Spleen

The spleen is functionally part of the lymphatic system, serving as an organ responsible for the uptake and destruction of aged erythrocytes. Lymphocytes are produced in the bone marrow and mature in the thymus but are activated in the spleen and lymph nodes. The spleen is the largest lymphatic organ and plays a key role in the defense against harmful substances and infections entering the bloodstream. Other functions of the spleen include the production of antibodies and immune cells, as well as serving as a reservoir for blood, which can be released into circulation by contraction of smooth muscle-like cells during periods of increased oxygen demand, such as prolonged physical activity. Spleen cells, particularly macrophages, have phagocytic abilities. During fetal development, hematopoiesis occurs in the spleen and liver in the hepatolienal phase.

Position and shape

Visceral side of the spleen

Spleen is an oval-shaped organ located obliquely from back to front, top to bottom, in the left hypochondrium (pars supramesocolica, or saccus lienalis), where it follows the curvature of the diaphragm. Its longitudinal axis aligns with the 10th rib/intercostal space, and the entire width of the spleen spans from the 9th to the 11th rib. In a living person, it appears dark red to purplish in color; in a deceased individual, it may appear bluish. The surface of the spleen is smooth, shiny, and covered by the visceral peritoneum. Its size can vary individually: length 10–13 cm, width 6–8 cm, thickness 4 cm; weight also varies, depending on the degree of blood filling: 140–160 g in men and 120–150 g in women (a weight of 200 g is not pathological). In a relaxed state, the spleen is larger, and its weight decreases with age—typically after the age of 40. Pathologically, spleen enlargement may occur in some blood disorders (splenomegaly). The surface of the spleen is very fragile and can tear easily from blunt trauma to the abdominal wall, leading to significant bleeding.

Spleen Surface Anatomy:

• Extremitas posterior: The posterior upper end, located 4 cm from the spinous process of T10 and 2 cm from the transverse process, is not palpable.
• Extremitas anterior: The anterior lower end, which should not extend beyond the costoclavicular line.
• Superior margin: The upper anterior border, characterized by a sharp edge with variously deep notches (crenae lienis), which vary individually in prominence. This was previously referred to as margo crenatus.
• Inferior margin: The lower, more rounded edge.
• Hilum: The longitudinal region for the entry and exit of blood vessels, located along the longitudinal edge on the visceral surface; divides the surface into two longitudinal areas.

Suspensory Ligaments of the Spleen:

• Lig. phrenicolienale and Lig. pancreaticolienale: Attach the spleen to the diaphragm and the posterior abdominal wall.
• Lig. gastrolienale: Connects the spleen to the stomach; both of these ligaments are part of the left wall of the bursa omentalis.

Syntopy

Topographic surroundings of the spleen

Spleen Surfaces:

• Facies diaphragmatica: The outer convex surface, facing the diaphragm.
• Facies visceralis: The opposite, concave surface, facing the abdomen, separated by edges.
• Facies renalis: The left, caudal surface, in contact with the kidney via the parietal peritoneum.
• Facies gastrica: The left, cranial surface, in contact with the stomach.
• Facies colica: An inconsistent flattening at the anterior end, located at the site of the left colic flexure (lig. phrenicocolicum).

Macroscopic structure of the spleen

The outer layer of the spleen consists of two layers: the superficial tunica serosa, formed by the visceral peritoneum, which fuses with the deeper tunica fibrosa (capsula lienis), a fibrous capsule beneath the peritoneal lining. The fibrous capsule contains elastic fibers and smooth muscle cells. From the fibrous capsule, trabeculae lienis (dense fibrous trabeculae) extend into the spleen, forming a spongy stroma and bringing blood vessels into the organ. The stroma of the spleen, as a peripheral lymphatic organ, consists of collagenous and reticular connective tissue interspersed with smooth muscle cells. The spaces between the trabeculae are filled with the spleen's red and white pulp. The white pulp is made up of lymphatic tissue nodules, which are macroscopically visible as grayish-white spots. The red pulp consists of a reticulum interwoven with blood vessels and their dilated segments (splenic sinuses); between the trabeculae, it forms structures known as Cords of Billroth.

