Cell and Tissue Biology

Lymphoid Tissue & Skin

The structure of lymphoid tissue is very complicated and far from being fully understood. Lymphoid organs exhibit a three dimensional network of epithelial cells (thymus only) or reticular tissue (loose connective tissue) filled with cells that participate in immune processes. The lymphoid organs are the thymus, spleen, and lymph nodes. There are smaller aggregations of lymphoid tissue present in the digestive tract (Peyer's patches, the appendix, tonsils), respiratory tract and urinary systems.

Some of the major types of lymphocytes:

1) "Classic" T lymphocytes (~90% of T cells): These cells express ab T-cell receptors and are involved in helping to rid the body of viruses and in stimulating antibody production. They are also referred to as ab cells.

2) gd T lymphocytes (10% of T-cells): These are a special subset of T-cells that express gd T-cell receptors. Mounting evidence suggests that these cells may be the first line of defense against invading pathogens crossing mucosal surfaces such as the the lining of the gut. There is evidence that "stressed", i.e. infected, epithelial cells display specific cell surface molecules that the gd T-cells recognize causing them to kill the infected cells. This permits the gd T-cells to rapidly fight a wide range of pathogens.

3) B lymphocytes: When stimulated these cells differentiate into plasma cells which secrete antibodies.

All lymphocytes originate in the bone marrow. ab T-cells mature further in the thymus. Recent evidence suggests that gd T-cells may mature in "cryptopatches", clusters of cells located just under the epithilia of the mucosal surfaces they protect, rather than in the thymus. The B-cells mature in bone marrow.

Lymphocytes continuously recirculate in the body via blood and lymphatic vessels, and complete their proliferation and differentiation in peripheral lymphoid organs.

You should recognize the following structures after completing this laboratory:

1) Lymphoid Tissue

Thymus: cortex, medulla, Hassall's corpuscle, macrophages

Lymph node: capsule, subcapsular sinus, cortex, medulla, lymph nodule with germinal center, high endothelial post-capillary venules, reticular fibers, macrophages

Lymphoid follicles within the appendix

Spleen: red pulp, white pulp

2) Skin (not part of the lymphoid system, but important for defense against foreign invaders)

Epidermis and its five layers

Dermis

Sweat glands and myoepithelial cells

Hair follicles and sebaceous glands

We will start with the thymus. You may want to refresh you memory by looking at the schematic diagrams of the thymus in the textbook or lecture slides.

W95 thymus, monkey

The thymus is divided into lobules by a connective tissue capsule that penetrates into the organ. Follow link [1] to see a labeled lobule. Click link [1] to see the connective tissue capsule. Zoom out once. Each lobule has a peripheral dark zone, the cortex, and a central light zone, the medulla. In this 50x image the cortex of each lobule appears dark purple, while the medulla appears light purple to pink. Note that there is no sharp border between the cortex and medulla.

Follow [1,1]. The cortex is packed with T lymphocytes and their precursors. They are characterized by a darkly stained nucleus and very little cytoplasm. Thymus epithelial cell nuclei are pale staining and larger that the T cell nuclei. Remember that the epithelial cells provide a 3-dimensional framework for the organ. There is also a well fixed macrophage visible in this picture. Click [1] to look at it more closely. The macrophage has plenty of cytoplasm and several granules of various sizes. Also note the mitotic figures. The thymic cortex produces more lymphocytes than any other lymphoid tissue (however most of them die there).

Zoom out twice. The medulla appears pink in contrast to the more darkly stained cortex. This is because the medulla contains fewer lymphocytes. If you can't spot the medulla in this picture, check the labels.

Occasionally a thymic corpuscle, or Hassall's body, can be seen in the medulla. Hassall's corpuscles are seen only in the thymic medulla and are composed of flattened epithelial cells that become filled with granules and filaments. Their function is unknown. Click the thumbnail and then follow [2,1,2] to see a close view of one Hassall's corpuscle. Zoom out once and click [3] for another example.

Practice on this unlabeled virtual slide: W95 thymus, monkey. Can you identify the connective tissue capsule, and the thymic lobules with distinct cortical and medulary regions? Can you zoom in and find a macrophage? How about mitotic figures? To be sure, try to find a mitotic figure in anaphase, which should have a very characteristic profile. Can you recognize the paler-stained, larger nuclei of the epithelial reticular cells? Now move to the medulla and find a thymic corpuscle (Hassel's body).

