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DEFINITIONS:
• Embryology – descriptive study of development
• Developmental Biology- study of processes and
mechanisms behind development; mostly
experimental
• Phylogenetic development- gradual historical
transformation of the species
• Ontogenetic development- transformation of a
rudiment derived from the parent organism into a
new adult individual
Types of ontogenetic development:
1. Embryogenesis- development of the egg
2. Organogenesis- determines subsections of the
body (e.g., vertebrate limb, Drosophila eye)
3. Blastogenesis- development of new individuals by
asexual reproduction
Phases of ontogenetic development:
Gametogenesis, Fertilization,
Cleavage and Blastula formation,
Gastrulation, Organogenesis, Growth
and Histological Differentiation
SCOPE OF DEVELOPMENTAL BIOLOGY:
Molecular Embryology, Biotechnology,
Ecological Developmental Biology, Clinical
Genetics, Evolutionary Developmental
Biology, Bioethics
A science with lots of questions to
be answered…
After viewing the film, face your nearest
neighbor and together, list on a ¼ sheet of
paper some of the questions that you
might want to ask regarding the processes
and mechanisms of development (+ pts.
for the intellectually challenging, - pts. for
the intellectually challenged) .
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 What are the major developmental processes
required for the formation of a multicellular
organism from a sperm and an egg?
 What mechanisms control development?
 Where are the plans or instructions for
development?
 How are they interpreted and used?
 Where are the raw materials for development?
 How did those raw materials get to the embryo?
 How are they used?
 How do different cells and tissues know what to
become?
 How do cells migrate? How do they know where to
go?
 How can we control development?
 To what goals can we apply controlled development?
Development DOES NOT happen by magic.
Information and mechanisms at the
cellular and/or molecular levels are
needed to accomplish development.
KEY PROCESSES IN DEVELOPMENT:
Gametogenesis
Fertilization
Embryonic cell divisions –Cleavage
Generation of diverse cell types: Cell fate
specification; Differentiation (altered gene
expression)
• Tissue organization (Patterning: 3-D
structure): Axis specification,
Gastrulation, Organogenesis
• Post-embryonic development:
Metamorphosis, Regeneration, Aging
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HISTORICAL BACKGROUND
1. Aristotle (340 BC)
 Do all parts of a developing organism come into
existence together and simply grow larger?
 Is development a stepwise process characterized by
progressive organization and an increase in
complexity?
• He observed that new structures arose progressively
in embryos (e.g. blood, blood vessels, heart, blood
vessels around organs). This supported epigenesis
(17th century), i.e., the organism develops in a
stepwise fashion from an unorganized state.
• The embryo was formed from menstrual blood
interacting with a male vital factor present in the
semen. This creative force forms the maternal
substance into embryonic body parts.
3.Caspar Friedrich Wolff (1759)
• Similar to epigenetic theory
but postulated that a
development force inherent
in the matter of the embryo
directs the laying down of
body parts in sequence.
• He laid the foundation for
the germ layer theory by showing that the
material out of which the embryo is
constructed is, in an early stage of
development, arranged in the form of leaflike layers.
2.Bonnet &
Swammerdam: (17th
century)
• Preformation Theory:
embryonic parts are
already present in the
sperm or egg
(animalculists or
ovists) which simply
grow in size in
development.
• Cite your counterarguments for this
theory!
Homunculus
Descriptive and Comparative Embryology
4. Karl Ernst von Baer (1828) most coherent embryological
data
• Baer’s law: More general
features that are common
to all members of groups of
animals are developed in the embryo earlier
than the more special features which distinguish
the members of the group.
• Ex: all vertebrates (brain & spinal cord,
notochord, segmented muscles, aortic arches);
vs. various classes (hair, feathers, limbs)
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5.Muller (1864), Haeckel (1868)
• Biogenetic law: features that are inherited
from the common ancestor of the group have
an ancient origin, and develop earliest
during ontogeny
• Example: “Big 4” characteristics of all
embryonic chordates despite differences in
adult appearance
• Divergent features are adaptations of the
embryo to its surroundings (e.g. placenta)
• However, an embryo does not “pass through”
the adult stages observed in lower animals.
