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Cleavage
September 20 & 22, 2005
Developmental Biology Biology 4361
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Cleavage Patterns Holoblastic (complete cleavage)
Meroblastic (incomplete cleavage)
Isolecithal
Mesolecithal
Telolecithal
Centrolethical
Radial (echinoderms, amphioxis)
Spiral (annelids, molluscs, flatworms)
Bilateral (tunicates)
Rotational (mammals, nematodes)
Radial (amphibians)
Bilateral (cephalopod molluscs)
Discoidal (fish, reptiles, birds)
Superficial (most insects)
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Figure 5.4
isolecithal eggs radial symmetry
equatorial meridional
axis of symmetry
Holoblastic cleavage Echinoderms
Sea urchin
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plane of cytokinesis
Figure 5.21
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spindle location and orientation determines cleavage pattern
unequal cytokinesis
equal cytokinesis
spindles (vegetal)
spindles (animal)
Holoblastic cleavage – sea urchin
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blastocoel formation
blastocoel
ion exchange/ water influx
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micromeres
macromeres
Holoblastic cleavage Echinoderms sea urchin
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Figure 5.2
Holoblastic cleavage amphibians mesolecithal eggs radial symmetry
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Figure 5.6
location and orientation of the spindle determine size and position of blastomeres spindle orientation is under genetic control position of specific blastomeres determines coiling orientation of snail shell
Holoblastic cleavage molluscs
“righthanded” dextral
“lefthanded” sinistral
isolecithal egg spiral cleavage
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Figure 5.7
Holoblastic cleavage ascidians isolecithal eggs bilateral symmetry
bilateral plane of symmetry
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Figure 5.9
first cleavage is meridional second cleavage is meridional in one blastomere, but equatorial in the other one
= rotation of cleavage plane divisions are NOT synchronized axis of symmetry??
Holoblastic cleavage mammals rotational cleavage
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Figure 5.10
compaction
formation of cell junctions fluid uptake from uterus
embryo
Figure 5.11
cavitation
trophoblast inner cell mass
hatching from zona implantation in uterus placenta formation
Compaction & cavitation mammalian embryo
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Figure 5.12
morula with zona pellucida
morula after compaction
early blastocyst
fully developed blastocyst
blastocyst hatches from the zona pellucida
enzymatic digestion of zona
inner cell mass
trophoblast
‘Hatching’ of the blastocyst from the zona pellucida
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CebraThomas, 2001
blastodisc forms at the animal pole
inner cells are continuous with the yolk
Meroblastic cleavage fish
zebrafish – Danio reria
telolecithal eggs discoidal cleavage
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Figure 5.15
chicken embryo
egg is laid during blastoderm stage
inner cells are continuous with the yolk
cleavage furrows appear at animal pole of the oocyte to form the blastodisc
telolecithal egg discoidal cleavage
Meroblastic cleavage birds
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Figure 5.16
embryo
extraembryonic endoderm
blastodisc develops into a blastoderm with a subgerminal space
chicken embryo
Discoidal cleavage birds
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Figure 5.17
cellularization – formation of cellular blastoderm
nuclei & cytoplasm form syncytial blastoderm
pole cells form at posterior pole become primordial germ cells
nuclei move towards the yolkfree periplasm
repeated mitosis without cytokinesis multiple nuclei in endoplasm
Drosophila Meroblastic cleavage insects
centrolecithal eggs superficial cleavage
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Figure 5.17
vitellophages
Superficial cleavage insects Drosophila
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Timing and regulation of the cell cycle
Figure 2.13 Figure 5.31 (reversed!)
mature somatic cell cycle
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cyclin/CDK cycle; MPF cycling
mitosis promoting factor MPF
Figure 2.16