Questions on stem cells and cell potency

Question 1

Evaluate the use of embryonic stem cells as a source of cells for treating human disease.

Accepted Answer

B1 for: embryonic stem cells are pluripotent, so a far wider range of tissues can be produced from them than from adult or cord blood cells B1 for: they divide in culture apparently without limit, so one cell line can supply large numbers of cells, and they offer treatment for conditions with no present cure B1 against: obtaining them ends the development of a blastocyst, which some hold to be a human individual from fertilisation, so that no benefit to a patient can justify it B1 against: the cells come from another individual and may be rejected, and undifferentiated cells remaining in a transplant may form tumours B1 alternatives: adult and umbilical cord blood cells raise no such objection but are multipotent, and induced pluripotent cells offer pluripotency from the patient's own tissue without using an embryo A1 a judgement is stated and supported by the points made above it; credit either conclusion provided the reasoning leads to it

Question 2

Describe what is meant by totipotent, pluripotent, multipotent and unipotent, giving one example of a cell at each level.

Accepted Answer

B1 totipotent: can become every cell type of the organism and the extra-embryonic tissues, for example a zygote or a cell of the first few divisions B1 pluripotent: can become every cell type of the body but not extra-embryonic tissue, for example a cell of the inner cell mass of a blastocyst B1 multipotent: can become a limited number of related types, for example a haematopoietic stem cell in bone marrow, which forms blood cells B1 unipotent: can become one type only, for example the cells that give rise to new cardiomyocytes in heart muscle

Question 3

Induced pluripotent stem cells are made by treating an adult body cell with four transcription factors. Discuss the advantages and the limitations of using these cells rather than embryonic stem cells.

Accepted Answer

B1 advantage: no embryo is used, so the ethical objection to ending the development of a blastocyst does not apply B1 advantage: the cells can be made from the patient's own tissue, so they carry the patient's own antigens and the risk of rejection is much lower B1 limitation: reprogramming succeeds in only a small proportion of the cells treated, and methods that insert the genes using a virus increase the risk of tumours B1 limitation: induced pluripotent cells have not been shown to be equivalent to embryonic cells, which are still used as the standard for comparison

Question 4

Compare embryonic stem cells with induced pluripotent stem cells as a source of cells for medical treatment, referring to their potency, where each comes from, and the ethical objection each raises.

Accepted Answer

B1 both are pluripotent, able to become any cell type of the body, whereas cells taken from adult tissue or cord blood are only multipotent B1 embryonic stem cells are taken from the inner cell mass of a blastocyst, whereas induced pluripotent cells are made by reprogramming an ordinary body cell, such as a skin fibroblast, with four transcription factors B1 obtaining embryonic stem cells ends the development of the blastocyst, which raises the ethical objection that a human embryo is destroyed, whereas induced pluripotent cells are made without an embryo and so do not raise that objection A1 induced pluripotent cells can also be made from the patient's own tissue, so they are not rejected by the patient's immune system, whereas embryonic stem cells come from another individual and may be rejected

Question 5

Outline two pieces of evidence that show a specialised cell retains a complete set of genes even though it does not express them all.

Accepted Answer

B1 in 1996 the nucleus from an adult sheep's mammary gland cell was transferred into an enucleated egg cell, and the resulting embryo developed into a normal sheep, showing the nucleus still held every gene needed to build a whole animal B1 the specialised mammary gland cell could not have produced a whole sheep on its own, so this outcome depended on the recipient egg's cytoplasm reprogramming the transferred nucleus B1 in 2006, treating an adult mouse fibroblast with four transcription factors returned it to a pluripotent state, and human cells were shown to respond the same way the following year B1 the four genes delivered were extra copies of genes the fibroblast already carried, so reprogramming worked by switching the cell's existing genes back on rather than by adding new genetic information

Question 6

Explain why researchers in the United Kingdom are not permitted to grow a human embryo in culture beyond fourteen days after fertilisation.

Accepted Answer

B1 research on human embryos in the UK requires a licence from the Human Fertilisation and Embryology Authority and the consent of the donors B1 the limit is set at fourteen days because that is when the primitive streak appears B1 before the primitive streak forms, the embryo can still split to give twins, so there is not yet a single developing individual A1 the primitive streak is treated as a landmark at which a line can reasonably be drawn, though some hold that a line drawn at any developmental landmark is arbitrary

Question 7

Identify which of the following are established stem cell treatments and which are still at the trial or research stage: a bone marrow transplant for leukaemia, a skin graft for burns grown from the patient's own cells, insulin-secreting cells for type 1 diabetes, and dopamine-secreting neurones for Parkinson's disease.

