MH 010 Mitochondria

Mitochondria, measuring only 0.1 to 0.2 μm in diameter, exist at the resolution limit of light microscopy. Despite this technical challenge, these organelles were successfully visualized using Iron Hematoxylin decades before the advent of electron microscopy, which confirmed their ultrastructural details.

Some formulations of iron hematoxylin stain mitochondria as dark black, thread-like, or granular structures within the cytoplasm. This staining method works through the binding of the positively charged iron-hematoxylin complexes to negatively charged phospholipids and respiratory complexes within mitochondrial membranes.

While this method has largely been replaced by mitochondria-specific vital dyes and antibodies, it still holds historical importance as one of the earliest methods to demonstrate mitochondrial distribution and morphology.

Iron hematoxylin is still used for staining nuclei and visualizing myelin, a fatty insulating sheath that covers axons of many neurons.

Small Intestine

The Small Intestine provides an excellent model for observing individual mitochondria due to the polarized organization of intestinal epithelial cells lining the villi. These absorptive cells exhibit high metabolic activity, making mitochondrial visualization particularly clear.

• Structural Organization:
  • Villi are finger-like projections extending into the intestinal lumen, covered by a single layer of absorptive epithelial cells
• Cellular Organization:
• Surface Absorptive Cells demonstrate distinct morphological polarization:
  • Basal Region: Nucleus is usually stained dark black
  • Apical Region: Faces the intestinal lumen, covered in small projections known as microvilli
    • Mitochondria appear as dark gray, thread-like structures running parallel to the cell's long axis
    • Individual mitochondria can often be resolved due to their elongated shape
  • Supranuclear Region: Golgi apparatus is visible as an unstained region adjacent to the nucleus
• Functional Correlation:
  • Reflects the high energy demands of active absorption and transport processes occurring at the luminal surface

Liver

Hepatocytes make up approximately 80% of the liver and represent one of the most metabolically active cell types in the body. These cells exhibit extraordinary mitochondrial density, making them ideal for studying mitochondrial abundance and distribution patterns.

• Structural Organization:
  • Liver is made up of lobules of radiating plates of hepatocytes surrounding a central vein.
• Cellular Organization () Toggle Label Visibility :
  • Nucleus: Remains unstained, providing central contrast
    • Nucleolus: Prominently stained, often with multiple nucleoli per cell
  • Cytoplasm:
    • Mitochondria completely fill the cytoplasm, creating a dark, granular appearance
    • Individual mitochondria are difficult to distinguish due to the high packing
• Functional Correlation:
  • Exceptional abundance reflects the liver's diverse metabolic functions, including gluconeogenesis, lipid metabolism, detoxification, and protein synthesis

Iron Hematoxylin

While understanding the underlying mechanism isn't essential for basic histology practice, the staining process is detailed below for those interested in the biochemical principles.

Iron Hematoxylin, particularly Heidenhain's iron hematoxylin, is a historical method for staining mitochondria:

• Staining:
  • Iron hematoxylin solution, often prepared with ferric ammonium sulfate ((NH₄Fe(SO₄)₂-12 H₂O) and hematoxylin, is used to stain the tissue
  • Stains not only mitochondria but also nuclei, the cytoskeleton, and other cellular components
• Differentiation:
• Differentiating solution is used to remove excess stain from non-target structures:
  • Nuclei are the last stained structures to be cleared when using a slightly acidic solution of ferric chloride (FeCl₃)
  • Cellular organelles are the last stained structures to be cleared when using a slightly basic solution containing Fe3+ (such as potassium ferrocyanide, K₃Fe(CN)₆)

The differentiation step must be carefully controlled to achieve optimal staining of mitochondria without over staining other structures.