CLEAVAGE
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Ciona intestinalis
http://www.mun.ca/biology/scarr/Garstang_Hypothesis.htm
http://biodev.obs-vlfr.fr/recherche/biomarcell/ascidies/Anim-fromegg.htm
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.07EmbIt9ELpT-YFA1rDieIO4AMQFlmlIwWj
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E. G. Conklin, 1905 - In Styela partida, cells that contain yellow crescent pigment form
muscle (mesoderm)
http://sdb.bio.purdue.edu/AboutThisSite/conklin.article.html
Swalla & Jeffery, 1995 - showed that a maternally derived mRNA was segregated into
cells containing yellow crescent pigment.
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Clear cytoplasm = ectoderm
Yellow cytoplasm = mesoderm
Dark gray cytoplasm = endoderm
Light gray cytoplasm = neural tube and notochord.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=.0f0icUJoQpTbPzZ-do7pAF-n3I2E18sYrYZ
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Frog
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Second polar body
15-30 min after
fertilization
Frog
Point of sperm fusion
with egg determines:
Dor
Ant
1.
2.
3.
4.
Position of grey crescent
Major axes of embryo
Position of dorsal lip
Position of first cleavage
furrow
5-10 min after
fertilization
Ven
Pos
Wilhelm Roux,
1885
Pronuclei will fuse
1.5 - 2 hr after
fertilization
L
R
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http://www.md.huji.ac.il/~yisraeli/embryology/embryo.html
What do these experiments tell us?
1. In some species, the cytoplasm of the zygote is very
organized.
2. That maternally derived cytoplasmic information is
sequestered into specific cytoplasmic compartments.
3. The end result - maternal mRNAs end up in specific
groups of cells - determines those cells eventual fate
a. the cells containing the yellow pigment
cytoplasm of Stylea’s embryos form muscle
b. the cells of the frog embryo that contain gray
crescent cytoplasm become the
chordamesoderm and eventually the
notochord.
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Purposes of cleavage
1. Decrease cell size
2. Increase cell number - allows for formation of blastocoel in many
species by a process called cavitation.
3. In some cases, distribution of maternal
cytoplasmic information to specific groups of cells
4. Beginning of Differentiation - as cleavage
proceeds, you reach a point where the
blastomeres are no longer totipotent
5. Synthesis of embryo derived mRNA for direction of
future development begins during cleavage.
Mammals - starts after a few cleavage divisions;
Frogs - starts during late blastula stage
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STARFISH CLEAVAGE - EQUAL HOLOBLASTIC
A
Meiosis complete at ovulation
Isolecithal, oligolecithal, microlecithal egg
Sagittal view
~32 cell stage
V
View of animal pole
A
View of vegetal pole
V
II
V
III
IV
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UNEQUAL HOLOBLASTIC (may be meroblastic cleavage, but not always)
Amphibians - meroblastic, unequal starting with 3rd cleavage division
Ovulated as primary oocyte. First
meiotic division completed in
oviduct, second after fertilization.
A
cellstage
stage
432
168cell
cell
stage
2
Sagittal view
V
Frog blastula
18View
hr
Mesolecithal vs telolecithal
of animal pole
A
IV
III
II
I
IV
1st - 3.5 hr
2nd - 4.5 hr
3rd - 5.5 hr
View of vegetal pole
V
http://www.luc.edu/depts/biology/dev/stages.htm
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http://www.its.caltech.edu/~bi12/2003/movies/xclvgtop.mov
Structure of the chicken (bird) shelled egg
Polylecithal or megalecithal egg (huge amount of yolk)
Blastodisc
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AVIAN CLEAVAGE - DISCOIDAL
1st - 3 hr
Meiosis I completed almost
immediately after ovulation.
Meiosis 2 completed after
fertilization.
2nd - 3.5 hr
3rd - 4 hr
Ovulated as primary oocyte.
Yolk
Looking down on the animal
pole.
4th - 4.5 hr
5th - 5 hr
A
Discoidal
cleavage is a
type of
meroblastic
cleavage
III
IV
II
V
Yolk
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A
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Delamination
Movie of delamination in digital lab manual
Polyinvagination
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SUPERFICIAL CLEAVAGE - INSECTS
Superficial cleavage is a type of
meroblastic cleavage
pole plasm
http://www.luc.edu/depts/biology/dev/insect.htm
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Human embryo cleavage and cavitation.
http://www.erin.utoronto.ca/~w3bio380/lecture/Lect11/L11.htm
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