Embryonic stem cells

Embryonic stem cells (ESCs) are derived from blastocysts. 4–5 days after fertilization, human embryos become blastocysts. Removal of the blastocyst, results in its destruction, therefore raising ethical issues (Hug., 2006). ESCs are pluripotent, which refers to their differentiative capacity. They can differentiate into more than 220 different cell types of the three primary germ layers: ectoderm, endoderm and mesoderm and have the ability to replicate indefinitely under defined conditions. This and ESC plasticity makes them potentially useful for research and regenerative medicine (Stocum, 2002). Figure 8 illustrates the process of ESC culturing to create cholinergic motor neurons that can be used for transplantation (Lindvall et al., 2004).

Figure 8 Formation of neurons from a blastocyst (Lindvall et al., 2004)

Adult stem cells

Also known as somatic stem cells (SSC) are multipotent cells that can be isolated from different sites of the body. SSCs can differentiate and proliferate by mitosis to regenerate damaged tissues. Multipotency refers to the adult stem cell's ability to generate into distinct cell types; however, research suggests that the transdifferentiation capacity of some SSC can be improved by modification of the growth medium used when cultured in vitro producing induced pluripotent stem cells (Yamanaka and Blau, 2010). Many different types of SSCs from different origins have been identified, including: mesenchymal stem cells, umbilical cord stem cells, neural stem cells, limbal stem cells and hematopoietic stem cells. Because these cells can be harvested from the patient, without ethical concerns, many researchers have focused on the SSC therapeutic potential for a variety of diseases. Yet, culturing SSCs up to the necessary numbers is still considered a challenge (Yamanaka and Blau, 2010). Figure 9 shows potential formation of neurons from cultured bone-marrow derived MSCs (Lindvall et al., 2004).

Figure 9 Formation of neurons from bone-marrow derived MSCs (Lindvall et al., 2004).

Induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) are specialized somatic cells, which have been genetically modified by injection of DNA or transcription factors to behave like ESC (Figure 10). IPSCs raise high hopes for future therapy, as they eliminate the risk of immune rejection by the patient (Mimeault et al., 2007). Table 2 illustrates potential applications of ESCs and SSCs as therapeutic agents (Mimeault et al., 2007).

However, due to the novel nature of the technology and the not well understood process of reprogramming, unfortunately iPSC therapy is only theoretical at the moment. A way to improve iPSC safety needs to be found, as sometimes after genetic modification iPSCs have been shown to form tumours (Liang et al., 2013).

Figure 10 Reprogramming adult stem cells into induced pluripotent stem cells usig Oct4, Klf4, Sox2 and c-Myc genes (Meregalli et al., 2011).

Table 2 Potential therapeutic applications of ESCs and SSCs.

- Brain and spinal cord disorders	- Hepatic diseases
- Cardiac diseases	- Diabetes
- Vascular disorders	- Lung disorders
- Eye disorders	- Hematopoetic and immunological disorders
- Skin disorders	- Gene therapy