BIOL 370 – Developmental Biology
Topic #9
Amphibians and Fish: Early Development and
Axis Formation
Lange
Lazzaro Spallanzani – (1729 -1799)
biologist and physiologist who made important
contributions to the experimental study of bodily
functions and animal reproduction.
Spallanzani’s most famous
work examines the process
of fertilization, and he
mechanically isloated
male gametes from female
gametes and was able to
induce fertilization in-vitro.
Various stages of
development in the
typical amphibian.
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.
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.
Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs
(Part 3)
Cleavage of a frog egg
Scanning electron micrographs of frog egg cleavage
Animal and vegetal
pole cell size
differences seen by the
fourth division c).
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.
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.
Cell movements during frog gastrulation
I will split this diagram up
to highlight specifics.
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
Cell movements during frog gastrulation (Part 2)
MID-GASTRULATION
Note the
development in
orange, this
endodermal tissue
will become the
BLASTOPORE.
Identified by the
formation of the
archenteron which
replaces the
blastocoel.
Cell movements during frog gastrulation (Part 3)
X
Later
Gastrulation….
note the
elimination of
the blastocoel.
Cell movements during frog gastrulation (Part 4)
Final Stage of
gastrulation…..
the design is now
called the
GASTRULA.
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?
Early movements of Xenopus gastrulation
Focus on cell
movement/migration that
leads to the formation of
the blastopore.
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.
Epiboly has been most extensively
studied in zebrafish as their development
allows for an easy visualization of the
process.
Xenopus gastrulation continues
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.
The most common place you may have heard this term is
in regard to the intercalated discs in cardiac
muscle tissue.
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
Epiboly of the ectoderm is accomplished by cell division and intercalation
Spemann’s demonstration of nuclear equivalence in newt cleavage
Hans Spemann’s 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.
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).
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.
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.
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.
End.