Stem Cells

Totipotent stem cells are able to mature into any type of body cell Multicellular organisms are made up from many different cell types that are specialised for their function, e.g. liver cells, muscle cells. All these specialised cell types originally came from stem cells. Stem cells are unspecialised cells that can develop into other types of cell. Stem cells divide to become new cells, which then become specialised. All multicellular organisms have some kind of stem cell. Stem cells are found in the embryo and in some adult tissues. Stem cells that can mature into any type of body cell in an organism are called totipotent cells. Totipotent stem cells are only present in mammals in the first few cell divisions of an embryo. After this point the embryonic stem cells become pluripotent. They can still specialise into any cell in the body, but lose the ability to become the cells that make up the placenta. The stem cells present in adult mammals are either: Multipotent stem cells - these are able to differentiate into a few different types of cell. Both red and white blood cells can be formed from multipotent stem cells in the bone marrow. Unipotent stem cells - these can only differentiate into one type of cell, e.g. there is a unipotent stem cell which can only divide to produce epidermal skin cells.

Stem cells become specialised because different genes are expressedStem cells become specialised because during their development, they only transcribe and translate part of their DNA: Stem cells all contain the same genes - but during development, not all of them are transcribed and translated. Under the right conditions, some genes are expressed and others are switched off. mRNA is only transcribed from specific genes. The mRNA from these genes is then translated into proteins. These proteins modify the cell - they determine the cell structure and control cell processes. Changes to the cell produced by these proteins cause the cell to become specialised. These changed are difficult to reverse, so once a cell has specialised it stays specialised. EXAMPLE Red blood cells are produced from a type of stem cell in the bone marrow. They contain lots of haemoglobin and have no nucleus. The stem cell produces a new cell in which the genes for haemoglobin production are expressed. Other genes, such as those involved in removing the nucleus are expressed too. Many other genes are not expressed, resulting in a specialised red blood cell.

Cardiomyocytes can be made from unipotent stem cells Cardiomyocytes are heart muscle cells that make up a lot of the tissue in our hearts. In mature mammals, it's thought that they can't divide to replicate themselves. This meant that people thought we weren't able to regenerate our heart cells at all. This is a major problem if our hearts become damaged. Recent research has shown that our hearts do have some regenerative capability. Some scientists now think that old or damaged cardiomyocytes can be replaced by new cardiomyocytes derived from a small supply of unipotent stem cells in the heart. Some researchers think that this process could be constantly occurring, but haven't yet agreed how quickly it happens. Some believe that it's a really slow process and it's possible that some cardiomyocytes are never replaced throughout a person's entire lifetime. Others think that it's occurring more quickly, so that every cariomyocyte is replaced several times in a lifetime.

Stem cells can be used to treat human disordersSince stem cells can divide into a range of specialised cell types, doctors and scientists think they could be used to replace cells damaged by illness or injury. Some stem cell therapies already exist Some stem cell therapies already exist for some diseases affecting the blood and immune system. Bone marrow contains stem cells that can become specialised to form any type of blood cell. Bone marrow transplants can be used to replace the faulty bone marrow in patients that produce abnormal blood cells. The stem cells in the transplanted bone marrow divide to produce healthy blood cells. This technique has been used to successfully treat leukaemia and lymphoma. It has also been used to treat some genetic disorders, such as sickle-cell anaemia and severe combined immunodeficiency (SCID): EXAMPLESevere combined immunodeficiency (SCID) is a genetic disorder that affects the immune system. People with SCID have a poorly functioning immune system as their white blood cells are defective. This means that they can't defend the body against infections by identifying and destroying microorganisms. So SCID sufferers are extremely susceptible to infections. Treatment with a bone marrow transplant replaces the faulty bone marrow with donor bone marrow that contains stem cells without the faulty genes that cause SCID. These then differentiate to produce functional white blood cells. These cells can identify and destroy invading pathogens, so the immune system functions properly. Stem cells could be used to treat other diseasesScientists are researching the use of stem cells as treatment for lots of conditions, including: Spinal cord injuries - replace damaged nerve tissue Heart disease and damage caused by heart attacks - replace damaged heart tissue Bladder conditions - grow whole new bladders which can then be implanted. Respiratory diseases - donated windpipes cna be striped down and then covered with tissue generated by stem cells. Organ transplants - organs could be grown from stem cells for people on the organ donor waiting list.

There are huge benefits to using stem cells in medicinePeople who make decisions about the use of stem cells to treat human disorder have to consider the potential benefits of stem cell therapies: They could save many lives - e.g. people waiting for organ transplants die before a donor organ becomes available. Stem cells could be used to grow new organs for people waiting for transplants. They could improve the quality of life for many people - e.g. stem cells could be used to replace damaged cells in the eyes of people who are blind.

Human stem cells can come from adult tissues or embryosTo use stem cells scientists have to get them from somewhere. There are three main potential sources of human stem cells:Adult stem cells These are obtained from the body tissues of an adult, e.g. bone marrow. They can be obtained in a relatively simple operation with minor risks but a lot of discomfort. Adult stem cells aren't as flexible as embryonic stem cells - they can only specialise into a limited range of cells, not all body cell types. Embryonic stem cells These are obtained from embryos at an early stage of development. Embryos are created in a lab using IVF. Once the embryos are 4-5 days old, the stem cells are removed and the rest of the embryo is destroyed. Embryonic stem cells can divide an unlimited number of times and develop into all types of body cells. Induced pluripotent stem cells (iPS cells) iPS cells are created in the lab. The process involves 'reprogramming' adult specialised cells so that they become pluripotent. The adult cells are made to express a series of transcription factors that are normally associated with pluripotent stem cells. The transcription factors cause the adult body cells to express genes that are associated with pluripotency. One of the ways these transcription factors can be introduced is by infecting the adult cells with specially-modified viruses. The virus has the genes coding for the transcription factors within its DNA. When the virus infects the adult cell, these genes are passed into the adult cell's DNA, meaning it is able to produce the transcription factors. iPS cells could become really useful in research and medicine in the future.

There are ethical issues surrounding embryonic stem cell use Obtaining embryos created by IVF raises ethical issues because the procedure results in the destruction of an embryo that could become a fetus if placed in a womb. Some people believe that at the moment of fertilisation an individual is formed that has the right to life - so they believe it is wrong to destroy an embryo. Some people have fewer objections to stem cells being obtained from egg cells that haven't been fertilised by sperm but have been artificially activated to start dividing. This is because the cells couldn't survive past a few days and wouldn't produce a fetus if placed in the womb. Some people think that scientists should only use adult stem cells because their production doesn't destroy an embryo. But adult stem cells can't develop into all the specialised cell types that embryonic stem cells can. This is where induced pluripotent stem cells could prove really useful. They have the potential to be as flexible as embryonic stem cells, but, as they're obtained from adult tissue, there aren't the same ethical issues surrounding their use. It's also possible that iPS cells could be made from a patient's own cells. These iPS cells, which would be genetically identical to the patient's cells, could then be used to grow some new tissue or an organ that the patient's body wouldn't reject.