Developmental Biology Biology 4361
Cleavage September 20 & 22, 2005 Holoblastic (complete cleavage) Radial (echinoderms, amphioxis) Cleavage Patterns Spiral (annelids, molluscs, flatworms) Isolecithal Bilateral (tunicates) Rotational (mammals, nematodes) Mesolecithal Radial (amphibians) Meroblastic (incomplete cleavage) Bilateral (cephalopod molluscs) Telolecithal Discoidal (fish, reptiles, birds) Centrolethical Superficial (most insects)
Holoblastic cleavage ­ Echinoderms ­ isolecithal eggs ­ radial symmetry meridional Figure 5.4 Sea urchin
equatorial axis of symmetry plane of cytokinesis Figure 5.21
spindle location and orientation Holoblastic cleavage – sea urchin
determines cleavage pattern spindles (animal) spindles (vegetal) equal cytokinesis unequal cytokinesis blastocoel formation ion exchange/ water influx
blastocoel ­ Holoblastic cleavage ­ Echinoderms sea urchin
macromeres micromeres Holoblastic cleavage ­ amphibians ­ mesolecithal eggs ­ radial symmetry
Figure 5.2 Holoblastic cleavage ­ molluscs
­ isolecithal egg ­ spiral cleavage
Figure 5.6 “right­handed” “left­handed” dextral sinistral ­ 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 ­ ascidians ­ isolecithal eggs ­ bilateral symmetry bilateral plane of symmetry
Figure 5.7 Holoblastic cleavage ­ mammals ­ rotational cleavage
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?? Compaction & cavitation ­ mammalian embryo
compaction Figure 5.10 ­ formation of cell junctions fluid uptake from uterus cavitation inner cell mass ­ embryo Figure 5.11 trophoblast ­ hatching from zona ­ implantation in uterus ­ placenta formation ‘Hatching’ of the blastocyst from the zona pellucida
morula with zona pellucida morula after compaction fully developed blastocyst blastocyst hatches from the zona pellucida ­ enzymatic digestion of zona trophoblast early blastocyst inner cell mass Figure 5.12 Meroblastic cleavage ­ fish ­ telolecithal eggs ­ discoidal cleavage
zebrafish – Danio reria ­ blastodisc forms at the animal pole ­ inner cells are continuous with the yolk Cebra­Thomas, 2001 Meroblastic cleavage ­ birds
­ telolecithal egg ­ discoidal cleavage chicken embryo ­ cleavage furrows appear at animal pole of the oocyte to form the blastodisc ­ inner cells are continuous with the yolk ­ egg is laid during blastoderm stage Figure 5.15 Discoidal cleavage ­ birds
chicken embryo blastodisc develops into a blastoderm with a subgerminal space embryo extraembryonic endoderm Figure 5.16 Meroblastic cleavage ­ insects Drosophila ­ centrolecithal eggs ­ superficial cleavage
­ repeated mitosis without cytokinesis ­ multiple nuclei in endoplasm nuclei move towards the yolk­free periplasm ­ pole cells form at posterior pole ­ become primordial germ cells nuclei & cytoplasm form syncytial blastoderm ­ cellularization – formation of cellular blastoderm Figure 5.17 Superficial cleavage ­ insects Drosophila
vitellophages Figure 5.17 Timing and regulation of the cell cycle mature somatic cell cycle
Figure 2.13 Figure 5.31 (reversed!) cyclin/CDK cycle; MPF cycling mitosis MPF ­ promoting factor Figure 2.16
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