Cotransplantation of embryonic mouse retina with tectum, diencephalon, or cortex to neonatal rat cortex

Retinae from embryonic mice were transplanted to the occipital cortex of neonatal rats together with their normal target regions, tectum or diencephalon, from embryonic mice or rats. In control experiments, retinae were cotransplanted with embryonic rat occipital cortex. In over 80% of the experimental animals, both transplants differentiated and grew. Ganglion cells in the retinae cotransplanted close to tectum or diencephalon survived for at least 15 weeks. Their survival was associated with the development of a distinct optic fiber layer and outgrowth of axons from the transplanted mouse retina. Specific innervation of distinct patches within the cotransplanted rat tectum or diencephalon was demonstrated by the use of an anti-mouse antibody. The innervated regions, which could be as far away as 1.3 mm from the retinae, were correlated with cytological features of the cotransplanted tectum or diencephalon. By contrast, the host cortex was never innervated by the transplanted retinae. In the control animals in which the retinae were cotransplanted with occipital cortex and in four animals in which the cotransplants lay more than 2.7 mm apart, no ganglion cells were identified and there was no evidence of an optic fiber layer, outgrowth of axons, or innervation. These results support the idea that in order to survive, retinal ganglion cells need to innervate an appropriate target region. Further, the specific innervation of regions within the cotransplanted tectum or diencephalon suggests that these target regions are able to exert a tropic influence on the axons of retinal ganglion cells, even in the absence of many of the normal structure cues.