Embryonic Development (Weeks 3-8): Study with Video Lessons, Practice Problems & Examples

Gastrulation, occurring between days 12-16 post-conception, transforms the bilaminar embryonic disc into a trilaminar structure, forming the ectoderm, mesoderm, and endoderm. The endoderm develops into the epithelial lining of the digestive, respiratory, and urogenital systems. The mesoderm forms the musculoskeletal system, cardiovascular system, and more. The ectoderm becomes the nervous system and epidermis. Organogenesis follows, with all organ systems recognizable by week 8, highlighting the intricate processes of human development.

Gastrulation: Germ Layer Formation (Days 12-16)

In this video, we're going to be talking about gastrulation or germ layer formation and this is happening approximately days 12 through 16 post conception. So a germ layer is just a primary or a very initial layer of cells that forms that will give rise to a specific group of tissues and organs. As we will see in some future videos, each of these layers is going to be highly specialized to go on to form very certain tissues and organs in the human body and the term gastrulation just refers to the actual conversion of our bilaminar or our 2-layered embryonic disc into a trilaminar or a 3-layered disc. So gastrulation forms the three primary germ layers, and those are the endoderm, which translates to inner skin, the mesoderm, which translates to middle skin—I always think of those 'm's going together, 'm' and the 'm' and mesoderm and the 'm' in middle—and then the ectoderm which is the outer skin. Now, the process of gastrulation is going to happen because of the formation of something called the primitive streak. So the primitive streak is a groove on the caudal, or the tail end, of the bilaminar embryonic disc and like I said that will facilitate gastrulation. And I will show you this in our image in just one second, but basically what happens is that cells will begin to migrate toward that primitive streak, and that migration essentially forms the third layer. So if you look at our image here, what we are looking at here is the bilaminar embryonic disc. We are looking straight down so we can only see the top layer which we have drawn here in blue and as you can see on the caudal end of it, we have this little groove that is our primitive streak. So if we were to take a little slice of that bilaminar embryonic disc and put it on its side, it would look something like this. You can see we have that blue layer right here and this yellow layer right there and you can see we have cells migrating toward that primitive streak and that migration is kind of bulking up and building up a middle layer and so over time this will turn into this and now we have a trilaminar embryonic disc or a 3-layered disc. So now you can see we have the blue ectoderm. We have this red mesoderm in the middle, and then we have that yellow endoderm along the bottom. And this can be kind of a hard thing to picture in your head and mentally rotate, so to kind of help you out we added one more image just showing what this looks like from the side over here. So, in the side view, you can see nicely how we have these three layers of the disc. So we have that blue ectoderm, that red mesoderm in the middle, and then that yellow endoderm along the bottom. And what this image shows really nicely is that the yolk sac is actually attached to the endoderm. So we can't see that over here but do be aware that the yolk sac is attached to the endoderm and you're going to see some images in future lessons with the side view and you will see the yolk sac and how that kind of evolves a little bit throughout development, in those videos as well. If you're wondering how this disc goes on to form anything that looks remotely like a human, what happens around week 4 is that this disc is going to begin to undergo folding and that folding is going to make it a bit more cylindrical and then from there it will go on to form a much more recognizable human shape. Alright. So that is gastrulation and I will see you guys in our next video. Bye-bye.

Embryonic Development (Weeks 3-8) Example 1

Alright, so which group-like feature on the embryonic disc facilitates the process of gastrulation? That is going to be the primitive streak, which, remember, is going to be forming on the caudal or the tail end of the embryonic disc, and that basically causes cells to begin to migrate toward that streak, which builds up the middle layer and turns that bilaminar disc into a trilaminar disc. So our answer here is b, the primitive streak.

Which of the following is the middle layer of the trilaminar disc?

• A. Endoderm
• B. Ectoderm.
• C. Mesoderm.
• D. Neuroderm.

Organogenesis

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.

Embryonic Development (Weeks 3-8) Example 2

This one asks us, if there was a malformation in the endoderm of the trilaminar disc, which of the following might you expect to see in the developing fetus? So, we have to keep in mind what structures the endoderm is going to go on to form, which we know is the epithelial lining of the digestive system, respiratory system, as well as the urogenital systems. And so, looking at these answers here, I can see a whole bunch of systems that the endoderm has nothing to do with. So the endoderm is not going to be forming anything related to the spinal cord, it's not going to be forming bones or skeletal muscle at all, and so our answer here is going to be c. If we had some kind of malformation of the endoderm, we may expect to see potential abnormalities in the epithelial linings of crucial systems such as the digestive, respiratory, or urogenital systems.

The embryonic period of a time of rapid organogenesis. By about what age are all organ systems recognizable?

