Histology of the spleen
The spleen is a fist sized organ located in the left upper quadrant of the abdomen. It is the largest lymphoid organ and thus the largest filter of blood in the human body. The spleen has a unique location, embryological development and histological structure that differs significantly from other lymphoid organs.
Special histological features define several important functions of the spleen, such as filtering blood, maintaining immune response balance and recycling iron. The spleen can also serve as a reservoir for additional blood in situations of acute or chronic blood loss (such as bleeding or anemia), as well as an alternative site for hematopoiesis (formation of blood cells and platelets) outside of bone marrow. Even though the spleen has a few unique functions that can't be replaced by other lymphoid organs, it is not a vital organ and people can live without it.
This article will discuss normal spleen histology and showcase labeled spleen microscopic slides.
|Definition||Non-vital lymphoid organ in the left upper quadrant of the abdomen|
|Structure||Red pulp, white pulp|
|Function||Blood filtering, immune response, recycling iron, blood storage, extramedullary hematopoiesis|
|Clinical relations||Splenomegaly, ruptured spleen|
- Location and anatomy
- Clinical relations
Location and anatomy
The spleen is located in the left upper quadrant region of the abdomen. More precisely, it is found posterior to the stomach and anterior and inferior to the left hemidiaphragm at the level of ribs 9-10. On the medial side, the spleen is in relation to the left kidney and inferiorly it sits on the left colic flexure (splenic flexure).
By being almost an entirely intraperitoneal organ, the spleen is mobile within the abdominal cavity. The hilum of the spleen is the only part of the spleen that is peritoneum free.
Being an intraperitoneal organ, the spleen is covered by a layer of visceral peritoneum. Underneath the peritoneum is the capsule of the spleen, encasing its parenchyma.
The capsule of the spleen consists of dense irregular fibroelastic tissue. The connective tissue of the capsule contains contractile cells called myofibroblasts. By producing weak contraction of the capsule, these cells help to discharge the blood stored within the spleen into the circulation. The capsule also allows the spleen to significantly increase in size when necessary and discharge a large amount of blood to contribute to the tissues oxygenation, like during physical exercise. At the level of the hilum, the capsule splits into several septae called trabeculae which penetrate into the parenchyma of the spleen and partly divide its tissue.
Like every other organ, the spleen consists of stroma and parenchyma. The stroma of the spleen is composed mainly of a network of reticular connective tissue. This mesh provides support for blood cells and cells of the immune system (lymphocytes, macrophages, and dendritic cells). The parenchyma of the spleen is divided into two functionally and morphologically distinct compartments (red pulp and white pulp) divided by a tissue layer called the marginal zone. Outside the marginal zone is the perifollicular zone which contains sheathed capillaries and blood-filled spaces without endothelial lining.
The red pulp occupies the majority of the stromal tissue of the spleen. It consists of the cords of Billroth and splenic sinusoids. The cords of Billroth (splenic cords) are the cellular aggregations supported by the reticular connective tissue. They appear as stripes and consist of of macrophages, plasmocytes and blood cells.
Splenic sinusoids are found between the cords of Billroth. They are filled with blood and give the red pulp its distinguishable red appearance. Blood slowly flows through the sinusoids where it is exposed to macrophages from the cords of Billroth, patiently waiting for foreign antigens that can appear in the blood. In a nutshell, the red pulp functions as a blood filter for various toxins, destroying them before they enter systemic circulation and get the chance to spread throughout the body and damage other organs.
The white pulp of the spleen is made of three different compartments: Periarterial lymphoid sheath (PALS), lymphoid follicles and the marginal zone.
The PALS consists of a central artery (a branch of the splenic artery) surrounded by a sheath of lymphoid tissue. Here, the lymphoid tissue organized into two layers: The inner layer and outer layer. The inner layer is mainly composed of T lymphocytes which is why it is also called the T-zone. The outer layer has a more diverse cellular morphology, containing T and B lymphocytes.
The branches of central arterioles are surrounded by the sharply defined areas of B lymphocytes, comprising the lymphoid follicles of the spleen. There are two types of lymphoid follicles depending on the features of the B lymphocytes that comprise them: Primary follicles and secondary nodules.
A follicle that consists mainly of small, immature lymphocytes is called a primary follicle. However, most nodules found in the spleen are secondary nodules that arise from primary follicles as the lymphocytes mature and increase in size. They differ from primary follicles by featuring a distinctive centrally positioned zone called the germinal center. The germinal centers are the sites where lymphocytes mature and acquire the ability to produce antibodies. So, seeing the germinal center is a sign that lymphoid tissue is responding to an antigen. Other than B lymphocytes, the germinal centers also contain follicular dendritic cells (FDC) which also increase in number after antigen activation. They support B lymphocytes, initiate and modulate their immune response.
The marginal zone is found on the very edge of the lymphoid follicles, containing different immune cells that are well equipped for starting an appropriate immune response.
