A theory for the coupling of ultrastructural transformations of mitochondria to the activity of glycolysis

Mitochondrial configurations in peripheral nerve suggest differential ATP production

Physiological states of mitochondria often correlate with distinctive morphology. Electron microscopy and tomographic reconstruction were used to investigate the three-dimensional structure of axonal mitochondria and mitochondria in the surrounding Schwann cells of the peripheral nervous system (PNS), both in the vicinity of nodes of Ranvier and far from these nodes. Condensed mitochondria were found to be abundant in the axoplasm, but not in the Schwann cell. Uncharacteristic of the classical morphology of condensed mitochondria, the outer and inner boundary membranes are in close apposition and the crista junctions are narrow, consistent with their function as gates for the diffusion of macromolecules. There is also less cristae surface area and lower density of crista junctions in these mitochondria. The density of mitochondria was greater at the paranode-node-paranode (PNP) as was the crista junction opening, yet there were fewer cristae in these organelles compared to those in the internodal region. The greater density of condensed mitochondria in the PNS axoplasm and in particular at the PNP suggests a need for these organelles to operate at a high workload of ATP production.

An analysis of the contribution of the preparatory technique to the appearance of condensed and orthodox conformations of liver mitochondria

The structure and volume of isolated mitochondria embedded for electron microscopy during different respiratory states were analyzed in thin sections. Three different embedding methods were compared; osmium tetroxide fixation/acetone dehydration, glutaraldehyde fixation/acetone dehydration, and glutaraldehyde fixation-osmium tetroxide postfixation/acetone dehydration. Analysis of fresh mitochondria, isolated in a sucrose medium, revealed the presence of a homogeneous population with respect to structure when any of the three methods were applied. After fixation with osmium alone, or in combination with glutaraldehyde, nearly 100% of the mitochondria were in a "condensed" conformation. Mitochondria fixed with glutaraldehyde alone resulted in a population of mitochondria that had large spaces separating the two membranes of the cristae which corresponds to the condensed conformation as observed after osmium fixation. Transfer of the mitochondria to the incubation medium led to the appearance of two classes of mitochondria with respect to size. One class had a volume close to that observed when suspended in sucrose, and another class was present that was 30-45% larger. In osmium fixed or in double-fixed preparations, these small and large classes corresponded to "condensed" and "orthodox" forms of mitochondria respectively. When glutaraldehyde was used alone as the fixative, the two size classes were also present. However, the mitochondria were homogeneous with respect to structure. In these mitochondria, the width of the space that separated the cristae membranes had become reduced when compared to mitochondria suspended in sucrose. The two size classes were also present in samples of mitochondria prepared during both states 3 and 4. State 4 conditions did not lead to any significant increase of the number of condensed mitochondria. In state 3 preparations, 65-70% of the population were condensed. The condensed and orthodox forms could be related to normal and swollen forms of mitochondria. Conditions that led to a swelling also led to an increase in the number of orthodox mitochondria in osmium-fixed material. The different appearance of the mitochondria is explained by the different conditions for fixation of the mitochondria that exist when nonswollen and swollen mitochondria are fixed. This difference is particularly crucial in the case of osmium tetroxide due to the unique way this fixative, among generally used fixatives, denatures proteins.