Transcript Chapter 20. Development of Multicellular Organisms

Chapter 21. Development
of Multicellular Organisms
Frog Development
Developmental process against
the 2nd law of Thermodynamics ?
Developmental processes cause
-Increase of orderliness?
-Decrease of entropy?
The 2nd Law of thermodynamics
Maxwell’s demon
http://cougar.slvhs.slv.k12.ca.us/~pboomer/physicslectures/maxwell.html
Information
is -ΔG
The Genome is The Maxwell’s demon
Life is open system
Life has information
Life consumes –ΔG,
(Erwin Schrodinger,887-1961)
Selective gene expression control
four processes by which the
embryo is constructed
1)
2)
3)
4)
Cell
Cell
Cell
Cell
proliferation
specialization
interaction
movement
Universal mechanisms of animal
development
• Similar gene usage
• Similar basic anatomical features
Universal mechanisms of animal
development-similar gene usage
Universal mechanisms of animal
development-similar basic structure
Unicellular vs. Multicellular
organisms
• Tranmembrane proteins (e.g. 2000 C.
elegans genes over yeast)
-Ion channels
-Cell adhesion molecules
-Cell surface receptors
• Gene regulatory proteins
(e.g. HLH gene family: 141 humans, 84 fly,
41 C. elegans, 7 in yeast)
How species can be different?
• Different animals utilize similar collection of genes
• Species identity genes (e. g 1% between human
and chimpanchee )
• non-coding, regulatory DNA sequences are highly
differential between species
Different genome causes different behaviors of cells
Urchin
Fly, Mouse
Urchin
Frog
Frog
8.5
Cell fate and morphogenesis
Cell fate determination
Determination (결정)
 Differentiation (분화된 상태)
= formation of specialized cell types
 Commitment (예정된 상태)
= biochemical changes in a cell that
restrict its developmental fate
Arms vs. Legs:
Differential Gene Expression
• Two transcription factors:
– Tbx5: Forelimbs
– Tbx4: Hindlimbs
• Expression dependant on
anterior/posterior location
Tbx4 in leg bud
Tbx5 in wing bud
Cell fate commitment by genes
Tbx5
Tbx4
How cell fate is determined?
Autonomous
specification
Regulative
specification
Commitment
 Three modes of initiating commitment
have been described.
Autonomous Specification
Conditional Specification
Syncytial Specification
Autonomous Specification
 Autonomous Specification
= cell fate is determined before fertilization my
morphogenetic determinants in ovum.
 Morphogenetic Determinants
= mRNA or proteins that cause cellular
commitment
 Mosaic Development
= embryo functions like a “mosaic” of
independent self-differentiating parts.
b-Catenin
positive cells
8.22
Asymmetric division
Mosaic Development
determinant
zygote
Mosaic Development
determinant
zygote
Dies or 1/2 Embryo Forms
Conditional Specification
 Conditional Specification = cell fate is
determined by the conditions surrounding
the cell.
 Morphogenetic determinants produced by
cells within the embryo. (signaling among
cells)
 Regulative Development = cells of an
embryo can change fate based on the
conditions within the embryo.
Regulative Development
zygote
Normal Embryo Forms
Regulative Development
Syncytial Specification
 Syncytial Specification = cell fate is
determined by the conditions affecting
nuclei in a single multinucleate cell.
 Syncytium = a cytoplasm containing
multiple nuclei.
 Morphogens may form a gradient
within the cytoplasm.
Basic mechanisms of cell fate
determination
•
•
•
•
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
Lateral inhibition
Basic mechanisms of cell fate
determination
•
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•
•
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
 Morphogen = Soluble molecule that
causes cellular commitment but is
secreted some distance from the target
cells.
 Morphogen Gradient = concentration
gradient of a morphogen.
