DOLPHIN PERIPHERAL VISUAL PATHWAY IN CHRONIC UNILATERAL OCULAR ATROPHY - COMPLETE DECUSSATION APPARENT

Components of the peripheral visual pathway were examined in two bottl enose dolphins, Tursiops truncatus, each with unilateral ocular degene ration and scarring of 3 or more years' duration. In both animals, the optic nerve associated with the blind eye (right eye in Tg419 and lef t eye in Tt038) had a translucent, gel-like appearance upon gross exam ination. This translucency was also evident in the optic tract contral ateral to the affected eye. In Tg419, myelinated axons of varying diam eters were apparent in the left optic nerve, whereas the right optic n erve, serving the blind eye, appeared to be devoid of axons. In Tt038, myelinated axons were associated with the right optic nerve (serving the functional eye) and left optic tract but were essentially absent i n the left optic nerve and right optic tract. Examined by light micros copy in serial horizontal sections, the optic chiasm of Tt038 was arra nged along its central plane in segregated, alternating pathways for t he decussation of right and left optic nerve fibers. Ventral to this p lane, the chiasm was comprised of fibers from the left optic nerve, wh ereas dorsal to the central plane, fibers derived from the right optic nerve. Because of this architectural arrangement, the right and left optic nerves grossly appeared to overlap as they crossed the optic chi asm with the right optic nerve coursing dorsally to the left optic ner ve. At the light and electron microscopic levels, the optic nerves and tracts lacking axons were well vascularized and dominated by glial ce ll bodies and glial processes, an expression of the marked glial scarr ing associated with postinjury axonal degeneration. The apparent absen ce of axons in one of the optic tract pairs (right in Tt038 and left i n Tg419) supports the concept of complete decussation of right and lef t optic nerve fibers at the optic chiasm in the bottlenose dolphin. (C ) 1994 Wiley-Liss, Inc.