Lymphatic System

Mike Clark, M.D.

Lymphatic System• Composed of lymphoid cells, tissues,

organs, and vessels

• The function of the lymphatic system is to (1) fight infection (2) carry lipids absorbed from the GI tract to the bloodstream and (3) return fluids and plasma proteins that escaped from the bloodstream back to the bloodstream

Figure 23.22b

(b)

Absorptive cells

Lacteal

Intestinal crypt

Mucosaassociatedlymphoid tissue

MuscularismucosaeDuodenal gland Submucosa

EnteroendocrinecellsVenuleLymphatic vessel

Goblet cellBloodcapillaries

Vilus

Microvilli(brush border)

Lipids absorbed from GI tract into CentralLacteal of Lymphatic

Cells and Fibers of the Lymphatic System

1. Lymphocytes

2. Macrophages

3. Dendritic Cells (Antigen Presenting Cells)

4. Reticular Cells

5. Reticular Fibers

Lymph is a connective tissue

Lymphoid Tissues1. Lymph follicles

2. Diffuse Lymphoid Tissue

MALT (Mucosal Associated Lymphoid Tissue)

GALT (Gut associated Lymphoid Tissue) –example

Peyer’s patches

BALT (Bronchial associated Lymphoid Tissue)

Lymphoid Organs• In order to be lymphoid organ – need a partial or

complete connective tissue capsule surrounding the organ

• Primary lymphoid organs – a lymphoid organ where the lymphocytes receive immunocompetence ( Bone marrow & Thymus)

• Secondary lymphoid organs – receive lymphocytes for residence that have received immunocompetence in the primary lymphoid organs (lymph nodes, spleen, vermiform appendix, tonsils)

Lymphatic Vessels• Lymph capillaries• Lymphatic collecting vessels• Lymphatic trunks• Lymphatic Ducts

Lymphatic Flow Vessels• Lymphatic flow is a one way flow – from

lymphatic capillaries through lymphatic collecting vessels to lymphatic trunks to lymphatic ducts – then into the blood stream

• Therefore lymphatic vessels drain the tissue fluids (interstitial fluids) and return substances to the bloodstream

• Unlike blood flow which is continuous and circular- lymph begins in blind capillaries

Lymphatic Capillaries• Similar to blood capillaries, with modifications:• Very permeable• Loosely joined endothelial mini-valves• Withstand interstitial pressure and remain open• The mini-valves function as one-way gates that:• Allow interstitial fluid to enter lymph capillaries• Do not allow lymph to escape from the

capillaries

Figure 20.1

Lymphatic Collecting Vessels

• Have the same three tunics as veins• Have thinner walls, with more internal valves

• Anastomose more frequently• Collecting vessels in the skin travel with superficial veins

• Deep vessels travel with arteries• Nutrients are supplied from branching vasa vasorum

Figure 20.2a

Lymphatic Trunks

Lymphatic trunks are formed by the union of the largest collecting ducts

Major trunks include: Paired lumbar, bronchomediastinal, subclavian, jugular trunks and a single intestinal trunk

Lymphatic Ducts

Lymph is delivered into one of two large ducts from the lymphatic trunks allowing lymph to enter the bloodstream

• Right lymphatic duct – drains the right upper arm and the right side of the head and thorax

• Thoracic duct – arises from the cisterna chyli and drains the rest of the body

Figure 20.2b

Lymph Transport

The lymphatic system lacks a pumping organ

• Vessels are low-pressure conduits

• Uses the same methods as veins to propel lymph:

• Pulsations of nearby arteries

• Contractions of smooth muscle in the walls of the lymphatics

Organization of Lymphatic System

• Cells

• Tissues – both diffuse and compact (nodules)

• Organs – must have a capsule

Lymph nodes, spleen, thymus, tonsils, appendix

Cells of the Lymphatic System

• Two main varieties of Lymphocytes:T cellsB cells

Other cells of the Lymphatic SystemMacrophages – phagocytize foreign substances and help activate T cells

