## 1 Introduction  
Neurodegenerative disorders (ND) are characterized by the progressive loss of structure or function of neurons, resulting from degeneration of selected neurons in the central nervous system (CNS). Neurological diseases have serious economic and societal impacts on patients, their family and society as a whole. There are no treatments for neurodegenerative diseases and the currently used medicines can only reduce the symptoms or slow down the progress of disease [1]. Successful design of therapies for a patient population needs careful consideration involving collaboration between clinicians, neuroscientists and bioengineers to cover both the disease aspects and clinical requirements [2]. The perception of neurogenesis have been developed during the recent cascades and the traditional concept of a static brain has been drastically altered with discovery of the presence of adult-born neurons and observation of the dynamic proliferation of progenitor cells and generation of new neurons [3]. Stem cells--which can be derived from many sources--have the potential to self-renew and differentiate to different cell types and are suitable candidates for cell therapy purposes [4]. The main purpose of cell-based regenerative therapy in the CNS is to improve neuroprotection, compensate for loss of cell function and enhance the ability to repair tissue. Cell therapy for CNS consists of cell injection into an injured brain tissue to retrieve a loss of neuron function [5]. In recent years, the application of stem cells in cell therapy for neurodegenerative diseases has attracted great interest in scientific societies [6]. Stem cells have great potential in inducing neuroprotection in a variety of neural diseases or brain injuries [7]. There are several reports of the beneficial effects of stem-cell transplantation on improvement of sensory motor and cognitive functions in stroke, Parkinson's disease (PD), Huntington's disease, Alzheimer's diseases (AD), amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy [8]. The quality of life has recently improved due to the development of science technologies and the discovery of novel methods such as cell therapy to treat degenerative disorders [1]. Despite promising results in preclinical trials, currently there is no stem-cell-based therapy. Furthermore, the application of stem cells on a clinical scale are limited due to their safety and ethical issues. Some of these concerns include tumorigenesis, stem-cell metastasis, unwanted differentiation, vital organ sequestration, irreversibility of treatment and long-term survival of transplanted cells [9; 10; 11; 12]. For addressing some of these challenges, combination of stem-cell therapy with other technologies can be a leading solution. Nanotechnology has the potential to collaborate with stem-cell therapy and improve the efficiency of cell-based therapy thanks to the unique characteristics of nanomaterials [13]. Moreover, the marriage of these technologies can form a novel interdisciplinary field with an area of intense research [14; 12]. Nanoparticles (NPs) and nanomaterials (NMs) can interact with pronerogenic factors within the stem-cell niche and thus, promote self-renewal, proliferation and differentiation of endogenous and exogenous neural stem cells (NSCs) [15]. Moreover, super-paramagnetic NPs labeled with functional peptides can be intravenously injected into injured area and significantly detected by MRI techniques [16]. The internalized modified-NPs would efficiently enhance neurogenesis and appears to be a promising approach for therapeutic purposes and drug delivery in the treatment of neurodegenerative diseases. In this review, we will provide an assessment of the recent advances in stem-cell therapy and the applications of nanotechnology in cell-based methods to boost the efficiency in the treatment of neurological diseases.