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- Course
- BIOL 120
- Institution
- Hunter College - Cuny
Lecture notes of 11 pages for the course BIOL 120 at Hunter College - Cuny (In depth notes)
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STEM CELLSWhat are Stem Cells?
• Stem cells are cells that are not terminally differen.ated, meaning they
have not reached their final state of maturity.
- They remain in undifferen8ated state, capable of developing into various cell types.
Characteris0cs of Stem Cells:
• A stem cell can divide without limit for the life8me of the animal.
• Stem cells exhibit a property called self-renewal:
o When a stem cell divides, it produces two daughter cells.
o One of the daughter cells maintains the stem cell-like characteris.c,
which means it retains its ability to con8nue dividing and remaining in
an undifferen8ated state.
o This feature allows the popula8on of stem cells to stay the same in number, as one daughter cell remains a stem cell
while the other can poten8ally proceed toward differen8a8on.
o Without this self-renewal process, if both daughter cells went down differen8a8on pathways, there would be one less
stem cell with each division. Thus, the self-renewal process allows a stem cell to produce differen8ated cells while
maintaining same amount of stem cells.
Uniqueness of Stem Cells:
• Stem cells are unique from other cell types because they have a type of “immortality.” They can divide without reaching a
terminally differen8ated state.
o ability to stay in a non-fully differen.ated state.
• Stem cells can con8nuously produce more of themselves while also giving rise to differen8ated cell types, a balance that is
essen8al for 8ssue maintenance and repair throughout the organism’s life.
Stem Cells in Adult Organisms
• Presence of Stem Cells in Adults:
o Yes, adult organisms have stem cells, referred to as adult stem cells.
o These cells are found in more types of 8ssues than once believed, although they are present only in small numbers within
these 8ssues.
• Example of Hematopoie0c Stem Cells (HSCs):
o Located in the bone marrow.
o HSCs become all the cells in the immune system and red blood cells, effec8vely popula8ng the bloodstream.
o Present in small amount; only 1 in 10,000 to 1 in 15,000 cells is an actual hematopoie8c stem cell.
o This small number is sufficient to con8nually renew all blood cells.
• Loca0ons of Adult Stem Cells:
o Bone Marrow: Produces all immune and red blood cells.
o Epithelial Cells of Skin: Allows con8nuous replacement of skin cells.
o Diges0ve System: Stem cells here replace cells along the diges8ve tract.
o Other Loca0ons:
§ Blood vessels.
§ Dental pulp.
§ Brain (especially in the hippocampus, involved in learning and memory).
§ In the brain, they replenish neurons and suppor8ng glial cells.
• Rate of Division:
o Unlike other cells, adult stem cells divide slowly and only in response to specific signals.
o This division is controlled.
• Why Controlled Division MaLers:
o Many cells in the body reach a mature state of differentiation where they no longer have the capacity to divide. These cells
must be replaced over time, and stem cells provide a means to do this.
o This replacement mechanism is safer than allowing mature cells to continue dividing, as mature cells accumulate mutations
over time. Starting fresh with new cells derived from stem cells minimizes mutation accumulation.
o The division of stem cells, which occurs only in response to signals, allows for a steady renewal of cells as needed,
maintaining a stable population over time.
Types and Potency of Stem Cells
• Embryonic Stem Cells:
o These cells can become all 8ssue types in the body and are considered pluripotent.
o The term pluripotent reflects their powerful plas.city, meaning they can differen8ate into any 8ssue in the body.
• Adult Stem Cells:
o While adult stem cells also have the ability to differentiate, they are less versatile than embryonic stem cells. This is
because they have already taken some steps towards differentiation.
, o Adult stem cells are considered multipotent, meaning they have a more limited differentiation potential compared to
pluripotent embryonic stem cells.
o Example: Hematopoietic stem cells in the bone marrow can differentiate into all cells of the bloodstream, such as
immune cells and red blood cells, but cannot differentiate into unrelated cell types, like stomach cells.
• Plas0city in Lab; Manipula0on (Inn Vitro):
o In laboratory conditions (in vitro), scientists can coax adult stem cells to show increased plasticity, or flexibility, allowing
them to differentiate into more cell types than they would naturally in the body (in vivo).
o However, in their natural in vivo environment, adult stem cells generally remain multipotent, with a limited
differentiation potential.
Stem Cell Differen0a0on Pathway
• Stem cells start as immortal cells that lack the mature characteris8cs of fully differen8ated cells.
• Through a series of controlled divisions, they gradually reach a fully mature, terminally differen0ated state, where they no longer
retain the ability to divide.
• Key Ques0on: “How can different cell types arise from a single stem cell?”
o There are two main mechanisms that allow a stem cell to produce daughter cells with different fates: divisional
asymmetry and environmental asymmetry.
Mechanisms of Asymmetry in Stem Cell Division
• Divisional Asymmetry:
o When a stem cell divides, it produces daughter cells that share
the same genome but have different cytoplasmic contents.
o The cytoplasmic contents, including various proteins and
gene-regula8ng factors, are unequally distributed between
the daughter cells.
o This unequal distribu8on leads to differences in gene expression
and dis8nct cell fates, even a]er the very first division.
o One daughter cell might inherit a set of proteins or regulatory
factors that the other daughter cell does not receive. This difference
leads to different behaviors and func8ons for each cell.
• Environmental Asymmetry:
o This mechanism is based on the environment surrounding the
cells a]er division.
o Example: One daughter cell remains a^ached to the extracellular matrix,
which sends signals to it, maintaining its stem cell-like characteris8cs.
o The detached daughter cell, no longer receiving signals from the extracellular
matrix, begins to move toward terminal differen8a8on.
o This environmental difference affects signaling and, therefore, the cell's "fate,"
or pathway it will take in development.
How Do Stem Cells Generate Large Numbers of Differen0ated Cells?
• Stem cells themselves do not divide frequently.
Given this, how can they produce large numbers of differen8ated cells?
• Transit Amplifying Cells:
o Stem cells can produce transit amplifying cells. These cells are capable of
undergoing several rounds of division before they reach a terminally
differen0ated state.
o Transit amplifying cells divide more rapidly than the original
stem cell, allowing for a controlled expansion of cell numbers
without uncontrolled or unlimited division, which
could lead to issues like cancer.
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