Lymphatic System 2017

Lymphatic System Emma Jakoi, Ph.D.

Overview The ability of the body to protect itself from pathogens, such as viruses and bacteria, is known as immunity. In the lectures on blood, you learned that phagocytic cells can mount a general pro-inflammatory response to pathogens, called innate immunity. The primary cells of innate immunity are macrophages and neutrophils and their weapons are oxidizing agents and phagocytosis. Some of these cells (i.e., macrophages) also signal the presence of foreign substances to lymphocytes, the mediators of acquired immunity. The small lymphocytes that recognize foreign substance (antigen) are called immunocompetent cells. They neutralize (or destroy) the pathogens and provide a memory of these encounters when re-exposed to the same pathogen. Their weapons include antibodies, cytokines, and cytolytic activities, and their actions are called the immune response.

There are millions of different types of lymphocytes each programmed to combat a different antigen. They serve three major functions:

1. Protection from disease causing invaders or pathogens2. Removal of dead or damaged tissue and cells3. Recognition and removal of abnormal cells

Once the foreign substance is removed, then a state of equilibrium (immune homeostasis) is regained by activation of negative feedback mechanisms which terminate the immune response and return the immune system to a basal state. These regulatory mechanisms also maintain unresponsiveness (tolerance) to ourselves (i.e., to self-antigens, molecules originating within the body).

The parenchyma (dominant cell type) of the organs and tissues of the immune system are lymphocytes. Unlike the parenchyma of other organs such as the heart and lung, small lymphocytes are migratory cells. They transiently lurk and often accomplish their jobs within tissues and organs distributed diffusely

throughout the body (Fig 1). Because small lymphocytes are the dominant cellular population in these tissues and organs, the immune system is also called the lymphatic system (or lymphoid system).

Lymphatic tissue lies within the connective tissue underlying epithelium and in non-lymphatic and lymphatic organs. It varies in appearance and can be classified by increasing structural and functional complexity from the simplest random array of cells (called diffuse tissue) to more organized aggregates (called nodular tissue). The organs of the lymphatic system also vary in complexity and include the partially encapsulated tonsils as well as the encapsulated thymus, lymph node, and spleen. Lymphatic organs may contain diffuse and/or nodular tissue.

Small lymphocytes migrate into and out of lymphatic tissues and organs over 4 to 6 hours as part of a general surveillance. However, once a foreign antigen is encountered, recruitment of other small lymphocytes to that region occurs within minutes from the blood and lymphatic circulations.

Figure 1. Tissues and organs of the lymphatic system

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In the next two lectures, we will deal with the cells, tissues, and organs that comprise this immune system. To gain an appreciation for how these organs function and their structure, we will consider briefly the effector cells and their interactions in the immune response. The details of acquired immunity will be presented in the Immunology component of Body and Disease.

The first lecture covers the thymus and lymph nodes; the second covers the spleen

Constituents and function in immune surveillanceLymphocytes are classified as one of two basic types established by colonization either from the thymus (T lymphocytes) or from the bone marrow (B lymphocytes) (Fig. 2). They are morphologically identical but can be distinguished by their functions and surface markers.

Figure 2. Differentiation and interaction of T and B lymphocytes. MHC is the major histocompatibility complex.

T LymphocytesThe stem cells that give rise to T lymphocytes reside within the bone marrow (Fig 2) and enter the blood circulation as immuno-incompetent cells (i.e., not responsive to antigen). They home to the outer region (cortex) of the thymus where they undergo clonal expansion and differentiation in response to endocrine secretions produced by the thymus. During this process, T lymphocytes that recognize “self-antigens” as foreign antigens are destroyed by apoptosis. T lymphocytes that do not recognize “self” as foreign leave the cortex to enter the medulla where they are further processed into mature T lymphocytes. Mature T lymphocytes exit the thymus by the post-capillary venules located at the cortex-medulla boundary.

Mature T lymphocytes can be classified by function as:

(1) Helper T cells (TH) initiate antigen response(s) by activating B lymphocytes.

