Lecture 5

8/10-cell stage

compaction

16-cell stage (morula)

ICM vs. trophoblast decision

32-cell stage

cavitation initiation

107-cell stage (blastocyst)

hatching from zona pellucida

Morula

cells know for the first time what fate they will take on

Maternal transcripts

from mother's follicle cells deposited into oocyte, done before fertilization

Maternal to zygotic transition

First mRNA are from mother, then as the zygote mitotically divides, zygotic mRNA takes over

Maternal and paternal contributions

are not equal

Transcription

regulated by chromatin level methylation status of DNA

Methylation

Dynamic change in rates, depending on where the cell is in development

Lots of methylation in genome

less expression of transcripts in genome; genes not open to RNA pol to transcribe

PGCs first migrating

methylation is super high; cells are only dividing and do not need active gene transcription as they don't need to become a specific type of cell

Fate determination

has low methylation rate as expression is needed since the cells are committing;

Females

Methylation rates drop much slower

Paternal genes

contribute to earliest transcription in embryo

Fertilization

has low methylation

Sperm contribution

methylation rates decrease dramatically right after fertilization

Imprinted genes

highly methylated at all times; stays inactive for one copy between male/female cells; male and female alleles are both transcribed but only one copy can be expressed

Transplantation experiments

used to discover parental imprinting; maternal and paternal pronuclei swapped between 2 embryos; embryos are not viable

2 paternal pronuclei

stunted embryo, normal placenta

2 maternal pronuclei

normal embryo, insufficient placenta

Vertebrates

imprinting only occurs in mammals

150

imprinted genes have been identified

Blastomere

decides whether to become trophoblast or ICM at 16 cell stage

Trophoblast

gives rise to extra-embryonic tissue

ICM

will generate all of the cells in the embryo

Pluripotent ES cells

come from ICM

Interior

cells fated to become ICM

Lineage trace experiments

moved position of blastomere around 16 cell stage; found position dictates fate and cells are plastic (undecided)

Peripheral

cells become trophoblasts

Undecided cells

location predicts fate

32 cell stage

fate is fixed

Cdx2 and Oct4

blastomeres initially express key fate determining transcription factors

Outer cells

down regulate OCt4 and up regulate Cdx2

Inner cells

down regulate Cdx2 and up regulate Oct4

Cdx2

trophoblast promoting transcription factors

Oct4

ICM-promoting transcription factor

Tead4

required to activate cdx2 which turns on trophoblast specific genes to make trophoblasts

ES cells

when oct4 and sox9 are on, the 2 turn on Nanog which together turn on the genes for ICM

OCt4, sox2, Nanog

amplify each other

Precedes implantation and gastrulation

formation of the embryonic shield

Epiblast

becomes 2-layered, forms embryo proper

Hypoblast

primitive endoderm lies below epiblast and contributes to yolk sac; becomes extra-embryonic

Hypoblast and epiblast

derived from ICM

Nanog

becomes epiblast

Gata 6

expressed in future hypoblast cells

Ted E

put genes in categories based on functions

Fraternal

2 oocytes rupture, 2 sperm fertilize the 2

Identical

If cells separate from each other when they are blastomeres or epiblasts, they are plastic enough (can make up lost cell numbers to give rise to 2 separate embryos)

Dividing cells

can take on any fate in the early stages; cells can compensate for lowered cell number by doubling again

32 cell stage morula

Cells get split in half 16 each; cells have enough plasticity to divide again to create 2 32 cell stage morulas; cells are resilient enough to recover from splitting of clump; both morulas go through ICM v trophoblast decision, both develop side by side; splitting time impacts how related the two identical twins are

Later cells split

the more embryonic structures they share

Earliest split

morula stage - come from 2 separate blastocyst and extra embryonic structures around them due to plasticity

Splits

can occur randomly, cells held by cell adhesion molecules

Genetic background

leads to cells to split easily during stages, cells aren't as tightly aligned and more likely to break off

Conjoined twins

incomplete splits; ICM splits, but a connection still remains

Embryonic shield

bilaminar disk

Genetic predisposition for fraternal twins

Ted E