noun, plural: promyelocytes
Hematopoiesis is the process of forming new blood cellular elements in vertebrates. It begins with a multipotent stem cell, a hemocytoblast, which gives rise to a common myeloid progenitor cell (CMP). The CMP, in turn, gives rise to granulocyte-monocyte colony forming units (CFU-GM), also called granulocyte-macrophage progenitor. The CFU-GM may partially differentiate into CFU-G (precursor in myelocyte-granulocyte series) or CFU-M (precursor in monocyte-macrophage series). The CFU-GM serves as the precursor for monoblasts and myeloblasts.
The myeloblast, in particular, is a cell that would develop into a promyelocyte. A promyelocyte, in turn, would develop into a myelocyte. In summary, the developmental stages are as follows: hemocytoblast → common myeloid progenitor (or CFU-GEMM) → CFU-GM → CFU-G → myeloblast → promyelocyte → myelocyte → metamyelocyte → band cell → granulocyte.
The promyelocyte is slightly larger than a myeloblast. Its size ranges from about 10 to 20 μ. The nucleus is slightly indented. It has nucleoli that when stained with conventional dyes appear pale staining areas in the nucleus. The chromatin is fine and coarser than that of the myeloblast. The cytoplasm is basophilic. When stained, it renders a bluish in color and contains reddish purple granules (primary azurophilic granules).