The development of the optic nerve in rodents.

In mammals, normal visual function depends upon both the retinotopic organization of visual nuclei and their interconnections. We have investigated in rodents some developmental mechanisms contributing to this organization. On embryonic day 14, in the rat, retinal ganglion cells first project axons through glial channels on the retinal surface before reaching the optic stalk. We suggest that the sequence in which axons enter the stalk (central before peripheral) and their prominent fasciculation impose some retinotopic order amongst the emerging optic nerve fibres. At birth (embryonic day 21) there are over 240 000 axons in the optic nerve, all non-myelinated. However, within one week, the number falls to the adult value (100 000) and myelination, complete in the adult, commences. Axons lost include some which misproject--to the opposite eye or to inappropriate parts of central visual regions. The number of surviving retinal ganglion cells depends on the amount of appropriate target tissue available. It is well established that removal of one eye early in development increases the survival of axons in the remaining optic nerve. However, in a group of adult mice with congenital unilateral anophthalmia, we counted only 21 000 optic axons in the remaining nerve compared with the 31 000 in normal mice of the same strain. Degenerating axons were observed, suggesting that the defect is not a simple developmental failure, but is associated with active degenerative processes throughout life.