Spleen

The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th‒11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. The spleen can also activate immune responses, produce antibodies, and function as a reservoir for platelet storage. There are 2 primary types of splenic tissue: red pulp, which contains dense fibrovascular networks for filtering the blood, and white pulp, which is primarily made up of lymphoid tissue surrounding the larger vessels. The spleen has a relatively weak capsule; thus, it can rupture more easily than other abdominal organs and lead to life-threatening hemorrhage.

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Development

The spleen is the largest lymphatic organ in the body.

Embryology

  • Spleen development begins during the 5th week of development.
  • Originates from the embryonic mesoderm (and not from the endoderm like the primitive gut tube)
  • Several mesenchymal buds form off the dorsal mesogastrium → fuse together to form the spleen
  • Fetal spleen has lobules that gradually regress as the organ matures → formation of the adult spleen that is divided into functional lobules
  • The fetal spleen acts solely as a hematopoietic organ until the 8th month of gestation.

Congenital variations

  • Congenital asplenia: congenital absence of a spleen
  • Congenital hyposplenism: congenital presence of a small, pathological spleen
  • Polysplenia: occurrence of multiple similarly sized smaller spleens (varies from 2 to 16)
  • Accessory spleens (supernumerary spleens):
    • Congenital occurrence of a primary spleen with 1 or more smaller spleens
    • Result from incomplete fusion of mesenchymal buds during development
    • Detected incidentally in 10%–30% of the population
    • When present, individuals will usually have 1–6 accessory spleens.
    • Tend to be about the size of cherries
    • Found in the splenic hilum in 75% of cases
  • Splenic lobulation: persistence of embryonic lobules in the adult 
  • Wandering spleen:
    • A rare condition in which the spleen changes positions in the abdomen
    • Caused due to malformation and excessive elasticity of the splenic ligaments that fix the spleen in the upper abdomen
  • Splenopancreatic fusion:
    • Fusion of the spleen with the tail of the pancreas
    • Usually associated with trisomy 13

Gross Anatomy

Location

  • LUQ of the abdomen against the left dome of the diaphragm
  • In contact with the 9th, 10th, and 11th ribs. The long axis of the spleen corresponds to that of the left 10th rib.
  • Positioned obliquely from posterior/superior (posterior pole) to anterior/inferior (anterior pole)
  • The spleen is an intraperitoneal organ:
    • Covered by visceral peritoneum
    • Suspended by mesentery folds of peritoneum
    • Only the hilum of the spleen, the site through which the splenic artery and vein pass, is not covered by the peritoneum.

Anatomic relationships

  • Anterior to spleen: stomach
  • Lateral to spleen:
    • Diaphragm
    • Ribs and intercostal muscles
  • Medial to spleen:
    • Left kidney and adrenal gland
    • Tail of the pancreas
  • Posterior to spleen:
    • Diaphragm
    • Left lung
    • 9th–11th ribs and intercostal muscles
  • Inferior to spleen: left colic flexure

Size

Average measurements in normal healthy adults:

  • Length: approximately 10–12 cm
  • Width (at its widest portion): 7–14 cm 
  • Circumference: 11 cm
  • Weight: 150–200 grams

Ligaments

The spleen is connected to adjacent organs via several important ligaments.

  • Gastrosplenic ligament: connects the spleen to the greater curvature of the stomach
  • Splenorenal ligament:
    • Posterior to the splenic hilum
    • Connects the spleen to the left kidney
    • Contains:
      • Splenic vessels
      • Tail of the pancreas
  • Splenocolic ligament: connects the spleen to the transverse colon
  • Phrenicocolic ligament:
    • A double fold of the peritoneum acting as a mesentery that connects the diaphragm and spleen
    • Part of the greater omentum

Surface anatomy

  • Hilum:
    • The point where splenic vessels (artery, vein, and lymphatics) enter the spleen
    • Located in the middle of the medial surface
  • Impressions: indentations in the medial spleen where it is adjacent to other organs
    • Gastric impression:
      • Posterior, superior aspect of the spleen
      • In contact with the stomach
    • Colic impression:
      • Anterior, superior aspect of the spleen
      • In contact with the transverse colon
    • Renal impression:
      • Inferior aspect of the spleen
      • In contact with the left kidney
Gross anatomy of the spleen

Gross anatomy of the spleen

Image by Lecturio.

