Stem cells

• In humans, the zygote (fertilised egg cell) has the potential to form all 216 different cell types needed for an entire new person. It is totipotent. A totipotent is an undifferentiated cell that can form any one of the different cell types needed for a new organism
• The first stage of embryonic development is cleavage. Cleavage involves a special kind of mitosis where cells divide repeatedly without interphase for growth between division. The result of cleavage is a blastocyst.

  
• Stem cells are undifferentiated cells that have the potential to develop into many different types of specialised cell from the instructions of DNA
• One large zygote cell forms many small cells in the embryo, known as embryonic stem cells.
• By around the 4th day after fertilisation, the very earliest cells in an embryo become a solid ball of 10-30 cells known as the morula. Each of the cells in a morula cell are totipotent. Within another day, the cells have divided into a blastocyst
• By the time the blastocyst has formed, the cells in the inner layer have already lost their ability to differentiate and the outer cells form the placenta. The inner cells are pluripotent embryonic stem cells. A pluripotent cell is an undifferentiated cell that can form most of the cell types needed for a new organism. They have become pluripotent as a result of some genes being permanently switched off
• Embryonic stem cells are totipotent. By the blastocyst stage, the inner walls are pluripotent. Pluripotent stem cells change to become specialised as the embryo develops. By around 3 months of pregnancy, the cells have become sufficiently specialised that when they divide, they form more of the same type of cell
• Umbilical cord stem cells are pluripotent. If the blood is frozen and stored, in theory, those stem cells will be available for the life of the child or if their family needs them later for stem cell therapy. However a lot of storage is needed and this is expensive. Also, there is some evidence that diseases like leukaemia are already present in the blood at birth
• Adult stem cells (somatic stem cells) are multipotent. They remain as undifferentiated cells and are found among the normal differentiated cells in a tissue or organ. They can differentiate when needed to produce any one of the major cell types. For example white bone marrow contains stem cells that can form white blood cells. There are only a small number of adult stem cells in each different tissue and they are difficult to extract and produce a very limited range of differentiated cells
• During human development, different versions of the haemoglobin genes are switched off. The gene controlling fetal gamma globin are very important during fetal growth but need to be silenced around the time of birth, and the adult beta globular genes need to be activated just before birth in the bone marrow
• Bone marrow transplants are now used regularely in the treatment of certain cancers and immune system diseases. These transplants need to be made with someone who has a matching immune system or the body will reject the implant
• It is very difficult to control the differentiation of cells.
• Some of the early treatments of stem cell therapy did result in patients being cured of one condition, but they later developed cancer.
• Adult stem cells have successfully been used to produce new body parts, like the trachea. Stem cells from a patient are seeded into a collagen-based framework, which may be from a donor or completely synthetic, and the stem cells grow to form the new trachea
• Adult stem cells can also be used to repair the damage from a heart attack
• Somatic cell cloning or therapeutic cloning is an experimental technique that can potentially be used to produce large quantities of tissue. Firstly, cloned cells are produced from the patient, and a mild electric shock is given si the nucleus fuses with the ovum that has previously been fused with stem cells. This will then grow and be able to be harvest and can be cultured in a suitable environment so that they can differentiate into the required tissue
• Japanese researches were able to make induced pluripotent stem cells (iPS cells) using cells from other people and genetic engineering techniques. This can overcome ethical objections from using embryonic tissue as a source of stem cells, and there is no risk of rejection. However, it is difficult to make the cells differentiate in the way you want
• Pluripotent cells could be used to help patients with Parkinson’s disease, type 1 diabetes, damaged nerves and to make organs for transplants
• The 4 ethical principles are:

  • respect for autonomy - respect for individuals (consent is needed)
  • beneficence - aim of doing good, relieving sufferance
  • non-maleficence - doing no harm
  • justice - treating everyone equally and sharing resources fairly
• Many religious groups think it is ethically wrong to use embryos in stem cell medicine. It is also difficult and expensive to store, and there are still not enough proofs that stem cell therapy is effective