Ultrastructure of hairy cell leukemia

Ultrastructural study on the bone marrow and peripheral blood from a case of hairy cell leukemia showed that the leukemic cells resemble lymphocytes rather than reticulum cells or histiocytes. The hairy cells have a dense nucleus, a few short lengths of rough endoplasmic reticulum, mitochondria with electrondense matrix, cytoplasm strewn with ribosomes and polyribosomes, and numerous well‐developed filopodia. Pinocytotic and micropinocytotic activity was minimal, and no lysosomes were seen in these cells. These hairy cells also frequently surrounded a macrophage and appeared to communicate with them as do normal lymphocytes. An unexpected finding was that erythrocytes, reticulocytes, and, to a lesser extent, normoblasts also show numerous club‐shaped processes arising from their surface. Only a few plasma cells were observed but they too appeared to be well endowed with cell processes. These findings suggest that hairy cell leukemia is a variety of lymphatic leukemia, and the hairiness of the leukemic cells is difficult to explain on the assumption that they are neoplastic reticulum cells or histiocytes, as has been done in the past.