MH/MHS/RnD Cerebellum

Hematoxylin & Eosin

The Cerebellum is a part of the brain involved in coordinating movement and balance. Its highly organized structure contains several distinct cell types, with Purkinje Cells being among the most distinctive neurons.

The cerebellar cortex is highly folded and consists of three distinct layers:

• Molecular Layer: Outer layer sparsely populated with cells
• Purkinje Cell Layer: Single row of large Purkinje cell bodies at the interface between the molecular and granular layers
• Granular Layer: Inner layer of densely packed small neurons

Several histological techniques will be used to visualize Purkinje cells, each highlighting different cellular components and features.

• Hematoxylin & Eosin (H&E): Structure of the cerebellum and distribution of Purkinje cells
• Toluidine Blue: Abundant Nissl substance
• Golgi's Method: Visualization of random, complete neurons
• Immunohistochemistry: Presence of proteins involved in their function
• Immunofluorescence: Spatial relationships of cell populations
• Intracellular Dye Injection: Visualization of the cell body and complete dendritic tree

Hematoxylin & Eosin

Principle: Hematoxylin & Eosin (H&E) is the most widely used routine histological stain. Hematoxylin stains nuclei blue-purple, while eosin stains proteins pink-red.

Purkinje Cell Features:

• Cell Bodies: Easily identifiable due to their large size and staining intensity
• Nuclei: Significantly larger than those of granule cells in the inner layer
• Cytoplasm: Exhibits blue staining due to the abundance of Nissl substance (rough endoplasmic reticulum and ribosomes)
• Dendrites: Proximal dendrites are visible for a short distance in some cells

Limitations: It is impossible to visualize the dendritic tree of Purkinje cells in a single thin tissue section

Functional Significance: The Nissl Substance reflects the high metabolic activity and protein synthesis characteristic of these neurons

Toluidine Blue

Principle: Toluidine blue is a basic dye that exhibits high affinity for negatively charged nucleic acids. It has metachromatic properties, binding preferentially to RNA-rich structures (orthochromatic, blue) and highly polyanionic structures (metachromatic, purple-red).

Purkinje Cell Features:

• Cell Bodies: Readily identifiable due to their large size and staining intensity
• Nuclei: Large, spherical nucleus with a fine, dispersed chromatin pattern indicating active euchromatin
  • Nucleolus: Intensely stained due to ribosomal RNA involved in ribosome biosynthesis
• Cytoplasm: Exhibits granular, dark blue (orthochromatic) staining due to the abundance of Nissl substance (rough endoplasmic reticulum and ribosomes)
• Dendrites: Not visible

Functional Significance: Abundant Nissl substance and prominent nucleoli indicate high protein synthesis rates, reflecting the metabolic demands of maintaining extensive dendritic trees and synaptic connections.

Golgi's Method

Principle: This classical silver staining technique randomly stains a small number (1-5%) of neurons completely, allowing detailed morphological analysis of individual cells against an unstained background.

Purkinje Cell Morphology:

Examine the cerebellum in this brain section for stained Purkinje cells.

• Purkinje Cells 1
• Purkinje Cells 2
• Purkinje Cells 3

Key Features:

• Cell Bodies: Large pear-shaped cell bodies located at the junction between the outer and inner layers
• Dendrites: Extensively branched dendrites confined to the outer molecular layer
  • Branches often missing depending on position in the tissue section
• Axon: Single axon extends downward for a short distance in the granular layer before it exits the tissue section

Functional Significance: The complete morphology of Purkinje cells with their extensive dendritic tree and single axon is visible. The branching gradually becomes finer and is covered with 150,000 to 200,000 synaptic connections.

Immunohistochemistry

Principle: Immunohistochemistry uses antibodies to detect specific proteins or antigens in tissue sections, revealing their distribution within cells and tissues.

As an example, Heat Shock Protein Family H Member 1 (HSPH1) is a chaperone protein involved in the folding of proteins, and the degradation of misfolded proteins is particularly important for quality control in Purkinje cells.

Staining Protocol:

• Primary Antibody: Incubate with a goat antibody that recognizes HSPH1
• Secondary Antibody: Incubate with an anti-goat antibody conjugated to the enzyme horseradish peroxidase (HRP)
• Visualization: Immerse in a solution of diaminobenzidine (DAB), which is oxidized into a brown precipitate
• Counterstain: Hematoxylin to stain nuclei

Results:

• Purkinje Cells 1
• Purkinje Cells 2
• Purkinje Cells 3

Key Features:

• Dark brown staining is only found in the Purkinje cell bodies and larger dendrites
• Light brown staining of the molecular layer is non-specific

Advantages: Immunohistochemistry can be used to detect specific proteins in Purkinje cells. The commercial availability of antibodies to the most widely studied proteins enables researchers to explore many aspects of cellular structure and function.

Courtesy of Alexander E. Kalyuzhny (R&D Systems, Minneapolis, MN).

Immunofluorescence

Principle: Like immunohistochemistry, different antibodies can detect multiple proteins or antigens at the same time by using secondary antibodies conjugated to different fluorescent dyes that produce various colors.

Staining Protocol:

• Primary Antibodies: Incubate with multiple primary antibodies from different species
• Secondary Antibodies: Incubate with species-specific secondary antibodies conjugated to different colored fluorophores (green or red in this example)
• Counterstain: Nuclei with the fluorescent dye DAPI (4',6-diamidino-2-phenylindole) that intercalates with DNA

Results:

• Purkinje Cells (green): This primary antibody recognizes a protein distributed throughout the cell body and dendrites
  • Dendritic trees are stained, but can only be assigned to a specific neuron where visibly connected to their cell bodies
• Radial Glial Cells (red): Specialized non-neuronal glial cells with cell bodies in the Purkinje cell layer and extend long radial processes through the outer molecular layer
• Nuclei (blue): Densely packed small granular neurons in the granular layer

Advantages: Multicolor immunofluorescence allows for the simultaneous visualization of different cell types and their spatial relationships. It is also widely used to compare the distribution of various proteins within the same cell.

Courtesy of Thomas Deerinck (National Center for Microscopy and Imaging Research, University of California, San Diego, CA).

intracellular Dye Injection

Principle: Lucifer yellow injection combined with confocal microscopy is a powerful technique for visualizing the entire neuronal structure in three dimensions.

Experiment Protocol:

• Preparation: Thick slices of tissue are cut from a living cerebellum
• Dye Injection: Glass microelectrode is used to inject Lucifer Yellow, a yellow fluorescent dye, which fills the cell
• Confocal Microscopy: A series of optical sections through the entire depth of the labeled neuron is collected with a laser scanning confocal microscope (LSCM)

Results:

• Three-dimensional reconstruction computed from the optical sections that can be viewed from any angle

Advantages: The detailed architecture of entire neurons, like Purkinje cells, can be examined in the three-dimensional reconstruction. This allows the visualization of the entire dendritic tree, which is difficult to appreciate in tissue sections.

Courtesy of Maryann Martone and Eric Bushong (University of California, San Diego, CA).