Structure and function of Spleen

Structure and function of spleen- anatomy, physiology

1. Roll no 0954 Structure and function of spleen

2. Spleen Lymphatic organ connected to blood vascular system Part of mononuclear phagocyte system. Wedge shaped

3. Spleen situated principally in the left hypochondriac region, but its superior extremity extends into the epigastric region

4. it lies between the fundus of the stomach and the diaphragm . the largest of the ductless glands, and is of an oblong, flattened form, soft, of very friable consistence, highly vascular, and of a dark purplish color.

5. 1" (2.5 cm) by 3“(7.5cm) by 5“(12.5cm) 150 grams (5.3 oz) to 200 grams (7.1 oz) lies between the 9th and 11th ribs on the left hand side.

6. Development Appears about the fifth week It arises in a series of hillocks, of the mesoderm in the dorsal mesogastrium above the tail of pancreas. With the change in position of the stomach the spleen is carried to the left, and comes to lie behind the stomach and in contact with the left kidney. The part of the dorsal mesogastrium between the spleen and the greater curvature of the stomach forms the gastrosplenic ligament .

7. Accessory spleens When the hillocks fail to unify 20% of persons. especially in the gastrolienal ligament and greater omentum , vary in size from that of a pea to that of a plum.

8. Relations D iaphragmatic surface ( external or phrenic surface ) convex, smooth Relation -under surface of the diaphragm, which separates it from the ninth, tenth, and eleventh ribs of the left side, and the intervening costodiaphragmatic recess of pleura and lung

9. Visceral surface divided by a ridge Gastric surface broad and concave is in contact with the posterior wall of the stomach ; and below this with the tail of the pancreas. It presents near its medial border a long fissure, termed the hilum . This is pierced by several irregular apertures, for the entrance and exit of vessels and nerves.   

10. Renal surface somewhat flattened, is considerably narrower than the gastric surface upper part of the anterior surface of the left kidney and occasionally left suprarenal gland.   

11. Colic surface flat, triangular in shape, and rests upon the left flexure of the colon and the phrenicocolic ligament, and is generally in contact with the tail of the pancreas.

12. anterior border - is free, sharp, and thin, and is often notched , especially below posterior border- more rounded and blunter than the anterior, corresponds to the lower border of the eleventh rib and lies between the diaphragm and left kidney. The intermediate margin is the ridge which separates the renal and gastric surfaces. inferior border- rounded

13. Peritoneal relations Gastrosplenic ligament : from the hilum of the spleen to the greater curvature of the stomach; it contains short gastric vessels and associated lymphatics and sympathetic nerves. Splenorenal ligament : from the hilum of the spleen to the anterior surface of the left kidney; it contains the tail of the pancreas and splenic vessels,ln Phrenicocolic ligament : from the splenic flexure of the colon to the diaphragm along the midaxillary line

14. Structure     Ext ernal or serous coat ( tunica serosa ) from the peritoneum thin, smooth, and intimately adherent to the fibroelastic coat. It invests the entire organ, except at the hilum and along the lines of reflection of the phrenicolienal and gastrolienal ligaments.   Fibroelastic coat ( tunica albuginea ) invests the organ, and at the hilum is reflected inward upon the vessels in the form of sheaths. From these sheaths, as well as from the inner surface of the fibroelastic coat, numerous small fibrous bands, trabeculae are given off in all directions

15. The fibroelastic coat, the sheaths of the vessels, and the trabeculæ , are composed of white and yellow elastic fibrous tissues spleen possesses a considerable amount of elasticity, which allows of the very great variations in size small amount of non-striped muscular fiber The spleen consists of a number of small spaces , formed by the trabeculæ which contain the splenic pulp.   

16. The spleen is made of red pulp and white pulp, separated by the marginal zone; 76-79% of a normal spleen is red pulp

17. red pulp Red blood–filled sinusoids and reticuloendothelial cell–lined cords of Billroth (connective tissue ) Splenic cords -reticular fibers, reticular cells, and associated macrophages Sinusoids wide vessels that drain into trabecular veins. Gaps in the endothelium lining the sinusoids mechanically filter blood cells as they enter the spleen.

