Biology 4361 – Developmental Biology
Commitment and Specification
June 11, 2009
Overview
What forces affect embryonic development?
The embryonic environment: external and internal
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
How do cells organize themselves into tissues and organs?
The Embryonic Environment
What is the “embryonic environment”?
External influences:
- light
- temperature
- humidity
- predators
- competitors
- intraspecific signals
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.
Differentiation
Differentiation – development of cellular specialization
Differentiation is a process; multi-step/multi-phase…
- preceded by the commitment of cell to certain fate(s)
Commitment is also a staged process:
1) specification
2) determination
Commitment Stages
undifferentiated
differentiated
specification
determination
1. Specification. Capable of differentiating autonomously when
placed in a neutral environment; not when placed in
non-neutral environment.*
- reversible
2. Determination. Capable of differentiating autonomously
even when placed into another embryonic region.*
- essentially irreversible
*Functional definition
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
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)
Autonomous Specification
Tunicate (sea squirt)
blastomere separation
Autonomous Specification - 2
Blastomeres are committed
at a very early stage in
mosaic development
If split, each dissociated
blastomere pair forms
original structures
Each blastomere contains
positional information
in the form of specific
proteins and genes
dissociated blastomeres
Removal Experiment - Mosaic
mitosis
early embryo
later embryo
Syncytial Specification
Syncytium – nuclear division without cell division; results in
cytoplasm with many nuclei
Drosophila
Cleavage
nuclei & cytoplasm form
syncytial blastoderm
FELICE FARBER
Syncytial Specification through
Morphogen Gradients
Drosophila egg
Maternal messages:
bicoid – anterior determinant
nanos – posterior determinant
U Irion & D St Johnson
Syncytial Specification through
Morphogen Gradients
Maternal messages:
bicoid – anterior
nanos – posterior
Bicoid & Nanos proteins =
morphogens
Each morphogen
establishes a gradient
throughout the
embryo (like a
diffusion gradient)
1:0
10:1
1:1 1:5
1) each region has a distinct Bicoid:Nanos ratio
2) Bicoid:Nanos determines anterior-posterior identity
Syncytial Specification through
Morphogen Gradients
Cells identity depends
on their position in
multiple gradients
Bicoid Protein = Head
Bicoid Manipulation
= morphogen gradient
Conditional Specification
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
Removal Experiment
differentiated cell
mitosis
later embryo
early embryo
?
Conditional Specification
Legs 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.
antenna
proximal leg
?
distal antenna
claws
I. Duncan
claw
Morphogen Gradients
Cell commitment and
differentiation are
programmed by various
morphogen gradients.
e.g. cells respond to protein concentration by turning different colors
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.
Conditional Specification
Leg-antenna transplant
I. Duncan
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.
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.
Stem Cells and Commitment
*
*
Totipotent* – ability to make all cell types; embryo and
trophoblast (fetal portion of the placenta).
Pluripotent* – uncommitted; makes many types of cells.
Multipotent* – committed; makes several different types of cells
*all stem cells regenerate copies of themselves
Stem Cell Derived Blood Cells
Overview
What forces affect embryonic development?
The embryonic environment: external and internal
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
How do cells organize themselves into tissues and organs?
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?
Cell Interactions
Cells interact with each other either through paracrine signaling
at some distance, or through direct contact.
Cell membrane protein components bind cells together; e.g.
Cadherin
Calcium-dependent adhesion
- multiple forms
Cadherin-Mediated Cell Adhesion
Different cells
have different
cadherins.
Different cadherins
have different
affinities for
each other.
Thus, cell types can
segregate
themselves
based on
membrane components.
actin
microfilament
system =
anchoring and
movement
Ca2+-dependent
binding:
Ca2+ can control
both strength
and reversibility
of binding