Moorman - Embrryology Flashcards
sperm combining with oocyte
the oocyte is stalled in meiosis 2. The sperm needs to be capacitated in order to release it’s enzymes into the oocyte. When the sperm gets to the uterus, the head of the sperm is altered so as to allow it to release it’s enzymes through the corona radiata and zona pellucida (capacitation - happens at 6 hours). It usually takes many sperm all releasing their contents to penetrate the zona pallucida. When the sperm fuses with the oocyte it will cause a massive increase in Ca2+ increase in the oocyte. This will cause the presynaptic sperm to release its contents into the postsynaptic oocyte. This Ca2+ increase will then cause a cortical reaction that prevents additional sperm from fusing to the zona pellucida. This causes the second meitic division to be complete and the begining of mitosis.
Where do the cellular components of the sperm/oocyte complex come from?
exclusively the mom because the sperm is just made up of a pronucleus, centrosome, and microtubules and no other organelles. Everything that happens in the first 3 weeks is from the mRNA that was in the oocyte at fertilization.
first 5 days
Cells will synchronously divide. All cells will be the same size and will be maintained in the zona pallucida. At 5 days they “hatch” from the zona pallucida
What happens on the 5th day?
There are too many cells in the zona pallucida so that it “hatches”. The mass of the cells get large enough so the cells in the middle begin to get hypoxic. In an attempt to mitigate the hypoxia they will migrate towards the ends mass of cells. This then makes an inner cell mass and a trophoblast. The trophoblast becomes the placenta, the inner cell mass becomes the embryo.
At 6 days?
When the trophoblast comes into contact with the body of the uterus it will rapidly divide and invade the lining of the uterus. This is implantation. It will turn into one large cell with many many nuclei. This allows for regional specificity of protein production. The main protein that the nuclei are making is HCG. It isn’t until implantation that HCG levels rise.
ectopic pregnancy
When implantation occurs anywhere other than the uterus. Ampulla of the uterine tube is the most frequent site of ectopic preg (this is where the egg is usually fertilized, but sometimes it stays there and implants).
2 common sites for ectopic prreg
uterine tube is most common in general
rectouterine pouch is most common in peritoneal cavity
embryoblast differentiation
dorsal surface is the epiblast. The hypoblast is the ventral. On top of the epiblast is the amniotic cavity.
3 layers of humans
ectoderm - outside
mesoderm - middle
endoderm - inside
primidive groove/node
In the 4th week we see a small groove/node beginning to form. This is the first sign of mRNA transcription in the fetus. Before it was all maternal mRNA. This begins the process of gastrulation. The primitive crest forms on the caudal end of the dorsal surface of the epiblast.
Sonic Hedgehog
Anything in the body that is morphologically asymmetric is due to Sonic Hedgehog. Like the difference between pinky and thumb.
Setting up sidedness of the embryo
Basically in the primitive pit/node, which is at the tip of the primitive streak/crest, there are cilia beating from the right to the left. This sweeps the fluid on the dorsal surface of the embryo from the right to the left. There are also vesicles in the pit that contain sonic hedgehog which get swept to the left as well. Therefore the cells on the left will get stimulated before the right due to the physical forces as well as the sonic hedgehog induction. This causes a downstream pathway and a sidedness.
what happens if you paralyze the asymmetry in the sidedness process?
Half of the embryos will be normal and half will have everything on the wrong side.
where does the mesoderm come from?
cells of the primitive streak will delaminate and migrate between the epiblast and hypoblast. These were the cells that were just involved in setting up sidedness in the Sonic Hedgehog pathway. Some of these cells will invade the hypoblast and replace it. Remainder of epiblast will be called the ectoderm.
where does notocord come from?
epiblast cells of the primitive node will delaminate and establish the midline notochord between the ectoderm and endoderm cranial to the primitive streak
