HISTOLOGY & EMBRYOLOGY Teaching PPT Dept. of Anat., Hist. & Embry. School of Medicine Xi’an Jiaotong University

1. HISTOLOGY & EMBRYOLOGY Teaching PPT Dept. of Anat., Hist. & Embry. School of Medicine Xi’an Jiaotong University

2. Dr. Jinsong Zhou Immune System I. Lymphocytes: Divided into 3 types based on the surface antigens. A.Thymus derived lymphocytes (T cells): Formed in thymus. Participate in cell-mediate immune responses. Divided into Tc, Th, Ts and memory T cells. B.Bone derived lymphocytes (B cells): Formed in bone marrow?. Participate in humoral immune responses. C.NK cells.

3. II. Mononuclear phagocyte system (MPS): Composed of the cells that are derived from monocytes in blood and as meanwhile have high capability to phagocytose. MPS includes macrophages, osteoclasts, microglia, Kupffer cells, dust cells and Langerhans cells, etc. III. Lymphoid tissue: Reticular tissue containing rich macrophages, lymphocytes and plasma cells, etc.

4. A. Divided into 2 types based on function: 1. Central lymphoid tissue: Irrelevant to immune response. a. develops earlier and perishes earlier. Makes up the central lymph organ, the thymus and bone marrow?. b. the site where the B lymphocytes (in bone marrow?) and T lymphocytes (in thymus) proliferate and differentiate into different types. 2. Peripheral lymphoid tissue: a. develops later and retains forever. b. the site where immune cells perform their functions. Makes up of peripherallymph organs, or distributes as tissue in other organs, e.g. the mucosa of digestive and respiratory tract, etc.

5. B.Divided into 3 types based on the composition and appearance: 1.Diffuse lymphoid tissue: Mainly contains T cells and has obscure boundary with surrounding CT. Has post venule capillaries, which are lined by high endothelium and are the important channels for lymphocyte recirculation. It is the place where the lymphoid nodule is formed when stimulated by antigens.

6. 2.Lymphoid nodule: Ball-like structure with distinct boundary, mainly contains B cells. a. primary lymphoid nodule: smaller. b. secondary lymphoid nodule: the primary one enlarges when Th cells and antigens exist. It has a cap and less densely stained region known asgerminal center, where the particular lymphocytes proliferate and differentiate into particular plasma cells to respond to the particular antigens. It disappears when the antigens disappear.

7. 3. Lymph cords: Cord-like lymphoid tissue. Mainly contains B cells and surrounded by rich capillaries.

8. IV. Lymph organs: A. Thymus: A central lymphoid organ. 1. Capsule: Composed of CT, which protrudes into parenchymato form the supporting trabeculaethat divide the parenchyma into incomplete lobules.

9. The incomplete lobule is composed of cortex and continuous medulla. The cortex is densely stained because the cells in it are closely arranged.

10. 2.Cortex: divided into outer and inner cortex. a. thymocytes: the T cells in different developing stages.The cells in outer cortex are larger, and in inner cortex are smaller but much more. b. epithelial reticular cells: provide micro support. Divided into 2 types:

11. The cells that support the thymocyte differentiation are called thymus stromal cells, which include epithelial reticular cells, macrophages, eosinophils, mast cells, fibroblasts, etc. Thymus begins to perish in puberty. The thymus in adult contains a lot of fat cells instead of thymocytes.

12. 3. Medulla: Contains mature T cells, macrophages and 2 types of epithelia: a. medullary epithelial cells: larger, desmosomes can be found among them. Secrete thymosin. b. thymic corpuscle epithelial cells: exist in Hassall’s corpuscles.

13. Hassall’s corpuscles: The acidophilic characteristic structures of thymus, which are composed of concentrically arranged, flattened thymic corpuscle epithelial cells that become filled with keratin filaments, degenerate and sometimes calcify. Function unknown.

