EXPERIMENTAL SYRINGOMYELIA IN THE RABBIT - AN ULTRASTRUCTURAL-STUDY OF THE SPINAL-CORD TISSUE

HYDROSYRINGOMYELIA WAS PRODUCED experimentally by the injection of kao lin into the cisterna magna of the rabbit, and the ultrastructural cha nges of the spinal cord surrounding the syrinx were investigated 2, 4, and 6 weeks after injection by transmission electron microscopy. The ependyma at the ventral part of the central canal was flat and stretch ed, whereas, in the dorsal part, it was split, and the syrinx extended through the dorsal median plane in most animals. Extracellular edema was found in the subependymal white matter and in and around the poste rior median septum. Many nerve fibers surrounding the syrinx were in v arying stages of axonal degeneration. Myelin sheaths were split, thinn ed, and completely lost in many nerve fibers. In some fibers, the axon s were totally lost, leaving the myelin sheaths as empty tubes. Astroc ytic processes containing a large number of glial filaments covered th e nerve fibers adjacent to the syrinx and partially replaced the edema tous area. The perivascular spaces were enlarged, especially near the syrinx and in the dorsal white matter. Oligodendrocytes remained undam aged, and the remyelination by oligodendrocytic processes was seen on some denuded axons. Sometimes, this further remyelination was abortive , especially where the edema was severe. The ultrastructural changes o f the neural tissue and their sequences were identical, in most respec ts, to those of hydrocephalus and noncommunicating syringomyelia. The oligodendrocytic remyelination with ongoing demyelination found in thi s model has many similarities to those in experimental hydrocephalus.