Neurological disorders can generally be divided into several types: those in which specific cells are lost over time such as Parkinson's,1
Alzheimer's disease and multiple sclerosis2
that can be classed as neurodegenerative diseases and those in which cells are lost in response to an ‘acute injury’ such as stroke, traumatic brain injury or spinal cord injury and those in which cell function is impaired but cell death may not occur such as epilepsy. The pathological characteristics of Parkinson's disease include the loss of the dopaminergic projection neurons of the substantia nigra pars compacta and the presence of α-synuclein-positive Lewy bodies, whereas Alzheimer's disease is characterized by the loss of neurons from the cortex and hippocampus and the presence of beta-amyloid plaques and tau-tangles. Multiple sclerosis involves the loss of the myelin sheath surrounding neurons and these are all progressive disorders. The more acute disorders such as stroke, traumatic brain injury or spinal cord injury involve the loss of cells in direct response to an insult such as ischemia or blunt trauma, though indirect cell loss with time also occurs. In epilepsy, cells fire abnormally which can result in seizures and changes in attention or behavior. The progressive neurodegenerative disorders also include diseases caused by a genetic mutation or deletion such as Huntington's disease, muscular spinal atrophy and Sanfilippo syndrome.
Treatments for these disorders would therefore be expected to replace the lost cells (of the substantia nigra, cortex or hippocampus), clear the pathological hallmarks (e.g. synuclein or beta amyloid deposition) or repair cell function. Alternatively, a treatment may be able to improve the localized environment to maintain the survival of cells and prevent additional cells dying by release of neurotrophic or anti-inflammatory factors. Stem cells are a potential treatment that once their full potential has been elucidated, may be capable of achieving the above therapeutic applications. In this review, we will be discussing stem cells and providing an overview of how they are developing as a therapy for neurological disorders. A number of important concepts need to be defined before they are likely to be successful and some of these are highlighted here. While much of this review focuses on adult-derived stem cells, the potential of embryonic and induced pluripotent stem cells are also relevant and touched upon, but have their own set of problems including ethical issues and possible tumorigenicity.