Amphibians and Fish: Early Development and Axis Formation

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Amphibians and Fish Early Development and Axis
Formation

• BIOL 370 Developmental Biology
• Chapter 7
• (Chapter 8 in the textbook)
• Lange

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Various stages of development in the typical
amphibian.
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Figure 7.1 Reorganization of the cytoplasm and
cortical rotation produce the gray crescent in
frog eggs
The Grey Crescent in frog eggs Due to the
reorganization of the cytoplasm and rotation of
the cortex In a) 50 of the cell cycle is
complete, but no polarity In b) (70 of the
cell cycle complete) we see how the microtubules
in the cells become parallel in the vegetal
hemisphere. Together, these movements create the
grey crescent.
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Figure 7.1 Reorganization of the cytoplasm and
cortical rotation produce the gray crescent in
frog eggs (Part 2)
The gray crescent is a region of intermediate
pigmentation where the first identifiable aspects
of gastrulation will be seen. (two slides from
now there is an even better rendition of this
crescent.
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Figure 7.1 Reorganization of the cytoplasm and
cortical rotation produce the gray crescent in
frog eggs (Part 3)
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Figure 7.2 Cleavage of a frog egg
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Figure 7.3 Scanning electron micrographs of frog
egg cleavage
Animal and vegetal pole cell size differences
seen by the fourth division c).
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Figure 7.4 Depletion of EP-cadherin mRNA in the
Xenopus oocyte results in the loss of adhesion
between blastomeres and the obliteration of the
blastocoel
The EP-cadherin (named because it appeared
initially similar to both the E-cadherin and the
P-cadherin) is required for adhesion in the
blastomere Without these proteins, the formation
of the blastocoel is not possible.
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Standardized Color Scheme Ectoderm outer germ
layer will become nervous system, tooth
enamel, epidermis, lining of the mouth, anus,
nostrils, sweat glands, hair and nails. Mesoderm
middle germ layer will become the muscle
(smooth, cardiac and skeletal), connective
tissues, dermis, hypodermis (subcutaneous layer
of the skin), bone, cartilage, red blood cells,
white blood cells, kidneys, and the adrenal
cortex. Endoderm inner germ layer will
become a variety of epithelia including the
alimentary canal (excluding specialized parts of
the mouth, pharynx rectum), the lining cells of
all the glands, trachea, bronchi, and alveoli of
the lungs, endocrine glands, auditory tube,
urinary bladder and parts of the urethra.
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Figure 7.6 Cell movements during frog
gastrulation
I will split this diagram up to highlight
specifics.
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Figure 7.6 Cell movements during frog
gastrulation (Part 1)
Gastrulation is a phase early in the embryonic
development of most animals, during which the
single-layered blastula is reorganized into a
trilaminar structure known as the gastrula.
EARLY GASTRULATION
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Figure 7.6 Cell movements during frog
gastrulation (Part 2)
MID-GASTRULATION Identified by the formation of
the archenteron which replaces the blastocoel.
Note the development in orange, this endodermal
tissue will become the BLASTOPORE.
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Figure 7.6 Cell movements during frog
gastrulation (Part 3)
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Later Gastrulation. note the elimination of the
blastocoel.
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Figure 7.6 Cell movements during frog
gastrulation (Part 4)
Final Stage of gastrulation.. the design is now
called the GASTRULA.
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Figure 7.7 Surface view of an early dorsal
blastopore lip of Xenopus
In this Xenopus example, which side is the
vegetal and which side is the animal region?
Why did you select the positions you did?
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Figure 7.8 Early movements of Xenopus
gastrulation
Focus on cell movement/migration that leads to
the formation of the blastopore.
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Figure 7.9 Epiboly of the ectoderm

• Epiboly
• a cell movement that occurs in the early embryo,
  at the same time as gastrulation.
• It is one of many movements in the early embryo
  that allow for dramatic physical restructuring.
• Movement is characterized as being a thinning and
  spreading of cell layers.

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Epiboly has been most extensively studied in
zebrafish as their development allows for an easy
visualization of the process.
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Figure 7.10 Xenopus gastrulation continues
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Figure 7.10 Xenopus gastrulation continues (Part
1)
The archenteron is the primitive gut that forms
during gastrulation in the developing embryo is
known as the archenteron. It develops into the
digestive tract of an animal.
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The most common place you may have heard this
term is in regard to the intercalated discs in
cardiac muscle tissue.
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Figure 7.10 Xenopus gastrulation continues (Part
2)
Radial intercalation - part of the process of
epiboly involves radial intercalation. Interior
cells of the blastoderm move towards the outer
cells, thus "intercalating" with each other. The
blastoderm begins to thin as it spreads toward
the vegetal pole of the embryo until it has
completely engulfed the yolk cell.
To intercalate means to insert (something)
between layers
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Figure 7.13 Epiboly of the ectoderm is
accomplished by cell division and intercalation
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Figure 7.14 Spemanns demonstration of nuclear
equivalence in newt cleavage
Hans Spemanns work in 1903 demonstrated the
concept of nuclear equivalence in this elegant
experiment partially constricting the fertilized
egg. The resultant development is associated
with twinning.
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Figure 7.15 Asymmetry in the amphibian egg
Notice how normal development only proceeds when
the cellular constriction occurs along the
correct plane. because the embryo is already
asymetrical (as seen with the grey crescent).
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Figure 7.16 Determination of ectoderm during
newt gastrulation
Notice how in the early gastrula the neural
ectoderm transplant retains plasticity in
development and becomes epidermis. By the time
the embryo reaches the late gastrula stage this
plasticity is lost.
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Figure 7.17 Organization of a secondary axis by
dorsal blastopore lip tissue
Speeman Mangold, in 1924 differentially colored
embryos and then studied the organization of a
secondary axis by transferring dorsal lip
tissues. This further shows how a twinning may
arise.
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Figure 7.19 Transplantation and recombination
experiments on Xenopus embryos
Vegetal cells lying under the prospective
blastopore lip begin gastrulation.
Transplanting a slice of very dorsal vegetal cell
in the 64-cell stage leads to twinning.