what are cell differentiation and cell specification?
TRANSCRIPT
What Are Cell Differentiation
and Cell Specification?
• Development of differentiated cell types
– different in function, cell shape, internal
structures, proteins and biochemical pathways
• Preceded by commitment to fate
– Specification: can differentiate in neutral
environment, but reversible in new
environment
– Determination: irreversible even in new
environment
What Are the Different Modes of
Specification?
• Autonomous Specification
– Independent of other cells in embryo (self-
differentiating parts)
– Determined by internal cytoplasmic factors
– If cleavage patterns are invariant, then cell fates will be invariant
– “Mosaic” development
– Characteristic of invertebrates
What Are the Different Modes of
Specification?
• Conditional Specification (“Non-Autonomous”)
– Depends on conditions (cell interactions)
– More variable (variations in which cells are assigned which fates)
– Capacity to “regulate”
– Characteristic of vertebrates (and some invertebrates)
What Are the Different Modes of
Specification?
• Syncitial Specification
– Interactions between regions of cells before
cellularization
– Cells have no specific cell fate before cellularization
– After cellularization, mostly conditional specification
– Most insects
Chabry’s Experiment 1887
• Mollusc
• Certain cells
form trochoblasts
(ciliated)
• In isolation
• autonomous
Roux’s Frog Cell Destruction
Experiments (1888)
• Defect experiment
• Frog embryos
• One blastomere makes
only half of embryo
• Support for
autonomous
specification?
Tunicate Specification is Mostly
Autonomous
• Tunicate embryo
– Isolation experiment
– Separate 8 cell stage
quadrants
– Each forms only what
it would have formed
and does so
autonomously
Cytoplasm Contains Determinants
Cytoplasmic Autonomy Extends
to the Molecular Level
• Tunicate Embryo
(1977)
– Isolated muscle forming cell lineage makes muscle acetylcholinesterase
– If cytoplasm injected into isolated b4.2 causes it to make the enzyme
Driesch’s Sea Urchin Isolation
Experiments: Regulation (1892)
• Sea urchin
• Each blastomere gives rise to complete larva
• Supports conditional specification
Conditional Specification in Frog
Transplantation Experiment
• Cell fate according to
new environment (A)
– fate not fixed
• Embryo makes up for
what was taken away
(B)
– regulation
Gradient of Fate-Determining
Molecules in a Syncytial Embryo
Stem Cells and Commitment
• Cells that divide to form one copy of itself and one different
– Pluripotent stem cells can lead to many cell types
– Committed stem cells can lead to fewer types
– Progenitor cells committed to one or few cell fates: not stem cells
• Early cells have most potential (embryonic, fetal)
Restriction of Potency with Time
• Committed cells usually don’t change in
new environment
• Blood cell lineage:
Mutants Identify Some Fate-
determining Molecules
• What would be effect of a mutation in a
fate-determining molecule?
• These molecules must be acting very early – Packaged by the mother
– Mutation in gene will influence the mother’s ability to
make viable embryos
– “Maternal-effect” genes