Spleen Anatomy, Embryology, and Clinical Correlations (Splenomegaly, Rupture, Accessory Spleens)
Direct Answer: The Spleen in One Paragraph
The spleen is the largest secondary lymphoid organ, positioned in the left upper quadrant between the 9th and 11th ribs along the midaxillary line, just under the diaphragm. It is an intraperitoneal organ about the size of a fist (11 × 7 × 4 cm, weighing 150-200 grams), suspended by two ligaments: the gastrosplenic ligament (containing short gastric arteries) and the splenorenal ligament (containing the splenic artery, vein, and tail of the pancreas). The spleen has a convex diaphragmatic surface and a concave visceral surface, with a central hilum where vessels enter and leave. Functionally, the spleen performs four main roles: (1) immunologic surveillance through the white pulp (B-cell and T-cell zones), (2) filtration of old and defective red blood cells through the red pulp, (3) storage of platelets (up to one-third of circulating platelets sequester in the spleen), and (4) extramedullary hematopoiesis in certain disease states. The spleen is embryologically derived from the dorsal mesogastrium and is supplied by the splenic artery (a branch of the celiac trunk). Its close anatomical relationships to the left 9-11 ribs make it prone to injury in rib fractures. The spleen is also famously 'friable' — its capsule is thin and its parenchyma soft, so blunt abdominal trauma commonly causes splenic rupture with significant internal hemorrhage. This content is for educational purposes only and does not constitute medical advice.
Surface Anatomy and Borders
The spleen sits under the left hemidiaphragm, contacting ribs 9-11 along the left midaxillary line. Its long axis runs parallel to the 10th rib. This topographical relationship makes the spleen clinically palpable ONLY when it is enlarged — a normal spleen is not palpable below the costal margin. Surfaces: - Diaphragmatic surface: convex, faces the diaphragm and covers the left lung/pleura superiorly. - Visceral surface: concave, divided into gastric impression (for the stomach), renal impression (for the left kidney), colic impression (for the splenic flexure of the colon), and pancreatic impression (for the tail of the pancreas). Borders: - Superior border: characteristically notched — these notches can be palpated during splenomegaly to confirm that a left upper quadrant mass is the spleen (not the left kidney or colon). - Inferior border: smooth. - Anterior border: sharp, directed toward the left iliac fossa. - Posterior border: rounded, near the left kidney. Hilum: on the visceral surface, where splenic vessels and nerves enter and leave. The tail of the pancreas abuts the hilum and travels within the splenorenal ligament. The spleen is mobile within its ligamentous attachments and moves with respiration — it descends with inspiration as the diaphragm pushes down. This respiratory motion is the basis for the standard clinical exam technique: ask the patient to take a deep breath while you palpate the left upper quadrant — an enlarged spleen will descend and be felt under the costal margin.
Ligaments and Peritoneal Attachments
The spleen is intraperitoneal, attached to neighboring structures by two main ligaments that are derivatives of the dorsal mesogastrium: Gastrosplenic ligament: connects the greater curvature of the stomach to the spleen. Contains the short gastric arteries (supplying the fundus of the stomach) and the left gastroepiploic artery. This ligament forms the left lateral boundary of the lesser sac. Splenorenal ligament (also called the lienorenal ligament): connects the spleen to the anterior surface of the left kidney and the posterior abdominal wall. Contains the splenic artery and vein (as they travel to/from the splenic hilum) and the tail of the pancreas. This ligament is important during splenectomy because dividing it can damage the pancreatic tail, causing postoperative pancreatitis or pancreatic fistula. Additional peritoneal attachments: - Phrenicocolic ligament: connects the splenic flexure of the colon to the diaphragm. Supports the inferior pole of the spleen (acting as a 'sustentaculum lienis'). This ligament also prevents the left paracolic gutter from communicating with the left subphrenic space. - Splenophrenic ligament: a small fold connecting the upper pole of the spleen to the diaphragm. During splenectomy surgery (for trauma, ITP, hereditary spherocytosis, lymphoma, or other indications), the surgeon must: 1. Mobilize the spleen by dividing the splenorenal, gastrosplenic, and phrenicocolic ligaments. 2. Ligate and divide the splenic artery and vein. 3. Carefully dissect the tail of the pancreas away from the hilum to avoid pancreatic injury. 4. Inspect for accessory spleens (present in 10-30% of people) — if incompletely removed, remnant splenic tissue can reconstitute splenic function, defeating the purpose of splenectomy for conditions like ITP.