(2) Killer T cells (TC) directly contact infected cells causing cell death.

(3) Regulatory T cells (TREG) regulate the magnitude and duration of the immune response.

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The mature T cells leave the thymus entering the circulating population of lymphocytes in the blood. About 80-90% of lymphocytes in the circulation are T cells. The T lymphocytes home to diffuse lymphatic tissues where they mediate discrimination between “self” and “non-self” in both cellular and humoral immune responses (Fig 2).

T lymphocyte mediated cellular immune functions encompass:(1) Removal of neoplastic cells (2) Rejection of transplanted organs or tissue (3) Mediation of certain autoimmune diseases (4) Recovery from intracellular microbial and viral infections

B LymphocytesThe B lymphocytes attain immunological maturity independent of the thymus (Fig 2). In birds, some stem cells reach immuno-competence in a lymphatic mass called the Bursa of Fabricius. In humans, there are no organs directly corresponding to the bursa. Instead, B-lineage cells within the fetal liver and bone marrow and in the adult bone marrow differentiate directly to become immunocompetent B lymphocytes. The B lymphocytes are produced continuously throughout life, their growth and maturation takes a few days. During their maturation, considerable genetic recombination occurs within the immunoglobulin genes to generate a wide variety of antigen-binding specificity. The antigen receptor on the surface of B cells is the resulting antibody. There is a single type of antigen receptor for each B lymphocyte.

About 10 to 20% of lymphocytes in the blood circulation are B cells. B cells home to nodules of lymphatic tissue.

B lymphocytes mediate humoral immunity. In most cases, the B lymphocytes need the help of an antigen presenting cell (APC) and/or TH lymphocyte to elicit a response (Fig 2). Once activated, the B cell undergoes clonal division to generate daughter cells, some of which differentiate into plasma cells while others remain as memory B cells (Fig 2). The plasma cells secrete soluble antibodies that match the specificity of the original immunoglobulin-receptor expressed on the B cell surface. This is called the humoral immune response because the antibodies are secreted into the blood for delivery.

Five different classes of immunoglobulins (antibodies) can be made:(1) IgG constitutes most (~75%) of serum immunoglobulin. (2) IgA is in colostrum, saliva, tears, and in secretions from nasal, bronchial, vaginal

and prostatic tissue.(3) IgM is important in early immune responses.(4) IgE is secreted by plasma cells but attaches to mast cells and basophils where it

serves as a receptor for allergens.(5) IgD serves as a receptor for antigens on the surface of B cells and has been

implicated in enhancing local and systemic surveillance against air-borne pathogens but its function(s) is not well understood.

Immunological memory Once stimulated, T and B lymphocytes undergo clonal division. This process produces effector cells and memory cells. The increased number of memory (T and B) lymphocytes is an important aspect of acquired immunity because it permits a more robust and rapid response to the antigen with a second challenge (exposure). The secondary immune response is the basis for acquired immunity from vaccines and of hypersensitivity (Nat Rev Immunol 2009, 9:153).

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Antigen presenting cells (APCs)

Antigen presenting cells (APCs) include macrophages, Langerhans cells (skin), and subsets of dendritic cells. These cells migrate from the bone marrow to peripheral tissues where they act as sentinels for foreign antigens. When APCs encounter pathogens, they ingest the foreign substances, degrade them into small antigens (called epitopes, e.g., short peptides and polysaccharides) and then display these small antigens on their cell surfaces loaded onto the major histocompatibility complex (MHC). These mature antigen-bearing APCs migrate to lymphatic tissues where they instruct and/or regulate the activation of T or B lymphocytes.

Lymphatic tissueLymphocytes in lymphatic tissue and organs are supported by a connective tissue stroma that is of mesodermal origin. The stroma consists of reticular cells which secrete extracellular reticular fibers (collagen type III). The reticular cells also direct small lymphocytes (B and T) to specific regions within the lymphatic tissue by exhibiting “address labels” on their cell surfaces. There is one exception: in the thymus, the stromal cells are derived from endoderm and have endocrine functions.

