Transcript Cell Birth, Cell Death - UCO

Cell Birth, Cell Death
An overview of Chapter 22
By Patty Eneff
Cell Birth
• Cell division implies production of identical
daughter cell that do not differ form the
parent cells.
• If this were always the case, hundreds of
differentiated cells would never be formed.
Differences Occur
• When two Identical daughter cell differ in
environmental signals received and
diverge into differentiated cells.
• Or daughter cells may divide
asymmetrically, and differ in size, shape
and/or composition, with different genetic
signals.
Stem Cells
• See hand out for
diagram
• Give rise to
specialized cells such
as progenitor cells
Properties of stem cells
include
Ability to reproduce
indefinitely (self
renewal)
Ability to divide
asymmetrically
Patterns of Cell Division
• Unipotent- One cell
type
• Pluripotent- More that
one kind of cells such
as hematopeietic cells
that produce various
types of blood cells.
• Totipotent- all cells
types
Patterns of Cell Division
• Totipotent = zygote (not technically a stem cell)
but gives rise to embryonic stem cells.
• The eight cell stage in mammalian cells are
totipotent.
• When separated each of the eight cells can give
rise to a total organism.
• Eventually divide into germ cells at 64 cell stage,
then into ectoderm, mesoderm, and endoderm.
Stem cells
• Give rise to all cells of the
body, either directly or
indirectly
• Form the basis of cell birth.
Cell Death
Regulated in two fundamentally different ways.
I. Requires signal to stay alive
Absence of signals called trophic factors,
activates a cellular suicide program.
II. In developmental context specific signals induce
a “murder” program that kills cells.
– Cell that are damage by injury e.g.
mechanical damage
exposure to toxic chemicals
Cell Death by Injury
Cells undergo a series of
changes in a process called
necrosis.
I.
II.
Cells and their organelles swell, because the plasma
membrane’s ability to control the passage of ions and
water is damaged.
The cell contents leak out, leading to inflammation of
surrounding tissues.
Cell Death by Suicide
• Call Apoptosis (Dropping off or falling off as leaves of tree)
– Cell are induced to commit suicide
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Shrink
Develop bubble-like blebs on their surface,
Chromatin in their nucleus is degraded
Cytochrome C is release from the mitochondria
Break into small membrane wrapped fragments
Phospholipids (phosphatidylserine) are exposed to surface
Phospholipids is bound by receptors on phagocytic cells
The phagocytic cells release cytokines that inhibit
inflammation.
Differences in Cell Death
Reasons for Apotosis
• Programmed cell death is needed for
proper development.
– The absorption of the tadpole tail at the time
of metamorphosis into a frog occurs by
apotosis
– Formation of fingers and toes of fetus requires
apotosis of tissue between them
– Sloughing off of the uterine lining occurs by
apotosis
Reasons for Apotosis
• Programmed cell death is needed to
destroy cells that represent a threat to the
integrity of the organism
– Cells infected with viruses
– Cells of the immune system
• Cell mediated responses must be removed to
prevent attacking self
• Defects in the apototic process are associated
with autoimmune disease.
Reasons for Apotosis
• Cells with DNA damage
Damaged DNA can cause cancer, birth defects
Cells respond by increasing p53, which is a
potential inducer of apotosis.
Radiation and chemicals used in cancer
therapy induce apotosis in some types of
cancer.
Mechanisms of Apotosis
• External signals = extrinsic pathway
– Fas and TNF receptor are integral membrane
proteins.
– Binding of the two to complementary death
cell activators transmit a signal to the
cytoplasm that leads to activation of caspase
8, which initiates a cascade of caspase
activation leading to phagocytosis of the cell.
Mechanisms of Apotosis
• Triggered by internal signals = the intrinsic
or mitochondrial pathway.
– Bcl-2 on surface of mitochondrial membrane
is present in healthy cells
• Damage causes Bcl-2 to release Apaf-1, a related
protein Bax penetrates mitochondrial membrane
and cytochrome C leaks out.
• The cytochrome C and Apaf-1 bind to molecules of
Caspase 9.
• The resulting complex is called apoptosome.
Addition Apotosis Info
• Caspases are named because they
contain a cystine residue and cleave
proteins at sites just C-terminal to
aspartate residues
• Bad = pro-apoptotic protein
– Binds to the anti-apototic protein Bcl-2 and
Bcl-xl
– Apoptosis
Summary
• In asymmetrical cell division daughter cells
differ from mother cells, as differentiation
of stem cells begin the process of cell
birth.
• Necessity of cell death protects and helps
form organisms.
• Both processes are essential to life.
Questions ????