How Do Embryos Develop?

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Chapter 7

• How Do Embryos Develop?

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Do Sperm and Egg Contain Tiny Preformed
Individuals?

• Preformationuntil 18th century miniature infant
  in sperm and egg
• 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

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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.

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At fertilization the sperm and nuclei fuse
together and make a zygote
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What Is the Pattern of Embryonic Development?

• Embryonic development is the result of cell
  divisions, cell differentiations, and cell
  growth.

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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 doesnt get
  bigger.
• Forms a hollow ball of cells called a blastula

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Gastrula

• Cells form three layers in a gastrula
• The three layers start to form different kinds of
  cellscell differentiation.

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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.

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How Do New Embryonic Structures Develop?

• Each cell should have every gene, but then how
  can the cells act differently?

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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 . . . Rouxs experiment showed normal
  development
• Therefore . . . it is likely that embryos are
  controlled by internal factors

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Is Cell Differentiation Controlled by the Eggs
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

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Does Cytoplasm also Control Cell Differentiation
in Sea Urchin Embryos?

• Hans Driesch did a variation of Rouxs 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

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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 . . . Rouxs experiment showed normal
  development
• Therefore . . . it is likely that embryos are
  controlled by internal factors

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What Did People Know by the Turn of the Century?

• Cell Differentiation Theory (1900)
• Chromosome Theory (1903)

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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 cells 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

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Which Cells Move During Development and Where Do
They Go?

• Gastrula1920s 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

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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.

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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

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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

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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

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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

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Do Eyes Also Develop Due to Induction?

• Lens and cornea turn out to be induced by the
  optic cup (Figure 7.10)

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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!!!!

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How Does Regeneration Occur?

• Some adult animals can grow back damaged tissues
  and organs
• Regeneration

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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

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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.

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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)

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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

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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

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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

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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