*2.6.1 Parkinson's disease (PD)*

The strategy of gene therapy has been applied to this disease in order to improve the advanced symptoms of PD. Gene therapy was applied to transfer 'Glutamic Acid Decarboxylase (GAD), a chemical produced by a gene into the basal ganglia. GAD showed an increased amount of a neurotransmitter called as Gamma-Aminobutyric Acid (GABA), responsible for inhibiting brain signals and decreasing activity in the nervous system Decreased GABA activity leads to certain brain-related disorders [34].

#### *2.6.2 Alzheimer's disease (AD)*

AD and other frontotemporal dementias (FTDs) are caused by the accumulation of amyloid-β peptide (Aβ) and protein tau in the brain. It is characterized by having memory loss, difficulty in learning and communicating along with the inability to organize things. The use of recombinant Adeno-Associated Viruses (rAAVs) has provided new ways for studying AD and other related neurological disorders [35]. Such strategies or approaches have added novel dimensions to medical treatments.

#### *2.6.3 Cystic fibrosis*

Cystic fibrosis is a disease known to affect the lungs primarily. Its symptoms include inflammation, airway obstruction leading to respiratory tract infection and deformity. Insertion of the Cystic Fibrosis Transmembrane Regulator (CFTR) gene directly into the epithelium cells of the respiratory tract bear the capability to lessen the symptoms but not totally cure the disease in patients suffering from cystic fibrosis [36].

#### **2.7 Cell therapy**

Cell-based therapy is one of the most important and well-known forms of all treatments in the fields of modern science & medicine. It is not only a curative option for treating deadly or threatening diseases but is also making 'Regenerative Medicine' the most vital technique in health care with the specific goal of replacing diseased cells, tissues or organs and thereby restoring their normal function(s) [4]. Over the years, there has been a gush of interests and work done in understanding the potential of stem cells. They are the cells found naturally in the living bodies, characterized by two defining properties of eternal self-renewal and the propensity to differentiate into an adult cell type. There are three main types of stem cells: Totipotent (a cell developing into a healthy organism independent of the permissive environment), Pluripotent (a cell developing into any type of adult cell) and Multipotent (a cell developing into a limited type of cell) [37].

Following is the account of different stem cells used for the treatment of various diseases:-

#### *2.7.1 Pluripotent stem cells*

Reportedly, pluripotent cells have been used successfully to treat animals per se. Animals diagnosed with diabetes are incorporated with cells containing insulin responsive to glucose levels. Additionally, the treatment of the animals suffering from acute spinal cord injury and visual impairment is performed with myelinated neurons and retinal epithelial cells, respectively. Researchers are still conducting studies with the use of pluripotent stem cells to cure several disorders such as Parkinson's disease, muscular dystrophy and heart failure.

*Two-Dimensional and Three-Dimensional Cell Culture and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.100382*