• A. Week 5
• B. Week 6
• C. Week 7
• D. Week 8

Specialization of the Mesoderm

Let's get into the specialization of the mesoderm. As a reminder, the mesoderm is the middle germ layer. If you look at the image here, we're looking at a cross-section of an embryonic disc, zoomed in, and this entire middle portion in red is the mesoderm. One of the first events of embryonic development is that mesodermal cells at the midline of the embryo will form a structure called the notochord. The notochord helps organize the embryo along a central axis. Eventually, during development, the spinal cord will develop along the notochord, which will in turn become the vertebrae along the spinal cord. You can see this clearly in our image, this central structure is a literal cord that extends down through the embryo along the central axis; that is our notochord. We'll label that there.

What will happen next is that the mesoderm, on either side of the notochord, will differentiate into something called somites. Somites are paired, almost cube-like structures. You can see them developing along the notochord. These somites will develop into the skeleton, the dermis of the skin, and also skeletal muscle. Thus, somites transform into the musculoskeletal system as well as the dermis of the skin. Yet, the mesoderm is not done yet. It will also go on to form the cardiovascular system, the kidneys, the gonads, the membrane of most body cavities, as well as the connective tissue of limbs. In this image, you can see that the mesoderm is much larger and bulkier than the other two blue and yellow layers, because the mesoderm forms the majority of your body, hence more cells and more tissue.

We will have a review video coming up soon to help you remember which structures each of these layers will form, as I know this is a lot to remember. Ok, see you guys in our next video. Bye-bye.

Embryonic Development (Weeks 3-8) Example 3

Okay. So during embryonic development, which of the following structures arises from the mesoderm? The mesoderm will develop into quite a few different structures in the human body, but predominantly, those somites are going to develop into the skeleton, skeletal muscle, and the dermis of the skin. And based on that, looking at our answer choices, I can tell that our answer is going to be A, the musculoskeletal system. Looking quickly at these other answers, both the central nervous system and the epidermis are going to arise from the ectoderm and portions of the urogenital system are going to be forming from the endoderm. So our answer here is going to be A, the musculoskeletal system.

Which of the following structures helps to organize the embryo around a central axis?

• A. Somites.
• B. Neural plate.
• C. Notochord.
• D. Allantois.

Specialization of the Ectoderm

In this video, we're going to be talking about the specialization of the ectoderm. So as a reminder, the ectoderm is the outer germ layer, and the ectoderm is going to go on to become the majority of the nervous system as well as the sense organs and the epidermis of the skin. And one nice way to remember that last point is that the epidermis is basically the outer layer of our skin and the ectoderm is the outer dermal layer. So those two go together kind of nicely. Now, the first major event of organogenesis is a process called neurulation and this is usually happening around week 3, so very early in development, and neurulation is what forms the nervous system.

We're going to spend the remainder of this video just focusing on neurulation because, you know, the nervous system is kind of a big deal. So, basically, the first event of neurulation is that a portion of the ectoderm is going to begin to thicken and form something called the neural plate. So you can see in image number 1 here we have that blue ectoderm running around along the outer layer there, and then here in the center right above that notochord, so like right center of that embryonic disc, we have these purple cells here and that is representing our neural plate. Okay, so that's happening at about week 3, and then over the next few days, what's going to happen is the neural plate will begin to fold inward. So you can see in image number 2 here how essentially that neural plate is now folding in on itself.

And like I said, that will happen over the course of a couple of days. And then by about week 4, what happens is that the edges of that neural plate fuse into something that is now called the neural tube. Okay. So the neural plate has now fused into the neural tube, and around the same time, something called neural crest cells are going to be forming between the ectoderm and the neural tube. So if you look over here at image number 3, you can see how our neural plate has now formed this circular structure, the neural tube there, and you can see how we now have these green cells here in between our ectoderm and our neural tube.

So those green cells represent our neural crest cells, and the neural tube is going to go on to create the brain and the spinal cord. So that is essentially going to form our central nervous system and then those neural crest cells are going to form the remainder of our nervous system structures. So the peripheral nervous system, the spinal nerves, the cranial nerves, any ganglia that will all be created by those neural crest cells. Alright, so that is the specialization of the ectoderm and I will see you guys in the next one. Bye bye.

Embryonic Development (Weeks 3-8) Example 4

Alrighty, this one reads the blank becomes the blank which develops into the brain and spinal cord. So if we're thinking about the structure that directly goes on to form the brain and spinal cord, that is going to be the neural tube. So our second blank has to be the neural tube; I'm going to put a little NT there, and then if we're thinking about the structure that forms the neural tube, that is going to be the neural plate. So our neural plate will start off as a literal kind of flat plate and then eventually it's going to be, essentially folding in on itself until it closes and fuses into that neural tube. So our first blank is the neural plate and based on that our answer is going to be d. So, the neural plate becomes the neural tube which develops into the brain and spinal cord. If you were looking at some of those other answer choices just keep in mind that the neural crest cells are going to go on to form structures of the peripheral nervous system, so things like cranial nerves, spinal nerves, ganglia, structures like that, and the endoderm, like we have in answer choice a here, is a completely different layer of our trilaminar disc. We are working within the ectoderm, so our answer here is going to be d. I'll see you in the next one.

Which of the following structures is NOT formed by neural crest cells?

