Spleen and Thymus Flashcards
Splenic Functions
Removal of unwanted elements from the blood by splenic phagocytosis
– Red cells, platelets, bacteria, cell debris, and abnormal macromolecules produced in some inborn errors of metabolism
Major secondary organ of the immune system
- Dendritic cells in the periarterial lymphatic sheath trap antigens and present them to T lymphocytes
- T and B cells interact at the edges of white pulp follicles, leading to the generation of antibody-secreting plasma cells (found mainly within the sinuses of the red pulp)
Source of hematopoietic cells (extramedullary hematopoiesis)
Sequesters a portion of the formed blood elements
- The human spleen lacks contractility limiting its function in this regard
- Spleen harbors approximately 30% to 40% of the total platelet mass
Congenital Abnormalities of Spleen
Congenital absence rare
Accessory spleens common = splenunliculi
Hypersplenism/Splenomegaly
Thrombocytopenia +/- anemia +/- leukopenia
Hyperplasia of the marrow precursors of the deficient cell type
Correction of cytopenia(s) by splenectomy
Splenomegaly
May undergo traumatic rupture*
- Rupture more common with certain conditions such as infectious mononucleosis, malaria, typhoid fever, and lymphoid neoplasms and less likely with chronic congestive splenomegaly
Nonspecific Acute Splenitis
Secondary to any blood-borne infection Splenomegaly (up to 200 to 400 gm) Soft and diffluent Acute congestion of the red pulp Neutrophils, plasma cells, +/- eosinophils
Congestive Splenomegaly
Systemic, or central, venous congestion
- cardiac decompensation involving the right side of the heart
- moderate enlargement of the spleen usually < 500 gm
Cirrhosis of the liver (schistosomiasis, etc.)
- striking massive congestive splenomegaly (1-5 kg.)
Obstruction of the extrahepatic portal vein or splenic vein
- spontaneous portal vein thrombosis with or without external compression or portal vein inflammation (pylephlebitis)
Spleen cut surface gray-red to deep red
- Red pulp is congested in early chronic congestion
- Becomes more fibrous and cellular (hypersplenism) late
- Gandy-Gamma nodules: foci of fibrosis containing iron and calcium salts secondary to hemorrhages
Gandy-Gamma nodules
foci of fibrosis containing iron and calcium salts secondary to hemorrhages
Splenic Infarcts
Caused by occlusion of the splenic arteries (cardiac emboli, sickle cell disease)
Can be septic with infectious endocarditis
Can lead to decreased splenic function
–> Increased risk of infections with encapsulated bacteria (pneumococcus, H. influenza, meningococcus)
Splenic Neoplasms
Lymphangiomas and hemangiomas most common tumors
Primary lymphomas
- Usually diffuse large B cell or splenic marginal zone lymphoma
- Hepatosplenic alpha beta or gamma delta T-cell lymphomas
- — Primarily involves spleen and liver and often involves bone marrow
- —- Rare (under 100 cases) neoplasms of cytotoxic post-thymic immature T-cells
- —- Poor prognosis
Secondary involvement by lymphoid and myeloid neoplasms and metastatic carcinomas
Splenic Neoplasms- mainly white pulp involvement
follicular lymphoma
CLL/SLL
Hodgkin lymphoma
Marginal zone lymphoma
Splenc neoplasms- mainly red pulp involvment
hairy cell leukemia
CML
T-cell prolymphocytic leukemia
Thymus
Epithelial organ secondarily infiltrated by lymphocytes and fat!