18. Venous sinuses → a lining of endothelial cells that are positioned in parallel and connected by stress fibres to annular fibres , Contractility of the stress fibres allows the formation of slits between the endothelial cells, regulating the passage of blood Worn-out or abnormal red cells attempting to squeeze become damaged, and are devoured by macrophages

19. Cells found in red pulp Splenic cells- Large rounded cells, ameboid movement, often contain pigment and red-blood corpuscles in their interior. Cells of the reticulum may contain pigment granules in their cytoplasm B and T lymphocytes Erythrocytes granulocytes plasma cells hematopoietic cells plasmablasts that migrate from the follicles and the outer PALS after antigen specific differentiation macrophages

20. White pulp white lymphoid follicles arrayed within the red pulp matrix Contains lymphocytes and macrophages, dendritic cells, plasma cells, arterioles, and capillaries in reticular connective tissue meshwork Periarterial lymphatic sheaths (PALS)- Lymphatic tissue sheath that surrounds central artery Most of the T cells are CD4+

21. Malpighian bodies, splenic lymphoid nodules: hemispherical accumulations of B lymphocytes attached to the PALS at intervals. 0.25 mm. to 1 mm. in diameter Secondary follicles Follicles with a germinal centre arise when antigen specific B-cell immune reactions occur in the spleen existence is of limited duration. attached to the smaller ramifications of the splenic artery.

22. Germinal centre Antigen specific larger B cells pale-staining nuclei and abundant cytoplasm Mantle zone or corona. The expanding germinal centre displaces the small, dark recirculating B lymphocytes to the periphery of the follicle Marginal Zone broad region primarily occupied by relatively large memory B cells. light impression after routine staining delimited from the PALS and the follicles by a very irregular capillary blood vessel called the marginal sinus .

23. white pulp of the spleen is seen in a lymphoid follicle, outlined by the dashed line found.  The lymphoid follicle has a pale-staining germinal center (GC) in which B cells are proliferating.  mantle zone ( ManZ ) contains small lymphocytes and an outer marginal zone that contains larger lymphocytes that are less densely packed than cells in the mantle zone. 

24. Blood supply Blood flow is about 150 mL /min Splenic A. large size in proportion to the size of the organ tortuous course

25. Passes through leinorenal ligament to reach the hilum where it divides into six or more branches receive sheaths the external fibrous tissue branches into numerous arterioles

26.   When arterioles acquire a coating of T cells, they are called central arteries and the surrounding lymphoid tissue is called the PALS, i.e., the periarteriolar lymphoid sheath.

27.  The arterioles end by opening freely into the splenic pulp; their walls become much attenuated, they lose their tubular character, and the endothelial cells become altered, presenting a branched appearance, and acquiring processes which are directly connected with the processes of the reticular cells of the pulp   

28. The spleen comprises many units of red and white pulp centered around small branches of the splenic artery, called central arteries. White pulp is lymphoid in nature and contains B cell follicles, a marginal zone around the follicles, and T cell–rich areas sheathing arterioles. The red pulp areas include pulp sinuses and pulp cords. The cords are dead ends . In order to regain access to the circulation, red blood cells must traverse tiny openings in the sinusoidal lining. Stiff, damaged, or old red cells cannot enter the sinuses.

29. Pitting Red cell–inclusion bodies such as parasites ,nuclear residua ( Howell-Jolly bodies ), or denatured hemoglobin ( Heinz bodies ) are pinched off in the process of passing through the slits The culling of dead and damaged cells and the pitting of cells with inclusions appear to occur without significant delay because the blood transit time through the spleen is only slightly slower than in other organs

30. Splenic vein and tributaries

31. Open circulation

32. Closed circulation

33. Nerve supply Sympathetic fibres derived from the celiac plexus They are distributed to the blood vessels and to the smooth muscle of the capsule and trabeculae .

34. Lymphatic vessels Splenic tissue proper has no lymphatics Lymphatics from connective tissue drain into pancreaticosplenic lymph nodes along splenic artery

35. Functions Red pulp- Mechanical filtration of red blood cells. white pulp - Active immune response through humoral and cell-mediated pathways. Production of opsonins , properdin , and tuftsin .

36. Creation of red blood cells. Hematopoietic functions up until the fifth month of gestation After birth, erythropoietic functions cease, except in some hematologic disorders. retains the ability to produce lymphocytes

37. Storage contains approximately one-third of the total body platelets and a significant number of marginated neutrophils . sequestered cells are available when needed to respond to bleeding or infection. the normal human spleen does not sequester or store red blood cells

38. Removal of Red Blood Cells Removes senescent and defective red blood cells through a unique organization of its parenchyma and vasculature After normal lifespan RBC will be broken down and some parts recycled. Removes all abnormal red blood cells.

39. The removal of antibody-coated bacteria and antibody-coated blood cells from the circulation. Phagocytes: Reticular cells and free macrophages of red pulp Modified reticular cells of ellipsoid Free macrophages and endothelial cells of venous sinusoids Surface reticular cells of lymphatic follicle

40. Assisting the host in adapting to its hostile environment. (1) clearance of bacteria and particulates from the blood (2) the generation of immune responses to certain pathogens (3) the generation of cellular components of the blood under circumstances in which the marrow is unable to meet the needs (i.e., extramedullary hematopoiesis ).