Central Nervous System Injuries

"In many ways the results which were obtained with animals has been misleading, because in the case of quadrupeds the physiological mechanisms are different, so that the kinds of data obtained from different systems - circulatory, the blood pressure and so forth, respiratory, the cardiac- are different from those obtained from human head injuries."---Paul Carrao, M.D., former head injury researcher with the U.S. Navy, analysing the head injury experiments on baboons conducted in the laboratory of the University of Pennsylvania in the 1980s.

The spinal cord is the major bundle of nerves that carries nerve impulses to and from the brain to the rest of the body and severe injuries in the thoracic region usually affect the chest and the legs and result in paraplegia whereas cervical injuries usually result in quadriplegia. Spinal cord injuries can result in a severe motor handicap and efforts have been made in the past decade in Canada to find therapeutic approaches in order to manage spinal cord injuries. Animals have been used to test these new avenues. With a great deal of creativity and resourcefulness, spinal cord lesions were made in several ways in an attempt to reproduce the condition observed in humans. However, this is a difficult task because spinal cord injuries depend on many clinical criteria, such as segmental level, complete or incomplete injury and mechanisms leading to injury and cross-species obstacles. For example, in most animal models, researchers used a posterior approach through a laminectomy (i.e. the spinal cord is dorsally exposed after drilling in the vertebral column a hole), then a weight is dropped on, a scalpel sections through, or a clamp crushes the spinal cord of the animal. Then, the animal becomes paraplegic after recovering from the anesthesia. Rats, cats are the species of choice for that sort of experimental procedure.

When modern experimental spinal injury started more than 80 years ago, dogs were used by Allen AR who then substituted the weight-drop system, through which a lesion was created, for the method of cerebral dislocation, in which the neck of the dog was broken.(2)The shortcoming of the method is that most human spinal injuries involve anterior compression rather than posterior, and therefore this model does not reproduce the human spinal cord injury. Also, the great number of species used in research has made comparison difficult and created confusion. In contrast to mammals and birds, studies indicated that regeneration of lesioned nerves could occur in the spinal cord of fish, amphibians, a situation that was not understood, let alone the assumption made that lower vertebrates seemed to have an evolutionary advantage in their capacity of self-regeneration. In order to study the regrowth of spinal cord nerves, transplants of different materials were used to bridge the two stumps of the spinal cord of experimental animals. Grafts of peripheral nerves were implanted in the tract of injured nerves in different regions of the brain and in different species. By and large, it was observed that some regenerating nerve fibers could reach the graft, create a few synapses within the host tissue, leading to a poor functional recovery.

In addition to the use of other materials such as collagen, hydrogels, grafts of glial cells, Schwann and olfactory ensheathing cells are currently used in rat models of spinal cord injury.(3)A cell suspension is generally injected in a thoracic region where a transversal segment of the spinal cord was removed, leaving the animal unable to use the hindlimbs. Not only the cell migration to the site of injury is investigated because the migratory properties of cells is important to study, but also the ability to accompany the growth of axons, and the in situ delivery of some neurotrophic factors as well. A few reports are published every year stating the merit of one particular experimental procedure or announce the breaking news that one rat was able to climb a scale during a test. It was one rat, among hundreds of other rats that were unable to recover motor functions after transplantation.(4)

Studies on rodents have shown that they have a capacity of self-regeneration that humans do not have for unknown reasons. Furthermore, of the many compounds tested on animals, many were not properly evaluated and the data was obtained from different species under different experimental conditions. Also, behavioral assessment of functional recovery in rodents is difficult if not impossible to make and without relevant anatomical and histological data, such assessment has little value. Among the few promising compounds issued from animal experimentation, such as naloxone, dizocilpine and steroids that made it to the clinical level, they did not improve the motor functions and had important side effects. It is estimated that only 10% of surviving or reconnected axons are necessary to re-establish limited and partial locomotion functions. That is the reason why spinal cord injury patients who retained only a small proportion of their axons can be trained in rehabilitating facilities. But in severe cases, rehabilitation will never be enough to help these patients walk and enjoy a normal life. Spinal cord injuries although treated with drugs such as methylprednisolone and despite the current controversy over its use-since prolonged administration reduces the tissue repair-have no cure. Scientific tests continue to generate a great deal of animal data while anxious patients are waiting a cure.

