNS Regrowth:

    • Central axons usually do not regenerate due to:
    1. Activation of necrotic and apoptotic cell death pathways.
    2. Change in local environment lacks developmental signals for growth.

  • Protection of the Brain:

    • Physical barriers include the skull, meninges, and cerebral spinal fluid, while the blood-brain barrier regulates movement of ions and protects from toxins.

Chronic Traumatic Encephalopathy (CTE)

  • Symptoms of CTE:
    • Cognitive impairment, mood changes, memory deficits, nausea, blurred vision.
  • Long-term Impacts:
    • Can be confused with Alzheimer's disease due to overlapping symptoms despite differing pathology.

Molecular Mediators of Injury Responses

  • CNS Injury Response:

    • Involves glial scars formed by astrocytes, oligodendrocytes, and microglia, which inhibit regrowth of neurons across injury sites.

  • Neurogenesis in Mature CNS:

    • Stem cells in areas like the olfactory bulb and hippocampus can generate new neurons to replace lost ones.

  • Neurodegenerative Disease Mechanisms:

    • Factors include environmental stressors, genomic instability, mitochondrial dysfunction, neuroinflammation, and excitotoxicity.

Specific Disease Mechanisms

  • Excitotoxicity:

    • Excess glutamate leads to uncontrolled neuronal death by overactivating receptors causing calcium influx and mitochondrial dysfunction.

  • Oxidative Stress:

    • ROS can damage DNA and proteins, leading to cell death in diseases such as ALS, Parkinson’s, and Alzheimer’s.

  • Autophagy Disruption:

    • Impaired autophagic processes have been implicated in neurodegenerative diseases, affecting the removal of damaged mitochondria.

  • Parkinson's Disease:

    • Characterized by dopamine depletion, leading to motor symptoms like tremors and rigidity. Risk factors include age, environmental toxins, and genetic mutations.

  • Multiple Sclerosis (MS):

    • Immune-mediated demyelinating disease affecting the ability of neurons to conduct impulses efficiently. Symptoms are highly variable and include unpredictable neurological dysfunction.

  • Leber Hereditary Optic Neuropathy (LHON):

    • Caused by mitochondrial mutations leading to vision loss due to retinal ganglion cell death following rapid degeneration.

Summary of Treatments and Interventions

  • Current Treatments in Neurological Disorders:
    • Focus largely on symptom management due to lack of cures. Pharmacological options include various neurotransmitter modulators, neuroprotectants, and experimental therapies aiming to promote neurogenesis and mitigate neurodegenerative progression.
  • Therapeutic Approaches:
    • Various mechanisms from drugs to lifestyle adjustments that could help mitigate symptoms but require multidisciplinary approaches due to complexities of neurological disorders.