In this article we will discuss about the structure of spleen with the help of suitable diagram. Also learn about its functions.
Spleen (lien) is the largest lymphoid tissue in the body and specialised, been-shaped organ for filtering blood. It is a highly vascular haemopoietic organ situated in the left hypochondrium directly beneath the diaphragm, above the left kidney and descending colon, behind the fundus of the stomach and weighing about 150 gms in adult. It also plays an important role in the metabolism and defense mechanism of the body. There is no afferent lymphatic vessel.
Histologically it consists of (Fig. 5.7):
i. Capsule with its outer covering the peritoneum,
ii. Trabeculae with blood vessels or without blood vessels,
iii. Hilus (hilum),
iv. White pulp scattered throughout the red pulp,
v. Red pulp,
vi. Reticular meshwork, and
vii. Blood vessels.
Spleen is covered by a connective tissue capsule which is again enveloped by a serous membrane, the peritoneum. The peritoneum is closely adherent to the outside of the capsule. The capsule is deeply indented at the medial aspect of the organ and this indentation is known as hilus (hilum) of the spleen. Blood vessels, lymphatics and nerves pass through the hilus.
From the inner surface of the capsule and from the hilus many trabecular radiate into the substance of the spleen and subdivide or delineate the organ into many communicating compartments or lobules. Each lobule is supplied with blood vessels that run along with the trabeculae. The lobules are not distinct because these are not completely surrounded by trabeculae.
The parenchymal tissue which is enclosed within the capsule is the splenic pulp.
The splenic pulp is of two distinct types:
(1) White pulp, and
(2) Red pulp.
The white pulp is composed of typical lymphatic tissue whereas the red pulp is composed of an atypical lymphatic tissue.
1. White Pulp:
In a freshly sectioned spleen the white pulps are seen scattered all throughout the red pulp as grey patches. These grey patches at early periods were described as Malpighian bodies. But for confusion these terminology has been avoided in the literature in case of spleen and instead, white pulp has been used.
The white pulp is the accumulation of lymphatic tissue surrounding a major arterial vessel of the spleen. This lymphatic tissue is comprised of lymphocytes, plasma cells, macrophages or other free cells lying in the meshwork of reticular fibres. In the white pulp, two distinct components are seen.
As the arteries leave the trabeculae and enter the splenic parenchyma, the vessels lose its adventitia and are replaced by reticular tissue followed by invasion with lymphocyte. This constitutes the periarterial lymphatic sheath of the white pulp. At various points along the course of the vessels where the infiltration of lymphocytes is greater, it forms spherical or ovoid nodules, called as splenic nodules of white pulp. The splenic nodules may have typical germinal centres.
2. Red Pulp:
It is a modified lymphatic tissue and is mostly infiltrated with cells of the circulating blood.
It consists of two components:
i. Splenic Sinuses or Sinusoids:
These are long vascular channels having 35 to 40 µ in diameter. They may have an irregular course and may vary in diameter. They extend throughout the red pulp.
ii. Splenic (or Billroth) Cords:
They appear as continuous partitions in between the sinuses. These cellular cords ultimately form a spongy network of modified lymphatic tissue that gradually merges into the white pulp. In mammalian embryos the red pulp contains myelocytes, erythroblasts and also megakaryocytes. These types of cells are not present in adult spleen except in certain pathological conditions.
It is the junctional region in between the white pulp and the red pulp, and consists of a meshwork of branched reticular cells in association with extracellular reticulum, into which many arterial vessels open.
Blood Vessels and Nature of Blood Circulation:
At the hilus of the spleen, arteries enter and divide into several trabecular branches. The trabecular (interlobular) branches pass along the trabeculae and after coursing for a certain distance the arteries may enter the splenic parenchyma. After entering the splenic parenchyma the artery loses its adventitia and takes the character of reticular tissue and afterwards become infiltrated with lymphocyte.
In this way the vessels are en-sheathed with lymphocyte constituting the periarterial lymphatic sheath. Along the blood vessels and at various points there are greater infiltrations of lymphocyte forming the so-called splenic nodule of the white pulp.
Thus after leaving the trabeculae as central artery or arteriole passes through the white pulp where it gives off several branches. From here arterioles enter the red pulp. Here the arteriole is subdivided into several branches and as these vessels lie close like a brush, are called penicillar vessels.
These penicillar vessels have three distinct successive components; the first long portion having thin smooth muscle is known as pulp arteriole; the middle one having thick sheath is known as sheathed arteriole or ellipsoid or Schweigger-Seidel sheath, the terminal one is arterial capillaries one to two in number. The arterial capillary ultimately ends in the splenic venous sinuses.
There are two theories regarding the opening of the arterial capillaries into sinuses:
According to open theory the arterial capillary opens into the pulp reticulum or pulp cord and then blood gradually filters into the splenic venous sinuses.
According to closed theory the arterial capillary directly opens into the splenic venous sinuses.
