Lymphatic System

Atlas:
Ross & Pawlina (6th ed.), Plates 36-41, pp. 476-487
Text:
Ross & Pawlina (6th ed.), Ch 14, pgs. 440-474, Lymphatic System

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.

Tissue/Organ

Capsule?

Cortex-Medulla?

Germinal Centers?

Lobulation?

Notes

Lymph Nodule

No

No

Primary - No
Secondary - Yes

No

Present in lymph nodes, spleen, tonsils, and other regions of body such as GI and respiratory tracts

Tonsils

Covered on one side by strat. squa. epithelium (SSE)

No

Yes

Infoldings of SSE and underlying CT form tonsillar crypts

The lumens of tonsillar crypts can accumulate microorganisms, sometimes leading to inflammation.

Lymph Node

Yes

Yes

Yes

No -- 
Framework contains trabeculae = elongated dense CT spicules extending from capsule.

Sub-capsular sinus
Cortex - high endothelial venules
Medulla - medullary cords

Thymus

Yes

Yes

No

Yes --
Cortex is partially lobulated by septae (dense CT walls), whereas medulla is continuous.

Hassall's Corpuscles in Medulla
Fat in Cortex
Medulla ≠ Germinal Center

Spleen

Yes

No
Nodules spread
throughout organ

Yes

No -- 
Framework contains trabeculae = elongated dense CT spicules extending from capsule.

Venous Sinusoids in Red Pulp
Central Arteries in White Pulp
Cords of Billroth

 

I. Lymph Nodules

Webslide 0060A_E: Ileum, human, H&E 
[Aperio ImageScope] [Aperio WebScope]

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). Webslide 60 shows a series of nodules (called Peyer's Patches) which have germinal centers.

 

 

II. Tonsils

Webslide 0034_E:  Tonsil, H&E
[Aperio ImageScope] [Aperio WebScope]

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
[Aperio ImageScope] [Aperio WebScope]

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
[Aperio ImageScope] [Aperio WebScope]

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.

 

 

IV. Thymus

Webslide 0311_E (courtesy of U. Mich.): Thymus, neonatal, H & E
[Aperio ImageScope] [Aperio WebScope]

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
[Aperio ImageScope] [Aperio WebScope]

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, AF & TB-FG
[Aperio ImageScope] [Aperio WebScope]

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
[Aperio ImageScope] [Aperio WebScope]

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
[Aperio ImageScope] [Aperio WebScope]

Webslide 0073S_E:  Spleen, rabbit, Trypan blue infused
[Aperio ImageScope] [Aperio WebScope]

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.

 

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