Embryo Cleavage and Blastulation

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Fusion of pronuclei in zygote allow mixing of paternal and maternal chromosomes (syngamy)

Oocyte cytoplasm contains all enzymes/proteins that initiate cleavage and activate embryonic genome

Cleavage:

Mitotic division that increases cell number but not volume. 2 cell to 4 cell to 8 cell as embryo transverses oviduct

Genome Activation:

Transcripts and proteins which initiate embryo groeth are synthesised during oocyte growth

These degrade during cleavage, requiring activation of embryonic genome for new transcription. Timing of major activation varies between species. (2-cell mouse, 4-cell pig)

Compaction:

Morula stage:

Cluster of cells

Outer and inner populations formed

Gap junctions between inner cells

Tight junctions and desmosomes between outer cells

Outer cells strongly interconnect and flatten (compaction)

Blastulation:

Structural changes:

Na+ pumped into morula, causes water influx

Fluid accumulation forms blastocoel cavity

Increased CDX₂ in outer cells cause tropholast differentiations (foetal placenta - chorion)

OCT₄ in inner cells cause inner cell mass differentiation (embryo proper)

Rapid expansion and enzymes hatching from zona pellucida

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Molecular Control

Further differentiation:

ICM further differentiates into epiblast and hypoblast

In the mouse, Oct4 specifies inner cell mass, CDX2 specifies trophoblast -> foetal placenta, Nanog specifies epiblast -> embryo proper, GATA6 specifies hypoblast -> epithelium of yolk sac

Bilaminar embryonic disc:

Hypoblast expands to line epiblast and trophoblast, closes and forms primitive yolk sac

Erosion of polar trophectoderm overlying epiblast (only in domestic species) forms bilaminar embryonic disc

Blastocyst elongation:

Bilaminar embryo initially becomes ovoid

Elongation continues with embryo next becoming tubular.

This does not happen in horses. Horse embryos migrate.

Exponential elongation results in filamentous embryos (continue to elongate into string like structures)

Growth mainly in extra embryonic membranes. Occupies both uterine horns.

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Differentiation in Chick and Mouse Embryos

Chick Cleavage:

Chick egg is yolk rich, cleavage in discrete 2-3mm blastodisc of cytoplasm

Cleavage occurs horizontally, single layered blastoderm

Cells form along the expanding marginal zone with yolk at their base

Later verticalcleavages give rise to a blastoderm 5-6 cell layers deep

Chick Blastulation:

subgerminal cavity forms by secretion of water by blastoderm into yolk

Deep cells in middle of blastoderm die →1 cell thick area pellucida→embryo proper

Peripheral deep cells remain (area opaca)

Hypoblast layer forms from delaminating epiblast cells & migrating sheet of peripheral deep cells →future yolk sac

2-layered blastodermjoined at marginal zones & space between forms blastocoel

Mouse Blastulation and Egg Cylinder Formation:

Development into blatocyst stage quicker than domestic species

B 6 p.c. ICM differentiates into epiblast and hypoblast, which lines blastocoel

An amniotic cavity forms from epiblast (different from trophectodermal folds in domestic species)

Epiblast/hypoblast expand into blastocoel cavity, form into egg cylinder (instead of bilaminar embryonic disc)