Stem Cells

Question 1

Definition

Answer 1

A stem cell is an undifferentiated/unspecialised cell capable of undergoing proliferation and self-renewal and retains potential to differentiate to produce specialised cells upon receiving appropriate molecular signal.

Totipotent - all + EE
Pluripotent - all except EE
Multipotent - several related cell types, but restricted

Question 2

Why 8-cell mammalian embryo must have stem cell characteristics

Answer 2

(Zygotic) stem Cells up to the 8-cell embryo remains totipotent where each cell has to divide and differentiate for growth and form an entire organism on its own, forming all cell types including extraembryonic tissue such as placenta.

This is a result of differential switching on of genes which occurs when appropriate molecular signals are received.

Question 3

Zygotic stem cells vs embryonic stem cells

Answer 3

1. ZS is totipotent, ESC is pluripotent.
2. ZS can differentiate into all cell types that make up an organism including extraembryonic tissue such as placenta which nourishes embryo, ES can differentiate into all cell types that make up an organism except extraembryonic tissue such as placenta
3. ZS can form an entire organism while ES cannot
4. ZS are derived from a fertilised egg which becomes the zygote, produced within the first three divisions after it is fertilised. ES are derived from inner cell mass of a blastocyst at about 4-5 days post-fertilisation

Question 4

Blood stem cells

Answer 4

Blood stem cells are adult stem cells found in the bone marrow and umbilical cord.
They can undergo self-renewal by mitotic division to ensure a constant pool of blood stem cells. Since blood cells have a short lifespan, blood stem cells replace those lost by normal cell death.

Blood stem cells are multipotent and able to differentiate into several related cell types but is restricted to blood cells only. They can divide asymetrically after stimulation by molecular signals to produce stem cells to maintain stem cell pool or produce progenitor cells to increase or renew population of specialised blood cells.

B and T lymphocytes are derived from lymphoid progenitor cells while platelet producer and red blood cells are derived from myeloid progenitor cells.

Question 5

Why injection with own stem cells to which normal allele has been added is less effective

Answer 5

1. Stem cells from an unaffected donor is likely to have two copies of the functional allele while transduced stem cells are likely to have at most 1 copy of functional allele integrated into its genome, resulting in lower level of expression.
2. Normal functional allele may not be stably integrated into genome and is gradually lost with each subsequent cell division while stable integration is not an issue in donor.
3. Normal functional allele may be linked to, and is regulated by a weaker promoter.
4. Normal functional allele was under influence of enhancer/is now under influence of silencer that increased/decreased level of expression

Question 6

Why are owner cells removed before stem cell transplant

Answer 6

To prevent stimulation of patient’s immune system because donor cells are recognised as foreign and would cause stem cells to be destroyed by the immune system

To make space for donor cells to be eventually replaced by new cells differentiated from them.

Question 7

Why use own stem cells and not transplant a new organ/someone else stem cells

Answer 7

1. Own stem cells will not give rise to foreign tissues and antigens and will thus not be rejected by immune system
   No need for immunosuppressant
2. Stem cells are capable of proliferation and self-renewal and can thus be cultured in large amounts
3. Own stem cells eliminates need to find suitable donors
   No need to decide who is eligible or do blood/tissue match
4. Fewer ethical sourcing issues like from embryonic
   No destruction of embryo, not regarded as killing of life, no using embryo for spare parts

Question 8

Why stem cells must be treated with chemicals to stimulate proliferation

Answer 8

These chemicals stimulate mitosis to produce identical daughter cells capable of self-renewal, producing daughter cells that possess same developmental and differential potential as parent cell.
Stem cells are able to differentiate when given appropriate molecular signals
Chemicals cause changes in patterns of gene expression/switching on-off of different sets of genes conferring a particular function, resulting in specialisation

Question 9

Stem cells can divide/undergo mitosis infinitely

Answer 9

Each round of DNA replication results in the shortening of telomeres at the ends of chromosomes. Stem cells express telomerase gene to produce telomerase that extends telomeres to prevent them from reaching critical length and triggering apoptosis.