Hemopoietic sites and development of eosinophil granulocytes in the loach, Misgurnus anguillicaudatus.

Unique eosinophils, each of which contained only one eosinophilic granule, have been found in the peripheral blood of the loach (Misgurnus anguillicaudatus). Several loach organs have been studied by light and electron microscopy to determine the hemopoietic site of this cell type. Eosinophils are produced mainly in the spleen and to a small extent in the kidney, but not in other organs. Presumed myeloblasts are identified as large lymphoid cells containing a number of small-dense granules (diameter, 0.12-0.16 micron) in the cytoplasm. These granules have been observed throughout eosinophilopoiesis but they are most abundant in the promyelocyte stage. The largest cells have been identified as myelocytes which contain a number of large granules (diameter, 0.7-1.4 micron) with electron-dense crystalline cores. These large granules are present from the myelocyte to metamyelocyte stage. Metamyelocytes differ from myelocytes in having more large granules. Mature eosinophils are morphologically similar to metamyelocytes but are characterized by the presence of only one very large electron-dense granule (diameter, 2.5-2.8 microns) with a crystalline core. The nature of these granules has been studied by enzyme digestion using pepsin and trypsin. The results indicate that the crystalline cores are almost pure protein.