Cleavage

September 20 & 22, 2005

Developmental Biology Biology 4361

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)

Figure 5.4

­ isolecithal eggs ­ radial symmetry

equatorial meridional

axis of symmetry

Holoblastic cleavage ­ Echinoderms

Sea urchin

plane of cytokinesis

Figure 5.21

spindle location and orientation determines cleavage pattern

unequal cytokinesis

equal cytokinesis

spindles (vegetal)

spindles (animal)

Holoblastic cleavage – sea urchin

blastocoel formation

blastocoel ­

ion exchange/ water influx

micromeres

macromeres

Holoblastic cleavage ­ Echinoderms sea urchin

Figure 5.2

Holoblastic cleavage ­ amphibians ­ mesolecithal eggs ­ radial symmetry

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

Figure 5.7

Holoblastic cleavage ­ ascidians ­ isolecithal eggs ­ bilateral symmetry

bilateral plane of symmetry

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

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

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

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

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

Figure 5.16

embryo

extraembryonic endoderm

blastodisc develops into a blastoderm with a subgerminal space

chicken embryo

Discoidal cleavage ­ birds

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

Figure 5.17

vitellophages

Superficial cleavage ­ insects Drosophila

Timing and regulation of the cell cycle

Figure 2.13 Figure 5.31 (reversed!)

mature somatic cell cycle

cyclin/CDK cycle; MPF cycling

mitosis promoting factor MPF ­

Figure 2.16