Embryology: medulla derived from the third (+/- fourth) pharyngeal pouch (endoderm); cortex from pharyngeal cleft (ectoderm)
Birth 10 to 35 gm; puberty, 20 to 50 gm.; elderly 5 to 15 gm
- Age-related involution: replacement by fibrofatty tissue
- Thymus can also involute with severe stress, including HIV infection
Have antigen independent T-cell maturation, negative selection of self-reactive clones and positive selection of MHC-recognizing clones
Epithelial cells in the thymus
Cortex – Large epithelioid with pale cytoplasm
Medulla – Spindled elongated cells and thymic corpuscles
Lymphoid cells in the thymus
= Thymocytes
Cortex – Medium sized blastic lymphocytes
CD1a+,TdT+,CD4+8+ = Immature
Medulla – Small mature appearing lymphocytes
CD1a-,TdT-,CD4+ or CD8+ = Mature
A few scattered B-lymphocytes (and granulocytes) may be present
Myoid (muscle-like) cells
in the thymus
Related to myasthenia gravis (musculoskeletal disorder with antibodies that cause acetylcholine receptor loss)
Other cells in the thymus
Macrophages and dendritic cells
Developmental Disorders of Thymus- Ectopic thymus
- Can occur in the neck or on the pleural surface
Developmental Disorders of Thymus- Ectopic parathyroids
Occasionally become enclosed within the thymic capsule
Developmental Disorders of Thymus- DiGeorge syndrome
(+/- 22q11 del syndrome)
- Thymic hypoplasia or aplasia accompanied (Severe deficits in cell-mediated immunity and variable)
- Parathyroid developmental failures (Hypoparathyroidism)
- Often associated with other developmental defects as part of the 22q11 deletion syndrome
Developmental Disorders of Thymus - Thymic cysts
Uncommon usually < 4 cm lesions usually discovered incidentally
Stratified to columnar epithelium and serous or mucinous contents
Neoplastic thymic masses often associated with cysts
In a symptomatic patient with a cystic thymic lesion ** must search for a neoplasm,** particularly a lymphoma or a thymoma
Thymic Hyperplasia
= Thymic Follicular Hyperplasia
Appearance of lymphoid follicles containing predominantly ** B lymphocytes** within the thymus
Most frequently encountered in myasthenia gravis, being present in about 65% to 75% of cases
Similar changes sometimes encountered in other autoimmune disorders
- Graves disease
- Systemic lupus erythematosus
- Scleroderma
- Rheumatoid arthritis
stuff that causes massive splenomegaly
Chronic myeloproliferative disorders Chronic lymphocytic leukemia Hairy cell leukemia Lymphomas Malaria Gaucher disease Primary splenic neoplasms
Thymomas
- Tumors of thymic epithelial cells
- Myasthenia gravis in 30% to 45%
Background non-neoplastic T cells (thymocytes) usually present
Usually occurs in adults > 40 years
20% to 30% of tumors of the anterosuperior mediastinum
Sometimes occur in neck, thyroid, pulmonary hilus, posterior mediastinum
40% present with symptoms from impingement on mediastinal structures
Paraneoplastic syndromes
Cortical thymomas rich in thymocytes are more likely to be associated with autoimmune disorders
15-20% of patients with myasthenia gravis have thymomas
Other syndromes:
- Hypogammaglobulinemia, pure red cell aplasia, Graves disease, pernicious anemia, dermatomyositis-polymyositis, and Cushing syndrome)
With minimal invasion and complete excision > 90% 5-year survival
With extensive invasion (often metastatic) under 50% 5-year survival
thymoma description
Lobulated, firm, gray-white masses up to 15 to 20 cm
Majority are encapsulated (20% to 25% penetrate capsule)
Benign (encapsulated) thymoma
cytologically and biologically benign
Medullary-type epithelial cells or a mixture of medullary and cortical type cells
Malignant thymoma
Type I invasive thymoma (20% to 25%): cytologically bland but biologically aggressive
Most commonly of the cortical-type
Type II thymic carcinoma (5%): cytologically malignant
Most are squamous cell carcinomas, either well or poorly differentiated.
2ND most common variant is lymphoepithelioma-like carcinoma
Mixed thymoma
(benign or malignant) - admixture cortical-type thymic epithelial cells and a very rich in thymocytes
