| Definition |
This term refers to the earliest marrow cells identified by the light microscope. Blasts represent about 1 percent of normally developing marrow cells. They are largely myeloblasts, which are cells that will develop into neutrophils. In normal lymph nodes, blasts are lymphoblasts, that is, cells that are part of lymphocyte development. In the acute leukemias, blast cells, similar in appearance to normal blast cells, accumulate in large numbers, perhaps up to 80 percent of all marrow cells. In acute myelogenous leukemia, myeloblasts accumulate and in acute lymphoblastic leukemia, lymphoblasts accumulate. Sometimes the distinction between myeloblasts and lymphoblasts can be made by examination of stained marrow cells through the microscope. Often, immunophenotyping or use of special staining of marrow cells is required to be sure of the distinction.
In speaking about leukemia, “blast” is the short name used for lymphoblasts, the immature white blood cells. There are normal blasts and leukemic blasts. Normally, blasts compose less than 5 percent of the cells made by the bone marrow and grow to form mature white blood cells with certain typical features visible under the microscope. Leukemic blasts are abnormal because they remain immature and do not function like mature white blood cells. In many cases, they look different from normal blasts when viewed under a microscope.
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