14. 4. Bloodthymus barrier: Composed of endothelial cells and their basal lamina, space surrounding capillary in which exist macrophages, endothelial reticular cells and their basal lamina. Prevents circulating antigens from reaching the thymus where T lymphocytes are being formed.

15. B. Lymph node: A peripheral lymphoid organ. Locates in the pathway of lymph circulation. 1. Capsule: Composed of CT. Contains one efferent lymph vessel that leaves at hilum and several afferent lymph vessels. Trabeculae, which have the function of support, are the protrusions of capsule into the parenchyma.

16. 2. Cortex: Beneath the capsule. a. superfacial cortex (outer cortex): locates at periphery, usually contains a lot of lymphoid nodules. b. paracortex zone (inner cortex): Composed of diffuselymphoid tissue. Also called deep cortex unit. It is the thymus dependent area in lymph node.

17. Deep cortex units have semi-spherical appearance. The plane surface is opposite to an afferent lymphatic vessel, and the other surface is continuous to medulla. The inner part of the unit is occupied by T cells and its periphery contains many post capillary venules and small blind sinus, which are the beginnings of medullary sinus.

18. The post capillary venules are lined by simple cuboidal/columnar epithelium, from which the lymphocytes in blood enter the lymphoid tissue. They, together with small blind sinus, are the important channels for lymphocyte re-circulation.

19. 3. Cortical sinus: a. subcapsular sinus exists between the outer cortex and capsule, and receive lymph from the afferent lymph vessels. b.intermediate sinus: usually have blind ends and are parallel to trabeculae. The ones between deep cortex units are called thin channels, which communicate the medullary sinus.

20. In the sinus, there are satellite epithelial cells that provide support, lymphocytes, veiled cells and more macrophages. The slowly running lymph gives more chances for immune cells to detect antigens.

21. 3. Medulla: Located in the center and connects hilum directly. a. medullary cords: cord-like lymphoid tissue in which B lymphocytes predominate. They are the branched extensions of inner cortex. b. medullary sinus: connect the thin channels and contain more macrophages. Recirculation of lymphocyte: Lymphocytes in inner cortex → small blind sinus → efferent vessel → right lymphatic duct and thoracic duct → blood vessels → post Cap venules → inner cortex. One circulation circle needs 1~2 days.

22. Results in a constant monitoring of all parts of the body by recirculating lymphocytes, which can inform the immune system of the presence of foreign antigens ASAP.

23. 5. The pathway of lymph fluid: Afferent vessels → subcapsular sinus →thin channel →medullary sinus→hilum →efferent vessel. 6. Function: Participates in filtration of lymph, immune response and lymphocyte re-circulation.

24. C. Spleen: A peripheral lymphoid organ. Situates in the blood circulation pathway. 1. Capsule: Thick serous membrane containing DCT and smooth muscles. Penetrates into parenchyma to form trabeculae, which have support function and contain trabecular arteries/veins.

25. 2. White pulp: Isolated and randomly distributed. a. periarterial lymphatic sheath (PALS): diffuselymphoid tissue surrounding the central artery, which is companied by a lymphatic vessel. b. splenic corpuscles: lymphoid nodules that append to the PALS.

26. 3. Red pulp: Among white pulp. a. splenic cords: lymphoid tissue cords containing many blood cells. b. splenic sinus: sinusoidal capillaries. Occupy the spaces among the splenic cords. Lined by rod-shaped endothelia with their long axis parallel to the long axis of sinus and surrounded by circular reticular fibers.

27. 4. Marginal region: Locates at the boundary of white and red pulp, and is the unique way for lymphocytes in blood and in white pulp to exchange. Plays a major role in splenic immune responses. Between the marginal region and white pulp, there are many dilated ends of branches of central atrery, the marginal sinus.

28. 5. Blood circulation pathway: Spleen artery → trabecular arteries → central arteries (white pulp) → splenic cords (& sinusoid) → sinusoid → trabecular veins → spleen vein. 6. Function of spleen: filtration of blood. Immune response. Formation of blood when as fetus.