Term
Where does fertilization occur?
Where does implantation occur?
How many days does it take for implatation to occur?
What is that thing that implants called? |
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Definition
Fertilization occurs at the distal end of the oviduct (fallopian tube).
Implantation occurs in the endometrium of the uterine wall (you hope!).
It takes around five days from fertilization to implantation.
The blastocyst hatches from the zona pellucida and implants. |
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Term
| Tell me about the structure of a fertilized egg as it floats down the gentle stream of the fallopian tube to reach the promised land of the uterus. |
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Definition
Within 24 hours of fertilization, regulated mitotic divison (aka 'cleavage') occurs. The first few divisions subdivide the big zygote into smaller daughter cells. By 4 days, there are 16-32 cells. We call this a morula. The whole things is surrounded by a zona pellucida.
The outer cells of the cell-ball are called the embryoblast. The inner cells of the cell-ball are called the trophoblast. Around day 5 the solid cell-ball hollows some fluid accumulates in the middle. Now we have what's called a blastocyst. That's what implants (after it hatches from the ZP)! |
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Term
| What's the difference between the embryoblast and the trophoblast? |
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Definition
The embryoblast comes from the center of the morula, and its cells give rise to the embryo proper.
The trophoblast comes from the outsides cells of the morula and its cells give rise to the placenta (and other such things that won't actually be a part of the fetus). |
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Term
Is this a dinosaur? If no, prove it by identifying some features.
[image] |
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Definition
[image]
Definitively not dinosaur. |
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Term
| Once touch-down into the uterine wall occurs, what becomes of the trophoblast? |
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Definition
Some of the trophoblast cells become the synctiotrophoblast (merging of mutliple cells to create a multinucleated mass). The synctiotrophoblast penetrates down into the uterine wall and digests the endometiral lining.
The trophoblast cells that do not touch the endometrium are the cytotrophoblast. These cells retain their membranes. This is sort of a reserve set of cells that will help the syncytiotrophoblast grow as it expands further into the uterine wall. |
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Term
| What's going on with the embryoblast from day 5 to day 8 or so? |
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Definition
The embryoblast differentiates into a bilayer embryonic disk. The cells that are closer to the invading synctiotrophoblast are the epiblast (aka primary ectoderm) and the cells beneath them are the hypoblast (aka primary endoderm).
By day 8, some fluid accumulates within the epiblast, forming the amniotic cavity! The remaining space is now called the primative yolk sac. (The space exists because, at first, the trophoblast-derived structures grow more quickly than the embryoblast-derived structures). |
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