Morphogen Threshold
Concentrations
embryo
Morphogen
conc.
position
p. 63
Morphogen Threshold
Concentrations
embryo
Morphogen
conc.
position
p. 63
Morphogen Threshold
Concentrations
embryo
Morphogen
conc.
position
p. 63
Influence of Other Cells
 Morphogen Receptor Gradient =
frequency gradient of the receptors for
a morphogen in target cell cell
membranes.
Morphogen gradient
Morphogen receptor gradient
Activin Gradient
 Activin = morphogen in frog blastula,
notochord
blood
heart
cells
muscle
morphogen gradient of activin commits cells
as a type of mesoderm.
No activin = ectoderm
Frog Blastula (section)
ectoderm
blastocoel
mesoderm
endoderm
vegetal pole
Morphogenetic Field
 Morphogenetic Field = a group of cells
whose position and fate are specified with
respect to the same set of boundaries.
Within a morphogenetic field fate is not yet
specified.
 The limb field will form a limb.
If divided the limb field will form two limbs.
Sonic Hedgehog
Ahn and Joyner (2004) Cell, 118,505-516
Gli1-CRE + LacZ conditional expression
+/+
Gli2-/-
Gli3-/-
Basic mechanisms of cell
differentiation
•
•
•
•
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
Basic mechanisms of cell
differentiation
•
•
•
•
Inductive interaction
Morphogen
Extracellular inhibitor
Intrinsic program for time course
-Time keeping mechanisms
-Cell division associated
-Glial progenitor cells become oligodendrocytes
after 8 divisions
Morphogenesis
 Commitment
 Cell shape changes.
 Cell movement.
 Cell death.
 Changes in cell membranes or
secreted products.
Sequential induction makes
complex patterns
General Cell Types
 Epithelial Cells = cells connected
together in sheets (attached to each
other and an acellular basal lamina).
fold, elevate, expand, involute,
intercalate
 Mesenchymal Cells = cells
unconnected together and operate
independently.
ingress, migrate
Epithelial Fold
Epithelial Fold
Epithelial Fold
Cell Affinity
Cell Affinity
Cell Affinity
 Selective Affinity = Disassociated
cells will group together with
(positive affinity) or will not group
together with (negative affinity)
only certain other cells.
 Homotropic Aggregation =
Disassociated cells of the same
type group together. (positive
afinity)
Cell Adhesion
 Differential Adhesion
Hypothesis = explains patterns
of cell sorting based on
thermodynamics of affinity
between adhesion molecules.
Surface tension.
 Different adhesion molecules.
 Different amounts of the same
type of adhesion molecules.
Cell Affinity
2 different
adhesion
molecules
2 different
amounts of the
same adhesion
molecule
Cell Adhesiveness
 Adhesion molecules = proteins in cell
membrane.
 Cadherins (5 classes)
calcium dependent adhesion molecules
binds to other cadherins (same type)
connected to cytoskeleton by catenins
 Homophilic binding = adhesion molecules
attach to the same class of adhesion
molecule.
Cadherins
cell
membrane
Ca2+
actin (cytoskeleton)
catenins
Cell Affinity
N-cadherin
E-cadherin
Methods for developmental biology
• Descriptive embryology
• Experimental embryology
• Developmental genetics
Origins of Descriptive
Embryology
• Epigenesis vs. Preformationism
– preformationism argued for species
continuity and constancy
– to some, epigenesis implied a need for
a mysterious vital “life force” that was
required to create life de novo
– careful observations on the anatomical
development of embryos eventually
required acceptance of epigenetic
development
Classical Embryology
• Kaspar Wolff (1767): studies of chick
embryogenesis
– Where did the instructions to build the
embryo come from?
– Were they internal or external?
– ‘vital force’ [vis essentialis] needed to
explain embryonic organization?