Lymphatic tissue is classified in terms of the relative densities of lymphocyte aggregates as either diffuse (loose aggregates) or nodules (highly organized aggregates). A nodule often contains a light staining central region called the germinal center. Germinal centers are areas of active lymphocyte proliferation and differentiation. Located within the germinal center are the follicular dendritic cells. The follicular dendritic cells secrete cytokines to promote clonal division of the B lymphocytes. They can bind antigen-antibody complexes to their cell surfaces but unlike APCs, the follicular dendritic cells do not process the bound antigen and instead present the intact antigen without the MHC complex. If the nodule has a germinal center, then it is called a secondary nodule. The darker staining periphery of the secondary nodule (called mantle) contains small, non-responding B lymphocytes. Lymphatic nodules can be found nearly anywhere in the body, but they are especially common in the gastrointestinal and respiratory tracts (called Mucosa Associated Lymphatic Tissue, MALT).

Lymphatic organsOrgans of the lymphatic system are divided into two categories:

Primary lymphatic organs are sites in which T and B lymphocytes differentiate from precursor stem cells to immuno-competent cells independent of antigens. These include the thymus (T cells) and bone marrow (B cells) in the adult.

Secondary lymphatic organs are seeded with immuno-competent lymphocytes. Theseinclude lymph nodes, Peyer’s patches, tonsils, and spleen. These organs respond to antigenand are the principle source of continuing lymphocyte production and function in the adult.

Primary lymphatic organ: ThymusThe thymus is a bi-lobed organ located in the thorax just above the heart. Like all compact organs, a thin, connective tissue capsule surrounds the stroma and parenchyma (Fig. 3). Each lobe is subdivided into partial lobules by connective tissue septa (walls) that carry blood vessels in from the capsule.

The stroma consists of an epithelium formed by stellate-shaped cells called epithelial reticular cells. These cells are attached to each other by desmosomes and have an intracellular system of tonofilaments (intermediate filaments) for support. There are no extracellular connective tissue fibers.

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The epithelial reticular cells are endodermal in origin. These cells secrete hormones (e.g., thymosin) critical for the maturation and proliferation of immunocompetent T lymphocytes.

The parenchyma of the thymus is separated into a peripheral cortex and a central medulla. The cortex is composed primarily of small T lymphocytes actively dividing and differentiating. The medulla is lighter in appearance since there are fewer lymphocytes but more epithelial reticular cells in this region. Also seen scattered throughout the medulla are unique structures of epithelial reticular cells layered concentrically upon one another in swirls. These structures are called thymic corpuscles or Hassall’s corpuscles. Their function has been identified (Nature 2005) to be endocrine. They secrete lymphopoietin, a hormone critical for differentiation of T cell subclasses. Dendritic cells, macrophages and mast cells are present in the medulla as well.

Figure 3. Diagram depicts the cellular organization of the thymus.

The thymus does not have a hilus. Instead small arteries penetrate the capsule and then follow the interlobular septa into the interior of the organ. At the cortex-medulla boundary, arterioles give off capillaries which enter either the cortex or the medulla. Within the cortex and medulla, these capillaries anastomose extensively and then return to the cortex-medulla boundary where they drain into venules. The capillaries are continuous, sealed with tight junctions, and are surrounded by phagocytic epithelial reticular cells to ensure no leakage of antigen. This is known as the blood-thymus barrier.

Figure 4. Diagram of blood flow in thymus.

The blood-thymus barrier is essential for the exclusion of non-self (foreign) antigens from the thymus so that proper selection of immunocompetent T lymphocytes can occur. If a foreign antigen gains access to the thymic cortex, then T lymphocytes capable of recognizing this antigen as foreign would be destroyed.

