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