Lymphatic System



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Figure 8. Diagram of the spleen. © Copyright 2000 Department of Biology, Davidson College, Davidson, NC 28036

The organization of the spleen can best be understood by describing the blood flow through this organ (Fig. 9 and 10). The splenic artery enters at the hilus, branches within the capsule and trabeculae, and enters the parenchyma as a central artery of the white pulp. Lymphocytes cluster around the central artery forming a concentric sheath called the periarterial lymphatic sheath (PALS, primarily a T cell locus). Eccentric to the PALS, lymphatic nodules (called follicles or Malpighian corpuscles) may form. The lymphatic nodule is the primary locus for B lymphocytes. It is called a primary follicle if there is no germinal center or a secondary follicle if there is a germinal center. The central artery gives off many small arterioles which end in the marginal zone surrounding the white pulp [PALS and follicle(s)]. The marginal zone is a unique interface between the white pulp and red pulp which often contains macrophages (i.e., APC rich zone). Most lymphocytes enter the spleen at the marginal zone.






Figure 9. Diagram depicts the cellular organization and blood flow of the spleen.

Once the central artery leaves the white pulp and enters the red pulp region, it is called the pulp artery. The pulp artery often terminates in the red pulp as branched vessels termed penicilli (sheathed arterioles) (Fig 10). Here most of the blood is released into a meshwork of cells called the Cords of Billroth. Subsequently the blood cells enter the venous sinusoids, which drain into veins within the trabeculae and then veins within the capsule. Blood exits at the hilus via the splenic vein.

There are two types of circulation in the spleen:

(1) Open circulation in which the arteriolar vessels terminate in the red pulp in the Cords of Billroth (Fig. 10).

(2) Closed circulation in which the arterioles in the red pulp connect directly into venous sinuses (Fig. 10)

In humans, at least 90% of the blood circulation in the spleen is via open circulation. Despite its open nature, blood flow through the spleen is very efficient: 98 to 99% of red cells (labeled with chromium-51) entering the spleen flow through it in 30 sec. The entire cardiac output is filtered in 20 minutes.




Figure 10. Diagram depicts open and closed circulation of the spleen.
The spleen has structurally distinct sites for filtration of plasma and of cells.

Antigens in the plasma are examined in the white pulp. Because the central artery branches at right angles, “cell-enriched” blood is separated from “plasma-enriched” blood (plasma skimming). The plasma-enriched blood is distributed to the marginal zone of the white pulp where resident lymphocytes and macrophages examine soluble antigens.

The cell-enriched blood is distributed to the red pulp where mechanical filtration and cell surface antigens are monitored as they move from the Cords of Billroth into the venous sinuses. The walls that line the venous sinuses are specialized endothelial cells called littoral cells that have intercellular pores of a fixed diameter (2-5µm). The trick is to move blood cells (RBCs and others) from within the Cords of Billroth through this endothelium lining the venous sinuses. In order to do this the blood cells must be deformable. Those cells or parts of cells that are rigid remain in the Cords of Billroth and are removed by macrophages lurking there. The littoral cells themselves are nonphagocytic.

From the venous sinuses, filtered blood drains to the pulp vein, then to the splenic vein and leaves at the hilus, entering the portal system of veins en route to the liver. Drainage of the white pulp (plasma-enriched blood) and of the red pulp region (cell-enriched blood) reconstitutes the blood hematocrit so that its composition in the venous sinuses matches that of the peripheral circulation.

Lymphatic drainage


The spleen has only efferent lymphatic drainage which is poorly defined and lacks HEV. Lymphatic vessels of the spleen are not of major functional importance. Blind lymphatic capillaries start in the marginal zone where lymphocyte- and plasma- enriched blood is dumped. Here lymphocytes can exit the blood circulation and directly enter the lymphatic circulation.

Given what you know of the lymphatic drainage in the spleen, predict the most likely route for breast cancer cells to enter the spleen in metastasis.


TABLE 1. Summary of Distinguishing Features of Lymphatic Organs




Tonsil

MALT

Thymus

Lymph node

Spleen

Cortex/medulla

No

No

Yes

Yes

No

Lymphatic nodules

Yes

Yes

No

Yes

Yes

Capsule

Partial

No

Yes

Yes

Yes

Cords & sinuses

No

No

No

Yes

Yes

Lymphatic vessels

efferent

efferent

efferent

afferent & efferent

efferent

HEV

Yes

Yes

No

Yes

No


For identification, ask if there are:

Thymus Lymph node Spleen Tonsils

1. CT capsule? yes yes yes partial

2. nodules? no yes yes yes

3. cortex and medulla? yes yes no no


To confirm, ask if there are:

Thymus Lymph Node Spleen Tonsils

4. subcapsular sinus? no yes no no

5. Hassall’s corpuscles? yes no no no

6. white pulp/red pulp? no no yes no



7. artery associated with nodule? no no yes no


Common terms:

Lymphatic Tissue Organization

Diffuse-random grouping of lymphocytes

Nodule or follicle-concentric grouping of lymphocytes

primary-no germinal center

secondary- has germinal center
Thymus

Cortex –outer region of parenchyma

Medulla-inner region of parenchyma

Capsule- CT

Septae- CT divides the parenchyma into partial lobules

Stroma- epithelial reticular cells (endoderm derivative) connected by desmosomes, endocrine

Hassall’s corpuscles- concentric swirls of reticular cells, eosinophilic, endocrine

Lymph Node

Cortex- follicles or nodules surrounded by diffuse (tertiary) cortex

Medulla- cords of cells and lymphatic sinuses

Capsule- CT pierced by afferent lymphatic vessels

Stroma- reticular cells (mesoderm derivative), APC

Subcapsular sinus- large diameter lymphatic vessel lying beneath the capsule has perforated endothelium

Trabeculae- CT stroma supports reticular cells & fibers

Trabecular sinus- lymphatic vessels along trabeculae have perforated endothelium

Hilus- exit and entry of blood vessels, exit of lymphatic vessel
Spleen

Capsule- CT, elastin fibers, some smooth muscle, myofibroblasts, covered by mesothelium

Trabeculae- CT

Stroma- reticular cells (mesoderm derivative), APC

White pulp- central artery, PALS, follicles (Malpighian corpuscles), marginal zone

Red pulp- Cords of Billroth (meshwork of cells), venous sinuses (lined by littoral cells)

Hilus- entry and exit of blood vessels, exit of lymphatic vessel


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