Development of the optic nerve of the opossum (Didelphis virginiana).

The development of the optic nerve of a marsupial, the North American opossum, was examined in 24 animals from postnatal days 5 to 78 (P5-P78): gestation is 13 days. The estimated number of axons increased from 24,000 at P5, to 267,000 at P27, approximately 2.7 times the mean number in the adult. Following P27, axon numbers decreased rapidly to 140,000 at P40, then decreased more slowly, attaining adult values between P50 and P59. Thus, the opossum is similar to placental mammals examined in evidencing an overproduction and later attenuation to adult values in the number of axons in the optic nerve during development. Monocular enucleation of 3 animals at P17, 10 days before peak axon counts, resulted in a mean population increase of 24,000 (range 19,000-30,000) above the normal adult mean. Additionally, a 4th animal monocularly enucleated on P7, 3 days prior to the arrival of migrating fibers to central target sites, had a similar value of 26,500 supernumerary axons. Our findings in the opposum, when coupled with previous reports in other mammals, suggest that binocular interactions during development account only for optic nerve axon loss approximately equal in magnitude to the ipsilateral projection from one eye.