Classical Embryology
• Christian Pander (1774-1865)
– studied the chick embryo and identified
primary germ layers found in triploblastic
embryos
• ectoderm: gives rise to outer layer of embryo
and nervous system
• endoderm: gives rise to innermost layer and
gives rise to digestive tube and associated
organs
• mesoderm: middle layer that gives rise to bones,
connective tissues, kidney, gonads, heart and
hematopoietic system
– primary germ layers interact to form organs
Classical Embryology
• Karl Ernst Von Baer (1792-1896)
– “enwicklungsgeshicte”: extended Pander’s
observations; discovered notochord
– his work on chick embryogenesis was
death knell to preformationism (also
discovered mammalian egg)
Classical Embryology
• Von Baer’s laws:
– the general features of a large group of animals
appear earlier in development than specialized
features in a small group
– within embryos, specialized structures develop
from more generalized structures
– an embryo does not “pass through” the adult
stages observed in lower animals: ontogeny
does not recapitulate phylogeny
– early embryos share characteristics in common
and become more and more divergent as
development proceeds
Classical Embryology
• Wilhelm His (1831-1904)
– one of the major antagonists to Haeckel
– developed the microtome, allowing for serial
sectioning and much better anatomical resolution
and reconstruction
– focused on the the mechanics of development
and the importance of morphogenic movements,
foldings and cellular interactions in the process
of development.
The birth of experimental
embryology
 Defect = destroy part of embryo.
 Isolation = remove part of embryo and
observe its development in culture.
 Recombination = replace part of an embryo
with a part of the same embryo.
 Transplantation = replace part of an embryo
with a part from a different embryo.
Birth of Experimental
Embryology
• Laurent Chabry (1887) ‘Qualitative mosaic’
– experiments performed by isolating specific cells
in developing tunicate embryos
– each blastomere was responsible for producing
a particular set of larval tissues
– the blastomeres were apparently developing
autonomously
– mosaic development: embryo constructed of
individual modules capable of self-differentiation
Birth of Experimental
Embryology
• Wilhelm Roux (1850-1924) ‘Quantitative
mosaic’
– student of Haeckel who performed ablation experiments in
frogs
– Result of fate mapping in frogs implied that the destruction
of certain regions in the early blastula would preclude
development of certain structures
• destroyed right or left halves of frog embryos at 2 and 4 cell
stages
• obtained “half embryos” having a complete right or left side,
arguing for a mosaic model of development
Birth of Experimental
Embryology
• Hans Driesch (1867-1941): ‘Regulative
development’
– each of the blastomeres from a two cell embryo
developed into a complete larvae
– some of the later stage cells also developed into
complete larvae
– conflicts with experiments of Roux and Chabry:
first example of regulative development
Birth of Experimental
Embryology
• Hans Driesch (1867-1941): pressure plate
experiment; by compressing the developing
embryo between two plates, he could force
a change in cleavage plane from equatorial
to meridional, resulting in a different pattern
of cleavage from normal. This reshuffled
the position of the nuclei in the embryo…did
it alter the fate map?
– Embryos were normal
Birth of Experimental
Embryology
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Pressure plate experiments implied:
nuclear equivalence
cytoplasmic/nuclear interactions
Driesch left science as a result of these
experiments; he could not explain these
results relative to the physics of his day and
came to the philosophical view that living
things can not be explained solely through
physical laws
Experimental Design Matters!
• J. F. McClendon (1910) Repeated
experiments in frog development using
Driesch’s isolation technique relative to
Roux’s ablation technique
– noted regulative development NOT mosaic
development
– isolated frog blastomeres developed into a whole
frog
– ablated blastomeres were still in contact with
intact blastomeres; they still were providing
information for developmental programming
Fate Mappng
• Fate maps do not necessarily imply
commitment; not maps of potency or
states of determination
– clonal restriction does not imply
determination: allocation: clonal
restriction in a population regardless of
state of commitment
– commitment: intrinsic aspect of a cell
that makes it follow a particular
developmental path
– ‘commitment’ vs. ‘determination’?
The birth of developmental genetics
C. elegans, Drosophila, Frog,
Mice, Plant as model systems