Download Cleavage

Cleavage follows fertilization.
Functions of cleavage:
Multicellular for differentiation
The zygote is partitioned into blastomeres.
Each blastomere contains different regions of the undivided
cytoplasm and thus different cytoplasmic determinants.
Restores Somatic Nuclear to Cytoplasmic Ratio
1:500
-> ->->
1:6
sea urchin fertilized egg
at end of cleavage - somatic cell
Controls on # of cleavage divisions?
2n = 6 divisions
4n = ?
n=?
first two cleavages are vertical.
The third division is horizontal.
The result is an eight-celled embryo with two tiers of four cells Yolk is most concentrated at the vegetal
pole and least concentrated at the animal pole.
In animals with less yolk there is complete division of the egg: holoblastic cleavage.
Holoblastic equal
Holoblastic unequal where there
is more yolk at the vegetal pole.
Cleavage occurs more
rapidly in the animal
pole than in the
vegetal pole.
A blastocoel forms within the morula  blastula
In birds the yolk is so plentiful that it restricts cleavage to the
animal pole: meroblastic cleavage.
What Controls Cleavage?
Information in Cytoplasm – Source?
Spiral Cleavage in Snails
Rt = Dextral (dominant)
Left= recessive
Female dd X male DD
F1 genotype vs. phenotype
F1 cross
dD X dD
Prediction? Genotype
Phenotype
Gastrulation rearranges the
blastula to form a three-layered
embryo with a primitive gut
 Gastrulation rearranges the embryo into a
triploblastic gastrula.
– The embryonic germ layers are the ectoderm,
mesoderm, and endoderm.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fate map
Ability of the micromeres to induce presumptive ectodermal cells to acquire other fates
8.10 Ability of the micromeres to induce a secondary axis in sea urchin embryos
Normal sea urchin development
Fate map – vital stain
Invagination
Involution
Epiboly
Avian Development.
Cleavage is meroblastic, or incomplete.
Cell division is restricted to a small cap of cytoplasm at the animal pole.
Produces a blastodisc, which becomes arranged into the epiblast and
hypoblast that
bound the
blastocoel, the
avian version
of a blastula.
During gastrulation some cells of the epiblast migrate (arrows) towards the
interior of the embryo through the primitive streak. The primitive knot is where
the future notochoral mesoderm forms.
Ingression
(immigration)
Once again, the embryonic membranes – homologous
with those of shelled eggs.
Chorion: completely surrounds the embryo and
other embryonic membranes.
Amnion: encloses the embryo in a fluid-filled
amniotic cavity.
Yolk sac: found below the developing embryo.
Develops from the hypoblast.
Site of early formation of blood cells
which later migrate to the embryo.
Allantois: develops as an outpocketing of the
embryo’s rudimentary gut.
Incorporated into the umbilical cord, where it
forms blood vessels.
Activation of embryonic genome
Mexican axolotl o-mutant strain
The “o” gene is a recessive gene
“O” gene is the normal, dominant gene
In embryos obtained from female axolotls homozygous for
gene “o”, development is always arrested during gastrulation.
WHY?
“O” protein is necessary to activate the embryonic genome.
PROOF?
The eggs can be rescued by injecting eggs of o/o females
with cytoplasm from normal eggs.
Where is the "corrective component" (O protein) produced?
Rescue Experiments
Cytoplasm from Egg
vs.
Primary Oocytes Cytoplasm
vs.
Primary Oocyte fluid from germinal vesicle
What is the “O” protein doing?
activation of the embryonic genome.
How can we prove it?
When is mammalian embryonic genome activated?
Two-cell stage determined by radioactive labeling experiments
Imprinting of mammalian gamete’s nucleus
Experiments combining:
Female pronucleus combined with Female pronucleus
Male pronucleus combined with Male pronucleus