W95a thymus, young human

This is human material and thus is not well fixed. The cortex is packed with T lymphocytes and their precursors, characterized by a darkly stained nuclei and very little cytoplasm. Follow [1,2,1] to see a few labeled examples of T-cell precursors. There are also poorly fixed macrophages visible in this picture. They appear swollen, pale, almost clear with some intensely staining purple to blue granules. There are also numerous blood vessels in the cortex. These appear bright pink because they are filled with red blood cells (many of which have lysed, making it hard to see individual erythrocytes).

Zoom out twice. The medulla of each lobule appears pink, while the cortex appears purple. As mentioned already, this is because there are fewer lymphocytes in the medulla. Follow [1,1]. The thymus epithelial cell nuclei are paler staining and larger that the T-cell nuclei. A few examples are labeled. A Hassall's corpuscle is also seen in the center of this picture. Zoom out once to see several more examples of Hassall's corpuscles.

Practice on this unlabeled virtual slide: W95a thymus, young human. Find the T-cell precursors, some of the poorly fixed marcrophages, and several examples of Hassel's corpuscles.

This thicker H & E paraffin section Thymus, dog is nice becasue of the clear difference in the appearance of the cortex (darker) and the medulla (lighter). Numerous blood vessels in the cortex are also easy to identify.

Now we move to the lymph node. You may want to briefly refresh your memory using the lymph node schematic diagrams in the textbook or lecture slides.

W07-2 lymph node, monkey, silver stain

The main purpose of this slide is to demonstrate the abundance of type III collagen (reticular) fibers in the lymph node. These fibers form a scaffold for the lymph node, and appear yellow brown with this stain. Follow [1,1,2] to find labeled examples of the reticular fibers. The lymphocytes themselves are stained only faintly. Notice that the arteriole is also supported by reticular fibers.

Zoom out once and follow link [1]. You can see several examples of high endothelial cell post-capillary venules. We will see better examples in the next two slides.

Use this comparable virtual microscope slide W07-2 lymph node, monkey, silver stain to practice finding type III collagen fibers and HEVs.

W07-1 lymph node, monkey

This slide is also silver stained, but in addition has been stained with methylene blue to contrast the cells. Follow [1,2,2,1] to see reticular fibers (type III collagen fibers), high endothelial post-capillary venules and lymphocytes. The endothelial cells forming the walls of the venule are tall and bulge into the lumen. In this view you can see several endothelial cells. We'll cover these in more detail in the next slide.

Zoom out 3 times and click link [1]. Mouse-over to find a lymph nodule and the germinal center within. The germinal center is the place where B-cells are produced (the rapidly dividing cells in the germinal center have a higher ratio of cytoplasm to nucleus, and therefore this region stains less intensely).

W90 lymph node, monkey

Follow link [2]. In the upper left of this slide is a lymphatic vessel. Follow link [1]. The walls of lymphatic vessels, like those of the blood vessels, are composed of simple squamous epithelium or endothelium. Lymphatic vessels have relatively little smooth muscle surrounding them. Follow [1] to find the valve leaflets and lymphocytes within the lymphatic lumen. Zoom out once and click link [2] to find the endothelium and adipocytes labeled.

Click the "thumbnail" and then link [1]. The cortex of the lymph node is labeled in this picture, but is hard to distinguish from the medulla in this specimen. The cortex contains germinal centers and many high endothelial post-capillary venules. Follow link [4]. The germinal center in this nodule appears as a lighter-stained region. It appears this way because the rapidly dividing lymphocytes have more cytoplasm than the non-dividing lymphocytes outside of the germinal center. Use "Mouse-over" to confirm that you have found the germinal center. Note also the lymph node capsule. We will take a closer look at the capsule shortly.

Many high endothelial cell post-capillary venules are also labeled in this picture. Notice the bumpy nature of the endothelial lining of the venules. These bumps are the nuclei of the endothelial cells protruding into the vessel lumen.

Zoom out once and then follow [5,1,1] to see a better example of a high endothelial cell venule. As blood flows through these venules, the lymphocytes enter the lymph node by passing between the endothelial cells and into the lymph node. Note the lymphocytes passing through the endothelium. If the lymphocytes encounter antigens and co-stimulatory factors, they will begin the process of proliferation and differentiation. The lymphocytes leave the lymph node by way of the efferent lymphatic vessels and recirculate into the blood through the thoracic duct.

