In this video, we're going to be defining organogenesis as well as talking about the specialization of the endoderm. So organogenesis refers to the process of the 3 primary germ layers differentiating into organs and organ systems. As we go through the next few videos and talk about the specialization of each of those layers, what we describe is essentially organogenesis. And fun fact, by the end of the embryonic period or by the end of week 8, all organ systems are recognizable and several are even functional. To give you more context, by that point, the embryo is about 1 inch long. Isn't it amazing to think about how it already has all of its organs? Just, like, mind blowing. Human development is amazing, you guys. Just can't get over that. Alright.
Now, switching gears to talk about the specialization of the endoderm. As a reminder, the endoderm is the inner primary germ layer closest to the yolk sac, and it is going to go on to form the epithelial lining of the digestive system, respiratory systems, as well as the urogenital systems. I want to be clear, we are just talking about the epithelial lining here. It is not going to go on to form the entirety of the stomach or the entirety of the lungs, just the epithelial lining of those structures.
So, if we look at our image here, we are showing development in week 4 and week 5. Week 4, we've drawn in our more traditional color-coded format to help you figure out what you're looking at. Then, week 5 is a bit more realistic to show you how much progress has actually been made here in development. So, you can see here in week 4, our embryonic disc has begun to fold a little bit, get a little bit more cylindrical, if not yet quite human-shaped, and our endoderm is pretty much similar to how we last saw it, just sort of making up that inner layer right there. But by week 5, we have made quite a bit of progress, and you can see how it's already forming the epithelial lining of quite a few different organs. We have the trachea and the esophagus here, the lungs, we have the stomach, the liver, the intestines, as well as the urinary bladder beginning to form over here. So, we'll label that as our endoderm. And, as I mentioned, the yolk sac is attached to that endoderm. So, we can see that here at week 4 and we can see how by week 5, it's already getting noticeably smaller there.
Just to give you a little sense of how some of those extraembryonic membranes are developing, we have our allantois developing here at week 4. And then you can see how by week 5, it gives you a nice sense of how this is going to go on to form a portion of the urinary bladder. So, we can see that that little green structure right there. Alright. So, that is organogenesis as well as the specialization of the endoderm, and I will see you guys in our next video. Bye, bye.