Blood flow through spleen

The splenic artery (a. lienalis) enters the hilum of the spleen with 4-6 branches, which further divide into the ramus superior and ramus inferior, giving rise to segmental arteries that divide the spleen into segments. From the segmental arteries, trabecular arteries emerge, running along the trabeculae of the spleen. Small branches, the central arteries (aa. centrales), with a diameter of approximately 0.2 mm, extend from the trabeculae into the pulp. The central arteries are surrounded by a layer of lymphatic tissue known as the periarteriolar lymphoid sheath (PALS), which is typically excentric. PALS is a thymodependent zone, meaning it is populated by T lymphocytes. Malpighian bodies, or B-dependent lymphatic follicles (folliculi lymphatici lienales), are embedded along the central artery. These follicles, together with the surrounding marginal zone, form the peripheral white pulp of the spleen.

After exiting the white pulp, the artery branches into the penicillar arteries (aa. penicillatae), which have a diameter of about 25 μm. Each of these arteries forms a short, thickened segment known as Schweigger-Seidel's sheath, composed of reticular cells. In this region, the artery is lined only by endothelium, and the cells of the sheath replace the other layers of the vessel wall. Nerve endings are found within the sheath. Beyond this point, the artery becomes a thin pre-capillary arteriole, opening into the sinus lienis, or splenic sinuses, which are dilated sections of the vascular bed (diameter 80-150 μm). These sinuses densely permeate the red pulp, and their endothelial cells are long, narrow, and arranged longitudinally. The discontinuous basal lamina and reticular fibers contribute to the characteristic appearance of the sinuses, which resemble barrels with hoops.

As the sinuses expand, their endothelial cells separate, forming slits (2-3 μm wide) that allow blood cells and plasma to pass into the red pulp and back into the sinuses. This mechanism is unique to the spleen and the placenta. From the sinuses, small postcapillary venules begin, which transition into the medullary veins. These veins carry blood from the red pulp towards the trabeculae of the spleen, where they drain into trabecular veins. The wall of trabecular veins consists only of endothelial cells, with the other layers being replaced by the connective tissue of the trabeculae. Trabecular veins converge into the hilum and join within the ligament pancreaticolienale to form the splenic vein (v. lienalis), which exits at the upper border of the pancreas and enters the portal vein (v. portae).

Summary:

Splenic Artery → Trabecular Arteries → Central Arteries → Penicillar arteries → Splenic Sinuses → postcapillary venules → medullary veins → trabecular veins → splenic vein → portal vein

Microscopic structure of the spleen

White pulp

The white pulp makes up 20% of the internal volume of the organ. It is bound to the course of blood vessels - after branching of the a. Lienalis in aa. trabeculares and then aa. centrales, the central arteries are enveloped by a thymodependent periarterial lymphatic sheath (PALS). PALS as thymodependent regions contain T-lymphocytes. Malpighian bodies can form around these sheaths - lymph nodes containing B-lymphocytes, which as standard lymph nodes are divided into primary (without antigen stimulation) and secondary (with germinal centers created by antigen stimulation). There is a marginal zone with B-lymphocytes, macrophages and dendritic antigen presenting cells at the white and red pulp interfaces. The marginal zone plays an important role in the development of the immune response to antigens in the blood - dendritic cells take up antigens from the blood and pass them on to immunocompetent nodular cells. These are activated and form the germinal center of the nodule. Activated B-lymphocytes and plasma cells pass into the red pulp. The plasma cells in the red pulp remain and deliver their antibodies to the sinus blood; activated B-lymphocytes leave the red pulp and enter the bloodstream.

Red pulp

The red pulp is formed by a reticular ligament arranged in the marrow cord (cords of Billroth ) interwoven with sinuses. Fixed and free macrophages, monocytes, lymphocytes, plasma cells and blood elements (erythrocytes, granulocytes, platelets) occur in the reticulum mesh. Erythrocytes penetrate the sinuses - viable erythrocytes return to the sinus and circulate through the veins, surviving erythrocytes are retained in the pulp, destroyed and phagocytosed. The principle of retention is based on the fact that aged erythrocytes have a thinned cell membrane - by pressure from inside the cell they arch into spherical shape (spherocytes), which prevents them from passing back to the sinuses. Macrophages are programmed to capture spherocyte-sized spherical structures.

