Cellular Mechanisms of Development

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Cellular Mechanisms of Development

• Chapter 19

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Overview of Development

• All three multicellular kingdoms exhibit cells
  that express genes at different times.
• mammals - complex bodies
• insects - intricate development cycle
• nematodes - very simple design

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Vertebrate Development

• Cleavage
• Within an hour of fertilization, the zygote
  divides rapidly into large number of blastomeres.
• no increase in overall size
• two ends referred to as animal pole (form
  external tissues) and vegetal pole (form internal
  tissues)
• cleavage slows after about 12 divisions, and
  transcription of key genes begins

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Cleavage Divisions
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Vertebrate Development

• Formation of blastula
• water drawn into cell mass forming a hollow ball
  of cells - blastula or blastocyst
• Gastrulation
• some cells of blastula push inward, forming an
  invaginated gastrula
• invagination creates main axis of vertebrate body
• embryo now has three germ layers
• What are the 3 germ layers?

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Vertebrate Development

• Neurulation
• zone of ectoderm thickens on dorsal surface of
  embryo
• neural tissue rolls and forms neural tube (brain
  and central nervous system)
• Cell migration
• variety of cells migrate to form distant tissues
  (gonadal cells move through the embryo to
  specific location to form sex organs somites
  move to form skeletal muscles)

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Vertebrate Development
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Vertebrate Development

• Organogenesis
• basic body plan established
• tissues develop into organs
• embryo will grow to be a hundred times larger

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Vertebrate Development
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Insect Development

• Many insects produce two body types
• larvae - gathers food
• adult - capable of flight and reproduction
• Developmental stages
• larval instars
• three larval stages over four days

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Insect Development

• Imaginal disks
• committed to form key parts of adult fly
• Metamorphosis
• larvae transformed into pupa

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Insect Development
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Nematode Development

• Nematode Caenorhabditis elegans develops 959
  somatic cells from a single fertilized egg.

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Cell Movement and Induction

• Cell movement
• Cells can move by pulling themselves along with
  adhesion molecules.
• Cell migration is largely a matter of changing
  cell adhesion patterns.
• Cadherins protein that connects cells
• Integrins fingerlike protein projections
• that help cell moves

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Cell Movement and Induction

• Induction
• Occurs when a cell switches from one path to
  another as a result of interaction with an
  adjacent cell.
• Experiments of removing cells from animal pole
  (ectoderm) and one from vegetal pole (endoderm)
  of blastocyst

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Cell Movement and Induction

• Inducing cells secrete proteins that act as
  intercellular signals.
• In some cases, organizer cells produce diffusible
  signal molecules (morphogens) that convey
  positional information to other cells.
• Is this another possible way of regenerating cell
  tissue?

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Determination

• Mammalian cells are totipotent - potentially
  capable of expressing all their genes.
• If separated, any cell can produce a normal
  individual. This is where stem cell research
  comes in.
• If cells from two different eight-cell embryos
  are combined, a normal individual results
  (chimera).

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Determination

• Mammalian cells begin to differentiate after the
  eight-cell stage.
• Determination is the commitment to a specialized
  developmental path. (Ex. cells might be
  determined as gastrointestinal cells.)
• Differentiation is the cell specialization that
  occurs at the end of the developmental path.
  (This is where cells develop into stomach, etc.)
• Determination is reversible.
• Given proper technique, fate of fully
  differentiated cell can be altered.

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Mechanism of Determination

• Positional labels - In a chick embryo, cells
  become partially committed to a certain type of
  tissue, but the position of the cell on a body
  structure makes a difference as to what the
  tissue will develop into. If a cell from the
  thigh region of a chick embryo is implanted into
  the tip of the wing area, the cell will
  eventually form a toe at the tip of the wing!

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Pattern Formation

• Within three hours after fertilization,
  orchestrated cascade of gene activity produces
  fly embryos basic body plan.
• Gene activation depends on free diffusion of
  morphogens through cellular spaces
• Anterior/posterior axis and dorsal/ventral axis
  are determined in this way.

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Expression of Homeotic Genes

• After pattern formation has been established in
  Drosophila, a series of homeotic genes determine
  the forms these segments will take.
• code for proteins that function as transcription
  factors
• Mutations in homeotic genes can cause normal body
  parts in unusual places.
• Homeotic genes have also been found in mice and
  humans

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Programmed Cell Death

• Apoptosis - process of cells shriveling,
  shrinking, and dying in a preprogrammed timeframe
• program regulated by gene activation
• humans
• bax genes encodes program
• oncogene bcl-2 represses program (important in
  research being done on aging and Alzheimers
  disease)

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Programmed Cell Death
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Theories of Aging

• Accumulated mutation hypothesis
• as they age, cells accumulate mutations
• Telomere depletion hypothesis
• telomeric regions shortened by repeated
  replications

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Theories of Aging

• Wear and tear hypothesis
• Over time, disruption, wear, and damage,
  eventually erode cells ability to function
  properly.
• no inherent limit to aging
• Gene clock hypothesis
• genes regulate rate of aging

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