Download The Sea Urchin (invertebrate)

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CHAPTER 47: ANIMAL DEVELOPMENT
I.
THE STAGES OF EARLY EMBRYONIC DEVELOPMENT
A.
From egg to organism, an animal’s form develops gradually: the concept of epigenesist
1.
Preformation: an 18th century concept stating that the sperm or egg containing a
miniature infant and all that had to occur was for that “infant” to grow larger. It was
also believed that within that same infant were all future infants or descendants.
2.
Epigenesis: states that the egg is basically formless and that animal development
emerges gradually. Once microscopic work was able to observe embryonic
development, scientists could observe these progressive steps.
3.
Current thought: although preformation is not accepted, there must be something in
the zygote that controls development, the genome or cytoplasmic organization,
protein distribution.
a)
Differential gene expression must be regulated by something in the zygote’s
environment.
b)
Cell signaling, division and differential
B.
Fertilization activates the egg and brings together the nuclei of sperm and egg
1.
Introduction
a)
Function of fertilization
2.
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The Acrosomal Reaction
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3.
The Cortical Reaction
a)
Calcium ions are released by the egg’s ER.
b)
This occurs initially at the site of sperm entry and then spreads over the entire
egg.
c)
The presence of calcium ions causes cortical granules in the cytosol to move
towards and fuse with the egg’s plasma membrane and release the calcium
ions into a space between the vitelline layer and the plasma membrane, which
is called the perivitelline space.
d)
Water enters the space expands
e)
The vitelline layer hardens and is now called the fertilization envelope that
prevents further sperm entry.
f)
This hardened fertilization envelope is part of a process called Slow Block to
Polyspermy
4.
Activation of the Egg
5.
Fertilization in Mammals
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C.
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Cleavage partitions the zygote into many smaller cells
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D.
Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut
1.
Gastrulation: this basically involves taking the blastula and rearranging the cells into
three distinctive layers. Each layer will be responsible for producing specific types
of tissues and organs.
a)
Changes in the cells during gastrulation are: (1) the cells move (cell motility);
(2) cells change shape; (3) cells will adhere to new cells and other cellular
material that will hold them in place (extracellular matrix) and therefore
position them in the correct locations.
b)
The end product of gastrulation is the formation of a 3-layered embryo called
a gastrula
2.
The Three Embryonic Germ Layers
a)
Ectoderm: outer part of gastrula
(i)
eventually becomes the nervous system and skin
b)
Endoderm: lines the inner layer of the embryo’s digestive tract
(i)
eventually develops into liver and pancreas and cells lining the dig.
tract
c)
Mesoderm: cells located between the ecto- and endoderm.
(i)
forms the kidneys, heart, muscles and inner layer of skin
3.
The Process of Gastrulation
The Sea Urchin (invertebrate)
a)
cells at the vegetal pole migrate (cells are motile) inwards into the blastocoel.
(i)
this produces an invagination at the vegetal pole and becomes what is
called the blastopore.
(ii) The invagination extends into the blastocoel and is called the
archenterons.
(iii) the archenteron will become the gut that connects the mouth to the anus
The blastopore will become the anus (sea urchin)
b)
Complete gastrulation: formation of a tube (gut) from mouth to anus)
The Frog (amphibian)
c)
The wall at the blastopore is thicker and the dorsal (top or back side) is called
the dorsal lip.
d)
Cells move over the dorsal lip and into the archenterons by involution.
e)
These mobile cells can now migrate and form the endoderm and mesoderm.
f)
Three layers are developed just like the sea urchin
g)
A yolk plug develops at the blastopore and moves inward. This serves as
nutrient supply for gastrula and cell development.
E.
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Organogenesis, the organs of the animal body form from the three embryonic germ layers.
1.
Notochord, neural tube, and somites
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F.
Amniote embryos develop in a fluid-filled sac within a shell or uterus
1.
Introduction
a)
What was needed when animals moved onto land?
2.
Avian Development
a)
Meroblastic Cleavage: embryo doesn’t divide equally but only in a small area
of the egg. The yolk of the bird’s egg allows for just a small area of the
cytoplasm to divide.
b)
Blastodisc: cap of cells at the top of the yolk, which will develop into the
embryonic chick sitting right next to all the yolk, it needs for development.
c)
Blastomeres: these are cells of the blastodisc
d)
Epiblast: upper layer of blastomeres
e)
Hypoblast: lower layer of the blastomeres
f)
There is a cavity between the epi- and hypoblast
g)
Gastrulation:
(i)
Some cells of epiblast move along the blastodisc and then tuck inwards
and produce what is known as the primitive streak.
(ii) Primitive Streak: marks the head to tail line of the bird
Just as the cells move inward at the blastopore of the sea urchin’s
development, cells move inward at the primitive streak.
(iii) Epiblast: these cells form the embryo of the bird
(iv) Cells migrate inward at the primitive streak and form the 3 germ layers.
(v)
As organs develop from the 3 germ layers, the embryo is pinched from
the yolk. This pinching produces a yolk stalk that connects the nutrient
rich yolk to the embryo.
h)
Tissues outside the embryo or 4 extraembryonic membranes
(i)
Yolk sac: contains the yolk; digests yolk; carries nutrients to embryo
(ii) Amnion: contains the amniotic fluid around the chick and maintains a
moist environment
(iii) Chorion: membrane that encloses the chick; shock protection
(iv) Allantois: sac for waste material (uric acid); also is part of the
respiratory apparatus for the chick as it presses against the shell.
Oxygen is transported in blood vessels in the allantois.
The amnion, chorion and allantois are terrestrial adaptations.
3.
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Mammalian Development
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II.
THE CELLULAR AND MOLECULAR BASIS OF MORPHOGENESIS AND
DIFFERNETIATION IN ANIMALS
A.
Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
1.
Changes in cell shape require changes in the cytoskeleton
a)
Formation of neural tube requires elongation of cells and reorientation of
microtubules
b)
Invaginations
2.
The cytoskeleton and the movement of cells
a)
cells move during embryonic development
b)
cells move by extending and retracting cytoskeletal fibers
3.
Extracellular matrix
a)
may function as t racks upon which migrating cells move.
b)
May also provide anchorage for cells as they move.
c)
Chemical messages may inhibit cell movement hence directing cells in a
particular path.
4.
Cell Adhesion Molecules
a) glycoproteins that hold cells together at their destination
b) Cadherins
(i)
CAM that requires calcium ions
(ii) are expressed at different times during development
B.
The developmental fate of cells depends on cytoplasmic determinants and cell-cell
induction
C.
Fate-Mapping can reveal cell genealogies in chordate embryos
D.
The eggs of most vertebrates have cytoplasmic determinants that help establish the body
axes and differences among cells of the early embryo
1.
Polarity and the Basic Body Plan
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2.
E.
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Restriction of Cellular Potency
Inductive signals drive differentiation and pattern formation in vertebrates
1.
Introduction
a)
Induction
2.
The”Organizer” of Spemann and Mangold
3.
Pattern Formation in the Vertebrate Limb
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