Lymphatic system- Thymus and MALT

About the histology of thymus and other lymphoid organs.

1. Lymphatic System: 2 Dr. Komal Parmar

2. Previously we discussed: The microanatomy of: Lymphatic Channels Lymph nodes Spleen

3. Today’s Objectives: Microanatomy of Thymus Tonsils Lymphoid tissue associated with epithelial surface Tertiary lymphoid organs

4. Thymus O ne of the two primary lymphoid organs. It is an encapsulated soft, bilobed organ, the two parts being joined in the midline by connective tissue that merges with the capsule of each lobe. The thymus is largest in the early part of life, particularly around puberty , and persists actively into old age despite considerable fibrofatty degeneration which sometimes hides the existence of thymic tissue .

5. Site of mature T lymphocyte production. A cquiring immune tolerance to the body’s own components. A lso part of the neuroimmunological and neuroendocrine axes of the body

6. General Architecture Embryological Consideration : The thymus gland is formed from the ventral part of the third pharyngeal pouch on each side. Cardiac Neural Crest mesenchyme forms connective tissue septa which produce the lobulated architecture of the gland . Angiogenic mesenchyme, including lymphoid stem cells, invades this local mesenchyme

7. Capsule Both thymic lobes have a loose fibrous connective tissue capsule, from which septa penetrate to the junction of the cortex and medulla, and partially separate the irregular lobules, which are each 0.5–2.0 mm in diameter. The connective tissue septa form a route of entry and exit for blood vessels and nerves and carry efferent lymphatics. Most migrant cells enter or leave the thymus by this route.

8. Cortex Mainly composed of lymphocytes, supported by a network of finely-branched epithelial reticular cells. Rich Capillary plexus Medulla   Network of reticular cells is coarser than in the cortex, T he lymphoid cells are relatively fewer in number   Hassall's corpuscles Few vessels

9. In each lobule, the cortex is composed of a superficial subcapsular cortex (a narrow band of cells immediately beneath the capsule), and the main cortex, which is much more extensive. The central medulla of both thymic lobes is continuous from one lobule to the next.

10. Epithelial framework Unlike other lymphoid structures, in which the supportive framework is chiefly collagenous reticular tissue, the thymus contains a network of interconnected epithelial cells. They create an appropriate microenvironment , by cell–cell contact and the release of paracrine factors , in which thymic lymphocytes (T cells) develop and mature. common origin from pharyngeal endoderm

11. They vary in size and shape according to their positions within the thymus. Typically they have pale, oval nuclei, a rather eosinophilic cytoplasm and intercellular desmosomal attachments . Intermediate filament bundles of cytokeratin lie within their cytoplasm. S ubcapsular cells: Blood-Thymus Barrier M edullary epithelial cells tend to form more solid cords as well as thymic or Hassall’s corpuscles . T hymic nurse cells

12. Hassall’s corpuscles whorls of fl attened , concentrically layered medullary epithelial cells 30 to 100 μm in diameter characteristic features of the thymic medulla. contain keratohyaline granules

13. Thymic Nurse Cells Large epithelial cells may be associated with around 50 or more thymocytes . F ound in the cortex of thymus and also in cortico -medullary junction, Myoid Cells Thymic myoid cells correspond to a muscle-like cell population present in the thymic medulla. They are large, rounded cells, and possess a central nucleus surrounded by irregularly arranged bundles of myofilaments . Their functions are unknown, although it has been suggested that their contractions might aid the movement of lymphoid cells across or out of the thymus.

14. Epithelioreticular Cells Epithelial reticular cells  (or  epithelioreticular cells ) are a structure in both the cortex and medulla of the thymus. However, histologically, they are more easily identified in the medulla. These cells contain secretory granules which are thought to contain the thymic hormones . There are six different types: Types 1-3 are in the cortex, and types 4-6 are in the medulla

15. Thymocytes The cortex is densely packed with small thymocytes ( thymic lymphocytes, presumptive T cells ). D istinct subcapsular zone houses the thymic stem cells and lymphoblasts undergoing mitotic division. The processes of thymocyte development and maturation to generate T cells depend on the microenvironment provided by epithelial cells, dendritic cells, macrophages and fibroblasts.

