Disorders of spleen and thymus
The spleen has an important and unique role in the function of haemopoietic and immune
system , as well as directly involved in a many diseases of these systems, a number of
important clinical features are associated with hyposplenic and hypersplenic states.
The anatomy of the spleen:
The spleen lies under the left costal margin, has a weight of 150-250 g, and a length of
between 5-13 cm. It is normally not palpable but becomes palpable when the size
increased to over 14 cm.
The circulation of the spleen:
Blood enters the spleen through the splenic artery which then divides into trabecular
arteries which permeate the organ and give rise to central arterioles, the majority of the
arterioles end in cords which lack endothelial lining and form an open blood system
unique to the spleen.
The blood re-enters the circulation by passing into venous sinuses ,blood then passes into
the splenic veins and so back into general circulation
The cords and sinuses form the red pulp which form 75% of the spleen and has essential
role in monitoring the integrity of red cells.
The central arterioles are surrounded by a core of lymphatic tissues known as white pulp.
The functions of the spleen:
1-Controles of red cells integrity : spleen has an essential role in removing excess DNA ,
nuclear remnant, sidrotic granules and aged RBC.
2-Immmune function: the lymphoid tissue in the spleen responds to antigen filtered from
the blood and entering the white pulp.
Extramedullery haemopoeisis: the spleen undergo a transient period of haemopoeisis at
3-7 months of fetal life , but it is not a site of haemopoisis in the adult.
Haemopoeisis may be re-established in the spleen as extramedullery haemopoisis in
disorders such as myelofibrosis , megaloblastic anaemia and chronic haemolytic anaemia.
Splenomegaly :
Splenic size increased in a wide range of conditions, splenomegaly can be classified
according to the degree of enlargement :
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1-Massive splenomegaly (> 20 cm length,>1000 gm weight ) is caused by
myeloproliferative disorders ( chronic myeloid leukemia and myelofibrosis ),lymphoma,
chronic lymphocytic leukemia ,hairy cell leukemia, Gaucher disease, malaria,
leishemaniasis and schistosomiasis.
2-Moderate splenomegaly (500-1000 gm weight) is caused by chronic congestive
splenomegaly (portal hypertension),acute leukemia, hereditary spherocytosis ,thalassemia
major,amyloidosis, tuberculosis and typhoid fever.
3-mild splenomegaly is caused by acute splenitis ,acute splenic congestion and
infectious mononucleosis.
Hypersplenism :normally only 5% of the total red cell mass is present in the spleen and
up to half of the total marginating neutrophil pool and 30% of the platelets mass may be
located there. As the spleen enlarges ,the proportion of haemopoitic cells within the organ
is increases up to 40% of red cell mass and 90% of neutrophil. Hypersplenism is a
clinical syndrome that can be seen in any form of splenomegaly ,it is characterized by :
●Enlargement of the spleen.
●Reduction of at least one cell line in the blood in the presence of the normal bone
marrow function.
●Evidence of increased the release of premature cells such as reticulocytes or immature
platelets ,from the bone marrow into the blood .
Splenectomy may be indicated if the hypersplenism is symptomatic ,It is followed by
rapid improvement in the peripheral blood count.
Hypoplenism: functional hyposplenism is revealed by the blood film finding of HowellJolly bodies ,siderotic granules and target cell. The most common cause is surgical
removal of the spleen but hyposplenism may occur also in sickle cell anaemia, glutin –
induced enteropathy ,inflammatory bowel disease and splenic arterial thrombosis.
Splenectomy: surgical removal of the spleen is indicated in :
●splenic rupture.
●chronic immune thrombocytopenia.
●haemolytic anaemia for example hereditary spherocytosis, autoimmune haemolytic
anaemia, thalassemia major and intermedia.
●chronic lymphocytic leukaemia.
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●myelofibrosis.
●lymphoma.
Patients with hyposplenism and after splenectomy are at lifelong increased risk of
infection
from
a
variety
of
organisms
such
as
streptococcas
pneumonia
,haemophilus influenza and neisseria meningitidis and this is seen particularly in children
under the age of 5 years and those with sickle cell anaemia, so vaccination against these
organisms with prophylactic antibiotics should be given to all patients with hypoplenism.
DISORDER OF THE THYMUS
The thymus is a central lymphoid organ that has an essential role in T-cell differentiation
and the two most frequent disorders of the thymus are :thymic hyperplasia and thymoma.
Thymic hyperplasia:
hyperplasia of the thymus is often associated with the appearance of lymphoid follicles
or germinal centers within the medulla, these germinal centers contain reactive B cells
,which are normally present in only low numbers in the thymus .Thymic follicular
hyperplasia is present in most patients with myasthenia gravis and is sometimes found in
other autoimmune diseases such as SLE and rheumatoid arthritis.
Thymoma:
The term thymoma is restricted to tumors in which epithelial cells constitute the
neoplastic element. Scant or abundant precursors Tcells (thymocytes) are present in these
tumors ,but these are non-neoplastic.
Several classification systems for thymoma have been proposed on the basis of cytologic
and biologic criteria, one simple and clinically useful classification is as follows:
♦ benign or encapsulated thymoma :cytologically and biologically benign.
♦ malignant thymoma
Type I:cytologivally benign but biologically aggressive and capable of local invasion and
rarely distal spread.
Type II also called thymic carcinoma: cytologically and biologically are malignant .
Morphology:
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Macroscopically :thymomas are lobulated , gray white masses ,most appear
encapsulated but about 20 % there is penetration of the capsule and infiltration of
perithymic tissue and structures.
Microscopically: all thymomas are made up of a mixture of epithelial cells and a
variable proportion of non-neoplastic thymocytes ,in benign thymoma the epithelial cells
are spindled or elongated and resemble those that normally populate the medulla.
Malignant thymomas type I are composed of varying proportions of epithelial cells and
reactive thymocytes ,the epithelial cells are resemble those present in the cortex ,the
neoplastic epithelial cells are found around blood vessels ,the distinguishing feature of
this type is the penetration of the capsule and the invasion of the surrounding structures.
Malignant thymoma type II are usually invasive masses sometime accompanied by
metastases to the lung ,most cells are resemble well or poorly differentiated sequameous
cell carcinoma.
Clinical features of thymomas: are rare tumors and may arise at any age but typically
occur in middle adult life ,could be asymptomatic or produced local manifestation such as
mass demonstrated by CT scan associated with cough and dyspnea and the reminder of
the tumors are associated with systemic diseases which are myasthenia gravis, SLE and
pure red cell aplasia.
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