[Axonal composition of the marginal and basal optic tracts of the grass frog]

An electron-microscopy study of the marginal optic tracts (the medial and lateral divisions) and basal optic tract was performed for the frog Rana temporaria. The resemblance between the ultrastructure and composition of the marginal tracts is revealed. The basal tract is characterized by a relatively loose structure and considerable content of the myelinated large diameter fibres. The corresponding tract contains 3900 (4.7%), 4700, (4.9%), 700 (28%) of myelinated fibres and 79 800 (95.3%), 91500 (95.1%) and 1800 (72%) of unmyelinated ones. The spectra of the myelinated fibre diameters of the marginal tract are from 0.4 to 2.6 mum with the main peak at 1.0 mum and additional peak at 1.6 mum; the spectrum of the myelinated fibre diameters of the basal tract is from 0.4 to 4.0 mum with the main peak at 1.8 mum and the small additional peak at 3.2 mum. The spectra of the unmyelinated fibre diameters of all tracts are from 0.1 to 0.5 mum; 60% of them have a diameter of about 0.2 mum. 85 days after enucleation undegenerated myelinated fibres were not found in the basal tract of the frog kept at a temperature 18-20 degrees C. The medial and lateral marginal tracts contained a significant number of the undegenerated myelinated fibres, 33% and 13%, the diameter spectra being 0.6-1.4 mum and 0.6-1.0 mum, respectively. It is possible that these fibres participate in the organization of the ipsilateral visual projection on the tectum. Unmyelinated fibres remain unchanged in all the studied tracts.