Spleen

The spleen (lien, s.splen) performs the functions of immune control of the blood. It is located on the path of blood flow from the main vessel of the systemic circulation - the aorta to the portal vein system, branching in the liver. The spleen is located in the abdominal cavity, in the left hypochondrium, at the level of the IX to XI ribs. The mass of the spleen in an adult (20-40 years) is 192 g in a man and 153 g in a woman, length - 10-14 cm, width - 6-10 cm and thickness - 3-4 cm.

The spleen has the shape of a flattened and elongated hemisphere. It is dark red in color and soft to the touch. The spleen has two surfaces: diaphragmatic and visceral. The smooth convex diaphragmatic surface (facies diaphragmatica) faces laterally and upward toward the diaphragm. The anteromedial visceral surface (facies visceralis) is uneven, and the hilum of the spleen (hilum splenicum) and areas adjacent to adjacent organs are distinguished on it. The gastric surface (facies gastrica) is in contact with the fundus of the stomach; it is visible in front of the hilum of the spleen. The renal surface (facies renalis), located behind the hilum of the organ, is adjacent to the upper end of the left kidney and the left adrenal gland. The colonic surface (facies colica) at the point of contact of the spleen with the left flexure of the colon is located below the splenic hilum, closer to its anterior end. Slightly above the colonic surface, immediately behind the hilum, there is a small area to which the tail of the pancreas approaches. The upper (anterior) edge of the spleen (margo superior), separating the gastric surface from the diaphragmatic, is sharp. Two or three shallow notches are distinguished on it. The lower (posterior) edge (margo inferior) is more obtuse. The spleen has two ends (poles): posterior and anterior. The posterior end (extremitas posterior) is rounded, facing upward and backward. The anterior end (extremitas anterior) is sharper, protrudes forward and is located slightly above the transverse colon.

The spleen is covered on all sides by the peritoneum, which is firmly fused with its fibrous membrane. Only in the area of the gate, where the tail of the pancreas is directed, is there a small area free of the peritoneum. Between the visceral surface of the spleen on one side, the stomach and diaphragm on the other, the sheets of the peritoneum, its ligaments (gastrosplenic, diaphragmatic-splenic) are stretched, therefore, a change in the position of these organs (excursion of the diaphragm during breathing, filling and emptying of the stomach) is reflected in the topography of the spleen.

From the fibrous membrane (tunica fibrosa), located under the serous cover, connective tissue crossbars - trabeculae of the spleen (trabeculae splenicae) extend into the organ. Between the trabeculae is the parenchyma - the pulp (pulp) of the spleen (pulpa splenica). There is a red pulp (pulpa rubra), located between the venous sinuses (sinus venuldris) of the spleen and consisting of loops of reticular tissue filled with erythrocytes, leukocytes, lymphocytes, macrophages. White pulp (pulpa alba) is formed by splenic lymphoid periarterial cuffs, lymphoid nodules and macrophage-lymphoid cuffs (ellipsoids), consisting of lymphocytes and other cells of the lymphoid tissue located in the loops of the reticular stroma. The mass of white pulp in children and adolescents is 18.5-21% of the total mass of the spleen.

Periarterial thymus-dependent cuffs surround the pulp arteries from the point where they emerge from the trabeculae to the ellipsoids. Each pulp artery is surrounded by 2-4 rows (layers) of lymphoid cells consisting of small and medium lymphocytes, plasma and reticular cells, macrophages, and single large lymphocytes and cells with a mitotic pattern. T-lymphocytes are predominantly located near the arterial walls. This is the internal thymus-dependent zone of the periarterial lymphoid cuffs. The external zone of the periarterial lymphoid cuffs consists of T- and B-lymphocytes and other cells. This is the so-called marginal (border) zone. It separates the thymus-dependent zone from the red pulp.

Lymphoid nodules with a diameter of 300 µm to 1 mm are located along the lymphoid cuffs, forming their thickenings. In this case, the periarterial lymphoid cuff with its artery is part of the lymphoid nodule, occupying its periphery. The part of the lymphoid cuff located inside the nodule is called its periarterial zone. The pulp artery passing through the lymphoid nodule is always located eccentrically. Lymphoid nodules can have a center of reproduction, which is localized on the side of the artery of the lymphoid nodule. In the center of reproduction there are large lymphocytes, T- and B-cells, macrophages, plasma and mitotically dividing cells. The mantle (peripheral) zone surrounding the center of reproduction, consisting mainly of small and medium lymphocytes, has a thickness of 40 to 120 µm.

Macrophage-lymphoid cuffs (ellipsoids) are located in the area of the terminal branches of the pulp arteries. They consist mainly of macrophages and lymphocytes, as well as reticular cells surrounding the blood capillary. The length of such a lymphoid cuff reaches 50-100 μm, the diameter is 25-50 μm. Upon exiting the macrophage lymphoid cuffs, the capillaries are divided (branched) in a brush-like manner and flow into the wide (up to 40 μm) sinuses of the spleen, which are the beginning of the venous bed of the organ.

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Development and age-related features of the spleen

The spleen rudiment appears in the 5th-6th week of intrauterine development as a small cluster of mesenchyme cells in the thickness of the dorsal mesentery. Soon, lymphoid cells appear in the mesenchymal rudiment, and slits are formed - the future vessels of the spleen, around which tissue differentiation of the organs occurs. In the 2nd-4th month, venous sinuses and other blood vessels are formed. At the same time, strands of cells - future trabeculae - grow into the spleen from the capsule. At the end of the 4th month and in the 5th month, clusters of lymphocytes are found in the spleen - future periarterial lymphoid cuffs and lymphoid nodules. Gradually, the number of lymphoid nodules in the spleen increases, and reproduction centers appear in them.

In a newborn, the spleen is round, has a lobular structure, and weighs approximately 9.5 g. During this period, the white pulp makes up 5 to 10% of the organ's weight. In the 3rd month of postnatal development, the spleen weight increases to 11-14 g (on average), and by the end of the first year of life it reaches 24-28 g. In a 6-year-old child, the spleen weight doubles compared to a one-year-old, by the age of 10 it reaches 66-70 g, and at 16-17 years it is 165-171 g.

The relative amount of white pulp (82-85%) remains almost unchanged throughout a person's life. The content of white pulp (periarterial lymphoid cuffs, lymphoid nodules and macrophage-lymphoid cuffs, or ellipsoids) in the spleen of a child aged 6-10 years is on average 18.6%, by the age of 21-30 it decreases to 7.7-9.6%, and by the age of 50 it does not exceed 6-5% of the organ mass.

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Vessels and nerves of the spleen

The splenic artery approaches the spleen and divides into several branches that enter the organ through its gates. The splenic branches form 4-5 segmental arteries, and the latter branch into trabecular arteries. Pulp arteries with a diameter of 140-250 μm are directed to the parenchyma of the spleen, around which are located lymphoid periarterial cuffs and the periarterial zone of splenic lymphoid nodules. Each pulp artery divides into arterioles with a diameter of about 50 μm, and then into capillaries surrounded by macrophage-lymphoid cuffs (ellipsoids). The capillaries formed by branching arteries flow into wide splenic venous sinuses located in the red pulp.

Venous blood from the spleen parenchyma flows through the pulpal and then trabecular veins. The splenic vein formed at the organ gate flows into the portal vein.

The spleen is innervated by branches of the vagus nerves and sympathetic fibers that approach the spleen as part of the plexus of the same name.