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)
– also made strong arguments against the
“biogenetic” concepts of his contemporary,
Ernst Haeckel
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
After Haeckel, 1874...
After Richardson, 1997...
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.
Birth of Experimental
Embryology
• Entwicklungsmechanik: “Developmental
mechanics”: prompted by the hypothesis that
internal factors were programming
development, embryologists began to test this
hypothesis through experimental intervention
– ablation experiments
– isolation experiments
– transplantation experiments
Birth of Experimental
Embryology
• Laurent Chabry (1887)
– 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
• Development of Fate Maps
– continuing on in the anatomical tradition,
embryologists of the late 19th/early 20th century
began to trace cell lineages in the developing
embryo
• Living embryos (e.g. the tunicate Styela): natural
pigmentation within the embryo could be followed: these
pigments would be passed to the descendants of earlier
cells, allowing cell fates to be mapped. (plates from these
experiments are on line)
• use of vital dyes to mark fate: cells can be stained with non
lethal pigments to follow fate
• molecular tools: use genetic techniques (genotypes) to
mark regions, or follow gene expression
Birth of Experimental
Embryology
• Wilhelm Roux (1850-1924)
– 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): instead of using an
ablation technique like Roux, he performed
isolation experiments on sea urchin blastomere
cells isolated at 2, 4 and 8 cell stages
– 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
• 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
Regional Specification in
Animal Development
• Please read first reading assignment for
next time- on electronic reserve and
physical reserve at library
• Slack will define some terms that we will
use throughout the course…his definitions
are not universal…but precise
• Please pay attention to the semantic
distinctions…these are operational
definitions
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‟?
Forms of Commitment
• How do you „measure‟ states of
commitment?
– As we will see, it is only in the past 15 years
that the molecular tools have been developed
to do this…regulatory gene expression:
• paracrine factors, receptors, signal transduction
pathways, transcription factors
• Slack „defines‟ 3 ways using the tools of
experimental embryology
Forms of Commitment
• Specification
– cell or tissue explant is „specified‟ to become a
structure if it will develop autonomously into
that structure after isolation from the embryo.
– Fate need not be the same as that in embryo
– „specification‟ maps can be compared to fate
maps
• maps the same….”mosaic”?
• maps are different….”regulative”?
Isolated animal
cap: atypical
epidermis
Amphibian
Blastula
Normally gives
rise to
epidermis and
neural
structures
Mesodermal
derivatives:
(notochord,
muscle, etc.
Forms of Commitment
• Determination
– A determined region will also develop
autonomously, but its commitment is
irreversible, regardless of its environment
– determined state can be tested by grafting
experiments
• does tissue develop the same or differently
when introduced into new position?
• If same, „determined‟; if different, „not
determined‟
Forms of Commitment
• Potency
– the complete range of developmental
options a tissue can have depending on
environment
– obviously hard to measure…since testing
is environmentally determined
– „competence‟ vs. „potency‟?
– Changes in state of „potency‟? Nuclear
transplantation?
• Mintz experiments (see handout), Gurdon
experiments, Dolly
Forms of Commitment
• Development is a hierarchy…fig 1.1 of
Slack!
– Is tissue „commited‟ to form mesoderm, or
somite, or muscle?
– Hard to test using tools of experimental
embryology, since the „state‟ itself is often
the measure…mesoderm „becomes‟
muscle
– molecular markers are becoming useful in
defining states of hierarchy.
Evaluation of Commitment
• „Mosaic‟ vs. „Regulative‟ mechanisms
– mosaicism: isolation experiments will
determine if region is specified (remember:
this test requires knowledge of fate map)
– if pattern does not correspond to fate map,
the implication is that development is
„regulative‟
– this can be studied through a variety of
experimental manipulations…
Evaluation of Commitment
• Twinning and fusion experiments:
– tests for whether there has to be a
symmetrical deposition of cytoplasmic
factors determining commitment
– excludes localization of determinants
– implies some control of in change of scale,
since dimensions/boundaries are altered
– also implies mechanism of growth control...
stopping at the right time...
Evaluation of Commitment
• Defect Regulation
– ablation experiments can reveal whether
some regions can be removed without
developmental consequence
• sea urchin…equatorial cells in morula
• equivalence groups in C. elegans vulval
development
• Inductive Reprogramming
– grafting of signaling center to new regions
– Spemann and the organizer…new
handouts
Aquisition of Commitment
• Cytoplasmic Determinant: an „entity‟
that guaranties assumption of a
particular state of commitment by cell
which inherits it
– Jeffery article describes early attempts to
isolate/identify these factors
Aquisition of Commitment
• Cytoplasmic Determinant: how might
they function?
• Regulatory molecules that lead to gene
expression (localization of bicoid)
• change in a metabolic states? (role of Toll
activation in ventral cell fate; universal
dispersal, local activation)
• bias in cytoskeletal archetecture (swallow,
exuperantia, pumilio in ant./post. Drosophila
embryonic axis)
• NOT mRNAs governing terminal differentiation
(„molecular preformationism‟?)
Aquisition of Commitment
• Induction: Signaling centers sending out
inductive signals to competent regions
– instructive induction: the responding
tissue has a choice of fates, more than one
outcome
• response to morphogenetic gradient:
concentration of factor triggers different tissue
responses (TGF-b signal transduction; dpp in
Drosophila d/v axis)
• appositional induction: two cell layers come into
contact: (lens formation in vertebrate eye)
Morphogen
Aquisition of Commitment
• Induction: Signaling centers sending out
inductive signals to competent regions
– permissive induction: the responding
tissue has only a single outcome in
response to a signaling factor (e.g. late
developmental events like formation of
kidney and pancreas
factor
Aquisition of Commitment
• Slack: “It is the state of activity of a group
of homeotic genes which encodes the
developmental commitment of a cell.”
– homeotic (selector) gene: transcription factor,
expressed during developmental process, which
can trigger hierarchy of gene activation
– the “off” (repressed) state of the gene may be as
important as the “on” state
Aquisition of Commitment
• Let morphogen gradient turn on three
selector genes at different concentrations
– four different states: 111, 011, 001, 000; this
could conceivably give rise to 4
developmental fields; suppose 2nd gene
mutates….
• a null (loss of function) mutant changes coding to
101, 001, 001, 000
• a constitutive (gain of function) mutant changes
coding to 111, 011, 011, 010
Aquisition of Commitment
• Epigenetic coding: particular combination
of states of activity for selector genes
– how is coding set?
• Maintenance of nuclear equivalence?
• Chromatin modification?
– How stable is epigenetic coding?
• Genomic imprinting?
• Dosage compensation mechanisms?
Conklin‟s Experiments in
Styela (1905)
• Tunicate cells are specified autonomously:
show mosaic development
• different regions of fertilized egg show a
characteristic pigmentation (p. 248 Gilbert)
• during meiosis following fertilization,
cortical movements are triggered that
partition pigments in the cytoplasm to
characteristic locations
Conklin‟s Fate Mapping
• The movements are triggered by
microtubules generated by the sperm
centriole and a calcium ion flux
• different colored regions give rise to
different structures
– clear cytoplasm: ectoderm
– yellow cytoplasm: mesoderm
– slate gray: endoderm