Stem Cells

Figure 1. Representing the main types of stem cells, pluripotent embryonic stem cells, multipotent adult stem cells, and induced pluripotent stem cells (Naqvi & McNamara, 2020).

What Are Stem Cells?

Stem cells are the building blocks of all tissues of the body. They contain two key properties that differentiate them from all cells of the body (Slack, 2012). Differentiation is the process by which an unspecialized cell undergoes maturation to become a specialized cell, while self-renewal is the ability of the cell to undergo unlimited cell division that produces cells with the same abilities as the parent cell (Slack, 2012). Note that self-renewal is different from cell proliferation which is the process by which all cell types divide as cell proliferation is finite and as the cell divides it does not retain the developmental abilities of the parent cell (Kolios & Moodley, 2013).

Figure 2. Representing the embryonic stem cells (top of the figure) which differentiate into multipotent stem cells and finally differentiate into specialized cells such as bone cells, muscle cells, and others. Differentiated cells at the multipotent stem cell stage are only capable of further differentiating into their respective lineages, meaning that if a pluripotent stem cell differentiates into a multipotent stem cell at the mesoderm layer it is not capable of differentiating into other germ layers such as endoderm or ectoderm (Vogel et al., 2019).

Stem Cell Classification

While all stem cells share the two key properties that differentiate them from other cell types, there is another factor that differentiates one type of stem cell from another: Not all stem cells contain the ability to differentiate into any cell type, rather, Stem cells are divided into two groups those that are pluripotent (capable of differentiating into any cell type) and those that are multipotent (capable of differentiating into any cell type of a specific tissue group) (Kolios & Moodley, 2013).

Pluripotent and Multipotent Stem Cells

Pluripotent stem cells (PSCs) also known as embryonic stem cells have been characterized by their potentiality of unlimited self-renewal and the ability to differentiate into all different cell types of endodermal, mesodermal, and ectodermal origins (P. De Miguel et al., 2010). PSCs are created following the process of fertilization and the development of the blastocyst. The inner cell mass of the blastocyst contains the pluripotent stem cells as seen in Figure 1 (P. De Miguel et al., 2010).

Multipotent stem cells (MSCs) also known as "adult" or "somatic" stem cells are similar to pluripotent stem cells in that they are capable of renewal and differentiation (Vogel et al., 2019). However, MSCs differentiation ability is limited to the type of tissue in which the adult stem cell resides. For example, mesenchymal stem cells are multipotent stem cells capable of only differentiating into mature cells of several mesenchymal tissues. Hematopoietic stem cells are also multipotent stem cells that are only capable of differentiating into cells of the blood system (Vogel et al., 2019).

Figure 3. Representing the process of generating induced pluripotent stem cells. The process begins by isolating somatic cells most often skin cells derived from the ectoderm. Using transcription factors that are responsible for differentiation and proliferation, researchers reprogram these somatic cells into induced pluripotent stem cells. These induced pluripotent stem cells are then used in a variety of therapies such as cancer research and drug development (Papapetrou, 2016) .

Induced Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) are not normally found in nature. Through genetic reprogramming and forced expression of genes and factors specific to embryonic stem cells, researchers have been able to reprogram a differentiated adult somatic cell into an induced pluripotent stem cell-like state seen in Figure 3 (Naqvi & McNamara, 2020).

Stem Cell-Based Therapies

Over time, stem cell-based therapies have gained enormous popularity in the field of biology. The differentiative ability of induced pluripotent stem cells have proven extremely beneficial to cancer research, drug development, disease prevention, and cellular regeneration (Papapetrou, 2016). The infinite proliferative activity of iPSCs resembles the ability of cancer cells to continuously divide and so researchers are able to use iPSCs to study any form of cancer. Furthermore, the proliferative ability of iPSCs renders them extremely useful in understanding disease pathology as iPSCs are capable of providing a human model of any disease (Kolios & Moodley, 2013).

Limitations and Ethical Issues of Stem Cell Research and Therapies

Stem cell research provides a great promising understanding of the basic mechanisms of human development and differentiation, as well as avenues for new treatments of diseases such as diabetes, spinal cord injury, Parkinson’s disease, and myocardial infarction (Lo & Parham, 2009) . However, there has been a rise of ethical and political concerns in human stem cell (hSC) research. Deriving pluripotent stem cell lines from oocytes and embryos is charged with disputes concerning the onset of human personhood and human reproduction (Lo and Parham, 2009). The reprogramming of somatic cells to produce induced pluripotent stem cells (iPSC cells) prevents the ethical problems specific to embryonic stem cells. However, with any hSC research, there are difficult dilemmas, including consent to donate materials for hSC research, early clinical trials of hSC therapies, and oversight of hSC research (Lo and Parham, 2009). Moreover, the medical risks of retrieving oocyte include ovarian hyperstimulation syndrome, bleeding, infection, and complications of anesthesia. These risks may be reduced by excluding donors at high-risk for these complications, careful monitoring of the number of developing follicles, and adjusting the dose of human chorionic gonadotropin administered to induce ovulation or canceling the cycle (Lo & Parham, 2009). Also there has been reported concerns in Genomic instabilities and epigenetic variations of iPSCs, such as aneuploidy, subchromosomal copy number variations, single nucleotide variations, variations in X Chromosome inactivation , and aberrant DNA methylation (Kim & Park, 2017). Such variations can affect the properties of iPSCs and undermine their use in cell-based regenerative medicine iPSCs has some ethical and political issues due to the risk of teratoma formation after transplantation (Kim & Park, 2017). Moreover, Human Mesynchymal Stem cells (hMSCs) are characterized by a limited lifespan in an in vitro culture condition. The short life span can alter various cell functions including proliferation, differentiation, and migration, that are essential for their therapeutic efficacy (Kim & Park, 2017).

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