Course: SAT 170 Introductory Biotechnology

Unit 8 Reading and Multimedia Assignment

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“Stem Cells,” by Boundless, CC BY SA 3.0

https://www.boundless.com/biology/biotechnology/stem-cells-and-cloning/stem-cells/

Reading Assignment: Stem Cells

Scientists hope that one day therapeutic cloning can be used to generate tissues and organs for transplants. To do this, DNA would be extracted from the person in need of a transplant and inserted into an enucleated egg. After the egg containing the patient's DNA starts to divide, embryonic stem cells that can be transformed into any type of tissue would be harvested. The stem cells would be used to generate an organ or tissue that is a genetic match to the recipient. In theory, the cloned organ could then be transplanted into the patient without the risk of tissue rejection. If organs could be generated from cloned human embryos, the need for organ donation could be significantly reduced.

Stem cells are biological cells found in all multicellular organisms that can divide and differentiate into diverse specialized cell types. Stem cells can also self-renew to produce more stem cells. In mammals, there are two broad types of stem cells: embryonic stem cells that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.

The classical definition of a stem cell requires that it possess two properties. The first is self-renewal — the ability to go through numerous cycles of cell division while maintaining the undifferentiated state. The second is potency — the capacity to differentiate into specialized cell types. In the strictest sense, this requires stem cells to be either totipotent or pluripotent (to be able to give rise to any mature cell type), although multipotent or unipotent progenitor cells are sometimes referred to as stem cells.

Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell. Totipotent stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism. These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent. Pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, (i.e., cells derived from any of the three germ layers).

A number of adult stem cell therapies already exist, particularly bone marrow transplants used to treat leukemia. In the future, medical researchers anticipate being able to use technologies derived from stem cell research to treat a wider variety of diseases. However, there still exists a great deal of social and scientific uncertainty surrounding stem cell research, which could possibly be overcome through public debate, future research, and further education of the public.

The limitation of adult stem cells is that they can only develop into a few cell types. For this reason, they are not as useful as embryonic stem cells, which can develop into many different cell types under the right conditions. Embryonic stem cells (ES cells) are pluripotent stem cells derived from the inner cell mass of the blastocyst, an early-stage embryo. Isolating the embryoblast or inner cell mass results in destruction of the fertilized human embryo, which raises ethical issues.

In 2007, scientists took skin cells from mice and added viruses containing four specific genes. That caused the skin cells to be reprogrammed to behave like embryonic stem cells. Since this time, cells from many organisms, including humans, have been reprogrammed in a similar fashion. These reprogrammed cells are called induced pluripotent stem cells, or iPS. iPS are not adult stem cells, but rather reprogrammed cells (e.g., epithelial cells) given pluripotent capabilities. Using genetic reprogramming with protein transcription factors, pluripotent stem cells equivalent to embryonic stem cells have been derived from human adult skin tissue.

Two major avenues of research are being followed with respect to iPS. One of these avenues is the conversion of diseased cells back to iPS, therefore becoming a model system to study the disease. Secondly, these cells have the potential to be used to replace damaged or diseased cells in humans. In this method, an adult cell such as a skin cell could be taken from a patient, converted to an iPS cell, and then reprogrammed to develop into a specific type of cell necessary for treating a disease (see above figure).

Many challenges must be overcome before "cloned organ" transplants become reality. More effective technologies for creating human embryos, harvesting stem cells, and producing organs from stem cells would have to be developed.