Primary Lymphoid Organs - Video Tutorials & Practice Problems

Red Bone Marrow

In this video, we're going to begin our lesson on primary lymphoid organs by focusing in on red bone marrow. Now recall from our previous lesson videos that red bone marrow serves as the primary site of hematopoiesis, which recall is blood cell formation and this is going to include lymphocytopoiesis, which is lymphocyte formation or the formation of both T and B lymphocytes or T and B cells. So what this means is that both T and B cells are going to originate and initially be produced in the red bone marrow. However, recall that after being initially produced, the B cells will remain in the red bone marrow to fully mature and become immunocompetence. Whereas the T cells on the other hand will migrate away from the red bone marrow in their immature states to the thymus where they will mature and become immunocompetence. And so notice that this is exactly what we're showing you down below in this image. So on the far left, we've got this long bone and at the end, you can see the red bone marrow where again, both the T and B cells originate and are initially produced. But again, after being initially produced, the B cells remain in the red bone marrow to fully mature and become immunocompetence. And after becoming fully mature and immunocompetence in the red bone marrow, these mature B cells will migrate from the red bone marrow to the secondary lymphoid organs such as the lymph nodes, for example. So down below here, we can fill this box in with secondary lymphoid organs and the secondary lymphoid organs is where they are going to encounter their antigen for the first time and ultimately become activated to carry out an immune response. Now, again, on the other hand, the T cells, although they are initially produced in the red bone marrow, they do not mature and develop in the red bone marrow. Like the B cells do. Instead, the T cells will migrate away from the red bone marrow in their immature states using the bloodstream to the thymus and in the thymus, they will mature and become immunocompetence. And then after becoming mature, those T cells are going to migrate from the thymus to the secondary lymphoid organs using the bloodstream where again, they will encounter their antigens for the first time and carry out immune responses. So the last thing I'll leave you off with is that hematopoiesis and lymphocytopoiesis processes of the red bone marrow are actually covered in detail in other videos. So be sure to see those videos if you haven't yet seen them. And so this year concludes our brief lesson on the red bone marrow, one of the primary lymphoid organs. And in our next lesson video, we'll get to talk more about the thymus, which is the other primary lymphoid organ. So I'll see you all there.

Introduction to the Thymus

In this video, we're going to continue to talk about primary lymphoid organs as we begin our introduction to the thymus. Now, the thymus is actually a bi lobed organ, meaning that it has two lobes, a right lobe and a left lobe. And it's actually found in a region of our body known as the mediastinum, which is this upper chest area that is behind the sternum between the lungs and above the heart. And the thymus is actually a part of three body systems. It's a part of the lymphatic system as a primary lymphoid organ. It's a part of the immune system as it's the site of the maturation and development of T cells which are leukocytes or white blood cells involved in immunity. And it's a part of the endocrine system as it can serve as a gland to secrete signaling chemicals and hormones. Now, the thymus is not to be confused with the thyroid gland. And so notice that in this image down below here of the adult, the thymus is right here in this upper chest region, right where we described, however, the thyroid gland is in this neck region and it's part of the endocrine gland as it secretes hormones that regulate metabolism. But again, although they are in close proximity to each other, they are distinct organs and they should not be confused with each other. Again, here in this video, we're focused on the thymus. Now, the thymus is functionally almost exclusive to T cell maturation and selection, essentially making T cells immunocompetence, which recall means making them capable of carrying out an effective and appropriate immune response toward a specific pathogen or foreign substance. However, recall that although they are capable of carrying out an immune response, they don't actually carry out the immune response in the thymus. Instead. After becoming immunocompetence, they will migrate to secondary lymphoid organs such as lymph nodes, for example, where they will first encounter their antigen and carry out immune responses. Now, later in our course, when we're discussing immunity, we'll talk a lot more about this T cell selection process. However, this selection process is pretty intense and the selection actually eliminates any potentially problematic T cells leaving only immunocompetence T cells. And this essentially makes the thymus a T cell graveyard as only about 1% of all of the T cells that arrive to the thymus survive this selection process. And that 1% accounts for all of the T cells that provide us immunity. Now, it's important to note that the thymus is enlarged and highly active, especially during early life during the newborn and childhood stage. So notice that over here on the far left of our image, we're showing you an image of early life and notice that the thymus is very enlarged and again, highly active. However, it's important to note that over time, the thymus is going to gradually atrophy or gradually atrophies, which means that it decreases in size and activity over time. And so notice that over here in this adult, the thymus is much, much smaller than in what it was during early life. Now, although the thymus does atrophy decreasing in size and activity, uh uh you know, as life progresses, it still can continue to produce immunocompetence T cells throughout our lifetimes. However, it does. So at a decreased rate and as we age, the thymus tissue is replaced with fibrous and fatty tissue. And over time, the thymus starts to become really difficult to distinguish from surrounding tissues. Now, if we zoom into this thymus over here in the adult, which you'll notice is again, it is a bi lobed organ. So you can see the right lobe is over here and the left lobe is over here. And you'll notice that within each of these lobes, there are these smaller uh pockets and these smaller pockets, we call them lobules or thymic lobules. And so later in our next listen video, we'll get to talk a lot more about the structure of the thymus and its functions as well. Now, what you'll also notice is that in this adult thymus. There's also going to be some adipose tissue shown as this yellow uh structures that you see there. So this here concludes our brief introduction to the thymus. And as we move forward in our course, we'll be able to continue to learn more and apply these concepts and problems. So I'll see you all in our next video.

Which of the following systems does the thymus NOT play a direct role in?

• A. Lymphatic system.
• B. Immune system.
• C. Endocrine system.
• D. Respiratory system.

