Cytochrome oxidase (cox) is an oxidative enzyme in the electron transport chain. Staining for cox is a sensitive method to assess capacity for oxidative metabolism.
Type I muscle fibers have a higher capacity for oxidative metabolism than type II cells, which have a higher capacity for anaerobic metabolism. This means that the type I muscle cells will stain darker on cox, but should still show some uptake of the stain.
If a third population of very pale fibers is present, this may indicate a subset of fibers which have little or no capacity for oxidative metabolism, suggestive of a metabolic disorder.
