Morphometric analysis of axons myelinated during adult life in the mouse superior cervical ganglion.

In experimental studies addressing the regulation of myelin formation and maintenance by Schwann cells, the sympathetic nervous system of young adult rodents has served a key role as an essentially nonmyelinated yet modifiable control tissue. Nevertheless there is clear evidence of substantial myelination in the superior cervical ganglion (SCG) of normal mice and rats of more advanced age. Against this background, interpretation of experimental outcomes in particular sympathetic tissues will require detailed quantitative control data taking account of animal age. To provide a baseline for future investigations on myelin remodelling, an ultrastructural morphometric study of myelinated fibres in the SCG was undertaken in 4 strains (QS, Balb/C, C57 and CBA) of adult male mice aged 32-72 wk. Numbers of myelinated fibres in SCG cross-sections varied substantially between individual animals, and the mean numbers for QS (132), Balb/C (165) and CBA (254) were significantly higher than that for C57 (32). Both axonal and fibre diameter were distributed unimodally (means for the 4 strains ranged from 2.3-2.4 microns and 3.2-3.6 microns respectively). Myelin spiral length was distributed unimodally and skewed to the right (range of means = 227-357 microns) and was significantly greater in QS mice as compared with the other 3 strains. While the mean g ratio (axonal diameter/fibre diameter) was significantly lower in QS mice than in the other 3 strains, the range for mean g ratio was 0.64-0.73, indicating that myelination had proceeded appropriately even though late in onset in this tissue. The index of circularity was high in all strains, both for axons (range of means = 0.80-0.88) and fibres (range of means = 0.84-0.89). The small axonal and fibre diameter and unimodal distribution are consistent with the characteristics of autonomic myelinated fibres and it is probable that most are postganglionic sympathetic fibres arising within the SCG. In terms of providing a sufficient population of myelinated fibres for future experimental studies, the QS, Balb/C and CBA strains would be preferable to C57 mice.