stem cells

• a stem cell is a cell that can divide (by mitosis) an unlimited number of times
• totipotency: totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta and umbilical cord)

  the zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage of human embryo development
• pluripotency: pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into cells forming the placenta and umbilical cord

  
• embryonic stem cells can be one of two potencies

  totipotent if taken in the first 3-4 days after fertilisation

  pluripotent if taken on day 5
• embryos used for research are often the waste (fertilised) embryos from IVF treatment

  so they have the potential to develop into human beings which is the reason for ethical objections to using them in research or medicine
• adult stem cells are only able to produce a limited range of cell types

  bone marrow - used to produce different types of blood cell

  brain - used to produce different types of neural and glial cells
• use of adult stem cells is less controversial than embryonic stem cells because the donor is able to give permission e.g many people donate bone marrow to help treat leukaemia patients
• if adult stem cells are being donated from one person to another they need to be a close match (blood type, other body antigens)
• what will happen if there is no close match in terms of blood type and other body antigens (donating adult stem cells)

  chance the cells used are rejected by patient's immune system (recognised as foreign and attacked by immune system)

  so ideally patient's own adult stem cells are used to treat them - lower chance of rejection
• benefits of using stem cells

  great potential to treat a wide-variety of dieases from diabetes and paralysis
• benefits of using stem cells

  organs developed from a patient's own stem cells reduces the risk of organ rejection and the need to wait for an organ donation
• benefits of using stem cells

  adult stem cells are already used successfully in a variety of treatments acting as proof of benefits
• risks / issues of using stem cells

  stem cells cultured in the lab could become infected with a virus which could be transmitted to the patient
• risks / issues of using stem cells

  risk of cultured stem cells accumulating mutations that can lead to them developing into cancer cells

  low numbers of stem cell donors
• social issues

  possible for embryonic stem cells to be collected before birth (amniotic fluid) or after birth (umbilical cord blood) and stored by a clinic - but too expensive and not an option for everyone
• social issues

  lack of peer reviewed clinical evidence of the success of stem cell treatments
• social issues

  educating the public sufficiently about what stem cells can and cannot be used for
• ethical issues

  stem cells may be sourced from unused embryos produced in IVF treatment - is it right to use them? who gives permission?
• ethical issues

  is it right to create embryos through therapeutic cloning and then destroy them? who owns the embryo?
• ethical issues

  should an embryo be treated as a person with human rights? or as an object?
• 5 days after conception, a hollow ball of cells called the blastocyst has formed.

  outer blastocyst cell layer goes on to form the placenta

  inner cell of 50 or so cells forms developing embryo (pluripotent embryonic stem cells)
• any embryo older than 14 days can't be used for stem cell research as it reaches the 'primitive' stage where it begins to form 3 germ layers that give rise to different tissues and organs in the body which is considered a significant milestone in human development