ALL arises from a neoplastic stem cell committed to the lymphoid lineage. As with AML, the blasts accumulate in the marrow, crowding out normal progenitors, and they will spill out into the circulation. The peripheral blood will typically show anemia and thrombocytopenia with a high WBC - consisting predominately of blasts.
For diagnosis of ALL, blasts must comprise at least 20% of the nucleated marrow cells or blood leukocytes (same criterion level as for AML).
Lymphoblasts can look quite a bit like myeloblasts! Typically 20-30µ in diameter, they have bland, finely reticulated chromatin. Nucleoli may not be prominent, cytoplasm is sparse, and in most cases lacks granules. (If granules are present they will be positive for PAS, and negative for MPO.)
Immunohistochemistry and flow cytometry are used to distinguish AML from ALL and to subtype the disease. DNA studies are also important for classification and prognosis. Morphology is classified as L1, L2, and L3. (For your information only)
L1 smaller, rounder, scant cytoplasm
L2 slightly larger, slightly convoluted nuclear contours, more cytoplasm than L1
L3 Burkitt cells. Large and round with dark blue cytoplasm showing several "punched-out" vacuoles. The dark blue cytoplasm reflects abundant RNA, protein synthesis. These actually are a more mature cell than L1 and L2 cells because the Burkitt cells express surface immunoglobulin. Burkitt leukemia/lymphoma is now classified as a mature B-cell neoplasm (WHO, 2002).
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