Blood Supply and Circulation
Arterial supply: The splenic artery, one of the three branches of the celiac trunk, supplies the spleen. It arises from the celiac trunk and runs tortuously along the superior border of the pancreas (the tortuosity is characteristic — it allows the artery to accommodate diaphragmatic movement during respiration without stretching). Before reaching the spleen, the splenic artery gives off: - Pancreatic branches: supply the body and tail of the pancreas. - Short gastric arteries: 5-7 small branches supplying the fundus of the stomach. Travel through the gastrosplenic ligament. - Left gastroepiploic artery: runs along the greater curvature of the stomach, anastomosing with the right gastroepiploic artery from the gastroduodenal system. At the splenic hilum, the splenic artery divides into 4-6 segmental branches, each supplying a distinct splenic segment. This segmental anatomy allows partial splenectomy (removing only the injured segment) in select trauma cases. Venous drainage: Blood from the spleen drains through the splenic vein, which runs posterior to the body and tail of the pancreas. The splenic vein joins the superior mesenteric vein posterior to the neck of the pancreas to form the portal vein — meaning all splenic blood passes through the liver via the portal system before returning to the systemic circulation. Clinical implications of portal drainage: - Liver disease and portal hypertension cause back-pressure on the splenic vein, leading to splenomegaly (congestive splenomegaly). - Splenic vein thrombosis (often from pancreatic disease) causes 'left-sided portal hypertension' with isolated gastric varices (short gastric and gastroepiploic veins become engorged). - In portal hypertension generally, splenic enlargement is common due to venous congestion. Innervation: The spleen is innervated by autonomic fibers from the celiac plexus, traveling along the splenic artery. These are primarily sympathetic vasomotor fibers. The spleen has no clinically relevant somatic innervation — but the surrounding peritoneum does, which is why splenic injury causes referred pain to the left shoulder (Kehr sign) through diaphragmatic peritoneal irritation.
Microscopic Anatomy: White and Red Pulp
On the cut surface of a fresh spleen, two distinct tissue types are visible: white pulp and red pulp. White pulp: about 25% of splenic volume. Appears as whitish specks scattered throughout the spleen on gross examination. Composed of lymphoid tissue organized around central arterioles: - Periarteriolar lymphoid sheath (PALS): T-cell zone surrounding central arterioles. - Lymphoid follicles: B-cell zones, sometimes with germinal centers (sites of B-cell activation and antibody production). - Marginal zone: transitional region between white and red pulp, containing specialized B cells that respond to blood-borne pathogens. White pulp is where the spleen performs immunologic functions — filtering blood-borne antigens, activating lymphocytes, and producing antibodies. Encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) are cleared primarily in the splenic marginal zone, which is why asplenic patients are at high risk for sepsis from these organisms. Red pulp: about 75% of splenic volume. Contains: - Splenic cords (of Billroth): reticular meshwork with macrophages. - Splenic sinusoids: specialized capillaries with gaps in the endothelium, lined by elongated 'stave cells'. Blood enters the red pulp through central arterioles and percolates through the splenic cords before entering the sinusoids. The sinusoids have fenestrated endothelium and large gaps (2-3 μm) between stave cells, forcing red blood cells to squeeze through. Aged, damaged, or spherocytic red cells can't deform enough to pass through and are retained in the cords, where they're phagocytosed by macrophages. This filtering function is why certain diseases produce characteristic splenic findings: - Hereditary spherocytosis: abnormally shaped red cells are trapped in the cords, leading to chronic hemolysis and eventual splenomegaly. - Sickle cell disease: sickled red cells are trapped repeatedly, causing splenic infarcts and eventually 'autosplenectomy' by early adulthood. - Malaria: parasitized red cells are filtered out, and the spleen enlarges dramatically.
