Cell%20Birth,%20Cell%20Death

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Cell Birth, Cell Death

• An overview of Chapter 22
• By Patty Eneff

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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.

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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.

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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

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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

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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.

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Stem cells

• Give rise to all cells of the body, either
  directly or indirectly
• Form the basis of cell birth.

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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

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Cell Death by Injury

• Cells undergo a series of
• changes in a process called
• necrosis.
• Cells and their organelles swell, because the
  plasma membranes ability to control the passage
  of ions and water is damaged.
• II. The cell contents leak out, leading to
  inflammation of surrounding tissues.

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Cell Death by Suicide

• Call Apoptosis (Dropping off or falling off as
  leaves of tree)
• Cell are induced to commit suicide
• 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.

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Differences in Cell Death
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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

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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.

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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.

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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.

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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.

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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

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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 ????