Zoom out twice and locate the lymph node capsule. Click link [2] for a closer look at the capsule and the subcapsular sinus.

Use this virtual slide W90 lymph node, monkey to practice finding HEV postcapillary venules and a germinal center. Can you spot the valve leaflets in the afferent lymphatic? Do you recognize the subcapsular sinuse?

W90a lymph node, human

You saw this slide in lab 2. Follow link 2 and "Mouse-over" to review the capsule, subcapsular sinus, cortex and medulla. The cortex is rather thin in this node, and the germinal centers are not clearly visible (this is human tissue, and is probably from an old person).

Follow link 5 to look at the medulla more closely. It has cords of cells (lots of macrophages, lymphocytes, and plasma cells) that are supported by reticular fibers. It also has sinusoids that eventually direct the lymph into efferent lymphatics. A cord and several sinusoids are labeled in this picture. You better spot the macrophages! If not, Zoom out once, and then follow [4,1,1].

This thicker paraffin section of a human lymph node b90 lymphnode, human is very nice becasue you can see many examples germinal centers. Also, the cortical (darker) and medullary (lighter) regions are easily distinguished. There is also a nice example of a lymphatic valve in one of the afferent lymphatics. Can you find it?

W43 appendix, human

The appendix is an example of diffuse lymphoid tissue. The layered structure of the appendix wall is the same as elsewhere in the GI tract.

Within the mucosaare aggregations of lymphocytes within large lymphoid follicles or nodules. Some nodules extend almost to the epithelium. Click link [1] to find the layers of the appendix wall and a lymph nodule labeled. Follow [2,1] to review lymphocytes and simple columnar epithelium.

The structure of the spleen is complex and poorly understood. You may want to refresh your memory by looking at schematics of the spleen in the textbook and lecture slides.

W91b spleen, degus.

Traditionally, the parenchyma of the spleen was divided into white pulp (lymphoid tissue) and red pulp (sinusoidal vessels and their supporting framework).

Click link [1]. Note the splenic capsule and the connective tissue trabeculae extending from it. These protect the organ. Click link [1] to see the capsule and a trabecula more closely. The red pulp and white pulp are labeled in this picture. Zoom out once to an overall view of red and white pulp.

This spleen was perfused extremely well, so the venous sinuses are empty of blood and are wide open. Thus the red pulp has been largely removed. This animal was brought up in fairly sterile conditions, hence the white pulp is relatively scanty. However, you can see small lymphocyte sheaths or sleeves around small arteries (central arteries). Follow [4]. The lymphocyte sheath houses T-cells and is referred to as the thymus-dependent zone in the spleen. Click [1] for a high magnification view.

Zoom out twice, and follow [3,1]. The splenic capsule is covered with a simple squamous epithelium. Note the empty sinuses.

Click link [1]. Locate the basal lamina, which is fenestrated in spleen sinuses. (This is REALLY visible only at the EM level.) The endothelial cells are long and spindle-shaped, but are cut in cross-section in this view and, thus, appear round.

Use this virtual slide W91b spleen, degus to practice finding the capsule and trabeculae. Now, find the red pulp and white pulp. Can you find examples of lymphatic shathes around th central arteries? Do you recongnize the splenic sinuses?

W91c spleen, human

This is human material and is, therefore, poorly fixed. (Forewarned is forearmed!)

Click link [1]. Mouse-over to find the white pulp and the trabeculae of connective tissue (these are extensions of the capsule; the capsule is not visible in this picture). The red blood cells in this slide are stained purple, and consequently the red pulp appears as darker stained regions. Follow [1,1] to find a high magnification view of red and white pulp. The white pulp has many lymphocytes (cells with little cytoplasm toward the right). The red pulp has sinusoids that are full of erythrocytes. This is human tissue, and not well fixed, so many erythrocytes have lysed. The red pulp also contains reticular fibers (not visible) supporting free cells. Normally there are many macrophages and plasma cells in the cords, although the poor fixation has rendered them unrecognizable. Zoom out once and click link [2] to see a central artery and lymphocyte sheath.

Zoom out twice and follow [2,1] to look at a trabecula (connective tissue). Zoom out once and then click link [2] to see some examples of neutrophils within one of the larger sinusoids.

This thicker paraffin section b91 spleen, human shows extensive white pulp and lots of germinal centers. The trabecula show up very clearly.