Accepted Answer

B1 a bone marrow transplant for leukaemia is an established treatment, in routine clinical use for decades B1 a skin graft for burns grown from the patient's own epidermal stem cells is an established treatment B1 insulin-secreting cells for type 1 diabetes are still at the trial stage, not yet a routine treatment B1 dopamine-secreting neurones for Parkinson's disease are also still at the trial stage, not yet a routine treatment

Question 8

A neurone contains the gene coding for insulin but never produces insulin. Explain how this is possible.

Accepted Answer

B1 all body cells are derived by mitosis from the same zygote, so they contain the same genes B1 cells differentiate by transcribing some genes and not others, rather than by losing genes A1 in a neurone the insulin gene is not transcribed, so no mRNA and no insulin is produced from it, although the gene is present and intact

Question 9

Compare stem cells obtained from adult bone marrow with stem cells obtained from an early embryo.

Accepted Answer

B1 bone marrow cells are multipotent and can form only blood cells, whereas embryonic cells are pluripotent and can form any cell type of the body B1 bone marrow cells can be taken from the patient themselves so carry the patient's own antigens and are far less likely to be rejected, whereas embryonic cells come from another individual and may be rejected B1 obtaining bone marrow cells does not end the development of an embryo, whereas obtaining embryonic cells does, so the two raise different ethical objections

Question 10

A patient is treated with nerve cells that have been grown from embryonic stem cells. Suggest three difficulties that this treatment may present.

Accepted Answer

B1 any cells that have not differentiated may continue to divide after transplant and form a tumour B1 the cells carry antigens from another individual, so they may be rejected by the patient's immune system unless immunosuppressant drugs are given B1 it is difficult to control differentiation so that only the required cell type is produced, and difficult to make the cells connect correctly with existing tissue

Question 11

A cell population is unspecialised and divides without limit, but none of its cells ever differentiate. Explain why this population would not function as a useful source of stem cells for tissue replacement, and explain why transplanting it into a patient could do harm.

Accepted Answer

B1 a stem cell must be able to differentiate as well as renew itself, because differentiation is what replaces the specialised cells the tissue actually needs B1 a population that only self-renews and never differentiates would form a growing mass of unspecialised cells rather than a tissue A1 cells transplanted into a patient that go on dividing without differentiating can form a tumour, the same risk that undifferentiated cells left behind in any stem cell transplant carry

Question 12

A single parenchyma cell taken from a carrot root can be grown in culture into a complete new carrot plant, but a single skin cell taken from a human cannot be grown into a complete new human. Explain this difference.

Accepted Answer

B1 the carrot root cell is totipotent, retaining the ability to form every cell type of the plant B1 in most plants, unlike in most animals, a differentiated somatic cell can be induced to revert to a totipotent state and, given the right conditions, regenerate a whole new organism A1 the human skin cell is only unipotent or, at most, multipotent, since differentiation in an animal removes access to earlier developmental programmes far more completely than it does in a plant cell

Question 13

Cord blood collected at a birth is frozen and stored rather than used immediately. Suggest one advantage of collecting stem cells this way rather than from adult bone marrow, and suggest the main limitation on how the stored cells could later be used.

Accepted Answer

B1 collection at birth carries no risk to the mother or the infant, unlike aspirating bone marrow from an adult donor under anaesthetic B1 the cells can be frozen and stored for years, so they are available immediately when needed rather than requiring a fresh donation A1 the main limitation is quantity: one cord yields only a small volume of blood, which may hold too few cells to treat an adult patient

Question 14

State the two properties a cell must have if it is to be described as a stem cell.

Accepted Answer

B1 it is unspecialised and can differentiate into one or more specialised cell types B1 it can divide repeatedly by mitosis to produce more cells like itself, which is self-renewal

Question 15

A cell is removed from the inner cell mass of a five-day-old human blastocyst. Name the level of potency this cell shows.

Accepted Answer

B1 pluripotent; totipotent is not accepted, because the cell cannot form extra-embryonic tissue such as the placenta