Lecture 13 - Development III Flashcards
Eureka moment
- What happens with the sperm and oocyte?
- What happens with the oocyte?
- What day # is this?
Eureka Moment - Day -1
- sperm and oocyte plasma membranes fuse
- contents of sperm go into the oocyte
- oocyte completes Meiosis II (mature oocyte)
- What event starts fertilization?
- What event precedes the completion of fertilization?
- How long does the overall process take?
- Is the product haploid or diploid? What happens next (mitosis or meiosis)?
- What day # is this?
- Eureka moment: Official start of fertilization: contact and fusion of sperm and oocyte plasma membranes
- Fertilization complete - Day 0
- Chromosomes of the egg and sperm decondense. Each forms their own pronuclei, which fuse.
- Fusion creates a diploid cell, which then undergoes mitosis!
On what day does the first embryonic cleavage happen? How many cells are there? Where is the embryo?
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Day 1 - 2 cell stage - Fallopian tube
On which day is the embryo 4 cells? Where is it?
Day 2 - 4 cells - Fallopian tube
How many cells does an embryo have for biopsy/PGD?
What is PGD?
What cell stage is this called? Are the cells differentiated yet?
Are they expanding in size yet? Why?
Where is the embryo?
Day 2.5 - 3: 8 cells (blastomere)
- no cell differentiation yet
- no expansion in size, just shape changes; the zona pellucida is still around, which restricts size
- the blastomere is still traveling from the Fallopian tube down to the uterus
Morula
- What # day(s)?
- What occurs at this stage?
- Is the zona pellucida still around?
- Does differentiation occur yet? Explain what happens.
- Where is it?
Day 3 - 4: 8 cells (morula)
- cells change shape
- -> compaction: into a compact ball of cells which still has the zona pellucida
- differentiation begins
- polarity is established
- half becomes the inner cell mass (–> embryo)
- the other half becomes the trophoblast (—> placenta)
- The morula is still traveling to the uterus from the Fallopian tube.
Blastocyst
- What # day(s)?
- Where is it?
- What occurs at this stage?
- What is blastocyst hatching?
- Is the zona pellucida still around?
Day 4 - 14: 8 cells (blastocyst)
- Day 4-5: morula arrives in uterus and cleaves/gets bigger
- the morula forms the blastocoel (fluid-filled cavity), which identifies it as a blastocyst
- blastocyst hatching: increasing pressure of the blastocoel (fluid filled cavity) causes the zona pellucida to burst and it is shed
- The embryo is now ready for implantation as a blastocyst
Gastrula
- What # day(s)?
- Where is it?
- What occurs at this stage?
- What is blastocyst hatching?
- Is the zona pellucida still around?
Day 15 - 17: Gastrulation
Day 8-12: The inner cell mass of the trophoblast differentiates into a bilaminar disc which has 2 sides (epiblast, hypoblast) as well as the trophoblast and becomes a syncitiotrophoblast, which produces hCG (sustains corpus luteum/pregnancy test) and moves to the endometrium.
Day 15-17:
The epiblast then differentiates into the 3 germ layers (ectoderm, mesoderm, and endoderm).
The hypoblast delaminates to form the egg yolk/non-embryonic tissue and allows for endoderm formation.
The trophoblast becomes the placenta.
Name the 7 key events (re: the sperm/oocyte) that must occur for fertilization to be initiated and completed.
- sperm ejaculation
- sperm capacitation
- acrosome reaction (ZP3 protein-oocyte binds ZP3 receptor-sperm)
- membrane fusion (oocyte/sperm) - Eureka moment - official start of fertilization
- oocyte completes meiosis II
- Cortical reaction - block polyspermy
- fusion of sperm/oocyte pronuclei –> diploid cell –> mitosis
Sperm capacitation
- What step(s) precede and follow this step?
- What happens?
- What is the analogy to this reaction in IVF? Why is it necessary?
- Ejaculation –> 2. Sperm capacitation –> 3. acrosome reaction
- glycoprotein/seminal protein removal; improves motility/makes acrosome more accessible
- IVF: thorough washing
What does ACE do? Where does it come from? What does it act upon? Where in the fertilization process does it act?
- Sperm capacitation –> 3. acrosome reaction –> 4. Eureka moment - fertilization initiated
Step 3. ACE (angiotensin converting enzyme)
After sperm capacitation,
- the sperm’s ZP3 receptor binds the oocyte’s ZP3 protein.
- ACE in the sperm acrosome break down the sperm membrane
- other enzymes are released, such as hyaluronidase, which break down the oocyte matrix.
- Sperm penetrates through follicular cells and zona pellucida
- The acrosome reaction is complete
- contact and fusion of membranes - Eureka moment - Fertilization initiated
Cortical reaction
- Where in the fertilization process does this occur?
- What does Ca2+ do?
- Describe what happens.
- When does the oocyte complete MII?
Cortical reaction
- plasma membranes fuse (Eureka moment)
- oocyte completes meiosis II
- cortical reaction - block polyspermy
- Rapid, strong Ca2+ pulses from the oocyte cytosol trigger granule release (need correct amplitude and frequency)
- sperm-oocyte membrane fusion triggers rapid exocytosis of cortical granule contents (lysosomal derivatives) which cross-link and harden the zona pellucida
- lysosomal enzymes break down ZP3 proteins on other sperm - can’t get in the oocyte
- The remaining trapped sperm die / apoptosis
- This blocks triploidy (lethal) and polyspermy
- Then the sperm/egg chromosomes decondense. Each forms their own pronuclei, which fuse –> diploid cell (fertilization complete - 24 h total)
