principlesof experimentalembryologyweb 000
TRANSCRIPT
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Commitment and Specification
Biology 4361 – Developmental Biology
June 11, 2009
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Overview
How do cells organize themselves into tissues and organs?
How do forces in the embryo cause the differentiation of cells?
Differentiation - definition
Specification, commitment, and determination - concepts
Types of specification
Morphogens and morphogen gradients
Stem cells and commitment
The embryonic environment: external and internal
What forces affect embryonic development?
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The Embryonic EnvironmentWhat is the “embryonic environment”?
Internal influences (e.g. intrauterine):- chemicals (e.g. maternal hormones, caffeine, nicotine)- competitors (e.g. litter-mates)
Environmental regulation pathway:- external stimulation triggers signaling event in embryo- signal stimulates an embryonic pathway (e.g. endocrine),
that changes the developmental pathway.
External influences:- light- temperature- humidity- predators - competitors- intraspecific signals
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Differentiation
Differentiation – development of cellular specialization
Differentiation is a process; multi-step/multi-phase…
- preceded by the commitment of cell to certain fate(s)
1) specification
2) determination
Commitment is also a staged process:
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Commitment Stages
undifferentiated differentiated
specification determination
- reversible
Capable of differentiating autonomously when
placed in a neutral environment; not when placed in
non-neutral environment.*
1. Specification.
- essentially irreversible
Capable of differentiating autonomously
even when placed into another embryonic region.*
2. Determination.
*Functional definition
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Specification Types
I. Autonomous (mosaic)
- cells develop only according to early fate
e.g. Ceanorhabditis elegans
II. Syncytial
- cell fate dependent on exposure to cytoplasmic
determinants in a syncytium
e.g. insects
III. Conditional (regulative)
- fate depends on context
e.g. vertebrates
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Autonomous Specification
- cells are specified by differential distribution of cytoplasmic
components during cleavage of the egg and early embryo.
- proteins
- RNA
Fabio Piano (Cornell University)
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Autonomous Specification
Tunicate (sea squirt)
blastomere separation
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Autonomous Specification - 2
Blastomeres are committed
at a very early stage in
mosaic development
Each blastomere contains
positional information
in the form of specific
proteins and genes
dissociated blastomeresIf split, each dissociated
blastomere pair forms
original structures
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Removal Experiment - Mosaic
mitosis
early embryo
later embryo
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Syncytial Specification
Syncytium – nuclear division without cell division; results in
cytoplasm with many nuclei
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Drosophila
Cleavage
FELICE FARBER
nuclei & cytoplasm form
syncytial blastoderm
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Syncytial Specification through
Morphogen Gradients
Drosophila egg
bicoid – anterior determinant
nanos – posterior determinant
Maternal messages: U Irion & D St Johnson
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Syncytial Specification through
Morphogen Gradients
bicoid – anterior
nanos – posterior
Maternal messages:
1) each region has a distinct Bicoid:Nanos ratio
1:0 10:1 1:1 1:5
2) Bicoid:Nanos determines anterior-posterior identity
Bicoid & Nanos proteins =
morphogens
Each morphogen
establishes a gradient
throughout the
embryo (like a
diffusion gradient)
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Syncytial Specification through
Morphogen Gradients
Cells identity depends
on their position in
multiple gradients
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Bicoid Protein = Head
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Bicoid Manipulation
= morphogen gradient
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Conditional Specification
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Conditional Specification
Cell fate depends on
interactions with
neighboring cells
Embryonic cells can change
fates to compensate for
missing parts = Regulation
Conditional specification produces
Regulative Development
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Removal Experiment
differentiated cell
mitosis
early embryo
later embryo
?
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Conditional SpecificationLegs and antennae: structurally-related; different morphologies.
Experiment: Transplant embryonic cells that would produce
proximal leg (close to the body) to an area that would
ordinarily produce antenna tip.
claws
proximal leg
distal antenna?
I. Duncan
antenna
claw
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Morphogen Gradients
e.g. cells respond to protein concentration by turning different colors
Cell commitment and
differentiation are
programmed by various
morphogen gradients.
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Conditional Specification
Cell commitment and
differentiation are
programmed by various
morphogen gradients.
Transplants of flag “cells”
shows that they retain
their identity (nationality),
but grow according to
the cells around them.
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Transplanted leg cells keep “leg” identity
- but modify development from their original location (proximal
to the body), to that of their new location (the distal-most point).
Morphogen gradient started at the body (source) specifies proximal structures.
I. Duncan
Conditional Specification
As the morphogen concentration decreases more distal structures form.
Therefore, while transplanted leg cells kept their identity, they were
“conditioned” by the low morphogen concentration at the tip (sink) to
form the most distal leg structures – claws.
Leg-antenna transplant
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Stem Cells and Commitment
Pluripotent* – uncommitted; makes many types of cells.
Totipotent* – ability to make all cell types; embryo and
trophoblast (fetal portion of the placenta).
Multipotent* – committed; makes several different types of cells
*all stem cells regenerate copies of themselves
* *
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Stem Cell Derived Blood Cells
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Overview
How do cells organize themselves into tissues and organs?
How do forces in the embryo cause the differentiation of cells?
Differentiation - definition
Specification, commitment, and determination - concepts
Types of specification
Morphogens and morphogen gradients
Stem cells and commitment
The embryonic environment: external and internal
What forces affect embryonic development?
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Morphogenesis and Cell Adhesion
How are tissues formed from populations of cells?
How are organs constructed from tissues?
How do organs form in particular locations and how do
migrating cells reach their destinations?
How do organ and their cells grow, and how is their growth
coordinated throughout development?
How do organs achieve polarity?
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Cell Interactions
Cell membrane protein components bind cells together; e.g.
Cadherin
Cells interact with each other either through paracrine signaling
at some distance, or through direct contact.
Calcium-dependent adhesion
- multiple forms
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Cadherin-Mediated Cell Adhesion
actin
microfilament
system =
anchoring and
movement
Different cells
have different
cadherins.
Different cadherins
have different
affinities for
each other.
Thus, cell types can
segregate
themselves
based on
membrane components.
Ca2+-dependent
binding:
Ca2+ can control
both strength
and reversibility
of binding