Parkinson's disease is a very common neurodegenerative disorder that tends to present late in life. It is a disease with a lifetime incidence of 2.5% and a prevalence of at least 2% in individuals over 65 - 70 years of age. Millions of humans are victims of this neurodegenerative disease . It is caused by a progressive degeneration and loss of nigrostriatal dopamine-containing neurons. Sometimes degeneration also occurs in systems of non-dopaminergic neurons . The common symptoms of Parkinson's disease are tremor, rigidity, and hypokinesia (abnormally decreased mobility). Together with these problems, various degrees of cognitive, autonomic, and psychiatric abnormalities may also be present .
The dopaminergic neurons have their cell bodies located in the substantia nigra pars compacta (SNpc). These special neurons send axons to the caudate and putamen which is collectively known as the corpus striatum. The gradual degeneration of these cells initiates a chain of disorders. Degeneration results in the ongoing decrease over time of striatal dopamine levels which is essential for normal functioning. This, in turn, produces a decline in striatal output to the thalamus. These alterations result in a decrease in cortical motor output. This decrease can account for some of the observed motor symptoms, particularly bradykinesia and rigidity, but other features such as a resting tremor probably have a largely non-dopaminergic component .
Till date, there are several treatments available for Parkinson's disease however medical science has failed to find a complete cure for this disease. Treatments involving drugs target the dopaminergic nigrostriatal pathway. Unfortunately, it is found that over time the efficacy of these medications is limited by the development of profound motor fluctuations and dyskinesias. The common drug is the oral administration of l-dopa and dopamine receptor agonists and deep-brain stimulation in the subthalamic nucleus. These treatments are effective for some symptoms, but are associated with side effects and do not stop the progression of the disease .
Therapies that completely cure patients of Parkinson's disease is yet to be found, but stem cell therapies offer exciting possibilities [1, 3]. Stem cells have the incredible potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells as long as the person is alive. To be clinically competitive, a stem-cell-based therapy must lead to long-lasting, significant improvement in mobility, ameliorate currently intractable symptoms, or counteract disease progression. Besides, to make stem-cell therapy work, dopaminergic neurons with the characteristics of substantia nigra neurons must be produced in large numbers .
Today, stem cell-based therapies might be a turning point in medical science that might represent in the near future a plausible alternative strategy for these disorders. However, it is important to consider the challenges before envisaging any human applications of stem cell-based therapies in neurological diseases. For instance, the questions such as which is the ideal stem cell source for transplantation, what are the most appropriate route of stem cell administration, and, last but not least, which is the best approach to achieve an appropriate, functional, and long-lasting integration of transplanted stem cells into the host tissue.