In this video, we're going to be going over extraembryonic membrane development, kind of zoning in on weeks 2 through 8. So extraembryonic membranes are specialized membranes that develop outside of the embryo, and they're going to be helping to protect the conceptus as well as assisting with vital functions like nutrient and gas exchange and waste removal. Some will even go on to form structures in the human body. So we're going to begin with the yolk sac, which forms from the cells of the primitive gut. When we think of a yolk sac, we often think of how a yolk sac would work in a chicken egg where it's going to be the main source of nutrition for that embryo throughout development. But that's not quite how yolk sacs work in humans. These really only have a nutritive function for about 2 to 3 weeks or until the placenta can take over that function. In humans, what we are going to see is that the yolk sac will eventually go on to form part of the digestive tract. It will also be helping to produce some very early blood cells and will be producing some very early germ cells, also known as gametes. And I know that that is a little bit of an eclectic list. The yolk sac is kind of a jack of all trades. And, you know, looking at these, it's pretty easy to remember the digestive tract one because, you know, you often think of, like, yolk, like, nutrition, digestion. They all kind of go together pretty nicely, but these other 2 sort of come out of left field. So to help me remember this, I like to think of the phrase delightful baby giggles, where the first letter of each of those words will stand for one of these functions. So we have that 'd' for digestion, the 'b' for blood, and then 'g' for gametes or germ cells, if you prefer. When you're thinking about the function of the yolk sac, think of those delightful baby giggles.
Now moving on, we're going to be talking about the amnion. So the amnion is a transparent membrane that develops around the conceptus and this will create the amniotic cavity. The amnion secretes amniotic fluid, which I'm sure we've all heard of before, and amniotic fluid is derived from maternal blood, but once the fetus has functioning kidneys, fetal urine will also contribute to that fluid volume as well. Amniotic fluid has a variety of functions. It's going to help to protect the conceptus from trauma by kind of acting like a shock absorber if there's any trauma to the pregnant person's abdomen. It will also help to maintain a consistent temperature which is important because fetuses cannot thermoregulate on their own and it also helps to support symmetrical muscle development. And it does that by making sure that the conceptus is free-floating. And when a conceptus is free-floating, it's going to have a full range of motion for all of its arms and legs, and so that allows for that nice symmetrical muscle development to happen.
And then finally, we have the allantois. So the allantois is a small pocket of embryonic tissue around the base of the yolk sac, and it's going to go on to form portions of the umbilical cord as well as assist in the formation of some of that surrounding vasculature. And it will also go on to help form a portion of the urinary bladder. Alright. So if we look down at our image here, we are showing the development of these embryonic membranes at week 2 and at week 5. And I know our week 2 image is looking a little bit busy. Don't worry about most of this. Just to kind of orient you to what you're seeing, this right here is that bilaminar embryonic disc. So we have our blue layer and our yellow layer right there, and we are still embedded within that uterine wall here. Alright. So just like we saw previously, this blue portion right here is that amniotic cavity. And you can see how by week 5, this is looking much more recognizable like the amniotic sac that we've all seen before. So by week 5, it's completely surrounding that conceptus and the conceptus is free-floating within that amniotic fluid. So we'll label that as our amnion. And then this yellow structure right here is our yolk sac, and you can see how by week 5 that yolk sac is external to the amniotic cavity and you can see how it's kind of entering the conceptus right around the abdomen area where it's going to be forming part of that digestive tract. So that is our yolk sac. And then finally, we have our allantois. You can see here we have this little green structure kind of coming off the base of the yolk sac there. And you can see here this is a very, very early version of that umbilical cord, which is much more recognizable over here in week 5. So you see we have the umbilical cord and that little green allantois is kind of embedded in there. You can see how the allantois is entering the embryo right around that like navel-like urinary bladder region. And, again, that will go on to help form a portion of the urinary bladder, later on. So, we'll label that as our allantois. Alright. So that is our extraembryonic membrane development, and I will see you guys in our next video. Bye-bye.