Skin

The skin is the largest single organ of the body, accounting for approximately 16% of total body weight. It consists of the epidermis, an epithelial layer of ectodermal origin, and the dermis, a layer of connective tissue of mesodermal origin. You may want to briefly review skin structure by looking at the shematic diagrams in the textbook or lecture slides.

B87a thick skin, fingertip, human

The epidermis of thick skin consists of five layers of keratin-producing cells. From the outside in, these are: a thick stratum corneum (flattened, non-nucleated keratinized cells), stratum lucidum (not present in thin skin), stratum granulosum, stratum spinosum ("prickle cell layer"), and stratum basale (stem cells). Follow [1,2,1,1] to find all five layers labeled. As you zoom in, make sure to look for the skin layers at each magnification. Review the schematic diagram of the epidermal layers in the textbook and shown during lecture.

The somatic sensations (e.g., pain, pressure, vibration, heat) are transduced by free and/or encapsulated nerve endings. The best known of the encapsulated nerve endings are the Pacinian corpuscles. Click the "thumbnail" and follow link [1,3] to see a Pacinian corpuscle. The nerve ending is enclosed in a capsule containing 20 to 60 concentric layers of connective tissue separated by spaces that are filled with a gel-like material of low viscosity. In cross section, the corpuscle looks like a sliced onion. Zoom out once and click link [4] to view another Pacinian corpuscle.

Use this virtual slide of thick skin B85a Touch Corpuscles Skin Human to practice recognizing the stratum corneum, lucidum, granulosum, and basale. One nice example of a Pacinian corpuscles is visible, can you find it? There are also many nice examples of touch receptors, called Meissner's Corpuscles, present. They are located within fingerlike projections of the dermis into into the epidermis. Can you find them?

W85 thin skin, human

As you investigate the thin skin, note that the stratum corneum is much thinner than in thick skin, and there is no stratum lucidum. Follow [3,1,1,1]. Mouse-over and find three of the layers labeled. Note the desmosomes in the stratum spinosum. Zoom out 2 and follow [2,1] to find the stratum basale also labeled.

Within the skin are sebaceous glands associated with hair follicles. Click the "thumbnail" and follow [2,1] to find these structures.

Zoom out 1 and click link 2. This is a sweat gland. The sweat glands are simple, coiled tubular glands whose ducts open at the skin surface and whose secretory portion is embedded in the dermis. The secretory portion is surrounded by myoepithelial cells that contain actin and myosin and are specialized for contraction. Myoepithelial cells help to discharge the secretions. Follow [2,1] to see these cells.

Use this virtual slide W85 thin skin, human to proactice finding sweat glands and sebaceous glands. Can you find the myoepithelial cells in the sweat glands?

W83h eyelid, rat

Follow [1,1]. Labeled are the hair follicles, sebaceous glands and the keratinized epithelium (epidermis). Click the "thumbnail" and click link [2]. Activate "Mouse-over" and search for the large Meibomian (modified sebaceous) gland. Also labeled is the palpebral conjunctiva, which is composed of stratified non-keratinizing squamous epithelium. Follow [3,1] to see a higher magnification of the Meibomian gland.

Zoom out 2 and click link 1 to see a hair follicle in cross section.

B87 thumbnail, human

This slide shows four of the five layers of thick skin epidermis, as well as abundant sweat glands. Follow link [1] and note the location of the many sweat glands. Follow [1,1] for a labeled view of the 4 visible epidermal layers. If you click the "thumbnail" and go to link [2], the nail itself is labeled. The nail is composed of closely packed plates of hard keratin. These are formed in the "nail matrix", at the base of the nail, by proliferating epithelial cells that become keratinized.

Use the virtual slide B87 thumbnail, human to practice finding the layers of the epidermis, the sweat glands, and the thumbnail.

B87b scalp

The scalp has many hair follicles, sweat glands and sebaceous glands. It is covered in keratinized epithelium. Click link 3 to see hair follicles, sebaceous glands and hypodermis. The hypodermis, or subcutaneous tissue, contains many adipose cells and is not considered a part of the skin. It binds loosely to the skin and corresponds to the superficial fascia of gross anatomy.

Click link 1 to see the associated sweat glands. Zoom out once and click link 2 to see the hair follicle and the papilla. This papilla contains the capillary network that sustains the hair follicle.

Use this virtual slide B87b scalp to practice. Can you find hair follicles, sebaceous glands and hypodermis?