Spinal cord grafts: an intraocular approach to enigmas of nerve growth regulation

The possible usefulness of intraocular transplantation in studies of spinal cord growth and regeneration has been evaluated. Defined segments of fetal rat spinal cord were grafted to the anterior chamber of the eye of adult rats. Such grafts become vascularized from the host iris, grow and develop neuron types, myelinated fiber bundles, astroglial populations (as shown by GFA-immunoreactivity), and electrical activity reminiscent of such features in normal spinal cord tissue. The intraocular technique permits studies of intrinsic circuitries as well as conditions for formation of afferent and efferent connections with the host iris and with other central or peripheral tissues which can be grafted into contact with the spinal cord grafts. One example of an intrinsic system preserved in the grafts is a rich network of nerve fibers with enkephalin-like immunoreactivity. When combined with cerebral cortex, the enkephalin-positive neurons of the spinal cord graft are able to form only very limited projections to the cortex graft. Special emphasis was given the possible formation of adrenergic afferents to spinal cord grafts. No appreciable ingrowth of peripheral sympathetic nerves occurred. Locus coeruleus grafts have many organotypical electrophysiological characteristics and were able to innervate adjacent spinal cord grafts provided that the sensory innervation of the host iris was removed. Experiments such as these suggest that "negative neurotropic" factors may be present in spinal cord and possibly relate to the unique relationship between spinal ganglia and spinal cord.