1. Lymphatic System Mike Clark, M.D.

2. 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

3. Microvilli (brush border) Absorptive cells Lacteal Goblet cell Blood
Lipids absorbed from GI tract into Central
Lacteal of Lymphatic
Absorptive cells
Lacteal
Goblet cell
Blood
capillaries
Vilus
Mucosa
associated
lymphoid tissue
Enteroendocrine
cells
Intestinal crypt
Venule
Muscularis
mucosae
Lymphatic vessel
Duodenal gland
Submucosa
(b)
Figure 23.22b

5. 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

6. Lymphoid Tissues Lymph follicles Diffuse Lymphoid Tissue
MALT (Mucosal Associated Lymphoid Tissue)
GALT (Gut associated Lymphoid Tissue) –example
Peyer’s patches
BALT (Bronchial associated Lymphoid Tissue)

7. 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)

8. Lymphatic Vessels Lymph capillaries Lymphatic collecting vessels
Lymphatic trunks
Lymphatic Ducts

9. 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

10. 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

11. Figure 20.1

12. 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

13. Figure 20.2a

14. 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

15. 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

16. Figure 20.2b

17. 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

18. Organization of Lymphatic System
Cells
Tissues – both diffuse and compact (nodules)
Organs – must have a capsule
Lymph nodes, spleen, thymus, tonsils, appendix

19. Cells of the Lymphatic System Other cells of the Lymphatic System
Two main varieties of Lymphocytes:
T cells
B cells
Other cells of the Lymphatic System
Macrophages – 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

20. 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 cells
Germinal center composed of dendritic and B cells
Found in isolation and as part of larger lymphoid organs

21. Figure 20.3

22. 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 debris
Immune system activation – monitor for antigens and mount an attack against them

23. Figure 20.2a

24. Figure 20.4

26. Figure 20.4a

27. Figure 20.4b

28. 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)

29. Figure 20.5

31. 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

32. Figure 20.7

34. 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

35. 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

37. The thymus differs from other lymphoid organs in important ways
1. It functions strictly in T lymphocyte maturation
It 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

38. 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

39. 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

40. 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

41. Figure 20.6

43. 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

48. Ring of Waldeyer
Figure 22.3b

49. Figure 20.8

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

51. 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

53. Figure 20.9