Preimplantation Development

Errors

Hydatidiform mole Þ only paternal info available; error in M2 – all maternal info goes to 2^nd polar body; only placenta available

Correlation in mouse:

male/female chrom Þ gynogenetic Þ female/female Þ diploid parthogenetic embryo (won’t survive)

male/female chrom Þ androgenetic Þ male/male Þ extra-embryonic tissues look good but embryo is horrible

Must therefore have both male and female contribution (called genetic imprinting)

Takes ~One Week for Fertilization Þ Implantation

Cleavage

1^st cleavage occurs 20 hrs after fertilization

Activation of the Embryonic Genome

Occurs at 4-8 cell stage

RNA transcription begins (not working off reserves any longer)

Day 3: Compaction of Morula

8 cell stage

Rounded blastomeres flatten

Gap junctions form Þ permeability barrier between outside and inside of the embryo

Day 4: Blastocoel Formation (blastocyst)

64 cell stage

2 cell layers present

inner cell mass: mass of cells attached to interior of trophoblast Þ embryo proper

Day 6: Late Blastocyst Stage

Loses zona pellucida and attaches to uterine wall

Commitment Þ blastomeres lose totipotency (give rise to any tissue of the conceptus)

Inner cell mass forms epiblast and hypoblast