Function of the spleen

Lymphocyte formation

Lymphocyte formation takes place in the white pulp, lymphocytes then pass into the red pulp and then through the sinuses into the bloodstream. In the marginal zones, lymphocytes are taken up from the circulation and incorporated into the white pulp. The production of erythrocytes and granulocytes disappears with the end of the fetal period, but it reappears in adulthood, for example in [[leukemie|leukemias].

Defence functions

Antigens are taken up from the blood by macrophages, the surface of the reticular cells of the lymph nodes and their marginal zones. Some of these cells to enlarge the surface branched (dendritic). B-cells are activated by antigens, which proliferate and turn into immunoblasts, which turn into antibody-producing plasma cells. After the reaction subsides, some B-lymphocytes that have not reached the plasma cell stage remain as memory cells. Upon subsequent encounter with the same antigen, these cells allow for a rapid immune response and prompt antibody production. Macrophages actively phagocytose bacteria and viruses, but also lipid droplets, thereby filtering the spleen and cleansing the circulating blood. T-lymphocytes from the periarterial lymphatic sheaths proliferate, pass into the bloodstream, where they play a role in the so-called cellular immunity.

Erythrocyte destruction

The lifespan of erythrocytes is 120 days, their destruction takes place in the red pulp. Here, erythrocytes are retained, surrounded by macrophage protrusions, fragmented, phagocytosed, and lysed into lysosomes. Hemoglobin of destroyed blood cells is converted to bilirubin and ferritin. The bilirubin thus obtained is utilized in the liver as a bile pigment, and iron from ferritin is used in the bone marrow to form new erythrocytes.

Blood supply

The blood retained in the spleen can be passed into the bloodstream to increase the volume of circulating blood, for example during prolonged strenuous muscle work. The smooth muscle in the sheath and trabeculae is not enough to expel the blood, so the muscles of the spleen vessels are also necessary. The volume added to the bloodstream is not significant.

Examination of the spleen

Placement of the spleen, we feel the pathologically enlarged spleen during the X. rib (so-called spleen rib)

The spleen is not physiologically palpable, the percussion examination is inaccurate. X- ray examination is possible only by softer X-rays - splenoportography - X-ray examination on the basis of contrasting blood vessels. Examination by means of transverse sections by computed tomography or sonographic examination is also possible.

Development and variation of the spleen

The spleen arises as a bumpy thickening on the left side of the posterior mesogastrium in embryos 8 - 10 mm long. The superficial coelom epithelium is initially cubic, with a mesenchyme below the epithelium . With the growth and rotation of the arch to the left - enlargement is irregular, faster enlargement takes place in the districts around the vessels. The growing base has the character of low protrusions that merge quickly. In the 3rd month of development it reaches its final form, the mesenchyme of the interior of the base differentiates into the capsule and trabeculae and the next red pulp. Typical sinuses appear in the 6th month of development, hematopoiesis lasts until the 8th month of development.

• Shape variations:
  • lien lobatus: conspicuously lobed; the rest of the irregular embryonic growth
  • lienes accessorii: 1 and more in the surrounding curtains; the size of the cherry.

Lymphatic vessels of the spleen

They leave the larger beams along the vessels and the capsula lienis. Blind lymph capillaries begin in the white pulp, from where they lead to the trabeculae, from there to the capsule lienis, then to the hilum and then to the nodi lymphatici pancreaticolienales (group of nodes in the cauda pancreatis).

Splenic nerves

Autonomous fibers come from the celiac plexus.

Links

Related articles

• Spleen diseases • Splenomegaly • Hepatosplenomegaly • Spleen injury
• Diagnostic imaging methods in the examination of the pancreas, liver and spleen
• Development of lymphatic vessels, nodes and spleen
• Spleen (specimen) • Secondary amyloidosis of spleen (specimen) • Spleen / Spleen - Ag • Spleen / Spleen - Pearls • Spleen (SFLT)

References

• ws:Slezina

External links

• Spleen (Czech Wikipedia)
• Spleen (English Wikipedia)