Lecture 5: Mammalian pre-implantation development Flashcards
Give an overview of mouse development
Mouse development occurs over around 20 days.
• When discussing development people use E to denote embryo stage and then a number which represents dpc (days post coitum). For example: E3.5. I tis commonly indicated as 0.5 onwards as mating takes place at night.
• Before implantation, the egg cell has a thick glycoprotein layer called the zona pellucida. The layer persists for the first stages of cell division.
Mouse development occurs in a number of defined stages.
1) Egg cell is the unfertilised egg.
2) Zygote is formed when the egg is fertilised.
3) The embryonic genome is activated at the point where the blastomere forms and there are two cells.
4) There are two more rounds of cell division until there are 8 loose cells.
5) Cell adhesion proteins such as E-cadherin are expressed and the blastomeres huddle together and undergo compaction. The cells within the sphere form junctions for water and ions to pass through.
6) 8 cell embryo divides and forms a morula which has internal cells surrounded by external cells. Internal cells become inner cell mass (ICM) while external cells become trophoblast (also known as trophectoderm) cells.
7) Trophoblast cells do not produce embryonic structures, they become chorion (part of the placenta). ICM is found on one side of trophoblast ring. ICM becomes the embryo, it is pluripotent (it can’t form the TB). This is the first example of differentiation. The whole structure combined as known as the blastocyst, which implants. This is formed at about 4 days. A primitive endoderm forms around the ICM and there is a cavity as well.
What is a teratocarcinoma?
A teratocarinoma is a special type of cancer. It is caused by differentiation in the absence of correct positional information. It refers to a germ cell tumour that is a mixture of a teratorma (tumour made up of different tissue types such as hair and muscle, usually found in the testes/ovaries) with a carcinoma (or choriocarcinoma or both).
How is stem cell identity defined?
Stem cell identity must be very tightly regulated.
• Stem cell identity is defined by a transcriptional programme of turning genes on and off.
• Cell-type specific master transcription factors guide these programmes.
• Positive and negative feedback stabilises cell identity.
• Epigenetic pathways also contribute.
How can morphogens show positional information?
Morphogen gradients are used to establish positional information.
• Morphogens are released from signalling centres.
• Certain threshold concentrations will activate different transcriptional programmes or modified cell behaviours.
• The sensing of environment and cell-cell contacts can also do this.
How does mosaic development work?
- If one cell of a two-cell Xenopus embryo is removed, then a half embryo will form.
- If a cell is transplanted to another part of the embryo, then they will form the structure they were going to form originally.
- Relies on autonomous specification of cell fate. Fate is determined by cytoplasmic factors within the cell itself.
How does regulative development work?
- If one cell of a two-cell sea urchin embryo is removed, then a smaller but normal embryo will develop.
- If a cell is transplanted to another part of the embryo, then they will form the structure of where they are moved to.
- Relies on specific cell-cell interactions and the signals that it receives.
- Regulative development occurs in early gastrulation.
How are chimeras made?
Chimeras are organism which consist of cells from two or more individuals.
Chimeras can be made from two different 8-cell embryos.
1) Remove the zona pellucida from the embryos.
2) Aggregate in dish.
3) Culture in vivo to form a chimeric blastocyst.
4) Transfer to foster mother.
5) Chimeric progeny form.
ICM cells can also be transferred to another blastocyst to form chimeras.
What are master transcription factors?
There are different master transcription factors. They are at the top of the gene regulation hierarchy. They bind to the enhancers of other genes to activate them. They also bind to their own enhancers to maintain their own expression. They are Oct4, Nanog, Cdx2 and Gata6.