There are no afferent lymphatic vessels in the thymus. Immature T lymphocytes enter the parenchyma from the blood at the post capillary venules located at the cortex-medulla boundary. The immunocompetent T lymphocytes exit the organ by these same venules (Sci. 328:1129, 2010). Because

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B cells and foreign antigens do not enter the thymus parenchyma, there are no nodules and no germinal centers. The thymus does not participate in immune reactions.

At puberty, the thymus begins to involute in response to adrenocorticosteroids (steroid hormones secreted by the adrenals) and gonadal steroids. The lymphocyte population declines within the cortex and is replaced by adipose tissue. Hassall’s corpuscles in the medulla enlarge and increase in number. Despite post-pubertal involution of the thymic cortex, the thymus remains a functional organ well into adulthood. However, age-related T-cell mediated immunodeficiency can develop. Interestingly, the subset of naïve TC lymphocytes diminishes with age resulting in the elderly being more susceptible to severe infections.

PathologyFailure of T cell mediated immunity may have one of several etiologies. A few examples are:

(1) Severe Combined Immune Deficiency (SCID) can arise from the absence of either bone marrow stem cells or of immuno-incompetent T lymphocytes. The clinical spectrum is broad. Patients are at risk for fungal and viral infections (defective cell mediated immunity) and for bacterial infections (defective humoral immunity). The first known cause of SCID has been shown to be a deficiency in adenosine deaminase (ADA). In patients that lack a matched bone marrow donor, enzyme replacement (PEG-ADA) and gene therapy are used.

(2) In Acquired Immune Deficiency (AIDS), the HVTL III virus destroys the mature TH cells. These patients develop severe immune deficiency and are at risk for viral, fungal and bacterial infections.

(3) In Autoimmune diseases, such as Grave’s disease and rheumatoid arthritis, there is a failure of self-tolerance. The clinical manifestations are diverse and dependent on the antigen involved.

Secondary lymphatic organs The secondary lymphatic organs are sites that capture pathogens limiting their spread through out the body, as well as facilitate activation of acquired immune responses. These organs contain nodules (B cells) and diffuse lymphatic tissue (T cells) as well as APC rich regions in which antigens are bound for delivery to B cells and T cells. The secondary lymphatic organs include Peyer’s Patches of the ileum, tonsils, lymph nodes and spleen. Their development begins during fetal life and it is completed around the time of birth when they are populated by a mass migration of lymphocytes.

GALT and Peyer’s PatchesThe gut associated lymphoid tissue (GALT) consists of lymphocytes scattered throughout the epithelium and lamina propria, as well as organized lymphoid nodules including Peyer’s Patches (ileum) and their equivalent in the colon. The epithelium overlying Peyer’s Patches contains microfold (M) cells which are specialized for sampling the intestinal luminal content. The M cells enable antigens to cross the epithelium and enter an intra-epithelial pocket beneath their basal surface in which lymphocytes and dendritic cells reside. This enables an efficient transfer of the luminal antigens to the adjacent lymphatic nodule.

Figure 5. Diagram of Peyer’s Patch and M cell. Left image from Nature Reviews Immunol 2008, 8:767 and Right image from Mucosal Immunology (2013) 6, 666–677.

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Tonsils Tonsils are comprised of partially encapsulated groups of nodules supplied with lymphatic vessels and blood vessels. Three distinct tonsil masses form an incomplete ring at the oropharynx (entrance to the throat) and include the palantine, lingual, and pharyngeal (clinically the adenoids). The palantine and lingual tonsils are covered with stratified squamous epithelium. In the young, the pharyngeal tonsil is covered with pseudostratified ciliated columnar epithelium with goblets; in adults, it is covered by a stratified squamous epithelium. The surface epithelium of the tonsil invaginates into the underlying connective tissue to form crypts. The walls of these crypts are lined with nodules, primary and secondary. The tonsils are drained by efferent lymphatic vessels; there are no afferent lymphatic vessels.