Embryology
The spleen develops from mesenchyme in the dorsal mesogastrium during the 5th week of gestation. This embryologic origin has several clinically relevant consequences: 1. Splenic position and rotation. During fetal development, the stomach rotates so that the original left side faces anteriorly and the right side faces posteriorly. The dorsal mesogastrium rotates with it, carrying the developing spleen to the left upper quadrant. The rotation also brings the spleen into its current position relative to the lesser sac. 2. Accessory spleens (splenunculi): foci of splenic tissue that develop separately from the main organ and do not fuse. Present in 10-30% of people. Most are near the splenic hilum or along the splenic vessels, but they can occur anywhere in the abdomen (pancreatic tail, along the splenocolic ligament, in the mesentery, in the pelvis, even in the scrotum in rare cases). Accessory spleens are important because: - They can cause persistent symptoms after splenectomy for ITP or hereditary spherocytosis if not removed. - On CT scans, they can mimic tumors. - They can twist on their pedicle causing infarction and acute abdomen. 3. Polysplenia and asplenia syndromes: associated with heterotaxy syndromes and congenital heart disease. Polysplenia (multiple small spleens, typically on the right side) occurs with left atrial isomerism. Asplenia (absent spleen, 'Ivemark syndrome') occurs with right atrial isomerism and is associated with severe cyanotic congenital heart disease. 4. Wandering spleen: the spleen develops normal attachments to the dorsal mesogastrium but ligamentous laxity allows the spleen to move freely within the abdomen. Can present with abdominal pain from torsion of the splenic pedicle causing ischemia. More common in children and in women of reproductive age (ligamentous laxity during pregnancy). 5. Spleen shares embryologic origin with the pancreas. Both arise from the dorsal mesogastrium during the 5th week — the pancreas from endodermal outpouchings of the duodenum, and the spleen from mesoderm within the dorsal mesogastrium. Their close developmental and anatomical proximity explains why the tail of the pancreas is so intimately related to the splenic hilum in the adult.
Splenomegaly and Splenic Rupture
Splenomegaly (enlargement of the spleen) is clinically detectable when the spleen can be palpated below the left costal margin. It has many causes: Infectious causes: - Infectious mononucleosis (EBV) — classic cause in adolescents and young adults. Splenic rupture is a feared complication; patients are advised to avoid contact sports for 4-6 weeks. - Malaria (especially P. vivax and P. falciparum). - Tuberculosis. - Bacterial endocarditis (from splenic septic emboli). - HIV. - Visceral leishmaniasis (kala-azar). Hematologic causes: - Hemolytic anemias (hereditary spherocytosis, autoimmune hemolytic anemia, thalassemia). - Myeloproliferative disorders (CML, polycythemia vera, myelofibrosis). - Lymphomas and leukemias. - Idiopathic thrombocytopenic purpura (ITP) — though splenomegaly is usually mild. Congestive causes: - Portal hypertension from cirrhosis. - Splenic or portal vein thrombosis. - Right heart failure. Infiltrative causes: - Gaucher disease (and other lysosomal storage disorders). - Amyloidosis. - Sarcoidosis. Neoplastic causes: - Primary splenic lymphoma (rare). - Metastatic disease (uncommon — the spleen has some resistance to metastasis). Massive splenomegaly (spleen palpable below the umbilicus) narrows the differential to: chronic myeloid leukemia, myelofibrosis, chronic malaria, visceral leishmaniasis, Gaucher disease, and rarely hairy cell leukemia. Splenic rupture: the spleen is highly vascular and has a thin capsule, making it prone to rupture from blunt abdominal trauma or from pathologic enlargement (as in infectious mononucleosis). Presentation: left upper quadrant pain, left shoulder pain (Kehr sign from diaphragmatic peritoneal irritation), hypotension from blood loss, abdominal rigidity in severe cases. Classification (AAST grading): - Grade I: subcapsular hematoma <10% of surface area; capsular tear <1 cm deep. - Grade II: subcapsular hematoma 10-50% or intraparenchymal hematoma <5 cm; laceration 1-3 cm deep not involving trabecular vessels. - Grade III: subcapsular >50% or expanding; laceration >3 cm deep or involving trabecular vessels. - Grade IV: laceration involving segmental or hilar vessels, devascularization >25%. - Grade V: shattered spleen or complete devascularization. Management: - Hemodynamically stable, low-grade: observation in ICU with serial CT and hematocrit checks. - Hemodynamically stable, high-grade: splenic artery embolization by interventional radiology. - Hemodynamically unstable: emergency laparotomy, splenectomy or splenorrhaphy. Post-splenectomy considerations: - OPSI (overwhelming post-splenectomy infection): lifelong risk of sepsis from encapsulated organisms. - Vaccination: pneumococcal, meningococcal, and H. influenzae type b vaccines recommended at least 2 weeks before elective splenectomy or as soon as possible after emergency splenectomy. - Prophylactic antibiotics (penicillin or amoxicillin) for children and high-risk adults. - Patients should carry medical alert identification and seek immediate medical attention for any febrile illness. This content is for educational purposes only and does not constitute medical advice.
