a primitive, bone marrow-residing undifferentiated form of blood cell that gives rise to myeloid (e.g., granulocytes, monocytes, and platelets) and lymphoid (e.g., T and B lymphocytes) cell lines; they resemble lymphocytes and are considered to belong to the null cell population; they cannot be identified in normal blood or bone marrow smears. They have the capability for self-renewal.stem cell.
