Wheater's, Ch 11: Immune System

Junquiera, Ch 14: The Immune System & Lymphoid Organs

Overview:

The goal of this lab is to examine the organization of the major organs of the lymphatic system. By the end of the lab, you should be able to describe and distinguish lymph nodules, tonsil, lymph nodes, thymus, and spleen using the criteria given in the table below. Note that most of these distinguishing criteria can be observed at low magnification.

*Depending on the sample preparation, the serosa and mesothelium covering the spleen may or may not be visible.

I. Lymph Nodules

Webslide 0060A_E: Ileum, human, H&E  [DigitalScope]

Examine this slide using low power objective settings. Note that the lamina propria of this intestinal tissue contains many small lymphocytes within diffuse and nodular lymphoid tissue. There is no connective tissue capsule isolating the lymphoid tissue as in the lymphoid organs (tonsils, spleen, and lymph node). The uppermost section in Webslide 60 shows a series of nodules (called Peyer's Patches) which have germinal centers.

II. Tonsils

Webslide 0034_E:  Tonsil, H&E   [DigitalScope]

The tonsils contain aggregates of lymph nodules which are embedded in the wall of the throat. Note the lymph nodules which are found between the partial connective tissue capsule and the epithelium covering the tonsils which is continuous with the epithelium of the throat. This stratified squamous epithelium dips into the underlying tissue to form tonsillar crypts.

III. Lymph Nodes

Webslide 0077_E:  Lymph node, human biopsy, Masson stain   [DigitalScope]

First examine the thick section at low power. Here the differentiation between the peripheral cortex and lighter staining central medulla is striking. Surrounding the outer perimeter of the node is a capsule of dense connective tissue. The capsule is penetrated by several lymph vessels. Connective tissue (C.T.) trabeculae arise from the capsule and penetrate the organ in a radial fashion. Located immediately below the capsule is the subcapsular sinus. This sinus follows the trabeculae into the node and is now called the trabecular sinus.

Webslide 0029_E:  Lymph node, loris, 1.5 µm, GMA, AF & TB   [DigitalScope]

In this thin section, examine the subcapsular and trabecular sinuses for reticular cells (large, pale staining cells) and for free macrophages (large round cells with horse shoe shaped nuclei). Macrophages will be easier to observe in Slide 73 (see below) in which they have phagocytosed trypan blue. 

The cortex is characterized by densely packed lymphocytes in which dark blue staining nuclei predominate and by prominent nodules (called follicles) with lighter staining regions called germinal centers. Examine one of the dense lymphoid follicles of the cortex at low power. Within the germinal center there are numerous cells with large, pale staining nuclei. These are primarily lymphocytes that are dividing and differentiating into lymphoblasts. Can you identify any plasma cells within the germinal centers of follicles? Do you see any mitotic figures within the germinal centers?

The medulla is the central region of the node in which many small lymphatic vessels are evident. The walls of these lymphatic vessels are formed by an open meshwork of reticular cells. These cells gradually coalesce to form a continuous endothelium of a single efferent vessel which leaves the node at the hilus (a thickened area of the C.T. capsule).

The hilus is also the site where blood vessels enter and leave the node and can be seen on the lower left of webslide 29. As the artery enters the node, it divides sending branches into the cortex to perfuse the follicles and others to travel within the capsule and trabeculae. The capillaries within the cortex drain into post-capillary venules (paracortical region) which drain into veins. A single vein leaves the node at the hilus.

The endothelial cells of the blood vessels are clearly demonstrated in Slide 29.  Do you see lymphocytes crossing the high walled endothelium of the post-capillary venules within the paracortical region? An example of a cell crossing a HEV is marked by an arrow.

UMich Slide 027: Lymph Node, human, H&E   [DigitalScope]

This slide provides an excellent "classic" view of a lymph node as it is a sagittal section that allows visualization of the capsule, cortex, medulla, and even the hilus of the lymph node. Use this slide to review all of the features discussed in the descriptions above.

IV. Thymus

UMich Slide 140 (courtesy of U. Mich.): Thymus, neonatal, H&E   [DigitalScope]

Note the C.T. capsule and septa that extend into the parenchyma of the organ dividing it into partial lobules. Because the surface of the bi-lobed thymus is highly convoluted, the organ may look as if it has a "multi-lobular" arrangement. Examine one of the larger, apparent "lobules" and you will be able to distinguish easily a basophilic cortex in which the nuclei of lymphocytes predominate and a medulla, which is lighter in staining (contains fewer nuclei). This section is from a neonate so the thymus is not involuted. Numerous thymic or Hassall's corpuscles which consist of isolated, eosinophilic masses of concentrically arranged reticular epithelial cells can be found in the medulla.

