Reinnervation, myelination and organization of iris tissue implanted into the rat midbrain--an ultrastructural study.

The ultrastructure of autologous irides implanted into the midbrain of mature Sprague-Dawley rats was studied over a time period of 4-14 days. Most features of the normal iris still persisted throughout this time, including typical iris blood vessels and amelanotic melanocytes. At 5-4 days after implantation, the normal innervation of the implanted iris had degenerated, except for the presence of intact Schwann cells and some myelin debris. Reinnervation, beginning about the seventh day, proceeded rapidly. By the fourteenth day, extensive reinnervation of the implant was evidenced by the presence of numerous small (0.1-0.6 micrometer) and large (2-4 micrometer) non-myelinated axons ensheathed in Schwann cell cytoplasm. Axonal varicosities, filled either with dense core or clear core vesicles, formed junctions with axons. These junctions were characterized by an accentuation of areas of the axonal membrane and pre- or post-junctional thickenings; however, we did not observe typical synaptic complexes. Some large axons within the myelinated iris developed thick myelin sheaths of the peripheral nervous system (PNS) type; we believe this is the first reported instance in which myelination of central axons by Schwann cells within the brain parenchyma has been produced by the implantation of PNS elements.