Regeneration and myelination in autonomic ganglia transplanted to intact brain surfaces

Fragments of superior cervical ganglia (SCG) from donor rats between newborn and three months of age were transplanted either into the fourth ventricle, onto the dorsal surface of the medulla or in contact with the area postrema of recipient rats aged 6--14 days (allografts) and 3--4 weeks (autografts). Except for the meninges, the entire brain surface and parenchyma was undisturbed. The regenerative capacity of the transplanted ganglia and its interaction with the brain surfaces was followed for post-operative periods between 1 h and six months. Both ependymal and glial cells reacted to the transplant even though there was no mechanical damage to the brain. Ependymal cells developed luminal fronds that projected into the ventricle and the subpial glia displayed a very subtle gliosis in the form of thin multi-laminated processes. Schwann cells from the transplant tended to cover the free surfaces of the brain. The transplants, often incorporated into the stroma of the choroid plexus, received an extensive vascular supply of both fenestrated and non-fenestrated vessels. In contrast to SCG in tissue culture, the perinatal explants quickly degenerated while all those from older donors, at least 3--4 weeks of age, regenerated briskly in the ambient cerebrospinal fluid. Thriving SCG neurons, which diminished in number over time, sprouted numerous neurites as early as one week; growth cones and synaptic contacts between cell processes were still evident at six months. The trasplanted mature SCG fragment underwent a redevelopment after an initial period of degeneration. It seems likely that the survival of the allografted ganglion cells depends on their acquisition of a target site in their new environment. By four to six months many axons became enclosed by myelin produced by SCG Schwann cells that normally do not form myelin in situ. Other Schwann cells appeared reactive in that they had a great increase in cytoplasmic filaments and formed gap junctions, two characteristics of C.N.S. astrocytes. It is possible that the proximity to the C.N.S. changes the character of certain Schwann cells or, alternatively, resulted in the migration of glial cells out of the brain. If the glial cells have migrated into the transplant, they may support alien neural tissue. This system in which the transplantation site is easily accessible with a minimum of trauma could lend itself to the study of some underlying mechanisms of the growth and regulation of both central and autonomic neurons and their supporting cells.