Ontogeny does not
recapitulate phylogeny;
ontogeny is a
shortened and
modified recapitulation
of phylogeny
ONTOGENY RECAPITULATES PHYLOGENY?
But what if a bird was
THE
evolutionary biologist…
This bird would note that: (1) adult humans lack
beaks, just as embryonic birds do (2) their adult
extremities are very much alike (having not
differentiated into wings and legs) just as is the
case of embryonic birds (3) mammals show
only the most primitive form of feather
production. Therefore, birds are far more
advanced than humans, and the adult human
resembles the embryonic bird.
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6. August Weismann (1883)
• Germ plasm theory - every germ cell during
early development receives a complete set of
units of heredity (“ids” or Mendel’s “genes”)
• Development involves orderly unpacking of an
embryo as dictated by ids; interactions
between parts make epigenetic development
possible
• Each egg nucleus contain discrete localized
determinants which result to unequal
distribution of nuclear components during
cleavage
• Cells cannot change their fate if a blastomere
is lost (mosaic model of development)
8. H. Driesch (1891), Endres
(1895), Spemann (1901),
Schmidt (1903)
• If cleavage cells of a sea
urchin were completely
separated, each develop
into a whole embryo
(regulative model of
development).
• Massive cell
rearrangements and
migrations precede or
accompany
specifications of
development, allowing
cells to acquire different
functions.
Experimental Embryology
7. Wilhelm Roux (1905)
• Heat-killed one of the 2 blastomeres of a frog’s egg
• Surviving cell developed half of a complete embryo
• Results either support both preformation and mosaic
development, or reflect use of crude techniques
which possibly caused defects in the other half
Analytical (modern) Embryology
9. T.H. Morgan (1919), Watson & Crick (1953) –
units of heredity composed of sequence of DNA
base triplets are transformed into an array of
proteins, which acting partly on their own or
through other chemical components, transforms
the system that is an adult organism.
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Summary Points:
A FEW LAST WORDS…
Development is epigenetic.
Although it is regulated by the nucleus,
it takes place primarily in the
cytoplasm.
It involves interactions between parts.
The parts arise within gradient patterns
or fields.
Differentiation is in essence the
development of the macromolecular
pattern within the cell.
1. Developmental biology is not only a scientific
discipline. It is also a social discourse that is
deeply embedded in cultural concerns.
2. Reproductive cloning, abortion, stem cell
differentiation, genetic enhancement, gene
therapy, environmental estrogens, sex
selection, and teratogenesis all converge on
developmental biology.
3. Thus, it is crucial that we should be educated
toward having more social responsibility
than we have ever wanted.
GROUP PROJECT:
2. Born without a cerebrum, anencephalic babies
often die within a week. At the time of birth, their
healthy organs could be used for transplants
that could save the life of a child who might then
live a long and productive life. However, by the
time anencephalic babies die naturally, their
organs have deteriorated. Should one save other
lives by taking organs from anencephalic babies
while they are still alive?
3. Who is responsible for the physically and
mentally disabled and what does it mean to a
woman and her family to choose not to abort?
Individual families face a terrible burden of
fighting for their children and fighting for the
disabled, if they choose not to abort.
Discuss with your group a developmental issue
with social dimensions, and generate public
awareness regarding your chosen topic. Ex.:
1. Consider etiology, classification, and
technical aspects of surgical separation, as
well as historical responses to conjoined
twins, highlighting some famous 19thcentury twins. Address ethical issues
concerning surgical separation, including
discussions of the ethics of abortion of
conjoined twins and the ethical dilemmas of
killing one twin in order to save the other.
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