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Developmental Mechanisms
Cell Specification
Cell Specification
 Differentiation
 The
process and the processes associated
with a cell becoming specialized
 Occurs in multiple steps
Cell Specification
 Commitment
 Specification
 Determination
 Terminal
Cell Specification
 Specification
A cell is said to be specified when:
Cells differentiate autonomously when removed
from normal environment (embryo) and placed in
a neutral environment (culture medium)
Placing cells into a non-neutral environment (a
different place in the embryo) will cause the cells
to follow the fate of other cells the new location
rather than their original fate
Cell Specification
 Determination
cell is said to be determined when:
 Cells
differentiate autonomously even
when placed in a non-neutral environment
 When
moved to a different location within
the embryo, the transplanted cells
differentiate according to their original fate
Cell Specification
Terminal Differentiation
When a cell can no longer change or be
changed into anything other than the cell type
it is
Can be associated with permanent changes
in DNA
Methylation is a prominent factor
B-cells (plasma cells) rearrange the
immunoglobulin (Ig) genes so that they can now
only form a single type of Ig
Autonomous Cell Specification
 Characteristic
of most invertebrates
 Differential acquisition of cytoplasmic
determinants (morphogenetic factors)
 Invariant cleavages and invariant
 Gives rise to mosaic development
 Specification precedes cell migrations
Autonomous Cell Specification
 Isolated
blastomeres will differentiate
into the same cell type they would have
differentiated into if left in the embryo
 Removal
of cells from the embryo
results in an embryo missing the cell
types the removed cells would have
differentiated into
 Embryo
is in essence a mosaic of cells.
Removal of any cell from the early
embryo results in a missing region of
the embryo
Autonomous Cell Specification
Autonomous Specification in Mollusc Embryos
Autonomous Cell Specification
differentiation of
blastomeres of
tunicate embryo
Autonomous Cell Specification
 Autonomous
specification implies
localized morphogenetic determinants
 Morphogenetic determinants become
partitioned into individual blastomeres
during cleavage stages
 Morphogenetic determinants are both
proteins and mRNAs
Conditional Cell Specification
 Characteristic
of all deuterostomes
 Specification via cell-cell interactions
 Cleavages are variable – no definite cell
 Development is regulative
 Cell rearrangements precede &
accompany specification events
Conditional Cell Specification
Transplantation experiment
Defect experiment
Conditional Cell Specification
 Conditional
specification implies the
interaction of cells in some manner
 Cells must recognize their neighbors
 Cells must influence each others
Conditional Cell Specification
 Cell-Cell
 Restriction
of fates
Inhibition by neighboring cells
 Inductions
Neighboring cells initiate induction events
 Morphogen
Soluble molecules which diffuse from source
cells and affect the differentiation of cells at a
Conditional Cell Specification
 Both head and tail can be removed
from planaria
 Head and tail will regenerate from
anterior and posterior ends respectively
of trunk
 An anterior – posterior gradient allows
middle piece to “know” what parts to
regenerate on each end
Conditional Cell
Regeneration in flatworms is
directed by an A-P morphogen
Conditional Cell Specification
Morphogen gradients
 Source – cell(s) that produce
 Sink – site where morphogen is used
 Cells are either responding to varying
concentrations of morphogen or to
varying lengths of exposure times
Conditional Cell Specification
Morphogen Gradients
& Thresholds
Cells in each flag differentiate
in response to the morphogen
gradient yet retain their original
specification as “French” or
Conditional Cell Specification
Amphibian Morphogen Activin
Conditional Cell Specification
Conditional Cell Specification
Morphogenetic Fields
 Limb fields
 Region
where cells are committed to form
a limb
 Cells within field are not determined to be
any particular part of the limb
 Other
 Eye
& heart in vertebrates
 Imaginal discs in Drosophila
Conditional Cell Specification
Limb Field
Conditional Cell Specification
Invasion and splitting
of limb field by
nematode parasites
in frogs results in
duplication of limbs
Syncytial Cell Specification
 Syncytium
 Cytoplasm
 Many
that contains multiple nuclei
insect eggs begin development as
a syncytium – multiple nuclear divisions
occur before cellularization
 The syncytial cytoplasm contains
gradients of localized proteins and
 Upon cellularization, unequal amounts
of morphogens becomes localized
within cells
Localized Maternal
mRNAs in Drosophila
establish protein
gradients in the
syncytial embryo
Syncytial Cell Specification
 The
morphogens typically functioning in
syncytial specification are transcription
 The unequal distribution of these factors
into cells of the embryo allow for the
specific activation or repression of
genes within the nuclei of the newly
formed cells
Cell Specification
No embryo uses only one of the types
 Regulative embryos use autonomous
specification frequently later in development
 Embryos that use autonomous specification
early use conditional specification for the
formation of some organs
 Syncytial specification can only be used for
very early events, both autonomous and
conditional specification occur later
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