Structure of the Thymus

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 recalls a bi lobed organ with two lobes. It's got a right lobe and a left lobe. And 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. Now, 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. Now, the trabecula or the septa are going to be inward capsular extensions. So notice down below in the image, we're labeling one of the trabecula and just to make it a little bit easier to see if we can go ahead and highlight it like. So and again, 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. And I'll go ahead and highlight a few of them so that you can see them a little bit easier. Now, these trabecular or septa or inward capsular extensions are going to be dividing the lobes of the thymus into these smaller lobules. Now, 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. Now, what's important to note is that each of these smaller lobules is going to contain an outer cortex and an inner medulla. Now, 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. And as these premature T cells mature, they migrate from the outer cortex toward the inner medulla. And so the inner medulla has mature T cells. And so you can think that the M in inner medulla is for the M in mature T cells. Now, in addition to T cells, both the outer cortex and inner mela 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. Now down below in the micrograph, notice that the outer cortex of each of these smaller lobules is stained in a darker purple color. So 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. Now, what's important to note is that 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. So 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 and 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. And again, we know that the thymus is functionally almost exclusive to T cell development. And so really, it's no surprise that really, it only contains larger populations of T cells and it doesn't really have large populations of B cells. Now, the thymus is also structurally unique in that it contains specialized epithelial cells that other lymphoid organs do not have. And so 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. And 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 into the outer cortex of this smaller lobule. And that's what we can see over here. And so notice that you can see some of the capillaries here in the outer cortex. And 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. Now, 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. So you can see them here uh labeled uh and they're going to be surrounding the capillaries and they prevent the antigens from the blood from diffusing into the outer cortex where again, they could disrupt and interfere with the T cell maturation process. Now, 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. And they're a little bit tough to see in this micrograph, but they're these uh lighter circles that you can see. And 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. And the thymic corpuscle is going to consist of these concentric worlds of these thymic epithelial cells and research is still undergoing what their function, uh what their functions are. 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. But 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. Now, uh 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 worlds that you see here of thymic epithelial cells that are the thymic corpuscle Now, the thymic epithelial cells of the thymus are also going to be important for secreting signaling molecules that can uh 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. So, uh another thing to notice here in this image is that these white gaps that you can see all throughout represent adipose tissue, which again, the adipose tissue will start to accumulate more and more as we age in the thymus atrophies again, decreases in activity and size. So this year really concludes our lesson on the structure of the thymus. And 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.

Primary Lymphoid Organs Example 1

So here we have an example problem that asks which of the following statements about the thymus is true and we've got these four potential answer options down below. Now, option A says it is located in the inferior thorax which is like the lower chest. However, recall from our previous lesson video that this is actually not true. The thymus is located in the upper chest in a region called the mediastinum behind the sternum between the lungs and slightly above the heart. And so we can eliminate answer option A since it's not true. Now, option B says like other lymphoid organs, it contains lymphoid follicles and B cells. However, recall from our last lesson video that the thymus is unique because it actually does not contain lymphoid follicles or lymphoid nodules which have large populations of B cells. So the thymus does not really have these lymphoid follicles or large populations of B cells. The thymus is extremely specific to the development of T cells. And so for that reason, we can eliminate answer option B. Now option C says it is structurally unique compared to other lymphoid organs due to its lack of lymphoid follicles and So recall from our last lesson video that this is actually true, the thymus does lack lymphoid fol follicles. And that is something that helps make it unique compared to other lymphoid organs which usually do have lymphoid follicles. So option C is going to be the correct answer to this example problem. So we can indicate C is correct. Now, option D says after puberty, it stops growing and remains the same throughout life. However, recall that the thymus reaches its maximum size right around puberty and after puberty, it doesn't just stop growing. It actually atrophies, meaning that it decreases in size and decreases in activity. And so it doesn't remain the same size. It actually decreases in size after puberty. So that's why we can eliminate answer option D. So again, see here is the correct answer that concludes this example problem and I'll see you all in our next video.

Why is it important for there to be a blood-thymus barrier?

• A. To prevent maturing T cells from encountering antigens, as it could disrupt their development.
• B. To prevent mature T cells from leaving the thymus.
• C. To prevent immature T cells from the red bone marrow from entering the thymus.
• D. To prevent thymic corpuscles from being exposed to too much oxygen.

Primary Lymphoid Organs Example 2

So here we have an example problem that says to label the indicated areas of the thymus micrograph. And of course, recall from our previous lesson videos that the thymus is a bi lobed organ, meaning it has two lobes. And here in this micrograph, we're showing you just one of the lobes and each of the lobes of the thymus is made up of a bunch of smaller individual lobules. And so notice that this bracket over here is labeling one of these lobules. So we can go ahead and label it as so, however, the lobules are found all throughout the lobe. Now, each of these lobules is going to have an inner region that is in a lighter purple color here in this micrograph that we call the medulla. And also each of these lobules has an outer region that in this micrograph is in a darker purple color and we call this the cortex. Now recall that the immature T cells that are migrating from the red bone marrow arrive on scene at the thymus in the outer cortex. And as those immature T cells are developing and maturing, they migrate toward the inner medulla. So the inner medulla has the mature T cells now surrounding or encapsulating. The entire lobule is a layer of dense, irregular connective tissue that we call the capsule. And the inward imaginations or the inward extensions of the capsule that separate out the lobules from each other are called the trabecula, which are also known as the septa or singular would be septum. And so this here concludes this example problem and I'll see you all in our next video.

____________ are projections of the external capsule that extend inward, & their primary function is to separate the thymus into distinct functional _____________.

• A. Capsules; lobules.
• B. Capsules; trabeculae.
• C. Trabeculae; capsules.
• D. Trabeculae; lobules.