For many years, a strong relationship between stem cells and telomeres has been studied and stablished. Stem cells are known to have elevated proliferation capabilities concomitant with long telomeres and telomerase activity, which are essential to maintain the regenerative capacity of the tissues.
The expression of telomerase is restricted in human cells, so telomeres shorten throughout our lives, providing a mechanism that limits cell proliferation. Along with the aging process, a reduction in the regenerative capacity of various tissues occurs, which also means a decline in stem cell functionality and a drop-in telomere reserve. It is known that stem cell functionality limits organ homeostasis, restricting the longevity of the organism.
STEM CELLS AND TELOMERES
Stem cells are undifferentiated cells that have the ability to differentiate into specialized cell types. There are two types:
Adult stem cells that are found in different tissues and are multipotent, meaning that their differentiation potential is limited by the tissue of origin and embryonic stem cells derived from embryos that are pluripotent, meaning that their differentiation potential is not limited and can give rise to all of the cell types that make up the body.
In specialized cells, telomerase activity is usually diminished after birth so telomere length is gradually shortened with cell divisions, and triggers cellular senescence. But in stem cells, telomerase remains active and maintains telomere length and cellular immortality.
Telomerase can prevent the loss of telomere length during DNA replication and cellular senescence in stem cells. It has been shown that telomerase controls tissue homeostasis and organism survival, which suggests that telomerase activity and telomere length can directly affect the ability of stem cells to regenerate tissues.
Furthermore, telomerase activity and telomere maintenance are also associated with the immortality of cancer cells, germ-line cells and embryonic stem cells. Moreover, cancer and ageing, two biological processes in which the role of telomerase has been implicated, are increasingly recognized as stem cell diseases. Cancer may often originate from the transformation of normal stem cells, and aging has been associated with a progressive decline in the number and/or functionality of certain stem cells.
INDUCED PLURIPOTENT STEM CELLS AND TELOMERES
Induced pluripotent stem cells (IPSC) are differentiated cells that can be reprogrammed to an embryonic-like state by transferring of nuclear contents into oocytes or by fusion with embryonic stem cells. During the reprogramming of these cells, telomerase activation and telomere length extension occur.
The fundamental role that telomerase activity and telomere length play in the generation and functionality of iPSCs, highlights the importance of telomere biology in this field. Telomere analysis could be very useful in vitro and in vivo studies related to the quality of cell-derived products and to accurately assess the differentiation potential of stem cells and iPSCs in R&D projects.