The splenic venous sinuses have got irregular anastomosing tunnels throughout the red pulp. The wall of the sinus is composed of specialized reticular cells of phagocytic type and belongs to reticulo-endothelial (macrophage) system.
The blood from the splenic venous sinuses empty into the pulp vein which combines to form the large veins and ultimately blood return in the trabecular vein. In the hilus many trabecular veins join to form splenic veins and as such leave the spleen (Fig. 5.8 and Fig. 5.9).
Functions of Spleen:
i. Formation of Blood:
In the embryo, the spleen functions as a haemopoietic organ of some importance but in normal adult life it never functions in the formation of R.B.C. In some pathological conditions, the spleen may undergo myeloid metaplasia and a large number of erythroblasts, megakaryocytes and myelocytes appear in the tissue.
ii. Blood Destruction:
The old red cells and white cells are destroyed by the R.E. cells of the spleen. Spleen has got some influence on the formation of platelets. In thrombocytopenic purpura, splenectomy causes increase in the number of platelets in the blood. Haemoglobin breaks down and formation of bilirubin also takes place in the spleen (Biligenic function). Thus, in the post-natal life, spleen acts as a filter, which removes the old useless cells and allows only the young active cells to pass into the circulation.
iii. Reservoir of Blood:
Spleen acts as a great reservoir of blood. It is observed that the spleen may release about 150 ml of blood (mainly erythrocytes) to circulation. In cat, as much as one-sixth of the total blood volume or one-third of the total red cells may remain in the spleen. In anoxic conditions (such as in haemorrhage, asphyxia, severe muscular exercise, high altitude, etc.), spleen contracts and sends out the stored blood into the general circulation. Adrenaline injections also have the same effect.
All these conditions stimulate the sympathetic system, which causes contraction of spleen. In carbon monoxide (CO) poisoning this is a great help. Due to the presence of slow circulation in the spleen, CO cannot saturate all the splenic red cells. When spleen contracts these healthy red cells are turned out into the circulation and are used for carrying oxygen.
iv. Relation to Storage and Metabolism:
a. Pigment Metabolism:
Haemoglobin is broken down into haem and globin, the haem is further split into iron and pigment haematoidin, which becomes bilirubin of the plasma.
b. Iron Metabolism:
The iron that is liberated from haemoglobin is at first stored in the splenic pulp cells. Then it is gradually transferred to other places, being carried by the monocytes and the detached R.E. cells. It is specially taken to the liver for storage and to the bone marrow for further haemoglobin synthesis. After splenectomy this storage function suffers and more iron is lost and iron-containing pigment haemosiderin (storage form) are deposited in the spleen.
v. Defensive Action:
a. Many pyroninophil cells (probably plasma cells) are found in the splenic red pulp and hence the spleen is a chief site of immune body formation. Splenectomised animals cannot be immunized against tetanus toxin. Such animals easily succumb under inter-current infection.
b. The R.E. cells engulf bacteria, parasites like those of Leishman-Donovan bodies in Kala-azar and foreign particles.
c. The pulp cells unite with certain toxins, especially of diphtheria and remove them from general circulation.
d. The lymphoid cells of spleen also react against infections.
vi. Manufactures Haemolysin:
a. When red cells of one species are repeatedly injected in another, a specific haemolysin is formed in the spleen.
b. Clinical cases have been reported, where spleen was found to elaborate a haemolysin—causing severe haemolytic anaemia in the patients. After splenectomy such cases were cured.
From such evidence it is thought that spleen is either normally concerned with actual haemolysis of old red cells or prepares them for final haemolysis.
Effects of Splenectomy:
i. Moderate degree of Hypochromic anaemia, being maximum after about one and a half months and recovering slowly during the following three to four months. The red cell count seldom fails below 3 million per cu. mm and the haemoglobin below 55%. Immature nucleated red cells appear in the circulation.
Since spleen normally destroys red cells, anaemia after splenectomy is the exact opposite to one’s expectation. This is one of the reasons which led to the suggestion that spleen manufactured something which normally stimulates the activity of the bone marrow. The recovery from this anaemia is due to the compensatory overgrowth of bone marrow.
ii. The animals cannot withstand severe anoxia.
iii. There is diminished resistance against infections.
iv. The leucocyte count increases, and may go up to 20,000-40,000 per cu.mm. This rise is chiefly due to the increase in the number of polymorphs. Later on, lymphocytes and in some cases eosinophils may also rise. This may be explained from the fact that in the absence of the spleen these cells fail to be destroyed.
Regarding the defensive action of spleen much of it can be taken over by the hyper-growth and hyperactivity of cells of the R.E. system elsewhere. Enlargement of spleen occurs in certain pathological conditions (Gaucher’s and Banti’s diseases, certain anaemias). Furthermore enlargement also accompanies Malaria, Hodgkin’s disease, leukaemia. Splenectomy is adopted as a treatment for certain pathological conditions.