The optic nerve of the cat: appearance and loss of axons during normal development.

The number of axons in the optic nerve has been estimated in cats ranging in age from mid-gestation to adulthood. At mid-gestation the number of axons present in the nerve (218,000) already exceeded adult levels. The number of axons, nevertheless, more than doubled over the next 10-20 days, reaching a maximum of 450,000-483,000. In the last prenatal week and the first postnatal week, the number of axons declined rapidly, stabilizing at adult levels 2-3 weeks after birth. Myelination began just before birth and reached adult levels (over 95% of axons myelinated) 6-8 weeks after birth. Several mechanisms which may underlie the loss of axons are discussed.