Lymph nodeAs you learned in the Blood Vessels lecture, lymphatic vessels comprise a one-way drainage system from the periphery of the body towards the heart. They start as blind capillaries under the skin and mucous surfaces, join with larger caliber drainage vessels and eventually merge with the blood circulation at the thoracic duct and right lymphatic duct (Fig 1). En route, the lymph, a filtrate of the blood plasma, passes through at least one lymph node. The lymph node acts as a filter. Here cell-bound and soluble antigens are removed from the lymph thereby limiting the spread of pathogens within the body. Lymph nodes range in size from 1 mm to over 1 cm in diameter and often occur in a group or chains (Fig 1).

Lymph nodes are compact organs encased by a connective tissue capsule (Fig. 6). At the concave aspect, there is a thicker area of connective tissue that partially penetrates the organ called the hilus. From the capsule, strands of connective tissue called trabeculae (finger-like) extend into the interior of the organ. These trabeculae, in turn are connected to a network of reticular cells and extracellular reticular fibers that support the parenchyma (small lymphocytes).

Multiple afferent lymphatic vessels pierce the capsule to deliver lymph to the node. Beneath the capsule and along the trabeculae there are sinuses (large diameter lymphatic vessels) with perforated endothelium permitting the lymph to percolate throughout the organ. The lymph enters first into the subcapsular sinus and then runs along the trabecular sinuses towards the interior of the node. These trabecular sinuses eventually anastomose into one (or two) efferent lymphatic vessel(s) which exits the node at the hilus.

The parenchyma, small lymphocytes, is separated into a cortex and a medulla (Fig. 6). The cortex lies immediately beneath the subcapsular sinus. The cortex contains nodules called follicles of B lymphocytes and diffuse lymphatic tissue called the paracortical region (or tertiary cortex). The paracortical region consists of T lymphocytes. Adjacent to the cortex is the medulla, the central region of the node.

Figure 6. Diagram of a lymph node. The left side of the diagram shows the parenchyma and lymphatic vessels. The

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right side shows the blood supply. Adapted from Ross, Kaye & Pawlina, Histology: A Text and Atlas.Lymph enters at the capsule via the afferent lymphatic vessels (Fig 6). The lymph percolates slowly across the cortex and then drains into the medulla. There are no nodules in the medulla. The medulla contains lymphocytes, macrophages, and plasma cells organized into strands, called medullary cords. The cords are separated by sinuses. The lymph exits the node via an efferent lymphatic vessel at the hilus.

An artery enters the lymph node at its hilus (Fig 6). Subsequently it branches into arterioles which pass through the medullary cords to enter the cortex as small capillaries. The capillaries perfuse the nodules and then drain into venules in the paracortical region. These post capillary venules are called high endothelial venules (HEV) because their cytoplasm is visible in sections. Here small lymphocytes can leave the blood, cross the HEV endothelium, and enter directly into lymph. Eventually blood cells exit the node at the hilus in a single efferent vein.

Lymph nodes function in immune reactionsThe lymph node is the site where pathogens are trapped and acquired immunity develops in a confined space as lymphocytes and immunoglobulins are added to the lymph and circulating small lymphocytes are recruited from the blood circulation (at HEVs). When you are fighting an infection, nearby lymph nodes get bigger (up to 2-3 times normal size) and become tender. They will slowly return to normal 2 to 4 weeks after the infection is resolved. The time course of changes in a lymph node after stimulation with a T cell-dependent antigen is shown in Figure 7. The timed events are as follows:

(1) Antigen presentation occurs 24-48 hours after infection: Foreign antigens that enter a lymph node via afferent lymphatic vessels are taken up by macrophages and APCs lurking in the subcapsular sinus, cortex and/or in the medulla within minutes after infection. Most of this foreign antigen becomes degraded by macrophages which display the antigen on their cell surfaces for presentation to T lymphocytes in the diffuse cortex. Some intact antigen may be retained by follicular reticular cells in the nodule region of the cortex for presentation to B cells.

