An introduction to stem cells and their transformative potential

This is Article 1 in a three-part series on stem cells. Next article: The role of mesenchymal stem cells.

Welcome to the first article in a series of three articles about stem cells, where I will introduce stem cells and how they differentiate. 

Stem cells are a remarkable type of cells that can become other types. They are divided into two main categories: adult stem cells (ASCs) and pluripotent stem cells. ASCs can differentiate into cells of specific tissues and organs. Pluripotent stem cells can differentiate into all cells in the human body and can further be split into embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs).

ASCs are also known as non-embryonic or somatic stem cells, referring to cells that come from non-reproductive cells, not egg or sperm cells. Some examples of ASCs include mesenchymal cells, epithelial cells and skin cells. These cells are mainly used to replace and repair dead or damaged tissues and organs damaged by disease, injury or ageing. They may stay non-dividing (quiescent) but promptly differentiate in different cell types when needed.

ESCs do not come from fertilised eggs but rather from the inner cell mass of a blastocyst. A blastocyst is a group of dividing cells originating from a fertilised egg 3-5 days after fertilisation. After scientists have received informed consent, the cells are fertilised in vitro, outside a living organism, such as in a laboratory. iPSCs are created in a laboratory by mixing ASCs and ESCs. Scientists generate them by transcription-factor transduction, a type of nuclear reprogramming.

Nuclear reprogramming and stem cell differentiation

Nuclear reprogramming is when the nucleus of a cell is introduced into the cytoplasm of a new cell. The transfer results in changes in gene expression. In 2010, scientists Shinya Yamanaka and Helen M. Blau published a review of three alternative approaches in nuclear reprogramming to restore a cell's pluripotent state: nuclear transfer, cell fusion and transcription-factor transduction.

Nuclear transfer involves moving the nucleus from a specialised cell into an egg cell with no nucleus. This can be done with oocytes or fertilised eggs during specific cell cycle phases. The reprogramming factors in the egg cell activate genes in the transferred nucleus, causing the nucleus to express genes typical of embryonic stem cells. Through this process, a specialised cell can adopt the characteristics of embryonic stem cells and potentially develop into any cell type in the body.

Cell fusion is when two different cells merge to form a single hybrid cell. During cell fusion, the membranes of the two cells join, allowing their contents to mix. This merging of cells can lead to combining genetic material and cellular components from both cells. Transcription-factor transduction involves introducing specific genes called transcription factors (Oct4, Sox2, Klf4 and c-Myc) into adult cells to reprogram them into iPSCs.

Conclusion

Stem cells have a huge potential in medicine and research due to the different types having different functions. While the process of nuclear reprogramming does pose some challenges, such as the difficulty in ensuring that reprogrammed cells are safe and don't develop into tumours, ultimately, a better understanding of the mechanisms behind this process will allow scientists to leverage the potential of these cells, allowing them to be used in regenerative medicine.

Watch out for the next article in the series, where I will discuss the role of stem cells in regenerative medicine!

Written by Naoshin Haque

Related article: iPSCs and organoids