Ultrastructural observations of organelle accumulation in the equine recurrent laryngeal nerve.

The research examines nerve damage in horses that often goes undetected (sub-clinical neuropathy) by taking a close look at the changes in the structure (ultrastructural observations) of the affected nerves, specifically the left recurrent laryngeal nerve. The study particularly focuses on how cellular components (organelles) accumulate in these nerves and the possible impact of this accumulation on nerve function.

Observing Accumulation of Organelles

• The research aims to look at the build-up of organelles in nerve cells. Organelles are the specialized parts of a cell that perform specific functions, like mitochondria which produce energy, or lysosomes which break down waste. This build-up is observed at two parts of the nerve cells: the paranodal (around the nodes of cells) and internodal (between the node of one cell and the node of the next) locations.
• These clusters of organelles are seen in scattered fibers that show split myelin sheaths. Myelin sheath is a layer that protects nerve fibers and promotes efficient transmission of signals. If this sheath splits or gets damaged, it can disrupt communication between cells.
• Upon closer examination, these clusters of organelles are found to result from a protrusion from the nerve cell, which is filled with dense lamellar bodies (a type of organelle) and mitochondria.

Demyelination and Lysosomes

• These organelle clusters are often associated with infoldings of the terminal loops of the myelin sheath and occasional paranodal demyelination (loss of myelin). The study also confirmed the presence of lysosomes, a type of organelle involved in waste disposal, within these protrusions, as indicated by their positive staining for cathepsin D activity, an enzyme associated with lysosomes.

Swollen Axons

• The study also found axons (nerve fibers) that were swollen over a long distance and also had clusters of organelles. There was a clear separation in some places between these organelles and the proteins that make up the cell’s structure (cytoskeletal proteins).
• Interestingly, these swollen axons were surrounded by the nuclei and processes of Schwann cells, cells that produce the myelin sheath, forming organized, onion-like structures around the axons. These “onion bulbs” are characteristic of certain types of nerve damage.

Implication of The Findings

• The researchers discuss the possibility that these observed changes – the paranodal accumulation of organelles and the swollen axons – may be due to a disturbance in the transport of materials within the nerve cells (axonal transport). Axonal transport is crucial for the normal function of nerve cells, as it carries materials to and from the cell body.