Study Notes on Glial Cells and Their Functions

Glial Cells

Overview of Glia

  • Glia - also known as neuroglia or glial cells, play a critical role in the brain and are abundant in number compared to neurons.

History of Glial Cell Discovery

  • The term 'neuroglia' was coined in 1856 by Rudolf Ludwig Karl Virchow, a German physician and scientist.

    • Derived from the term 'Nervenkitt', meaning 'nerve-glue', reflecting the perceived function of glia at that time.

    • The term 'glia' comes from Greek meaning slimy and sticky in appearance.

  • Rudolf Virchow:

    • Known as the father of modern pathology.

    • Introduced systematic autopsy and forensic hair analysis for criminal investigations.

    • Pioneer in public health – promoted free healthcare and standards for food safety.

    • Linked infectious diseases among humans and animals, creating the concept of "zoonoses".

    • Coined terms such as chromatin, neuroglia, and spina bifida.

  • Camillo Golgi made the first detailed description of glia in 1870:

    • Observed different types of glia and posited that glia feed neurons.

  • Santiago Ramon y Cajal developed a staining method enhancing the visualization of glia.

    • Noted that glia can divide in the adult brain and mediate neuronal communication.

Number of Glial Cells in the Brain

  • Suzana Herculano-Houzel (2009):

    • Brazilian neuroscientist who introduced Isotropic Fractionation, or "Brain Soup Technique".

    • Developed a method to study brain cell composition by homogenizing four brains and isolating nuclei through immunohistochemistry.

    • Findings suggest a glial to neuronal ratio of approximately 1:1.

    • (Video link: https://www.youtube.com/watch?v=d2Uhv0_Ji1k)

  • Ben Barres (1954-2017):

    • Neurobiologist who stated humans possess a finite number of cells.

    • Each cell contains about 6.5 picograms of DNA.

    • Most neurons are established by week 20, while glia continue proliferating until age 2.

    • Compared DNA in human brains:

    • At 20 weeks: 0.25 millimoles of DNA.

    • At 2 years: 2 millimoles of DNA.

    • Glia comprise 80% of all cells in the human brain.

Glial Functions

Neuroglia as Champs of Homeostasis

  • Glial cells maintain homeostasis in the brain, contributing to the regulatory processes essential for neuron function.

Types of Glial Cells

  1. Astrocytes

    • Star-shaped cells with intermediate filaments, forming the cytoskeleton.

    • Glial Fibrillary Acidic Protein (GFAP) is a common marker, but not all astrocytes express GFAP.

  2. Oligodendrocytes

  3. Microglia

Astrocytes
Morphology

  • Star-like appearance; Not all astrocytes exhibit this morphology.

Types of Astrocytes

  • Various astrocyte subtypes located in different parts of the brain:

    • Protoplasmic (type IV), fibrous (type VI), or specialized types in regions such as the cortex, olfactory bulb, and cerebellum.

Functions of Astrocytes

  1. Control of Extracellular K+ Concentrations

    • Astrocytes buffer and redistribute K+ ions, maintaining ionic balance.

  2. Removal of Glutamate

    • Astrocytes remove 80% of extracellular glutamate (a major excitatory neurotransmitter) using excitatory amino acid transporters (EAAT).

  3. Supply Glutamine to Neurons

    • Convert glutamate to glutamine, which is non-toxic and can be transported to presynaptic terminals for conversion back to glutamate.

  4. Metabolic Support

    • Take nutrients from blood, converting glucose to lactate for neurons.

  5. Inter-Astrocyte Communication

    • Astrocytes communicate through Ca²+ mobilization and gap junctions, potentially coordinating activities within the brain.

  6. Receive Messages from Neurons

    • Though not electrically excitable, they possess various neurotransmitter receptors that enable them to respond to neuronal signals.

  7. Send Messages to Neurons

    • Release gliotransmitters (e.g., glutamate, ATP) impacting neuronal communication positively and negatively.

Oligodendrocytes

  • Defined by Pio del Rio Hortega's new staining technique.

  • Myelination:

    • Each oligodendrocyte can myelinate 20-60 axons in the central nervous system (CNS) vs. Schwann cells in the peripheral nervous system, which myelinate 1 axon each.

Conductance Speed

  • Diameter: Larger diameter axons transmit signals faster due to lower axial resistance.

  • Myelination: Enhances conduction speed by reducing ion leakage.

  • Myelination allows for the miniaturization of axons, enabling high-speed conduction without requiring immense axon diameters.

Microglia

  • Act as the immune cells of the brain, equipped to respond to environmental threats.

  • Activation States:

    • M1 Activation: Pro-inflammatory, involves releasing signals to activate and recruit other immune cells, potentially causing cell death.

    • M2 Activation: Anti-inflammatory, promotes regrowth and return to homeostasis.

  • Aging impacts microglia, changing them to become more static and hyperactive, leading to reduced phagocytic capability and increased disease incidence.

Implications and Applications

  • Understanding glial cells and their functions help link to health issues such as multiple sclerosis and neurodegenerative diseases like Alzheimer’s and Parkinson’s.

  • The evolving knowledge about the role of glia underscores the importance of continuous research and the development of methodologies to further dissect their contributions to neuroscience.