Stem cells

• multicellular organisms are made up from many different cell types that are specialised for their function e.g. liver cells, muscle cells, white blood 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 form of stem cell:

  • stem cells are found in the embryo (where they become all the specialised cells needed to form a fetus)
  • and in some adult tissues (where they become specialised cells that need to be replaced e.g. stem cells in the intestines constantly replace intestinal epithelial cells)
• stem cells that can mature (develop) into any type of body cell in an organism, including the cells that make up the placenta in mammals 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 or unipotent:

  • multipotent stem cells are able to differentiate into a few different types of cell
  • e.g. both red and white blood cells can be formed from multipotent stem cells found in bone marrow
  • unipotent stem cells can only differentiate into one type of cell
  • e.g. there is a type of unipotent stem cell that can only divide to produce epidermal skin cells which make up the outer layers of your skin
• Becoming specialised:

  stem cells become specialised bc 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 (expressed)

  under one set of conditions, certain genes are expressed and others are switched off

  under different conditions, different genes are expressed and others are switched off
• genes that are expressed get transcribed into mRNA, which is then translated into proteins. these proteins modify the cell - they determine the cell structure and control processes (including the expression of more genes, which produces more proteins)
• changes to the cell produced by these proteins cause the cell to become specialised. these changes are difficult to reverse, so once a cell has specialised it stays specialised
• • genes expressed
  • mRNA transcribed and translated into proteins
  • proteins modify the cell
  • cell becomes specialised for a particular function
• • genes switched off
  • mRNA not transcribed or translated
  • proteins not produced
• example 1:

  • red blood cells ae produced from a type of stem cell in the bone marrow
  • they contain lots of haemoglobin and have no nucleus (to make room for more haemoglobin)
  • 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 (switched off) resulting in a specialised red blood cell
• example 2:

  • nerve cells have long axons and dendrites (branches) which connect them to other nerve cells
  • they are produced from stem cells in the neural tube
  • the stem cells produce new cells in which the genes that direct the axon to extend outwards are expressed
  • genes that direct the dendrites to form are also expressed
  • many other genes are switched off
• it is a mix of some genes being switched on and some being switched off that causes specialisation - differentiation
• 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 for a long time, everyone thought that we weren't able to regenerate our own heart cells at all
  • major problem if the heart becomes damaged e.g. by a heart attack, or the cells become worn out through age
• • recent research has suggested 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 on how quickly it happens
  • some believe it is a really slow process and that it is possible that some cardiomyocytes are never replaced throughout a person's entire lifetime
  • others think that it's occurring more quickly, so that every cardiomyocyte in the heart is replaced several times in a lifetime
• stem cells can be used in medicine to treat or cure various diseases and conditions - some stem cell therapies ae already being used others are still being developed
• stem cell therapies in existence:

  • since 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 for some diseases affecting the blood and immune system
• bone marrow transplants:

  • 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 and specialise to produce healthy blood cells
  • this technique has been used successfully to treat leukaemia (cancer of the blood or bone marrow) and lymphoma (cancer of the lymphatic system)
  • also been used to treat some genetic disorders, such as sickle-cell anaemia an severe combined immunodeficiency (SCID)
• SCID:

  • is a genetic disorder that affects the immune system
  • people with SCID have a poorly functioning immune system as their white blood cells (made in the bone marrow from stem cells) are defective
  • this means 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 morrow 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 cell therapies of the future:

  • as stem cells can divide into specialised cell types, scientists think they could be used to replace damaged tissues in a range of diseases
  • scientists are researching the use of stem cells as a treatment for lots of conditions including:
• • spinal cord injuries - stem cells could be used to replace damaged nerve tissue
  • heart disease and damage caused by heart attacks - stem cells could be used to replace damaged heart cells
  • bladder conditions - stem cells could be used to grow whole bladders, which are then implanted in patients to replace diseased ones
• • reparatory conditions - donated wind pipes can be stripped down to their simple collagen structure and then covered with tissue generated by stem cells - can then be transplanted into patients
  • organ transplants - organs could be grown from stem cells to provide new organs for people on donor waiting lists
• adult stem cells:

  • obtained from the body tissues of an adult
  • e.g. adult stem cells found in bone marrow
  • can be obtained in a relatively simple operation - very little risk involved - lot of discomfort
  • not as flexible as embryonic stem cells
  • ca only specialise into a limited rage of cells, not all body cell types
  • multipotent
  • although scientists trying to find ways to make adult stem cells specialise into any cell type
• embryonic stem cells:

  • obtained from embryos at an early stage of development
  • created in a lab using in vitro fertilisation (IVF)
  • egg cells are fertilised by sperm outside the womb
  • once the embryos are approx 4-5 days old stem cells are removed from them and the rest of the embryo destroyed
  • embryonic stem cells can divide an unlimited number of times and develop into all types of body cells - pluripotent
• induced pluripotent stem cells (iPS cells):

  • iPS cells are created by scientists in the lab
  • the process involves reprogramming specialised adult body 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 that these transcription factors can be introduced to the adult cells is by infecting them with a specially-modified virus
  • 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 that the cell is able to produce the transcription factors
• • induced pluripotent stem cells could be useful in research and medicine in the future
  • at the moment - more research into how similar they actually are to true pluripotent embryonic stem cells is needed before they can be properly utilised
• Ethical considerations:

  • procedure results in 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 - believe it is wrong to destroy embryos
  • some people have fewer objections to stem cells being obtained from eggs that haven't been fertilised by sperm, but have been artificially activated to start dividing
  • bc the cells wouldn't survive past a few days and wouldn't produce a fetus if placed in a womb
• • some people think that scientists should only use adult stem cells bc 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
  • have the potential to be as flexible as embryonic stem cells but as they are obtained from adult tissue there aren't the same ethical issues surrounding their use
• • possible that iPS cells could be made from a patient's own cells
  • these iPS cells 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 (rejection of transplants occurs quite often and is caused by the patient's immune system recognising the tissue as foreign and attacking it)
• • decision makers in society have to take into account everyone's views when making decisions about important scientific work like stem cell research and its use to treat human disorders
• embryos in the normal sense are made when a sperm fertilises an egg - but it is possible to create an embryo by artificially stimulating an unfertilised egg to divide
• transcription factors are proteins that control whether or not genes are transcribed
• adult stem cells taken from a patient's own tissue are also less likely to by rejected by the patient's body - won't be seen as foreign
• benefits of stem cell therapy: - potential benefits must be considered:

  • they could save many lives e.g. many people waiting for organ transplants die before a donor organ becomes available. stem cells could be used to grow organs for those awaiting transplants
  • might even be possible to make stem cells genetically identical to a patient's own cells - could then be used to grow some new tissue of an organ that patient's body wouldn't reject
  • could improve the quality of life for many people - e.g. could be used to replace damaged cells in the eyes of people who are blind