• A. Cranial nerves.
• B. Spinal cord.
• C. Spinal nerves.
• D. Sensory ganglia.

Review of Embryonic Layer Specialization

This is just going to be a quick review of embryonic layer specialization. We've put all this information together in one table for you to highlight all of the major structures that each of those embryonic layers will develop into. And this table is by no means exhaustive, but we did focus on the structures that students are typically expected to know.

So first up, we have the endoderm or the inner skin, and that will become the epithelial lining of many structures within the digestive, respiratory, and urogenital systems. One way to remember that is that you can think of how the endoderm, or this inner skin, essentially becomes the innermost lining of your body. So the inner skin becomes some of the innermost structures of your body.

Next up, we have the mesoderm or the middle skin, and that one is our heavy hitter. So that's going to be forming the skeleton, dermis of the skin, skeletal muscle, the cardiovascular system, kidneys, gonads, the connective tissue of limbs, as well as the membranes of many body cavities. One way to kind of think about this is that many mesodermic structures are literally in the middle of these endodermic and ectodermic structures. So, in between the epithelial lining of your body and the epidermis of the skin. This middle layer pretty much stays in the middle even after all that development has taken place.

And then finally, we have the ectoderm or the outer skin, and that will go on to form the majority of the nervous system, your sense organs, and the epidermis of your skin. And one nice easy memory tool there is that ectoderm literally translates to outer skin and that is what it forms, your epidermis. So that one's pretty easy. And then you can also think about how your brain and your sense organs are basically the parts of your body that help you interact with the outer world or with your environment. So, the outer skin helps you interact with your outer world.

Now, our recommended study strategy here is going to be to mainly just focus on studying the endoderm and the ectoderm because they are so much more specific. And by doing that, you're essentially only memorizing 6 things. So you've got, you know, 3 for your endoderm and then 3 for your ectoderm. So, you know, rather than than just rote memorizing this entire list, just memorize these six things and then if you see anything else on your quiz or your exam, then you'll know that that must be a mesoderm structure. So if you're if you memorize these 6 and then you take your exam and it's talking about what layer turns into the skeleton or the kidneys, you think, well, that's not on my list of 6 things, so that must be the mesoderm. So that's a little easier than trying to memorize all of these things at the same time. So that is our little tip for you there.

Alright, everyone. So best of luck and happy studying. Bye bye.

Here’s what students ask on this topic:

What is gastrulation and when does it occur during embryonic development?

Gastrulation is a crucial process in embryonic development that occurs approximately between days 12 and 16 post-conception. During gastrulation, the bilaminar embryonic disc, which initially consists of two layers, transforms into a trilaminar structure with three primary germ layers: the ectoderm, mesoderm, and endoderm. This transformation is facilitated by the formation of the primitive streak, a groove on the caudal end of the embryonic disc. Cells migrate towards this streak, forming the third layer and resulting in a trilaminar disc. These germ layers will eventually differentiate into various tissues and organs in the body.

What are the primary germ layers formed during gastrulation and what do they develop into?

During gastrulation, three primary germ layers are formed: the ectoderm, mesoderm, and endoderm. The ectoderm, the outer layer, develops into the majority of the nervous system, sense organs, and the epidermis of the skin. The mesoderm, the middle layer, forms the musculoskeletal system, cardiovascular system, kidneys, gonads, connective tissue of limbs, and membranes of body cavities. The endoderm, the inner layer, becomes the epithelial lining of the digestive, respiratory, and urogenital systems. These layers are foundational in the development of the body's tissues and organs.

What is organogenesis and when are all organ systems recognizable during embryonic development?

Organogenesis is the process by which the three primary germ layers (ectoderm, mesoderm, and endoderm) differentiate into organs and organ systems. This process occurs during the embryonic period, which spans from weeks 3 to 8 of development. By the end of week 8, all organ systems are recognizable, and several are even functional. At this stage, the embryo is approximately 1 inch long, and despite its small size, it has developed all its major organs, highlighting the complexity and rapid progression of human development.

How does the mesoderm contribute to the development of the musculoskeletal system?

The mesoderm, the middle germ layer, plays a pivotal role in the development of the musculoskeletal system. One of the first events in this process is the formation of the notochord, which helps organize the embryo along a central axis. Mesodermal cells on either side of the notochord differentiate into somites, which are paired, cube-like structures. These somites develop into the skeleton, dermis of the skin, and skeletal muscle. Additionally, the mesoderm forms the cardiovascular system, kidneys, gonads, and connective tissue of limbs, making it essential for the development of many body structures.

What is neurulation and how does it contribute to the formation of the nervous system?

Neurulation is the first major event of organogenesis, occurring around week 3 of embryonic development. It involves the formation of the nervous system. The process begins with the thickening of a portion of the ectoderm to form the neural plate. This plate then folds inward to create the neural tube by week 4. The neural tube will develop into the brain and spinal cord, forming the central nervous system. Additionally, neural crest cells form between the ectoderm and neural tube, giving rise to the peripheral nervous system, including spinal nerves, cranial nerves, and ganglia.