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RELEVANT LEARNING OUTCOME:
(b) Explain the normal functions of stem cells in a living organism, including embryonic stem cells and blood stem cells.
🕐Estimated Time: 30 mins
💡Zygotic Stem Cell
💡Zygotic Stem Cell
Example: Zygote, up to the 8-cell stage
Potency: Totipotent
Function: Form the entire organism
Telomerase*: Highest level
(*Stem cells requires telomerase to delay the end replication problem (details only required for H2), which allows them to divide indefinitely, however, as the potency drops, the level of telomerase expression also drops)
Recall that the zygote is formed from the fusion of gametes (sperm and egg).
This single cell has the potential to form an entire organism, which is made up of all possible cell types.
As such, a zygotic stem cell is considered to be totipotent.
Apart from the zygote, cells up to the 8-cell stage (see figure) are considered to be all totipotent.
Watch the video to see how the zygote is formed, and its role in developing an entire organism.
Think of the following questions as you watch the video:
How many divisions does the zygote undergoes to form 8-cells?
After 3 divisions, there will be 8 cells, remember that each round of cell divison doubles the number of daughter cells.
(1 > 2 > 4 > 8).
cells that come after this are no longer considered totipotent.
💡Embryonic Stem Cells
💡Embryonic Stem Cells
Example: Inner cell mass of blastocyst
Potency: Pluripotent
Function: To differentiate into all cell types (from all 3 germ layers) except for the extraembryonic cells (placenta and umbilical cord) for the growth and development of organism
Telomerease: High levels
Watch the first video to see the function of embryonic stem cells.
Watch the second video to get an understanding of the germ layers (germ here refers to germinal, which means the early layers of tissues that goes on to form the embryo) (note that germ layers are not required in the syllabus, but it provides an understanding to the development of the embryo). Think of the following questions as you watch the videos:
Where did embryonic stem cells come from?
How is the potency of embryonic stem cell different from zygotic stem cell?
Recall that all cells come from pre-existing cells, blastocysts were formed from cell division of the zygote, which means the embryonic stem cells (and all other cells) came from the zygote (zygotic stem cell).
Embryonic stem cells though can form all cells that make up the embryo, they are unable to form extraembryonic cells, which includes the placenta and umbilical cord (see previous section).
💡Adult Stem Cells
💡Adult Stem Cells
❗NOTE: Adult stem cells does not mean that these stem cells are only found in adults, but rather adult here refers to stem cells that have lower potential (multipotent). Adult stem cells also exists in babies and children.
Example: Blood stem cells (Myeloid and lyphoid)
Potency: Multipotent
Function: Differentiate into specific cell types for growth and development of organism, and also replacement of cells that were damaged or died due to injury or disease.
Telomerease: relatively lower levels
Adult stem cells are found in developed organs and tissues. For example, muscle stem cells are found in the muscle and liver stem cells are found in the liver.
Blood stem cells
Function:
Growth and development of blood cells
Regenerate and replace worn out blood cells
myeloid stem cells differentiates to cells in the myeloid lineage (see figure)
lymphoid stem cells differentiates to cells in the lymphoid lineage (see figure)
Location: Bone marrow, umbilical cord, and also peripheral blood (circulating blood)
Watch the video to see how blood stem cells can be donated, and think of the following question.
What kind of patients requires blood stem cell donation?
What challenges are there in blood stem cell donation?
Patients with leukemia (blood cancer) may undergo intense chemotherapy to kill off existing cancer blood cells. After which, they will require transfusion of healthy blood stem cells to regenerate a healthy pool (non-cancerous) of blood cells.
It requires a match in donor and recipient, else there might be a higher risk of graft versus host disease, where the patient's immune system destroys the donated stem cells.
❗Possible Misconceptions❗
❗Possible Misconceptions❗
The higher the potency (e.g, totipotent) of the stem cell, more genes are expressed.
This is not true! Stem cells (toti, pluri, and multi), and specialised cells (e.g., muscle cells) just express different set of genes, there is no correlation between stem cell potency and number of genes expressed!
Stem Cells repairs cells
This is also not true! Stem cells replace damaged or worn-out cells, this in turn repairs the tissues and organs! (recall that cells form tissues and tissues form organs)
🖊️Comparison between stem cells
🖊️Comparison between stem cells
What are 3 similarities between all stem cells?
(Hint: All stem cells have the same 3 features!)
1. All three can undergo extensive proliferation and self-renew through mitosis.
2. All there are undifferentiated and unspecialised and do not have specialised structure and function.
3. All three are capable of differentiating into specialised cell types upon receiving appropriate molecular signals.
What are the differences between zygotic, embryonic and adult stem cells, in terms of features and function?
🖊️Conceptual Check
🖊️Conceptual Check
Stem cells have the same DNA sequences/genome as skin cells.
True / False
True!
Recall that all cells come from pre-existing cell, and this is via mitosis (except for gametes), and mitosis results in genetically identical daughter cells.
Therefore all somatic cells (non-gametes) in the organisms are genetically identical.
Explain your answer in the previous question, explain how stem cells and skin cells have different functions.
All somatic cells (non-gametes) in human body has the exact same DNA sequences/genome as they are all derived from the zygote/zygotic stem cell via mitosis; (recall that all cells come from pre-existing cells)
Although all stem cells and skin cells have the same DNA, they express different genes, accounting for their different cellular functions.
Why is it important that stem cells express the gene coding for telomerase?
Telomerase is an enzyme that delay chromosome shortening after each round of DNA replication;
Important for stem cells to divide indefinitely/long period of time;
🖊️Check Your Understanding
🖊️Check Your Understanding
Attempt qns 12-25 of the stem cells learning goals in this SLS lesson.
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