Effect of acclimation temperature on the axon and fiber diameter spectra and thickness of myelin of fibers of the optic nerve of goldfish

The optic nerves of common goldfish acclimated to 5 and 25 degrees C were fixed with glutaraldehyde in either phosphate buffer or PIPES with EGTA, post-fixed with osmium tetroxide, and examined by electron microscopy. The axon diameter spectra, from axons measured in electron micrographs and those measured on the electron microscope screen, differ noticeably with acclimation temperature. At the lower temperature, there is a definite shift toward the occurrence of larger fibers compared with the spectrum of the 25 degrees C fish. Although the number of fibers assessed is small compared with the total number in the goldfish nerve, these results confirm our previous study. These findings could be attributed to an increase in the number of new fibers during the acclimation to the higher temperature. We discuss this possibility and on the available evidence find it unlikely. Other changes in the axon and fiber are also seen with acclimation temperature. The axon to fiber diameter ratio, made directly from the electron micrographs, shows that axons from the nerves of the higher acclimation temperature fish possess consistently thicker myelin sheaths than are found for axons in nerves of the lower temperature fish. This finding is also in agreement with results obtained by us from measurements independent of each other.