Frequently Asked Questions
Common questions about spleen anatomy, embryology, and clinical correlations (splenomegaly, rupture, accessory spleens)
Blood from a ruptured spleen irritates the diaphragmatic peritoneum on the left. The diaphragm is innervated by the phrenic nerves (C3-C5 cervical roots), which share the same dermatomal distribution as the supraclavicular nerves supplying the skin of the shoulder. The brain interprets diaphragmatic irritation as pain in the shoulder through referred pain along shared sensory pathways. Kehr sign is a classic manifestation of splenic rupture but also occurs with other causes of left diaphragmatic irritation (pneumoperitoneum, subphrenic abscess).
Accessory spleens (splenunculi) are foci of normal splenic tissue that develop separately from the main spleen and fail to fuse. They are present in 10-30% of people, most commonly near the splenic hilum or along the splenic vessels. Clinically, they matter because: (1) patients undergoing splenectomy for ITP or hereditary spherocytosis may fail to improve if accessory spleens are missed and continue to function; (2) they can appear on imaging as masses that mimic tumors in the pancreatic tail, retroperitoneum, or pelvis; (3) they can undergo torsion and infarction, causing acute abdominal pain. Surgeons systematically search for and remove accessory spleens during planned splenectomy for hematologic indications.
The spleen is the primary site for clearing encapsulated bacteria from the bloodstream. Encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) have polysaccharide capsules that resist phagocytosis unless they are opsonized. The splenic marginal zone contains specialized B cells and macrophages that efficiently recognize and clear these encapsulated organisms. Without the spleen, circulating opsonins (particularly IgM and complement) are reduced, and the body cannot rapidly clear blood-borne encapsulated bacteria. This predisposes asplenic patients to overwhelming post-splenectomy infection (OPSI), which has a mortality of 50% or higher once established. Vaccination and prophylactic antibiotics substantially reduce but do not eliminate this risk.
The classic differential for massive splenomegaly includes: chronic myeloid leukemia, myelofibrosis with extramedullary hematopoiesis, chronic malaria ('tropical splenomegaly'), visceral leishmaniasis (kala-azar), Gaucher disease (lysosomal storage disorder), and rarely hairy cell leukemia. Most other causes of splenomegaly produce mild-to-moderate enlargement (palpable to 4-10 cm below the costal margin) rather than massive enlargement. Imaging (CT or MRI) and bone marrow biopsy are usually required to narrow the differential further.
The tail of the pancreas travels within the splenorenal ligament and abuts the splenic hilum. When a surgeon mobilizes and removes the spleen, the pancreatic tail can be inadvertently clamped, cauterized, or transected. This causes postoperative pancreatitis, pancreatic fistula, or pseudocyst. To avoid this complication, surgeons must identify the pancreatic tail before dividing the splenorenal ligament and carefully dissect it free from the hilum. Postoperative amylase and lipase monitoring is routine after splenectomy, and a drain is often left to detect pancreatic leak early.
Yes. Snap a photo of any spleen diagram, CT scan, or clinical vignette and AnatomyIQ traces the ligamentous attachments, identifies the peritoneal relationships, explains the blood supply and venous drainage, and maps clinical syndromes (splenomegaly differential, Kehr sign, OPSI, accessory spleens) at appropriate depth for pre-med, medical school, nursing, and USMLE preparation. This content is for educational purposes only and does not constitute medical advice.