Gastrulation = forming of the 3 germlayers: endoderm, ectoderm and mesoderm.
, 1. Cell divisions
Somatic cleavage is the process of normal cells going through mitosis which takes about 16 hours.
The full process consists of G1, S, G2 and M – stage. In which the cell invests in growth and
production of proteins and organelles in stages G1 & G2.
Embryonic cleavage is the process of embryonal cells going through mitosis which takes only 30
minutes. These cells won’t bother investing in growth and/or production as there is no G1 & G2
stages. The embryonic cleavage only consists of S and M-stages. Where in S we get
synthesis/replication of DNA. And in M we get the actual cell division.
The embryo does NOT grow
in size it only grows in # of
cells. That until the blastula
stage where the embryo starts
growing and also starts
migration.
Examples:
- Frog: 1-37 000 cells takes 43 hours
- Fruitfly: 1-50 000 cells takes 12 hours
- Mouse and human: 1-4 cells take around 48 hours
How come humans and mice are so slow? We simply don’t need to hurry up. No need to be
independent as they are protected in the mother. Whilst fish and Xenopus are not protected and so
they need to develop asap to be independent and protect themselves from predators.
, Patterns of embryonic cleavage
Yolk very heavy & dense embryo develops only at one side on top of the yolk and there the cell
divisions happen (see zebra fish).
Xenopus has no yolk, so embryonic cleavage not to one side.
Mammalia only have small amounts of yolk so embryonic cleavage can happen normally.
In the drosophila we see something interesting as we see nucleic divisions but no or few membrane
divisions => forming a syncytium.
Activation of the zygotic genome
Maternal mRNAs are laid down in the oocyte during oogenesis, after fertilization new proteins are
synthesized from the maternal mRNA. First hours embryo uses maternal proteins & RNA’s, it
functions on these maternal proteins (especially in Xenopus & Danio rerio). But when does the
transcription from the embryo’s own genes, the zygotic genome start?
In mouse from the 2-cell stage, and humans from the 8-16-cell stage.
In comparison the frog/xenopus only from 12 cleavages onwards, mid-blastula transition, MBT, 4096
cells). Xenopus needs to be fast and independent to escape/protect from predators. Need to develop
fast to a stage where the zygotic genome can be activated => TXN of own genes. Therefore the
embryonic cleavages is so fast compared to humans and mice with no need to hurry up.
How come the zygotic genome isn’t activated? A repressor is present in the cells at a certain
concentration. And with each division/cleavage the repressor gets diluted to a lower concentration.
Until a point where the concentration is low enough where the zygotic genome can be activated. The
goal of the Xenopus and zebrafish is to get to the point where they can dilute the repressor till the
activation of the zygotic genome asap! To be independent asap!
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