(2) T and B cell activation occurs 2-3 days after infection: Small T lymphocytes enter the lymph node through high endothelial venules (HEV), encounter the processed antigen and differentiate to large T lymphocytes within the diffuse cortex. These cells divide and give rise to clones of memory T cells and TH cells. Some TH cells migrate into the nodule (follicle). Here, small B cells with appropriate receptor specificities react with the processed antigen of TH cells. These activated B cells divide giving rise to memory and effector cells of the B cell lineage. The focus of dividing TH and B cells, as well as active macrophages, builds up to form a germinal center that compresses the rest of the nodule into a crescent around it called a mantle (Fig 7).

Figure 7. Time course of an immunological response within a lymph node. See text for details. (Redrawn from Hood, Weissman, and Wood. Immunology).

(3) Germinal centers appear 3-5 days after infection: Nodules with germinal centers (secondary nodules) appear 4-5 days after introduction of foreign antigen and may remain for several days. During this time plasma cells begin to settle between the sinusoids of the medulla to form medullary cords. Other activated B cell progeny and activated T cell progeny percolate through the lymph node and exit via the

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efferent lymphatic vessels to spread to other lymphatic tissues and eventually the blood circulation. Thus, most memory and effector T cells and some memory B cells eventually re-enter the general blood circulation but effector B-cells (plasmablasts and plasma cells) are retained within the lymph node.

(4) Lymph node swells 6-8 days after infection: Activated T and B cells also release soluble factors that cause local blood vessels to dilate allowing leakage of plasma into the lymph node. Some of these factors attract macrophages while other factors retain lymphocytes within the node. The resulting accumulation of cells and fluid may plug the medullary sinusoids leading to the efferent lymphatic vessel. As a consequence, the lymph node enlarges rapidly causing the typical “swollen glands” of infection.

(5) The node returns to its original size only as the response to infection abates.

Dynamic surveillance and Pathology All lymphatic tissues except the thymus are part of a surveillance system in which lymphocytes migrate from the blood stream into interstitial space, then into lymphatic vessels and return to the blood stream via the right lymphatic duct and thoracic duct. This is a slow process taking 4-6 hours not a mass migration. This path can be short circuited in lymph nodes and in other lymphatic tissues (tonsils, Peyer’s patches, and MALT) at HEVs where T and B lymphocytes can move quickly from blood into lymphatic drainage often in large numbers (Fig. 8). Because of this migration, antigens throughout the body are examined and specific lymphocytes can be activated even though they are located far from the original invasion. In general, T lymphocytes are circulating while B lymphocytes tend to reside in the spleen, lymph nodes, and Peyer’s Patches.

Figure 8. Pathway of lymphocyte circulation. [Redrawn from J.L. Gowan’s Hosp. Pract.].

Does the lymphatic drainage deliver only lymphocytes to the blood circulation? No. Lymphatic drainage also provides a route for distributing pathogens and cancer cells. If these cells evade detection and removal in the lymph node, then they are delivered to the blood circulation (by thoracic duct or right lymphatic duct) for distribution throughout the body. Metastasis to the regional lymph node from lymphatic vessels draining an organ (or tissue) is a common step in the progression of many cancers (e.g., breast and prostate) and is an important prognostic indicator.

Spleen The spleen is a complex filter interposed in the blood stream that does two jobs:

(1) Removal of particulate matter and senescent red blood cells(2) Immune response to blood-borne antigens such as bacteria and protozoa.

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The spleen is a compact organ encased by a capsule of dense irregular connective tissue covered by a mesothelium. Multiple branching trabeculae extend into the organ from the capsule. These trabeculae, in turn are connected to a network of reticular cells and reticular fibers that support the parenchyma.

The parenchyma is referred to as splenic pulp. In the freshly dissected non-perfused spleen, the splenic pulp appears white and red (Fig. 9).

White pulp consists of lymphocytes organized into large nodules (called follicles) and diffuse lymphatic tissue separated from the red pulp by a relatively acellular region called the marginal zone. The white pulp examines mostly plasma- soluble antigens.

Red pulp consists of many sinuses filled with erythrocytes separated by a reticulum of cells called the Cords of Billroth. The red pulp examines the red blood cells for deformability.

