Maternal to zygotic transition (MZT)
Period during which zygotic genes are activated and maternal transcripts are cleared
What are the molecules involved in morula compaction?
- E-cadherin
* Occludin
Describe cell-cell adhesion via E-cadherin
Extracellular domain is attached to extracellular domain of neighbouring cell
Intracellular domain –> anchored to a and b catenins –> actin cytoskeleton
Describe expression of E-cadherin during embryo development.
Expressed from early cleavage
Expression relocated to regions of cell-cell contact at the time of compaction
Describe cell-cell adhesion via occludin
occludin –> ZO-1 and ZO-2 –> cingulin –> actin cytoskeleton
Describe JAM=1 expression during embryo development
No JAM-1 expression detected until pre-compact 8 cell stage
At the onset of compaction, there is intense JAM-1 expression at the apical poles of blastomeres and contact sites
Describe tight junction biogenesis
- From the pre-compact 8 cell stage to the 16-cell morula stage, the embryo is permeable:
- Pre-compact 8 cell stage expresses non-adhesive E-cadherin
- Compact 8 cell stage onwards expresses adhesive E-cadherin. Number of junctional proteins expressed increases as the embryo progresses further.
- JAM is expressed at the compact 8 cell stage
- At the early blastocyst stage, junctional proteins are localised at the apical surface of the cells, forming a tight junction. At this point, the embryo is sealed, which is helpful for blastulation
Describe energy metabolism in the pre-implantation embryo
- There is a switch from pyruvate metabolism to glucose metabolism which happens at the time of pre and post-compaction morula stage
- At the time of compaction (morula stage), the embryo switches from dependence on TCA cycle to glycolysis
- In the early stages of embryo development, there are high levels of pyruvate and lactate. High ATP:ADP ratio allosterically inhibits PFK enzyme (converts ATP to ADP), limiting flux of glucose through the glycolytic pathway before compaction
- After compaction, pyruvate energy source is used up and embryo expresses more GLUTs. Reduced ATP:ADP ratio, increase in ADP has a positive allosteric effect on PFK, facilitating higher flux of glucose through glycolytic pathway
Describe expression of glucose transporters during pre-implantation embryo development
- GLUT4 starts to be expressed at the blastocyst stage
* GLUT5 starts to be expressed from the 8 cell stage onwards
What could be used as biomarkers to predict the fate of embryos?
- Glycolysis profile - viable embryos had higher levels of glucose uptake and low levels of glycolysis. too much glycolysis is toxic for the embryos, indicates stress –> non-viable.
- Oxygen consumption/respiratory activity - higher O2 consumption resulted in pregnancy.
- Amino acid profile
- Amino acid depletion - steady increase in amino acid depletion from day 2-3 to morula to blastocyst stage rather than high consumption in cleavage stages = increased viability, reach blastocyst stage
Describe oxygen consumption during embryo development
During oviductal development (early embryo stages), the embryo has more O2 supplied to them
During uterine development (post-compaction stage), O2 transport in the embryo decreases
What is the impact of oxygen concentration on mammalian embryo development?
5% oxygen (low) improves embryo development - associated with fewer perturbations in global pattern of gene expression
(increased blastocyst formation and cell number)
What is the impact of oxygen concentration on amino acid turnover?
Cleavage stage embryos cultured in 20% oxygen (high) - amino acid turnover is higher than those cultured in 5% oxygen
Blastocyst stage embryos cultured in 20% oxygen (low) - amino acid turnover is lower than those cultured in 5% oxygen - high oxygen damages embryos at the blastocyst stage
What is the impact of oxygen concentration on glucose uptake?
glucose uptake is reduced in embryos cultured in 20% oxygen –> reduced viability
Give examples of amino acids used as energy sources by the early embryo
Glutamine
Aspartate
What are some methods of assessing embryo quality?
- Metabolomic analysis
- Staining - ratio of ICM:TE low = embryo quality poor. high number of apoptotic cells = embryo quality poor.
- Time lapse microscopy
What are 3 important amino acids made by the embryo during development?
- Glutamine
- Glycine
- Alanine
Time lapse microscopy
Captures images over time, providing a combination of morphological, dynamic and quantitative information about developmental events
What information can be gained from time lapse microscopy?
- Time taken for 1st cleavage to occur
- Regularity in duration of cell cycles
- Embryo morphology, fragmentation
What are some other biomarkers of embryo metabolism that are controversial?
- sHLA-G
* Follicular IL-15 and GM-CSF
What can stress in the embryo cause?
- Apoptosis and fragmentation
- Modified gene expression
- Modified metabolism
