Stem cells are unique cells within the body that continue to
divide an produce either additional stem cells or cells that
become specialized to function in the body.Image by
Terese Winslow and Lydia Kibiuk for Stem Cells: Scientific
Progress and Future Research Directions. Department of Health
and Human Services. June 2001.
COMMENTARY: The structure of the body is made up of by billions of individual microscopic cells that work together to complete the functions necessary for life. Many cells of the adult body are specialized to complete unique functions, and if damaged may die contributing in symptoms of disease or injury. Other cells of the body retain the potential to replace dead cells. These cells known as stem cells are a focus of scientific research and ethical debate.
Development: During normal human development, all of the adult structures are ultimately formed from the products of the fertilized egg. At the most basic level, development consists of two fundamental processes.
(1) Cells divide to create copies of themselves. The fertilized egg splits into two cells, those two cells divide into four cells, and process continues to generate the trillions of cells in the adult body. However, the body does not develop as a shifting, shapeless blob.
(2) Instead, in the process of differentiation the cells acquire unique characteristics that allow them to complete special functions within the body. As the cells are generated, they begin to become specialized with some cells becoming basic tissues such as nerves and muscles and ultimately become arranged in the creation of functional organs such as the skin or stomach.
In truth, these two processes oversimplify the process since development from a fertilized egg into an adult is an incredibly complicated series of events, many of which are just now beginning to be understood. Further complicating normal development is that some stages must be completed properly at a specific time in order for the next step to occur. Birth defects are often caused by disruption of the normal process of develop by radiation, chemical teratogens, metabolic imbalances, or viruses. An example of this is Fetal Alcohol Spectrum Disorders occur when the unborn fetus is exposed to the toxic effects of alcohol during important developmental stages. The disruption and often death of cells can cause long-lasting, growth deficiencies and deficits in brain development. Removal of the toxic agents after the damage is done does not reverse the effects and it is not currently possible to replace the lost cells at a later time.
Stem cells are the unspecialized cells capable of dividing to maintain a reserve of unspecialized cells. More importantly some of the products of stem cell division are able to follow a path of specialization to become the functional cells needed by the body.
Adult Stem Cells: A basic understanding of stem cells has been around for decades. There are collections of adult stem cells located throughout the body. These cells are capable of being recruited when needed throughout an individual’s lifetime.
The classic example of an adult stem cell are the blood-forming (hematopoietic) stem cells found in bone marrow. In an adult, the red blood cells carrying oxygen throughout the body only live for about 120 days. The blood-forming stem cells continue to divide (maintaining a reserve of stem cells) but also produce cells that will become specialize by developing into the functional red blood cells. Without these stem cells the body would be unable to replace old or damaged red blood cells. The blood-forming stem cells can also develop into other types of blood cells. Bone marrow transplants have been used since 1968 to treat patients with diseases such as leukemia, immune deficiency disorders, or Hodgkin’s lymphoma. In essence, bone marrow transplants are the first example of adult stem cell treatment.
Scientists have identified and isolated Adult stem cells from many regions of the body including brain, muscle, skin, liver, ovary, and testes. These cells are important in the normal body and a greater understanding of their properties is improving the potential of using these cells in the treatment of diseases. However, these cells are not completely unspecialized and will only become a subset of adult cells. These adult stem cells cannot be used to repair or replace all regions of the body.
Embryonic Stem Cells: In the field of biomedical science, embryonic stem cells are the Holy Grail, sought after because of their immense potential. By definition, embryonic stem cells are isolated from cells of early embryos formed through in vitro fertilization. Most often these embryos were no longer needed and donated for research purposes rather than being destroyed. Since these cells come from embryos less than four to five days old, the cells are almost completely unspecialized.
Once isolated these embryonic stem cells are grown in the laboratory using the process of cell culture. By controlling the conditions the cells can be kept unspecialized and dividing. Scientists are now studying the factors involved in gradually and specifically directing these cells to differentiate into the functional cells of the adult body. Once these processes are better understood, it may be possible to create replacement cells, tissues or organs for therapeutic purposes.
Monday: Part 1. The collision of science and politics
Tuesday: Part 2. Basic concepts of stem cell science
Wednesday: Part 3. Cell replacement therapy
Thursday: Part 4. Future applications of stem cell treatment
Friday: Part 5. The ethics of stem cell use.
Saturday: Part 6: Medical Tourism: seeking cures around the world.
You can email me at phillyscienceguy@gmail.com or visit my website Science in my life. This series is adapted from a book manuscript in progress on The History, Science, and Politics of Stem Cells.
Comments