Dendritic cells – spiny-looking cells with functions similar to macrophages

Reticular cells – fibroblast–like cells that produce a stroma, or network, that supports other cell types in lymphoid organs

Lymphoid Tissue

Diffuse lymphatic tissue – scattered reticular tissue elements in every body organ

Larger collections appear in the lamina propria of mucous membranes and lymphoid organs

Lymphatic follicles (nodules) – solid, spherical bodies consisting of tightly packed reticular elements and cellsGerminal center composed of dendritic and B cellsFound in isolation and as part of larger lymphoid organs

Figure 20.3

Lymph Nodes

Principal lymphoid organs of the body

Embedded in connective tissue and clustered along lymphatic vessels

Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body

Two basic functions:Filtration – macrophages destroy microorganisms and debrisImmune system activation – monitor for antigens and mount an attack against them

Figure 20.2a

Figure 20.4

Figure 20.4a

Figure 20.4b

Thymus

• A bilobed organ that secretes hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent

• The lobes are separate from one another – but are hooked together by dense irregular collagenous connective tissue

• Size of the thymus varies with age: In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart

• It increases in size and is most active during childhood It stops growing during adolescence and then gradually atrophies (involutes)

Figure 20.5

• The dense connective tissue capsule of the Thymus – sends septa into the two lobes of the Thymus – partitioning them into lobules.

• Each lobule is composed of an outer cortex and an inner medulla

Figure 20.7

Cells of Thymus• The thymus does not have a reticular fiber

stroma like the other lymphoid organs – instead it has “epithelial reticular cells”

Cortical Epithelial Reticular Cells are the• Type I, II, and III – these three types of

epithelial reticular cells form tight junctions around the blood vessels in the Thymus to form – the Thymic- Blood Barrier

Medullary Epithelial Reticular Cells

• Type IV, V and VI. The Type IV form the Hassall’s corpuscles of the Thymus

• The Hassall’s corpuscles used to be thought of as a place where T-cells were destroyed – recent evidence shows that this is the site of production of regulatory T-cells important in preventing autoimmune actions

The thymus differs from other lymphoid organs in important ways

1. It functions strictly in T lymphocyte maturationIt does not directly fight antigens (Blood – Thymus Barrier)

2. The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers)These thymocytes secrete the hormones that stimulate lymphocytes to become immunocompetent

Spleen

• Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm

• Fibroelastic connective capsule with some smooth muscle

• It is served by the splenic artery and vein, which enter and exit at the hilus

• No cortex or medulla – white pulp, red pulp and marginal zone

Functions:• Site of lymphocyte proliferation• Immune surveillance and response• Cleanses the blood – old red blood cells• Traps some platelets

Additional Spleen Functions

• Stores breakdown products of RBCs for later reuse

• Spleen macrophages salvage and store iron for later use by bone marrow

• Site of fetal erythrocyte production (normally ceases after birth)

• Stores blood platelets

Structure of the Spleen

Surrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes

Two distinct areas:White pulp – containing mostly lymphocytes suspended on reticular fibers and involved in immune functions

Red pulp – remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens

Figure 20.6

Tonsils

Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx

Location:• Palatine tonsils – either side of the posterior

end of the oral cavity• Lingual tonsils – lie at the base of the tongue• Pharyngeal tonsil – posterior wall of the

nasopharynx• Tubal tonsils – surround the openings of the

auditory tubes into the pharynx

Figure 22.3bRing of Waldeyer

Figure 20.8

MALT

MALT – mucosa-associated lymphatic tissue:

•GALT- Peyer’s patches, tonsils, and the appendix (digestive tract)

• BALT- Lymphoid nodules in the walls of the bronchi (respiratory tract)

MALT protects the digestive and respiratory systems from foreign matter

Aggregates of Lymphoid Follicles

• Peyer’s patches – isolated clusters of lymphoid tissue, similar to tonsils

• Found in the wall of the distal portion of the small intestine

• Similar structures are found in the appendix• Peyer’s patches and the appendix:• Destroy bacteria, preventing them from

breaching the intestinal wall• Generate “memory” lymphocytes for long-term

immunity

Figure 20.9