The mechanism is classified in two ways it includes:
Direct Repair Pathway. 2. Indirect Repair Pathway

Direct repair pathway:
It includes supplementing endogenous neurogenesis, DA neuron differentiation (Park et  al. 2012), DA release (Rodriguez-Gomez et al. 2007; Bouchez et al. 2008), striatum reinnervation (Kordower et al. 1995) and neural trails integration (Piccini et al. 2000; Bjorklund et al. 2002).

Indirect repair system through trophic factors: Stem cells express various neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), cerebral dopamine neurotrophic factor (CDNF), nerve growth factor (NGF), or glial-derived neurotrophic factor (GDNF), and facilitate DA neuronal differentiation and maintenance. These passerby effects are mainly likely to result from grafts comprised of NSCs and MSCs (Rafuse et al. 2005; Tolar et al. 2010; Yasuhara et al. 2006; Lu et al. 2003). However, it is still hard to distinguish clearly which pathway shows a dominant role, and, as a result, it is generally assumed that both direct and indirect pathways contribute to the beneficial effects after transplantation

Treatment Efficacy from Patient Perspective:
We need to detect the outcome measures that are meaningful not only to investigators and clinicians but also to patients and for that we will work in partnership with end users in order to safeguard our key outcomes which are related to patient perspective. As a mean, we will early on establish a “user board”. An ultimate challenge is also to explicitly identify/develop tools that imitate real-life challenges (e.g. outdoor mobility) in standardized settings to certify that potential treatment effects translate into “real-world” settings. Pilot tests are then needed well advance of a clinical trial in order to determine the acceptability, probability,and reliability of such assessments. Toobin-depth investigate the patient viewpoint, qualitative methods (e.g. interviews) will be used as a complement to quantitative methods that include both unbiased measures and patient-reported outcomes measures. Taken together, our measurements will hit the difficulty of health condition such as PD by covering both functions, activities of importance in daily life and participation, i.e. “involvement in life situations”(International Classification of Functioning, Disability, and Health, ICF, WHO, 2001)

From the above theories, it has been concluded that there is still no perfect treatment for Parkinson'sdiseasehas been developing yet. however, the outcomes from the clinical trials have not been proven consistent or convincing. This may be due to a combination of factors, such as patient selection, amount and mode of tissue engraftment and the level of immune-suppression. Moreover, another side effect to be considered is GID. Fortunately, grafted tissues were unaffected by PD progression within 10 years after transplantation, so the treatment of PD with stem cell grafts is still a favorable direction. The major advantage of this strategy is the restorative and trophic abilities of the grafted cells which reach far away from drugs prescribed in a recent practice.

Another thing is the regeneration of dopaminergic neurons and the maintenance of dopamine homeostasis which are important for successfully treating this disease as the patients with PD are greatly affected by the loss of dopaminergic neuron. However, the major problem to developing therapeutic strategies for neurodegenerative diseases is the brain complexity. Due to the reason that the transplanted stem cells can redevelop neurons and glia in an optimal microenvironment and produce neuroprotective molecules, the stem cell therapy has great potential to treat PD as well as other neurodegenerative diseases.

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