Human Fetal Spinal Cord Xenografted to the Eye of Athymic Nude Rats: Survival, Ultrastructural Differentiation, Glial Responses and Vascular Interactions

The ultrastructure of human spinal cord xenografts transplanted to the anterior chamber of athymic nude rats was studied. The grafts displayed long-term survival and expressed several organotypical characteristics of normal human spinal cord. Three cell types of different sizes--small, intermediate, and large--and two different types of neuropil, with and without myelinated axons, were observed in the central cellular layer. The myelin-free area was formed between and round small neurons and was composed of many nonmyelinated fibers and bundles of fine axons. The hostgraft interface, especially around vessels from host iris, exhibited marked interdigitations of astroglial processes and basement membranes. Thick, superficial glial layers containing many fibrous astrocytes and many glial processes were also seen in both the intermediate axonal and central cellular layers. Thin-walled vessels were clustered in the central layer, and their perivascular spaces contained collagen fibers. No central nervous system-type vessels were observed. In conclusion, both the organotypical features of normal spinal cord and the glial responses and abnormal vascular interactions observed in this study must be considered in relation to the long-term survival and functional potential of xenografted neural tissue.