The changes in red cell morphology include the presence of Howell-Jolly bodies and siderotic granules in some of the cells and target cell vacuoles due to pitting may be seen (see below). In a proportion of subjects, irregularly contracted or crenated, acanthocytic forms are also a feature (Figure 21.5). There is usually an increase in the number of reticulocytes in the circulation and occasionally isolated erythroblasts are seen. There is, however, no alteration in red cell survival.
As a rule, only some of the red cells leaving the marrow will contain siderotic granules and Howell-Jolly bodies. As the number of cells containing siderotic granules is related to the sideroblastic percentage in the bone marrow, the siderocyte count in the peripheral blood will be low in hyposplenism if there is no marrow pathology. In patients suffering from haemolytic anaemias, thalassaemia and sideroblastic anaemia, a major proportion of the circulating red cells may contain siderotic granules. The number of Howell-Jolly bodies is also variable and is most marked in conditions characterized by dyserythropoiesis.
Figure 21.4 Demonstration of residual splenunculus by a scan of heat-damaged isotope-labelled red cells: (a) anterior view; (b) posterior view. Uptake is predominantly in the liver.
Other red cell inclusions may be prominent in the hyposplenic state: Heinz bodies are found following oxidative injury by drugs and in patients who have glucose-6-phosphate dehydrogenase deficiency or an unstable haemoglobin; precipitated a-chains are found in P-thalassaemia; and crystalline deposits of haemoglobin C in HbC disease.
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