Microscopic Anatomy

The spleen consists of a capsule and inner tissue known as parenchyma. The parenchyma consists of 2 types of tissues, white pulp and red pulp.

Capsule

  • A fibroelastic capsule wraps the spleen.
  • Relatively weak:
    • Allows for an increase in size when necessary
    • Ruptures relatively easily compared with other organs
  • Numerous septa (trabeculae) extend from the capsule into the parenchyma of the spleen → divides the spleen into functional lobules

Parenchyma: white pulp

The white pulp makes up 25% of the spleen, surrounds the larger arterioles in the spleen, and contains:

  • A central arteriole
  • Periarteriolar lymphoid sheath (PALS):
    • Congregations of lymphoid tissue surrounding the central arterioles
    • Primarily composed of T cells
    • PALS distinguishes the spleen from other lymphatic organs.
  • Lymphoid follicles:
    • Nodules of lymphoid tissue near the central arterioles
    • Primarily composed of B cells (antibody production)
  • Marginal zone:
    • Located at the edges of the white pulp
    • Contains antigen-presenting cells: macrophages and dendritic cells
    • Blood to be filtered leaves the vasculature in the white pulp → travels through the marginal zone to the red pulp (being “cleaned” by macrophages along the way)

Parenchyma: red pulp

The red pulp makes up 75% of the spleen and is present between the white pulp surrounding the larger vessels.

  • Consists of:
    • Bands of connective tissue known as splenic cords (cords of Billroth)
    • Reticular network of capillaries and venous sinuses
  • White pulp transitions to red pulp as trabecular arterioles branch into the network of capillaries and venous sinusoids.
  • Venous sinusoids:
    • Lined with reticuloendothelial macrophages
    • Splenic cords help define the structure.
    • Blood flows through the venous sinusoids → “cleaned” of foreign and defective components by macrophages
    • Very small diameter:
      • Low flow rate
      • ↑ Contact with macrophages
      • The structure is optimized for phagocytosis.

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Neurovasculature

Blood supply

  • The spleen is a highly vascular organ.
  • Arterial supply: via the splenic artery
    • Arises from the celiac trunk
    • Runs laterally along the superior aspect of the pancreas within the splenorenal ligament
    • Branches into several smaller vessels called trabecular arteries upon entering the spleen
    • Each vessel supplies a different segment of the spleen:
      • The vessels do not anastomose with each other.
      • Allows for easy surgical removal of 1 of more splenic segments
  • Venous drainage: via the splenic vein
    • Sinusoids in the red pulp drain into collecting veins → splenic vein
    • Splenic vein:
      • Runs along the posterior aspect of the pancreas
      • Combines with the superior mesenteric vein (SMV) to form the hepatic portal vein

Patterns of blood flow

About 10% of the blood stays within the vessels, whereas approximately 90% is filtered out into the splenic tissue. Blood flows to, through, and out of the spleen as follows:

  • Aorta → celiac trunk → splenic artery → trabecular arteries → splenic central arteries → 
  • Blood is filtered out of the vessels →
  • Travels through lymphoid tissue in the white pulp (activates immune responses) → 
  • Travels through splenic cords/venous sinusoids in the red pulp (blood is filtered by macrophages) → 
  • Venous sinusoids → collecting veins → splenic vein → hepatic portal vein

Lymphatics

  • The spleen acts as a large lymph node.
  • Lymphocytes produced in the spleen exit via efferent lymph vessels → splenic lymph nodes

Innervation

Innervation of the spleen takes place via the ANS.

  • Sympathetic: celiac plexus
  • Parasympathetic: vagus nerve

Functions

The spleen is the largest secondary lymphoid organ in the body. The functions of the spleen are primarily immunologic and hematologic. Survival without a spleen is possible as it is not a vital organ.

Immune functions

  • Detection of foreign antigens in blood
  • Activation of immune responses
  • Production of antibodies
  • Phagocytosis (Spleen is the most effective phagocytic compartment of the body.)

