Neurogenesis, Gliogenesis, and Neural Tissue Histology

Neurogenesis:
  - Refers to the generation of neurons.
  - Involves the transformation of radial glial cells (neural progenitors) into neurons.
  - Key mechanism: Lateral inhibition
    - This process helps mediate the formation of neuroblasts while keeping some progenitor cells reserved for later use.
Gliogenesis:
  - Refers to the generation of glial cells, which occur after neurogenesis.
  - Gliogenic signals begin to increase over time while neurogenic signals decrease.
  - This shift blocks proneural signals, leading to the switch from neuroblast production to glial cell production.
  - Types of glial cells derived include:
    - Astrocytes
    - Oligodendrocytes
    - Ependymal cells (line the ventricles)

Central Nervous System (CNS):
  - Comprises the brain and spinal cord.
  - Divided into two main tissue types:
    - Gray Matter:
      - Contains a high amount of neuronal cell bodies.
      - Appears darker due to the presence of neuronal nuclei.
      - Examples include the developing cortex.
    - White Matter:
      - Composed mainly of axon tracts and oligodendrocytes (which myelinate the axons).
      - Appears lighter in color because of the lipids present in myelin (fatty composition).
      - Example: Corpus callosum (connects the two hemispheres of the brain).
  - Staining Differences:
    - Hemotoxylin and Eosin Staining:
      - White matter stains more strongly with eosin due to the fatty content, while gray matter stains positively with hematoxylin indicating the presence of nuclei.
Peripheral Nervous System (PNS):
  - Similar arrangement to CNS but with different terminology:
    - Ganglia:
      - Aggregations of neuronal cell bodies.
      - Contain satellite cells which function similarly to astrocytes.
    - Peripheral Nerves:
      - Composed of axons running to peripheral muscles and organs.
      - Instead of oligodendrocytes, Schwann cells myelinate axons in this region.

Neuronal Migration:
- Neurons migrate to the cortex along radial and tangential paths to form cortical structures.
Axon Guidance:
  - Occurs after neurons reach their final locations.
  - Involves growth cones which extend from axons and have receptors that respond to environmental signals.
  - Cytoskeletal Dynamics:
    - Involves changes to microfilaments and microtubules, allowing growth cones to navigate toward targets.
Formation of Synapses:
  - Growth cones form synapses upon reaching target sites, triggering:
    - Changes in adhesion,
    - Modifications in receptors and signaling molecules.
  - Importance of neurotrophins:
    - Provides a survival signal; if growth cones fail to reach targets, neurotrophins are not received, leading to cell death.
    - Neurotrophins are internalized, transported retrogradely along microtubules to regulate gene expression necessary for survival.

In CNS development:
  - Neurogenesis occurs first, followed by gliogenesis.
  - Glial cells in the brain are primarily astrocytes and oligodendrocytes; in the PNS, they are satellite cells and Schwann cells.
  - Gray matter consists of aggregations of cell bodies; white matter consists of axons and myelinating oligodendrocytes.
  - In the PNS, we use the terms ganglia for aggregations of neuronal cell bodies and peripheral nerves for axon tracts.