Figure 9. 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. 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.

Figure 10. Diagram depicts the cellular organization and blood flow of the spleen.

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) and is the entry site for lymphocytes which

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then migrate into the white pulp. Because the central artery branches at right angles, “plasma-enriched” blood is delivered to the marginal zone where lymphocytes and macrophages (APCs) examine soluble antigens. The antigen laden APCs and/or antigen-activated lymphocytes then migrate into the white pulp to either the follicle(s) or PALS region. The marginal zone is less evident in the human spleen.

The “red cell enriched” blood remains within the central artery to enter the red pulp region where the blood cells undergo mechanical filtration. In the red pulp, the central artery is called the pulp artery (Fig 10). The pulp artery branches into parallel arterioles (called penicilli) which are wrapped often by macrophages (sheathed arterioles). These arterioles end abruptly releasing the blood cells into a meshwork of cells called the Cords of Billroth. This is called the open circulation of the spleen because the red blood cells are outside of a vessel, free within the Cords of Billroth. The blood cells move through the Cords of Billroth to enter the venous sinusoids. The endothelium that lines the venous sinuses are specialized cells called littoral cells that have intercellular pores of a fixed diameter (2-5µm). The trick is to move blood cells (RBCs and other cells) from within the Cords of Billroth through this endothelium into the venous sinuses. 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 quickly by macrophages lurking there. The littoral cells themselves are nonphagocytic.

In humans, 100% of the blood circulation in the spleen is by open circulation. No direct connection of the red pulp arterioles to venous sinuses (closed circulation) has been found (Immunol 145:334-346, 2015). 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 secs! The entire cardiac output is filtered in 20 minutes.

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

Lymphatic drainageThe spleen has only efferent lymphatic drainage which is poorly developed 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 OrgansTonsil MALT Thymus Lymph node Spleen

Cortex/medulla No No Yes Yes NoLymphatic nodules

Yes Yes No Yes Yes

Capsule Partial No Yes Yes YesCords & sinuses No No No Yes YesLymphatic vessels

efferent efferent efferent afferent & efferent

efferent

HEV Yes Yes No Yes No

For identification, ask if there are:

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Thymus Lymph Node Spleen Tonsils 1. CT capsule? yes yes yes partial2. nodules? no yes yes yes3. cortex and medulla? yes yes no no

To confirm, ask if there are: Thymus Lymph Node Spleen Tonsils

4. subcapsular sinus? no yes no no5. Hassall’s corpuscles? yes no no no6. white pulp/red pulp? no no yes no7. artery associated with nodule? no no yes no

Common terms:Lymphatic Tissue OrganizationDiffuse-random grouping of lymphocytesNodule or follicle-concentric grouping of lymphocytes

primary-no germinal centersecondary- has germinal center

ThymusCortex –outer region of parenchymaMedulla-inner region of parenchymaCapsule- CTSeptae- CT divides the parenchyma into partial lobulesStroma- epithelial reticular cells (endoderm derivative) connected by desmosomes, endocrine Hassall’s corpuscles- concentric swirls of reticular cells, eosinophilic, endocrine

Lymph NodeCortex- follicles or nodules surrounded by diffuse (tertiary) cortexMedulla- cords of cells and lymphatic sinusesCapsule- CT pierced by afferent lymphatic vesselsStroma- reticular cells (mesoderm derivative), APCSubcapsular sinus- large diameter lymphatic vessel lying beneath the capsule has perforated

endotheliumTrabeculae- CT stroma supports reticular cells & fibersTrabecular sinus- lymphatic vessels along trabeculae have perforated endotheliumHilus- exit and entry of blood vessels, exit of lymphatic vessel

SpleenCapsule- CT, elastin fibers, some smooth muscle, myofibroblasts, covered by mesotheliumTrabeculae- CTStroma- reticular cells (mesoderm derivative), APCWhite pulp- central artery, PALS, follicles (Malpighian corpuscles), marginal zoneRed 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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