Binocular competition does not regulate retinogeniculate arbor size in fetal monkey

Binocular interactions play a prominent role in shaping the axonal arbors of geniculocortical fibers and the arbors of Y cells in the retinogeniculate pathway of the fetal cat. Fiber interactions between the two eyes have also been suggested to regulate the formation of retinal projections to the dorsal lateral geniculate nucleus (dlgn) of the fetal monkey, but whether this reflects structural refinements of retinal arbors has not been established. To address this issue, we quantified the morphologic properties of individual fibers in two macaque monkeys at embryonic day (E) 110 and E121 that had an eye removed at E69 and E61, respectively. Fibers were labeled by DiI crystals into the fixed optic tract and were visualized by confocal microscopy. Three measurements were made: the number of branch points within the axon terminal arbor, the total arborization length, and the incidence of axonal side branches on the preterminal axon within the confines of the geniculate. There were no significant differences with respect to these parameters between the prenatal enucleates and normal monkeys of comparable age. This was the case for retinal fibers innervating the magnocellular and the parvocellular segments of the dlgn. The arbors stemming from the remaining eye were widely distributed in the dlgn, with some terminating in territories normally innervated by the other (enucleated) eye. These results lend support to the hypothesis that the expanded projection from the remaining eye to the lateral geniculate nucleus of the prenatally enucleated monkey is due to the maintenance of a contingent of retinal fibers normally eliminated by ganglion cell death.