The postnatal development of the optic nerve of a reptile (Vipera aspis): a quantitative ultrastructural study
A quantitative investigation of the optic nerves of eight adult vipers was carried out, both at the level of resolution of the light microscope by stereological methods, and at the level of resolution of the electron microscope. The majority (87%) of axons are myelinated, and the fibers are organized in fascicles; each bundle of axons is disposed around a central column of glial cells of which the greater proportion (62%) are oligodendrocytes, 33% are astrocytes and the remaining 5% are microglial cells. Estimations of the total number of fibers varied, from 5.37 X 10(4) to 6.96 X 10(4), among the eight specimens. The distribution of diameters of myelinated fibers is well characterized (chi 2 = 16.84, df = 22, 0.9 greater than p greater than 0.5) by a three-parameter lognormal distribution with a mean of 0.735 micron, standard deviation of 0.239 micron, and origin of 0.21 micron. Axon diameter is linearly related (r = 0.60) to the number of electron-dense layers in the myelin sheath. The diameters of unmyelinated fibers are distributed (chi 2 = 1.95, df = 4, 0.9 greater than p greater than 0.5) as a two-parameter lognormal distribution with mean of 0.25 micron and standard deviation of 0.067 micron. A surprisingly large number of myelinated fibers, apparently dispersed at random throughout the nerve, showed anomalies of myelination; two possible explanations, between which it is impossible to discriminate on the basis of the available data, are offered for this state of affairs.