By Horace Freeland Judson Jannuary 2005
Anesthetize the rat. Lay it belly down. Shave a patch along its spine and cut to the bone. Do a laminectomy, that is, take the bone off a short length of the back of the spine, exposing the spinal cord. Suspend a 10-gram rod above the spinal cord, at a height of 12.5 millimeters, or 25, or 50 millimeters. Let it drop.
The result will be a bruise, or more technically, a contusion, of the rat’s spinal cord. The bruise interrupts nerve transmission, paralyzing some muscles and blocking sensation. The location and severity of the damage will depend on the site of the blow and the height of the drop—and the consequent behavioral changes are reproducible. The procedure was developed in the early 1990s in the laboratory of Wise Young, a neurologist then working at New York University and now at Rutgers. He wanted to create a model for spinal-cord injury, in order to test and evaluate proposed treatments to repair the damage and restore some degree of function. Not long before, three scientists at Ohio State University had devised a rating scale for precise scoring of loss of function in spinal-cord injury. Young adapted the scale to his rat model, based on how well or poorly an injured creature could walk. In 1995, he showed that the behavioral rating varies in direct proportion with tissue damage at the injury site. In a recent conversation, he said, “This was the first behavioral outcome measure that correlated with morphological damage in the spinal cord.” Although no one measure is universally accepted in spinal-cord-injury work, Young said, “This comes close.”
The spinal cord is remarkably well protected, by bone and by its tough outer layer, the dura. In humans, only about 10 percent of spinal-cord injuries, caused by mishaps like a bullet through the spine, interrupt the cord completely. Ninety percent are contusions. Nerves in the adult central nervous system, including the spinal cord, do not spontaneously regenerate. Some nerves in the peripheral system, however, can—importantly, in the presence of Schwann cells, a type of cell that provides an environment favorable to new growth of nerve axons. Many attempts have been made to transplant such cells into damaged spinal cords, to promote regeneration, but they have all failed.
Enter olfactory ensheathing glial cells—bearing the hope of a way to fix, or at least to ameliorate, spinal-cord injuries. In 1984, Ron Doucette, at the University of Saskatchewan, described a new kind of cell, which he had found in the olfactory nerve and the olfactory bulb. The olfactory nerve is the only central-nervous-system nerve that continually regenerates throughout adult life. It is made up of neurons that arise in the mucous tissue of the nose and run the short distance to the olfactory bulb, one of the most primitive parts of the brain.
We sniff substances all the time that are toxic to these neurons, which die and must be replaced.