Hematologic functions

  • Cleanses the blood of old RBCs
  • Storage compartment for platelets
  • Hematopoiesis:
    • In the fetus
    • In cases of extreme anemia in adults
  • Can compensate for hypovolemia by transferring plasma from the blood into the lymphatic system

Clinical Relevance

  • Splenomegaly: pathological enlargement of the spleen. Causes include infections, myeloproliferative disorders, hemoglobinopathies characterized by chronic extravascular hemolysis, infiltrative processes (e.g., sarcoidosis, neoplasms), and congestive disorders (e.g., portal hypertension, splenic vein thrombosis). The most common symptoms are abdominal discomfort or pain, increased abdominal girth, and early satiety due to compression of the stomach. Anemia and thrombocytopenia are often present.
  • Rupture of the spleen: a medical emergency that carries a significant risk of hypovolemic shock and death. The most common cause is blunt abdominal trauma, (e.g., motor vehicle accidents). For individuals with splenomegaly, however, even minimal trauma may result in splenic rupture. Affected individuals often present with LUQ abdominal pain, although the pain may be referred to the left shoulder. Management depends on hemodynamic stability and ranges from observation to splenectomy.
  • Asplenia: absence of splenic tissue or function. There is a distinction between anatomic asplenia, which is due to surgical removal of the spleen, and functional asplenia, which is due to conditions that lead to splenic atrophy, infarcts, congestion, or infiltrative disease. Individuals are at high risk of sepsis caused by encapsulated bacteria, thereby requiring adherence to a strict vaccination schedule and early antibiotic treatment when an infection is suspected. Thromboembolic events are common. 
  • Hypersplenism: increased functioning of the spleen that occurs with or without organ enlargement. Sequestration of blood elements leads to congestion, whereas activation of the reticuloendothelial system leads to thrombocytopenia and anemia. Hypersplenism is associated with an increased risk of splenic rupture. Individuals may present with splenomegaly, pancytopenia, and compensatory bone marrow hyperplasia.
  • Splenosis: an acquired condition characterized by auto-implantation of 1 or more deposits of splenic tissue in various compartments of the body, typically occurring after abdominal trauma or surgery. Multiple ectopic splenic foci are seen, which are typically small, sessile (growing on peritoneal surfaces), and sometimes functional. Symptoms depend on the location of the implants. Affected individuals are often asymptomatic and are usually observed for further changes.
  • Splenic abscess: a rare infection associated with high mortality if not treated properly. The etiology is usually due to the spread of infection from different sites and is commonly associated with endocarditis. Symptoms include abdominal pain, left pleuritic chest or shoulder pain (due to irritation of the diaphragm), fever, and malaise. Management includes antibiotic therapy, percutaneous drainage, and surgical excision.

References

  1. Chaudhry, S. (2021). Anatomy, abdomen and pelvis, spleen. StatPearls. Retrieved September 14, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/29374/ 
  2. Bajwa, S. (2021). Anatomy, abdomen and pelvis, accessory spleen. StatPearls. Retrieved September 2021 from https://www.ncbi.nlm.nih.gov/books/NBK519040/ 
  3. Bona, R. (2021). Evaluation of splenomegaly and other splenic disorders in adults. UpToDate. Retrieved September 14, 2021, from https://www.uptodate.com/contents/evaluation-of-splenomegaly-and-other-splenic-disorders-in-adults 
  4. Vancauwenberghe, T., et al. (2015). Imaging of the spleen: what the clinician needs to know. Singapore Medical Journal. 56 (3), 133–144. https://doi.org/10.11622/smedj.2015040
  5. Fremont, R.D., Rice, T.W. (2007). Splenosis: a review. South Medical Journal. 100 (6), 589–593. https://doi.org/10.1097/SMJ.0b013e318038d1f8
  6. Losanoff, J. (2020). Splenic abscess. Medscape. Retrieved September 1, 2021, from https://reference.medscape.com/article/194655-overview
  7. Vaishali, K., Wehrle, C.J., Tuma, F. (2021). Physiology, Spleen. Treasure Island (FL): StatPearls Publishing. https://pubmed.ncbi.nlm.nih.gov/30725992/
  8. Varga, I., Babala, J., Kachlik, D. (2018). Anatomic variations of the spleen: current state of terminology, classification, and embryological background. Surgical and Radiologic Anatomy. 40 (1), 21–29. https://pubmed.ncbi.nlm.nih.gov/28631052/

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