Quantitative and morphological studies on developing optic axons in normal and enucleated albino rats.

In the albino rat, the number of optic axons increases from 400 on embryonic day 15 to reach a peak of 240000 at birth, before declining to adult numbers (100000) by postnatal day 5. Throughout the period of loss of axons there are few signs of degeneration in the optic nerve, which does not change its diameter: the decrease in density of axons is matched by an increase in the cross-sectional area of individual axons. Myelination of the initially non-myelinated axons starts on day 5, when axonal numbers stabilize. Following neonatal removal of one eye, fewer axons than normal are present in the contralateral optic nerve up to day 5. The axons removed by enucleation may be retino-retinal axons, representing up to 40% of the 83000 fibres lost between postnatal days 2 and 5. There is no increase in the numbers of optic axons in the remaining nerve in adult animals; this appears to be due to the small absolute numbers of ipsilateral axons saved by enucleation. After enucleation, axons remain clear and undergo a "watery" degeneration after initially swelling, and the removal of degenerative products is accomplished within four days.