A biphasic sequence of myelination in the developing optic nerve of the frog

We have examined the sequence of myelination along the optic nerve of the frog Litoria (Hyla) moorei from early tadpole life to adulthood. Myelinated axons were counted in electron micrographs of transverse sections taken from behind the eye, at the optic foramen and the chiasm. In tadpoles, myelinated axon numbers were significantly higher at the foramen than at the other levels. By metamorphic climax, numbers had risen at all three levels but more so behind the eye and at the chiasm to become approximately equal along the nerve. After metamorphosis, there was a dramatic increase in myelinated axon numbers, but another pattern was seen; in frogs of 5 cm and 7 cm body length, counts were significantly higher at the chiasm than at the foramen and lowest behind the eye. Thereafter, myelinated axon numbers stabilized at the chiasm but increased behind the eye and at the foramen so that in the most mature stage for this species, 9 cm adults, counts were again similar at the three levels. In addition, total axon numbers, that is, myelinated plus unmyelinated, were assessed from electron micrographs and increased from approximately 8,500 in early tadpoles to 0.65 million in fully mature adults. The proportion of axons that were myelinated showed two peaks, one before and the other after metamorphosis. Measurements of axon diameters from electron micrographs suggested that there was a critical diameter for myelination of 0.3 microns before, and of 0.5 microns after metamorphosis. The data indicate that there is a biphasic sequence of myelination of optic axons, the first phase being pre-metamorphic and the second post-metamorphic. The first phase is initiated at the foramen, and then extends both towards the eye and chiasm and continues until metamorphic climax. During the second phase, myelination originates at the chiasm, spreads towards the eye, and is complete only in the most mature adults. The critical diameter for myelination is smaller in the first phase than in the second.