Fiber order in the opossum's optic tract.

The distribution of axons by size in the optic tract of the South American opossum, Didelphis marsupialis was studied. Thin and semi-thin sections were examined, and measurements of axonal diameter were made on electron micrographs taken from various locations across the optic tract of normal opossums. In order to determine the contributions of the different axon diameter classes to the crossed and uncrossed retinofugal pathways, measurements were also made from the tracts of opossums in which one eye had been enucleated 5 weeks previously. Within the opossum's optic tract, the axons are partially segregated by their size: the deepest parts of the tract contain only fine and medium-sized axons, whereas coarse axons are also present superficially. In the middle of the tract, all three size classes are present. At increasingly superficial positions, there is a steady reduction in the proportion of medium-diameter axons, and an increase in the number of the finest axons. Medium and coarse axons contribute to both the crossed and uncrossed pathways, and the uncrossed component is displaced superficially relative to the crossed component. The fine axons in the deeper parts of the tract arise from both retinae, while those in the superficial parts of the tract, near the pial surface, are virtually all crossed. The opossum's optic tract thus displays the segregation of axons by size found in placental mammals, and follows a pattern reminiscent of that found in carnivores. Such a common organizational plan, particularly the similarities between the didelphids and carnivores, is suggestive of an early acquisition of parallel visual pathways in mammalian phylogeny. Since the fiber order in the optic tract of eutherians is a chronological map of axonal arrival during development, these results suggest that a conserved developmental mechanism has led to a common organizational plan.