Transcript If - Saint Joseph High School

Chapter 7
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How Do Embryos
Develop?
Do Sperm and Egg Contain
Tiny Preformed Individuals?
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Preformation—until 18th century;
miniature infant in sperm and egg
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If . . . human sperm and eggs contain
tiny preformed individuals
and . . . a sperm and egg combine during
fertilization
then . . . twins should be produced
But . . . twins are not often produced
Therefore . . . sperm and eggs do not
contain tiny preformed individuals
Just like Needham, some
people modified the theory
Individual existed in the sperm
OR the egg
Called a homunculus
Dismissed because each
homunculus would then need
to have a homunculus inside a
homunculus inside a
homunculus, etc.
At fertilization the sperm and nuclei
fuse together and make a zygote
What Is the Pattern of
Embryonic Development?
Embryonic development is
the result of cell divisions,
cell differentiations, and
cell growth.
Cleavage
– Fertilization can be internal or external
(sperm and egg join to make a zygote)
– Early cell division (mitosis) is called
cleavage because cells cut in half, but
embryo doesn’t get bigger.
– Forms a hollow ball of cells called a
blastula
Gastrula
– Cells form three layers in a gastrula
– The three layers start to form different kinds of
cells—cell differentiation.
Differentiation
– Groups of differentiated cells form tissues
– Differentiation and growth continue to make
organs with specific forms and functions
– Eventually the organs are integrated to work
together within organ systems
e.g. digestive, circulatory, respiratory, etc.
How Do New Embryonic
Structures Develop?
– Each cell should have every gene, but then how
can the cells act differently?
Is Cell Differentiation Controlled
by the External Environment?
If . . . environmental factors control embryos
and . . . embryos are constantly rotated so that such
factors are disrupted
then . . . the embryos should not develop normally
but . . . Roux’s experiment showed normal
development
Therefore . . . it is likely that embryos are
controlled by internal factors
Is Cell Differentiation Controlled
by the Egg’s Cytoplasm?
If . . . newly fertilized eggs contain differentiated
cytoplasm that cause the cells to develop differently
and . . . one cell of a two-celled frog embryo is killed with a
hot needle
then . . . the remaining cell should develop into half an
embryo
and . . . surviving embryos developed into half embryos
Therefore . . . cell differentiation seems to be caused by
cytoplasm differences
Does Cytoplasm also Control
Cell Differentiation in Sea
Urchin Embryos?
Hans Driesch did a variation of Roux’s experiment
where instead of keeping the living and the dead
cells connected, he separated the two cells
Driesch observed all the separated cells developing
into complete, but smaller, embryos
What Pattern of Development
Occurs in Amphioxus?
If . . . environmental factors control embryos
and . . . embryos are constantly rotated so that such
factors are disrupted
then . . . the embryos should not develop normally
but . . . Roux’s experiment showed normal
development
Therefore . . . it is likely that embryos are
controlled by internal factors
What Did People Know by the
Turn of the Century?
♦ Cell Differentiation Theory (1900)
♦ Chromosome Theory (1903)
Cell Differentiation Theory
Hereditary instructions in nucleus guide embryonic development
The instructions initiate a series of changes in newly fertilized eggs to
produce differentiated cytoplasm
The degree of cytoplasmic differentiation varies from species to species
As cell division occurs, hereditary instructions duplicate so that identical
instructions are passed down
Each cell division splits parent cell’s cytoplasm into two regions, with
differentiated cytoplasm
The differentiated cytoplasm modifies, but does not destroy the
instructions
The modified instructions produce cells with different characteristics in
different embryonic regions
Which Cells Move During
Development and Where Do
They Go?
Gastrula—1920’s Walter Vogt tracked cell movements in
embryos
Stained part of embryo
Held embryos in place with bent glass
Mapped where the stain went
Some cells moved inward forming the dorsal lip
Cavity inside was formed that became the gut
Which Cells Move During
Development and Where Do
They Go?
Three distinct layers formed the
gastrula
Endoderm-became stomach,
intestines, bladder, respiratory tract
Ectoderm-became nervous system
and outer layer of skin
Mesoderm-became connective
tissues, muscles, blood, kidneys,
etc.
Does One Embryo Part
Influence Development of
Another?
If . . . neural-tube development is
induced by notochord cells
and . . . the ectoderm cells are
removed before the notochord
cells appear
then . . . the removed ectoderm
cells should not develop into a
neural tube unless the notochord
cells are nearby
Does One Embryo Part Influence
Development of Another?
If . . . neural-tube development is controlled by
something inside the ectoderm cells
then . . . removed ectoderm cells should still develop
into a neural tube
But. . . the removed ectoderm cells did not
Therefore . . . embryonic induction is supported
Do Other Embryonic Parts Develop
Due to Induction?
If . . . induction of the neural tube is caused by
notochord cells that come from the dorsal lip
region
and . . . the dorsal lip cells are grafted into
another embryo that already has dorsal lip cells
then . . . both sets of dorsal lip cells should
move into the embryo and two neural tubes
should develop
Do Other Embryonic Parts Develop
Due to Induction?
On the other hand,
If . . . the bottom half is the inducer
then . . . only one neural tube should develop.
but. . . two neural tubes developed
Therefore . . . embryonic cells differentiated
because they are induced by nearby cells
Do Eyes Also Develop Due to
Induction?
– Lens and cornea turn out to be induced by the
optic cup (Figure 7.10)
How Does Induction Work?
– Some kind of molecule
– Inducer cells make a lot of molecules
– These chemical signals are the HOTTEST
area of research in biology right now!!!!
How Does Regeneration Occur?
Some adult animals can grow back damaged
tissues and organs
– Regeneration
How Does Acetabularia
Regenerate a Lost Part?
If . . . genes in the nucleus guide regeneration
and . . . stalks of several Acetabularia cells are cut
then . . . a piece containing a foot with a nucleus should
regenerate a new umbrella, but a piece containing an
umbrella without a nucleus should not generate a new
foot
and . . . the cells with a nucleus regenerated the lost
part
Therefore . . . nuclear gene control is supported
How Does Acetabularia
Regenerate a Lost Part?
– This was contradicted later with other experiments,
so probably the chemicals made by genes can
control regeneration even without the genes being
present.
Do All Cells Contain the Same
Genes?
– Looking at chromosomes from same
organism, but different cells under a
microscope show the same banding patterns
(Figure 7.14)
Do All Cells Contain the Same
Genes?
– If . . . all genes are retained during embryonic
development
and . . . nuclei from a many-celled albino frog are
inserted into eggs from a green frog that lack nuclei
then . . . each egg should develop into a complete
albino frog
and . . . they did
Therefore . . . embryo cells (and presumably all cells)
have the same genes
Box 7.1 Can We Someday Clone
Humans?
How to clone
– Cell from adult taken
– Activate inactive genes
– Take an egg from adult female and remove nucleus
– Join the adult cell with a nucleus and the egg
without the nucleus and activate with a spark
– Genes duplicate and cell divides (embryonic
development)
– Implant embryo into surrogate mother to grow
through to birth
Box 7.1 Can We Someday Clone
Humans?
What Are Some Problems of Cloning?
– Clone mortality is high
– Perhaps damage to genes in adult donor
Box 7.1 Can We Someday Clone
Humans?
What Is a Benefit of Cloning?
– Transgenic animals important for
pharmaceuticals
– Can grow human proteins in high quantities
for those in need