16. Microcirculation Major blood vessels enter the gland at the corticomedullary junction and pass within each lobe, giving off small capillaries to the cortex and larger vessels to the medulla. Capillary Loops Cortical Capillaries: Blood Thymus Barrier Medullary blood vessels are not as well protected by epithelial cells

17. Thymic changes during life

18. Mucosa Associated Lymphatic Tissue (MALT)

19. unencapsulated lymphoid tissue exist in the walls of the alimentary, respiratory, reproductive and urinary tracts, and in the skin. the main subclasses are gut-associated lymphoid tissue (GALT ) and bronchus-associated lymphoid tissue (BALT ). located in the lamina propria and in the submucosa as discrete follicles or nodules More scattered cells, derived from these follicles, are found throughout the lamina propria and in the base of the epithelium. Macroscopically visible lymphoid masses, notably the peripharyngeal lymphoid ( Waldeyer’s ) ring of tonsillar tissue (palatine, nasopharyngeal, tubal and lingual), and the Peyer’s patches of the small intestine. Lymphocyte populations are supported mechanically by a fi ne network of fine type III collagen ( reticulin ) fibres and associated fi broblasts , as they are in lymph nodes.

20. F ollicles and parafollicular Zones The close proximity of lymphocytes within the MALT to an epithelial surface facilitates their access to pathogens. Lymphocytes migrate into MALT through its HEV and leave mainly via its efferent lymphatics , which drain interstitial fluid as lymph. MALT lacks afferent lymphatic vessels .

21. Changes in the overlying epithelium The main function of B lymphocytes in MALT is to produce IgA for secretion into the lumen of the tracts which they line . Epithelium samples and transfers these antigens to antigen-presenting cells. In the small and large intestine these specialized epithelial cells have characteristic short microvilli on their luminal surface and are known as microfold (M) cells. In the palatine tonsils they include modified stratifi ed squamous reticulated epithelial cells.

22. Gut Associated Lymphoid Tissue (GALT) The following comprise lymphoid tissue in the gut: Waldeyer's tonsillar ring Peyer's patches Lymphoid aggregates in the appendix and large intestine Lymphoid tissue accumulating with age in the stomach Small lymphoid aggregates in the esophagus Diffusely distributed lymphoid cells and plasma cells in the lamina propria  of the gut

23. Tonsils: MALT of Oropharynx

24. Palatine Tonsils stratified squamous nonkeratinized epithelium deep grooves called tonsillar crypts Lymphatic nodules are distributed along the lengths of the tonsillar crypts. A dense connective tissue underlies the palatine tonsil and forms its capsule. Sends trabeculae from the lower aspect ( hemicapsule )

25. Pharyngeal tonsil pseudostratified ciliated epithelium located in the upper posterior part of the throat

26. Lingual Tonsils small, individual tonsils, each with its own tonsillar crypt nonkeratinized stratified squamous epithelium excretory ducts

27. Peyer’s Patches characteristic feature of the ileum located in the wall of the ileum opposite the mesenteric attachment diffuse lymphatic tissue of the lamina propria Villi are absent in the area of the intestinal lumen where the nodules reach the surface of the mucosa . Disrupt the muscularis mucosae

28. Cross-section from the chicken's nasal cavity. (A) Panoramic scanning of the section, (b) concha nasalis media, (c) meatus nasi , (d) optic nerve of nervi trigeminus, (e) nasal septum, (f) inferior nasal meatus, (g) infraorbital sinus, ( i ) choanal cleft. (B) Diffuse lymphoid follicle covered by FAE located on the concha nasalis media.  (C) NALT located on the dorsal side of choanal cleft. (D) NALT located on the nasal septum.

29. Tertiary Lymphoid Organs Tertiary lymphoid organs (TLOs) are induced postnatally in non-lymphoid tissues such as those affected by chronic infections, autoimmune diseases, and chronic allograft rejection, and also in cancer tissues. TLOs are thought to provide important lymphocytic functional environments for both cellular and humoral immunity, similar to lymph nodes or Peyer’s patches.