In this video, we're going to talk about the structure of the thymus. So notice that in the image down below on the far left, we are highlighting the thymus, which resembles a bilobed organ with 2 lobes. It's got a right lobe and a left lobe. In this image, we are taking a cross-section of just one of the lobes of the thymus, and that's exactly what we can see over here in this micrograph is just one of the lobes of the thymus. Now each lobe of the thymus is covered with an exterior capsule made of dense irregular connective tissue that provides structural integrity to each lobe of the thymus and defines the boundaries of each lobe of the thymus. Down below in the image, you can see that the capsule is being labeled right here in this micrograph, but just to make it a little bit easier to see, we can go ahead and highlight it all the way around. The trabeculae or the septa are going to be inward capsular extensions. So notice down below in the image, we're labeling one of the trabeculae. And just to make it a little bit easier to see, we can go ahead and highlight it like so. This is an inward capsular extension, so it's also made of dense irregular connective tissue, and they're going to be found all throughout the lobes of the thymus. I'll go ahead and highlight a few of them so that you can see them a little bit easier. These trabeculae or septa or inward capsular extensions are going to be dividing the lobes of the thymus into these smaller lobules. When you take a look at the image down below, again, the entire thing that you can see highlighted here is one lobe of the thymus. However, notice that over here, we're labeling one of the smaller lobules, and over here, we're labeling another of the smaller lobules. But really, these smaller lobules are found all throughout each of the lobes of the thymus. It's important to note that each of these smaller lobules is going to contain an outer cortex and an inner medulla. The outer cortex is going to contain pre t cells or premature t cells. And so after being initially produced in the red bone marrow, the premature T cells migrate from the red bone marrow to the outer cortex of each of these smaller lobules, where the maturation process will begin. As these premature T cells mature, they migrate from the outer cortex toward the inner medulla. The inner medulla has mature t cells. So you can think that the 'm' in inner medulla is for the 'm' in mature t cells. In addition to T cells, both the outer cortex and inner medulla will have dendritic cells, which assist in the T cell maturation process, and macrophages, which clean up the debris from the dead and dying t cells from the t cell selection process. Down below in the micrograph, notice that the outer cortex of each of these smaller lobules is stained in a darker purple color. We can go ahead and label it as the outer cortex, and the inner medulla of each of these smaller lobules is stained in a lighter purple color, as you can see here. It's important to note that the thymus is structurally unique from all of the other lymphoid organs because all of the other lymphoid organs have populations of both t cells and b cells. But the thymus is unique in that it only contains populations of t cells and not b cells. The thymus is structurally unique in that it lacks lymphoid follicles or lymphoid nodules, which all of the other lymphoid organs that we'll talk about moving forward contain. Recall that these lymphoid follicles or lymphoid nodules contain large populations of b cells. And so, because the thymus lacks lymphoid follicles or lymphoid nodules, it also lacks larger populations of b cells. We know that the thymus is functionally almost exclusive to t cell development, and so, really, it's no surprise that it only contains larger populations of t cells, and it doesn't really have large populations of b cells. The thymus is also structurally unique in that it contains specialized epithelial cells that other lymphoid organs do not have. These specialized thymic epithelial cells form what is known as a blood thymus barrier, which as its name implies serves as a barrier to prevent antigens in the blood from diffusing into the thymus where they could potentially disrupt and interfere with the T cell maturation process. This blood thymus barrier is going to be found in the outer cortex of each of these smaller lobules. So notice that over here in this micrograph, we're zooming in to the outer cortex of this smaller lobule, and that's what we can see over here. You can see some of the capillaries here in the outer cortex. Again, the premature T cells from the red bone marrow migrate to the thymus to the outer cortex, where they begin the maturation process and start to migrate toward the inner medulla. The antigens that are in the capillaries that are in the blood do not diffuse into the outer cortex, and this is because of the blood thymus barrier, which is going to be made up of these thymic epithelial cells. You can see them here, labeled, and they're going to be surrounding the capillaries and prevent the antigens from the blood from diffusing into the outer cortex where they could disrupt and interfere with the T cell maturation process. Thymic epithelial cells are also going to form what are known as thymic corpuscles, which are going to be found in the inner medulla of each of these smaller lobules. They're a little bit tough to see in this micrograph, but there are these lighter circles that you can see. Down below in this inner medulla of this lobule, you can see that we're zooming in to show one of these thymic corpuscles, which you can see much more clearly over here. The thymic corpuscle is going to consist of these concentric whorls of these thymic epithelial cells, and research is still ongoing about their function. However, we know that they are involved in the development of a special type of T cell called regulatory T cells, which we will talk more about later in our course when we're talking about immunity. These regulatory T cells are going to be important for preventing autoimmune diseases, which are characterized by our immune system mistakenly attacking our own body's tissues. What you'll notice is that the t cells or t lymphocytes are going to be all in the medulla, and, again, it's these concentric whorls that you see here of thymic epithelial cells that are the thymic corpuscle. The thymic epithelial cells of the thymus are also going to be important for secreting signaling molecules that can have an impact on the development of t cells locally within the thymus, but also can have an impact on the activity of T cells systemically in other parts of the body. Another thing to notice here in this image is that these white gaps that you can see all throughout represent adipose tissue, which will start to accumulate more and more as we age and the thymus atrophies, again decreases in activity and size. This here really concludes our lesson on the structure of the thymus. As we move forward in our course, we'll be able to apply these concepts and problems. So I'll see you all in our next video.