Morphometric analysis of myelinated fibre composition

In a study involving 50 optic nerves isolated from 3 different strains of adult male mice, C57BL, CBA and (C57BL x CBA) Fl hybrids, and from adult female CBA strain mice, we observed that the mouse had a lower mean total myelinated nerve fibre count than other mammals such as the rat, cat, rabbit, monkey and man where similar information was available from the literature. The nerve fibre spectrum, however, which mostly consisted of small diameter fibres, was similar to the distribution seen in these other species. The largest myelinated nerve fibres observed in any of the strains of mice investigated had a diameter of not more than 1.92 gm. The C57BL optic nerve had the largest population of large diameter fibres, while the Fl had the largest population of small diameter fibres. In all the strains of mice investigated, the distribution of nerve fibres was unimodal, with a modal diameter of 0.48 gm. The mean nerve fibre diameter was 0.62 +0.02 pm (S.E.M.), 0.57+0.03 jim and 0.55+0.01 jim for C57BL, Fl and CBA, respectively. The Fl had the lowest population of fibres around the modal diameter. The myelinated nerve fibres were most densely packed in the CBA strain of mice, whereas the C57BL was the least densely populated. There was a significant interstrain difference in the parameters measured between the 3 strains of mice studied, whereas there was no significant intrastrain difference.