In experimental animals, it is possible to remyelinate axons of the spinal cord by transplanting glial cells, but the question as to whether this procedure is adequate and safe in humans is not answered yet. Some cells have an unusual ability to remyelinate axons. Will they make a candidate for transplantation in multiple sclerosis patients, whose myelin degenerates? These cells would have to divide, which they do not do in adults or collected in high quantity, which is not feasible. Results from animal studies amount to an accumulation of data and the formulation of concepts, which are often impossible to validate in the clinic. The final conclusion will be drawn from the experiments conducted in the patients or healthy volunteers if they are carried out. Even though some results may prove satisfactory in rodents or primates, such results may not be the same in humans due to cross-species variations. The first trial in humans will be actually the very first experiment and unfortunately the volume of information gathered through animal research, though publishable and exciting, is no guarantee of further success. This is why it becomes important to augment the probability of success by studying human models, that most closely resemble the human condition.

Remyelination by cell graft in animal models can be enhanced by creating artificial conditions such as X irradiation of the tissue to kill all the host cells; such experimental conditions contrast sharply with the natural development of disease and healing. Also, the remaining cell population that survives in the spinal cord will participate in the remyelination process at some extent, which makes them very useful. Furthermore, repair by such cells in a focal lesion is never fully achieved in animal models since regions of the nervous system remain devoid of myelin. Remyelination is limited to regions where cells can migrate and express a normal biology in an adequate cellular and extracellular environment. It may be difficult to correlate grafting to functional gain of motor activity. Electrophysiological activities of a set of remyelinated neurons can be recorded and show similar signals compared to control animals. However, such recordings are not enough; motor benefits can be only evaluated by behavioral studies, which are highly subjective in rodents.(5)Until now, no animal model has contributed to help our understanding of human spinal cord injuries to such extent that patients can be helped.

Another major obstacle to the use of transplantation strategies in SCI lies in the choice of the cells (e.g. olfactory ensheathing cells and oligodendrocytes), which are limited by their availability. Olfactory ensheathing cells may be harvested from the patient's own olfactory tract by surgery with minor damage, cultured in dishes and implanted into the demyelinated central nervous system of the same patients, thus reducing the risk of immune rejection. But it is unlikely to harvest a sufficient amount of cells in the hope of reversing the symptoms of the disease and restore normal motor function. Oligodendrocytes cannot be harvested because they are found in the spinal cord and studies indicate that it could be a better avenue to obtain them from the differentiation of adult stem cells.

The use of human embryos and most likely adult tissue will continue to pose ethical problems for some individuals and there isn't a wide agreement on the matter at the moment.(6)Nevertheless, it must be recognized that biomedical research would gain more relevant information through the use of human tissue and that the notion of sanctity of human tissue competes poorly with the obligation to support research in order to help those who suffer. Whatever our moral convictions, religious beliefs or level of mistrust towards the biomedical establishement, can we seriously claim to support medical research while opposing the collect, distribution and regulated use of human tissue? Accepting and financially supporting animal experiments cause the delay of medical progress. Restricting clinical research is also bad policy for medical progress.

A more balanced attitude is necessary. A look at the recent facts should compel to use more relevant methodologies, techniques and obtain principles in a different way. Obviously, a significant shift from the paradigm of animal-based experimentation is urgent.

• 1. Martin Schwab and Deborah Bartholdi. "Degeneration and regeneration of axons in the lesioned spinal cord." Physiol Rev 76, no.2 (1996)
• 2. Allen A. R. "Surgery of experimental lesions in the spinal cord equivalent to crush injury of fracture dislocation." Preliminary report. J Am Med Assoc 57:878-880 (1911)
• 3. Toshio Imaizumi et al. "Transplanted olfactory ensheathing cells remyelinate and enhance axonal conduction in the demyelinated dorsal column of the rat spinal cord." The Journal of Neuroscience. 18(16): 6176-6185 (1998)
• 4. Almudena Ramón-Cueto et al. "Functional recovery of paraplegic rats and motor axons regeneration in their spinal cord by olfactory ensheathing glia." Neuron vol. 25: 425-435 (2000)
• 5. Gilliam D. Muir and Aubrey A. Webb. "Assessment of behavioral recovery following spinal cord injury in rats." European journal of Neuroscience vol.12: 3079-3086 (2000)
• 6. Boer G. J. "Ethical issues in neurografting of human embryonic cells". Theoritical Medicine and Bioethics 20: 461-475 (1999).