Introduction To Pattern
Formation
Autonomous & Conditional Cell
Specification, Fields &
Morphogens
Cellular Differentiation
• All Cells have the same genes
• Some genes are expressed in all cells
– Housekeeping genes
• Some genes are expressed only in specific cells
– Luxury genes
• Zygotes are totipotent
– Can form any cell in the organism
• Embryonic cells are pluripotent
• Differentiated cells are restricted in potential
Fate Maps
Prospective Significance
Prospective Potency
Autonomous (Mosaic) Specification in a
Tunicate Embryo
Figure 3.8. Autonomous specification
in the early tunicate embryo.When the
four blastomere pairs of the 8-cell embryo
are dissociated, each forms structures
that it would have formed if it had
remained in the embryo.
(The fate map of the tunicate shows that
the left and right sides produce identical
cell lineages.)
(After Reverberi and Minganti 1946.)
Conditional Specification-Regulative
Development in a Frog
Figure 3.11. Conditional specification.
(A) What a cell becomes depends upon its
position in the embryo. Its fate is
determined by interactions with neighboring
cells.
(B) If cells are removed from the embryo,
the remaining cells can regulate and
compensate for the missing part.
Autonomous vs. Conditional
Specification
I.
Autonomous specification (Prospective potency=Prospective significance)
Characteristic of most invertebrates.
Specification by differential acquisition of certain cytoplasmic molecules present in the egg.
Invariant cleavages produce the same lineages in each embryo of the species. Blasto mere fates
are generally invariant.
Cell type specification precedes any large-scale embryonic cell migration.
Produces "mosaic" ("determinative") development: cells cannot change fate if a blasto mere is
lost.
II
Conditional specification (PP>PS)
.
Characteristic of all vertebrates and few invertebrates.
Specification by interactions between cells. Relative positions are important.
Variable cleavages produce no invariant fate assignments to cells.
Massive cell rearrangements and migrations precede or accompany specification.
Capacity for "regulative" development: allows cells to acquire different functions
Another Way of Looking At It
• Animal development can proceed according to
either the American or the European plan. Under
the European plan (autonomous specification),
you are what your progenitors were. Lineage is
important. Under the American plan (conditional
specification), the cells start off undetermined,
but with certain biases. There is a great deal of
mixing, lineages are not critical, and one tends
to becomes what one's neighbors are - Sydney
Brenner (quoted in Wilkins 1993)
The Limb Field
Figure 3.23. Prospective forelimb field of the salamander Ambystoma maculatum. The central area contains
cells destined to form the limb per se (the free limb). The cells surrounding the free limb give rise to the
peribrachial flank tissue and the shoulder girdle. The ring of cells outside these regions usually is not included
the limb, but can form a limb if the more central tissues are extirpated. (After Stocum and Fallon 1983.)
FGF10 Expression Defines The Limb Field
Figure 16.4. FGF10 expression and action in the developing chick limb. (A) FGF10 becomes expressed in the lateral plate
mesoderm in precisely those positions where limbs normally form. (B) When cells genetically constructed to secrete FGF10
are placed into the flanks of chick embryos, the FGF10 can cause the formation of an ectopic limb (arrow).
(From Ohuchi et al. 1997; courtesy of S. Noji.)
Limb Bud Formation
Figure 16.3. Limb bud formation. (A) Proliferation of mesodermal cells from the somatic region of the lateral plate
mesoderm causes the Limb bud in the amphibian embryo to bulge outward. These cells generate the skeletal
elements of the limb. Contributions of cells from the myotome provide the source of the limb's musculature. (B)
Entry of myotome cells (purple) into the limb bud. This computer reconstruction was made from sections from an in
situ hybridization to the myf5 mRNA found in developing muscle cells. If you can cross your eyes, the three
dimensionality of the stereogram will become apparent. (B courtesy of J. Streicher and G. Müller.)
The Skeletal Elements of The
Chick Wing
Tissue Interactions
• Induction- the presence of one tissue is
required for the formation of a structure in
another.
– Instructive interaction: The inducing tissue
informs the fate of the responding tissue.
