Paranodal Schwann cell mitochondria in spinal roots of the cat. An ultrastructural morphometric analysis

We have calculated the number of paranodal Schwann cell mitochondria in adult feline ventral and dorsal lumbar spinal roots using ultrastructural serial section analysis. Distinct accumulations of paranodal mitochondria were noted in nerve fibres more than 4-5 microm in diameter. The calculated number of paranodal mitochondria increased linearly with fibre diameter from a few hundred up to 20,000-30,000 per node. A linear increase in the number of paranodal mitochondria per node also appeared as a function of nodal variables such as 'nodal axon membrane area', 'nodal Schwann cell membrane area', and 'node gap extracellular volume'. In large fibres (D = 15-18 microm), a calculated number of about 20,000 paranodal Schwann cell mitochondria were accumulated at each node of Ranvier and related to nodal axon membrane area of about 20 microm2. Our calculations indicate that, on the average, 1000 paranodal Schwann cell mitochondria with a total volume of 6.7 microm3, a total outer membrane area of 250 microm2 and a total inner membrane area of 580 microm2 projected to each microm2 of the nodal axon membrane via the nodal Schwann cell brush border.