Location of Fertilization & Early Divisions: Implantation

00:00 This seems like a very complex process. Doesn't it? Where you can get a sperm to bind to an egg, go through a couple of processes to get it through the right spot to deliver it's DNA.

00:12 At this point, though, you should think of even though that's a very complex process it must happen because we have about 7 billion people in the earth. How do you go far and now get fertilized eggs into binding to the uterus and having a birth take place? The next step of the process is the fertilization that happened in the ampulla of the fallopian tubes. The egg and its now fertilized component will start dividing and migrating out of the fallopian tubes. This can be seen here in a couple iterations of going from a two-cell to a four-cell to an eight-cell zygote. The zygote then is released into the uterus, divides again to get sixteen cells and then you have a blastocyte. The blastocyte then adheres to the opposite uterine wall, meaning that if you released an egg from the left fallopian tube, it would bind to the right uterus wall. What binds? Well on a blastocyte, you have triple blast cells that are around it. It is these triple blast cells that bind to the epithelial tissue of the uterus and these triple blast cells do one step further. They start to infiltrate into the epithelium of the uterus. These will form precursor cells to what will be the placenta and it will have function besides this infiltrate. It will even have some endocrine function associated with it. So what is this endocrine response? The endocrine response that is done by this pre-placental cells are releasing hCG. hCG peaks by about week ten if you think about the weeks of gestation and this is what is sensed usually through pregnancy test. So people will test hCG. You will not have hCG if you did not have a fertilized egg that has implanted into the uterus. What other endocrine responses happen at this particular time? You also get the release of placental lactinogen. This allows for a more hormonal response and will be directly related to the size of the fetus. The larger the fetus, the more release you'll get, which is a little bit different than hCG. It will start to tail off after it peaks about at week ten but the placenta retains the ability to release endocrine hormones to the body. hCG does one other important thing. It helps preserve the corpus luteum. The corpus luteum will be important in the female to continue to produce progesterone and estrogen.