Promyelocyte vs. Myelocyte — What's the Difference?

Promyelocytes are immature white blood cells found in the bone marrow, representing an early stage in the granulocyte series of hematopoiesis. They follow myeloblasts in the development sequence. On the other hand, myelocytes are the subsequent developmental stage after promyelocytes, where these cells begin to differentiate more distinctly into neutrophils, eosinophils, or basophils.

In terms of morphology, promyelocytes are characterized by a large nucleus and the presence of primary (or azurophilic) granules in their cytoplasm, which are not specific to any type of granulocyte. Myelocytes, however, start to show specific secondary granules that determine their line of differentiation, indicating a further commitment to becoming a specific type of white blood cell.

The size of a promyelocyte typically ranges from 12 to 20 micrometers in diameter, making them one of the larger cells in the granulocytic series. Myelocytes are slightly smaller, generally ranging from 10 to 18 micrometers, as the cell begins to condense and mature.

Promyelocytes are primarily involved in the synthesis of granules that play a role in the cell's immune functions. As these cells mature into myelocytes, the focus shifts more towards the development of specific granules that will define the cell's specific function in the immune response, such as combating parasites or modulating allergic reactions.

In clinical terms, an abnormal increase in promyelocytes can be indicative of certain types of leukemia, such as acute promyelocytic leukemia (APL), which is a distinct subtype characterized by a specific chromosomal translocation. An increase in myelocytes, on the other hand, can be seen in various conditions, including chronic myelogenous leukemia (CML) and other myeloproliferative disorders, reflecting a disruption in the normal maturation process of blood cells.