6.5. STEM CELLS

Question 1

What are stem cells?

Answer 1

• undifferentiated cells

Question 2

What happens once stem cells become specialised?

Answer 2

• lose ability to divide
• enter G0 phase of the cell cycle

Question 3

Why does the activity of stem cells have to be strictly controlled?

Answer 3

• if they do not divide fast enough, tissues are not efficiently replaced, leading to ageing
• if division is uncontrolled, form masses of cells (tumours), which can lead to cancer

Question 4

What does “stem cell potency” mean?

Answer 4

• a stem cells ability to differentiate into different cell types is called potency
• greater the number of cell types it can differentiate into, the greater its potency

Question 5

What are the three types of stem cell potency?

Answer 5

• totipotent
• pluripotent
• multipotent

Question 6

What does totipotent mean?

Answer 6

• these stem cells can differentiate into any type of cell
• e.g. zygotes/ fertilised egg to eventually produce a whole organism
• also differentiate into extra-embryonic tissues like the amnion and umbilicus

Question 7

What does pluripotent mean?

Answer 7

• these stem cells can form all tissue types but not whole organisms
• present in early embryos
• origin of different types of tissue within an organism

Question 8

What does multipotent mean?

Answer 8

• these stems cells can only form range of cells within a certain type of tissue
• e.g. haematopoietic stem cells in bone marrow, they give rise to various types of blood cell.

Question 9

What have multicellular organisms like plants and animals evolved from?

Answer 9

• unicellular organisms (single-celled)

Question 10

Where are blood cells derived from?

Answer 10

• stem cells in bone marrow

Question 11

How long are erythrocytes’ lifespans?

Answer 11

• 120 days (approx.)
• so need to be replaced constantly

Question 12

How many erythrocytes are produced by stem cell colonies in the bone marrow?

Answer 12

• approx. 3 billion per kilogram of body mass per day

Question 13

How long are neutrophils’ lifespans?

Answer 13

• 6 hours

Question 14

How many neutrophils are produced by stem cell colonies in the bone marrow?

Answer 14

• approx. 1.6 billion per kg of body mass per hour
• increases during infection

Question 15

What are the sources of animal stem cells?

Answer 15

• embryonic stem cells
• tissue (adult) stem cells

Question 16

What are embryonic stem cells (animal cells)?

Answer 16

• cells are present at a very early stage of embryo development and are totipotent
• after approx. seven days a mass of cells (blastocyst) has formed and the cells are now in a pluripotent state
• remain in this state in the foetus until birth

Question 17

What are tissue (adult) stem cells (animal cells)?

Answer 17

• cells are present throughout life from birth
• found in specific areas such as bone marrow
• multipotent, however there is evidence they can be artificially triggered to become pluripotent
  -can be harvested from the umbilical cords of new-born babies
• scientifically can be stored for future use and would not be rejected in a transplant to the owner

Question 18

What is the source of plant stem cells?

Answer 18

• meristematic tissue (meristems)

Question 19

What is meristematic tissue (plant cells)?

Answer 19

• tissue is found wherever growth is occurring in plants, i.e. tips of roots and shoots (apical meristems)
• also located between phloem and xylem tissue, this is the vascular cambium.
• pluripotent nature of stem cells in meristems continues throughout the life of the plant

Question 20

What are some potential treatments from the use of stem cells?

Answer 20

• heart disease
• type 1 diabetes
• Parkinson’s disease
• Alzheimer’s disease
• macular degeneration (blindness in elderly and diabetics)
• birth defects
• spinal injuries

Question 21

What are some actual (already in use) advancements from the use of stem cells?

Answer 21

• treatment of burns
• drug trials
• developmental biology

Question 22

What are some ethical problems using stem cells?

Answer 22

• destruction of embryos due to removal of stem cells
• moral objections, is this murder?

Question 23

Using umbilical cord stem cells has overcome some ethical issues of using stem cells in biology, but what are the limitations?

Answer 23

• merely multipotent, not pluripotent like embryonic stem cells, thus restricting usefulness.
• adult tissue stem cells have also been used, but these do not divide as well as umbilical stem cells and are more likely to have mutations

Question 24

What are some ways to overcome ethical issues of using stem cells in science?

Answer 24

• using plant cells instead of animal cells
• using induced pluripotent stem cells (iPSCs), these are adult stem cells that have been genetically modified to act like embryonic stem cells and so are pluripotent

Question 25

How has gene therapy used stem cells?

Answer 25

• children born with rare genetic condition ‘Severe Combined Immunodeficiency (SCID)’ are extremely vulnerable to all infections and without treatment are unlikely to live for more than a year.
• produce no T cells so B cells don’t function.
• normally treated with bone marrow transplant but it depends on finding matching donor, gene therapy uses own bone marrow stem cells which are genetically modified.
• transplanted stem cells divide and differentiate into the white blood cells above for a healthy immune system.
• moderately successful, however sometimes damaged and causes leukemia.

Question 26

How have plant stem cells been used in medicine?

Answer 26

• plant stem cells can be cultured and are unlimited and cheap.
• Paclitaxel is a drug used in treatment of breast and lung cancer, this cannot be chemically synthesized and must be obtained from bark of yew trees (this is limited as trees have to be mature) .
• recently stem cells from the yew tree have been used to produce paclitaxel cheaply and in sustainable quantities.