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Since the spleen is a blood filter, one has to assume that it is a highly vascular organ. Blood from the splenic artery enters the spleen through the hilum. From there, the artery divides into smaller branches that enter the splenic parenchyma following the course of trabeculae. Together with the trabeculae, the arteries branch throughout the parenchyma and gradually decrease in diameter. Eventually, smaller arterioles leave the dense connective tissue of trabeculae entering the parenchyma, where they are surrounded by PALS.
The artery in the center of this region is referred to as the central artery. On a histology slide, the PALS has a similar appearance to a lymphoid nodule. The only way to distinguish the two is by the presence of a central artery. The endothelial cells of the central artery have finger-like extensions that spirally wrap around the lumen protecting the PALS from a direct antigen invasion.
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The central artery sends branches that enter the marginal zone of the white pulp. It then continues into the red pulp where it divides further and eventually transforms into arterial capillaries. These capillaries are surrounded by clusters of macrophages and are called sheathed capillaries. From this point on, blood travels freely through the venous sinuses of the red pulp. The endothelial cells of the sinusoids have special histological features; they are elongated, spindle-shaped and don’t have characteristic cellular junctions to adjacent cells.
Due to the lack of junctions, there are wide gaps between the cells that serve as a mechanical filter between the blood and splenic cords. When different antigens (e.g. microorganisms, cellular debris and aged and damaged erythrocytes) cross these gaps they can be phagocytosed and destroyed by macrophages that are waiting on the other side. This type of blood flow is called open circulation which is characteristic for human spleen. In some species (e.g. rodents), the blood from sheathed capillaries enters venous sinuses directly. This is known as closed circulation.
From the maze made of venous sinuses, the blood flows into trabecular veins and eventually exits the spleen through the splenic vein.
Reinforce your knowledge about the microcirculation of the spleen with our quiz below:
One of the most important functions of the spleen is blood filtering. It is considered a "graveyard for red blood cells" because it removes old and damaged erythrocytes from circulation. This function is mostly due to the unique structure of the blood vessels and macrophages present in the red pulp. The macrophages phagocyte and destroy erythrocytes and then recycle their iron from hemoglobin. Eventually, iron ends up stored and reused in bone marrow.
Despite being the best-known destroyer of blood cells, the spleen can also be a site of their production. During fetal development, before bone marrow fully develops, the spleen is an important site of hematopoiesis. After birth, blood is produced in the spleen only during some pathological conditions such as severe bacterial infections. The spleen also stores a certain amount of blood in its blood vessels that can be released in a state of acute and severe blood loss.
Being the largest lymphoid organ, the spleen is one of the major sites of initiating and modulating the immune response. It can detect and present specific pathogens in the blood and produce an immune response to defend the body against them. The specific structure of the spleen makes it a key spot for defense against encapsulated bacteria. This is why people without a spleen are predisposed for some bacterial infections like pneumonia.
Learn more about the spleen structure and master its functions with our article and quiz.
Splenomegaly is an enlargement of the spleen in size and weight. Routinely, the spleen is not palpable during a physical exam in healthy adults. If the doctor can feel the spleen under the ribs, this means that it is enlarged. An enlarged spleen usually doesn't cause any symptoms and it is usually noticed during the physical exam.
Splenomegaly can be present in many pathological conditions of the spleen (e.g. lymphomas), or even of other tissues and organs (e.g. liver diseases, bacterial and viral infections and metabolic diseases). The mechanism of enlargement is different depending on the cause. Two of the most common mechanisms are hyperplastic and congestive enlargement. In cases of acute infection, the cells of the spleen get activated and multiply which results in hyperplastic enlargement. In cases of liver disease, the cause of enlargement is an increased venous pressure that traps blood inside the sinusoids of the spleen causing congestive enlargement.
Given the fact that approximately one-third of circulating platelets are stored in the spleen, if splenomegaly happens, it can destroy more thrombocytes than usual. This can result in thrombocytopenia (a low number of platelets in circulating blood) which can be a life threatening complication.
The rupture of the spleen is one of the most urgent medical conditions. It usually occurs due to a traumatic injury in the upper abdominal region. People that have an enlarged spleen are more predisposed to this kind of injury, in comparison to healthy individuals. The rupture happens when a physical trauma cracks a capsule of the spleen and results in internal bleeding. The typical symptoms of a ruptured spleen are acute pain in the left upper quadrant of the abdomen, tenderness upon palpation, dizziness and disorientation. A typical clinical finding includes Kehr’s sign, which refers to the acute pain in the tip of the shoulder when a patient is lying in the supine position with their legs elevated.
Depending on the size of the rupture, different volumes of blood can be lost and that’s the main factor that determines how long you can live with a ruptured spleen. If not treated, a ruptured spleen causes death within an hour or two in most cases.
A ruptured spleen is usually treated surgically and in some severe cases by removal of the spleen (splenectomy). People that had their spleen removed are at risk of serious bacterial infections (e.g. sepsis), especially children in the first two years after the surgery.
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