The purpose of Immature Cells in the Granulocytic, Monocytic, and Lymphocytic Series is to describe the young cells of these lines, which normally are seen only in the bone marrow. However, many of the photomicrographs for this unit are of cells found in the peripheral blood of patients with leukemia and other hematologic disorders. This was done because the cells are separated and stain better in blood than in bone marrow. I usually begin my teaching course in bone marrow interpretation with a blood smear made from a patient with chronic myelocytic leukemia. If the cells on this slide are easily recognized, 65%-75% of the cells in a normal bone marrow will be familiar.
There are some signs of aging that generally apply to all cell lines. For example, cells become smaller as they grow older and the nucleus-to-cytoplasm ratio progresses from larger to smaller. These and other signs will be discussed more fully. It is often difficult to identify and classify young cells, but nucleoli, if present, can be extremely helpful. Lymphocytes generally contain only one or two nucleoli, two to six granulocytes, and two to four monocytes. Light-colored nucleoli are seen in only three cell lines: granulocyte, lymphocyte, and monocyte. Nucleoli are not prominent in the early stages of nucleated red blood cells and often are masked by the dark-staining chromocenters. Sky-blue nucleoli are seen in reticulum cells and phagocytic histiocytes.
Wright's-stained preparations are excellent for screening for hematologic disorders but the stain is not specific for identification of young cells. Generally, we can narrow our suspicions of cell type by cytochemical stains. The different stains for determining cell type will be included in the programs that describe the various leukemias.
All cell lines have stages of maturation, but those of granulocytes and nucleated red blood cells are the only ones we can recognize with any certainty in Wright's-stained preparations. We can confidently differentiate most mature monocytes, but it is difficult to tell young monocytes from promyelocytes and at times from basophilic lymphocytes. It is nearly impossible to differentiate between blasts. Lymphoblasts can be identified in the blood and bone marrow of patients with acute lymphocytic leukemia but many lymphocytes, such as transformed lymphocytes derived from small mature lymphocytes, are difficult to classify as to age. Large and small lymphocytes with basophilic cytoplasm, and abnormal, reactive, or atypical lymphocytes are hard to fit into their proper stage of maturation. What is extremely important is to be able to recognize a blast even though you cannot positively categorize its cell line. The most helpful aid in categorizing young cells in Wright's-stained smears is to "go by the company they keep," meaning the more mature cells.
Course Section: 04. Immature Cells in the Granulocytic - Monocytic - and Lymphocytic Series
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