– Permissive: The inducing tissue is necessary
for the response to occur but not sufficient to
specify ultimate cell fates (Cell fate
information is in the responding tissue)
Establishing the Proximal Distal Axis
AER Required for outgrowth
P/D Axis
AER not responsible for
Specification A/P Axis
AER is permissive
Mesoderm is instructive
AER Maintained by Mesoderm
AER secretes FGF
Figure 16.8. Summary of experiments demonstrating the effect of the apical ectodermal ridge (AER)
on the underlying mesenchyme. (Modified from Wessells 1977.)
The Apical Ectodermal Ridge
AER
FGF8 Signals Limb Outgrowth
Figure 16.12. FGF8 in the AER. (A) In situ hybridization showing expression of Fgf8 message in the ectoderm
as the limb bud begins to form. (B) Expression of Fgf8 RNA in the apical ectodermal ridge, the source of mitotic signals
to the underlying mesoderm. (C) In normal 3-day chick embryos, FGF8 is expressed in the apical ectodermal ridge
of both the forelimb and hindlimb buds. It is also expressed in several other places in the embryo including the
pharyngeal arches. (A and B courtesy of J. C. Izpisúa-Belmonte; C courtesy of A. López-Martínez and J. F. Fallon.)
Evidence that led to the progress zone
model:
Sequential removal of the AER results in progressively less disruption of development in a P/D fashion.
The Progress Zone Model For ProximalDistal Specification
Fig. 1. A cell´s proximo-distal positional value may depend on the time it
spends in the Progress Zone. Cells continually leave the zone at the tip of
the limb under the apical ectodermal ridge. Cells that leave early form proximal
structures while cells that leave last form the tips of the digits. (Wolpert 2000)
Specification by the Progress Zone
Figure 16.11. Control of proximal-distal specification by the cells of the progress zone.
A) Extra set of ulna and radius formed when an early-bud progress zone was transplanted to a late wing bud
that had already formed ulna and radius.
(B) Lack of intermediate structures seen when a late-bud progress zone was transplanted to an early limb bud.
The hinges indicate the locations of the grafts. (From Summerbell and Lewis 1975; photographs courtesy of D.
Summerbell.)
An alternative model: Early Allocation and
Progenitor Expansion Model (Pre-specification)
• Observations consistent with this model:
Cell division is seen throughout the limb bud
When AER is removed, cell death occurs about 200μm beneath the
AER
If the limb bud is a fully formed rudiment that grows as it develops,
then 200 μm of cell death will kill progressively smaller amounts of
the rudiment.
Anterior Posterior Specification
• Limb field prior to bud emergence is already
specified for anterior/posterior, dorsal/ventral
• How is this information maintained during the
outgrowth of the limb?
• Classical experiments:
• Extensive series of experiments transplanting pieces
of the limb bud from one region to another at
different times in bud outgrowth.
• Only one region gave consistent pattern alterations:
The Zone of Polarizing Activity
Figure 16.17. When a ZPA is grafted to anterior limb bud mesoderm,
duplicated digits emerge as a mirror image of the normal digits.
(From Honig and Summerbell 1985; photograph courtesy of D.
Summerbell.)
The “French Flag Model”
Figure 3.19. The French flag analogy for the operation
of a gradient of positional information.
(A) In this model, positional information is delivered by a
gradient of a diffusible morphogen from a source to a sink.
The thresholds indicated on the left are cellular properties
that enable the gradient to be interpreted. For example,
cells becomes blue at one concentration of the morphogen,
but as the concentration declines below a certain threshold,
cells become white. Where the concentration falls below
another threshold, cells become red. The result is a pattern
of three colors.
(B) An important feature of this model is that a piece of tissue
transplanted from one region of an embryo to another
retains its identity (as to its origin), but differentiates
according to its new positional instructions.
This phenomenon is indicated schematically by reciprocal
"grafts" between the flag of the United States of America
and the French flag. (After Wolpert 1978.)
How Could Cells A Interpret a
Morphogen Gradient
Anterior And Posterior Limb Fields Are
Specified by Position
Questions Which Need to be Answered
About Morphogen Gradients
1. What is the morphogen?
2. What is the source of the morphogen?
3. How do cells respond to the gradient?
•
•
How do they sense the gradient?
How do they change their fate in response
to morpogen?
4. What sets up the source and sink?
5. What happens to the gradient over time?