Webslide 0076_E:  Thymus, adult human, H&E   [DigitalScope]

This human thymus is in the process of involution. Most of the lymphocytes within the cortex have been replaced by adipose tissue and only the reticular-epithelial cells of the medulla remain. There are many well-developed eosinophilic Hassall's corpuscles.

V. Spleen

Webslide 0003_E:  Spleen, human, H&E  [DigitalScope]

Examine this slide at low power. Note the dense connective tissue (C.T.) capsule at one surface. The C.T. trabeculae are evident throughout the section. Arteries and veins are within the trabeculae. There is no cortex-medulla appearance. Instead the parenchyma is divided into white pulp and red pulp. 

The white pulp contains nodules of lymphocytes (many with prominent germinal centers) and diffuse lymphatic tissue surrounding an artery (central artery with a periarterial lymphatic sheath - PALS). The red pulp contains sinusoids (large blood vessels), arteries (pulp artery), veins and cords of cells called the Cords of Billroth. These cords appear as interlacing red strands due to the red blood cells trapped within them at the time of perfusion fixation. Try to find most of the histological features described in the text. Be sure to compare the endothelial cells (littoral cells) lining the sinusoids with the simple squamous endothelium of the arteries and veins.

Webslide 0025_E: Spleen monkey, 7.5 µm, AF & TB-FG   [DigitalScope]

Continue your observation of white and red pulp in this slide. Note in particular the structure of the sinusoids and the littoral cells lining its walls. Again be sure to compare the littoral cells lining the sinusoids with the simple squamous endothelium of the arteries and veins.

EXTRA SLIDES

Webslide 0073L_E: Lymph node, rabbit, Trypan blue infused   [DigitalScope]

Webslide 0073S_E:  Spleen, rabbit, Trypan blue infused   [DigitalScope]

Use these slides to visualize the macrophages in these sections of the lymph node and the spleen. These sections came from a rabbit injected with trypan blue which was phagocytosed by the macrophages.

UMich Slide 147B: Spleen, human, H&E   [DigitalScope]

This slide provides another view of a human spleen with its fibromuscular capsule (and even an intact mesothelium in some places), trabecula, red pulp, and white pulp.

Lymphatic System Practice Questions

1. What histological feature distinguishes a lymph node from other lymphoid tissues?

A) Presence of nodules
B) Presence of a subcapsular sinus
C) Presence of Hassall’s corpuscles
D) Diffuse lymphoid tissue
E) Presence of macrophages

[ANSWER]

2. Which lymphoid organ is primarily responsible for filtering bloodborne pathogens?

A) Lymph node
B) Tonsil
C) Thymus
D) Spleen
E) Bone marrow

[ANSWER]

3. The stroma of the thymus is comprised primarily of which of the following?

A) Reticular fibers
B) Macrophages
C) Epithelial reticular cells
D) Fibroblasts
E) Type I collagen fibers

[ANSWER]

4. A 3-year-old boy presents with frequent infections and is diagnosed with a rare congenital immune deficiency. Genetic testing shows a mutation affecting his bone marrow's ability to produce functional B lymphocytes. Which structure in the lymph nodes would be most affected by this deficiency?

A) Paracortex
B) Medullary sinuses
C) High endothelial venules
D) Superfical cortex
E) Subcapsular sinus

[ANSWER]

5. A 45-year-old woman is diagnosed with breast cancer and has a masectomy to remove the affected breast and lymph node dissection to determine the amount of metastasis. Analysis of subclavicular and axillary lymph nodes shows the presence of tumor cells. Which of the following best decribes the route by which the tumor cells passed into the subclavicular nodes and onward to the axillary nodes?

A) Subcapsular sinus → trabecular sinus → medullary sinus → efferent lymphatic vessel
B) Subcapsular sinus → medullary sinus → germinal center → efferent lymphatic vessel
C) Paracortex → subcapsular sinus → medullary sinus → efferent lymphatic vessel
D) Germinal center → subcapsular sinus → medullary sinus → efferent lymphatic vessel
E) Efferent lymphatic vessel → subcapsular sinus → medullary sinus → afferent lymphatic vessel

[ANSWER]

Organ Review

0015_R [DigitalScope]

Identify as specifically as possible the organ shown in the slide AND provide at least FOUR specific or characteristic features (e.g. specific cell types and/or arrangements of cells/tissues in the organ) supporting your identification. For EACH feature listed, provide a brief explanation regarding its functional significance and/or how that feature is helpful in terms of distinguishing the organ.

[ANSWER]