Medusa cells: cytostructure and cytochemistry of amoeboid eosinophils with pseudopod-like processes

Eosinophils having one or more pseudopod-like processes of various lengths are observed in bone marrow, peripheral blood, sputum, nasal smears, and in other exfoliative cytology and tissue specimens after fixation and staining for histochemical study; they are called medusa cells. Although the conformations and lengths of the processes vary, they resemble protozoal pseudopodia. The form which first called our attention to these cells is a conical determinate projection that tapers to a fine tip, much like a protozoal axopod. A basal specialization in the form of a vesicle or thickening may be frequently observed where the process protrudes from, or appears attached to, the cell body. The processes of these eosinophil variants appear morphologically specialized to interact with other cellular elements of the blood and are occasionally seen in contact with, or engulfing, erythrocytes, platelets or other leukocytes. Two hydroperoxidases have been elucidated in eosinophils and medusa cells by virtue of different substrate specificities, subcellular localizations and inhibitor sensitivities. One of these hydroperoxidases is shown by 3,3'-diaminobenzidine, is cyanide resistant, and is never observed in granules or rods in the medusa cell processes; it is frequently polarized to the sites of contact of medusa cells with other cellular elements of the blood. The other hydroperoxidase is revealed by p-phenylenediamine-pyrocatechol, is sensitive to cyanide and is frequently observed in granules and rods in medusa cell processes as well as in a population of larger granules in the cell bodies; the granules in the processes appear to be precursors to the rods, which may be related to Charcot-Leyden crystals. The extrusion of medusa cell processes is facilitated by the divalent cations calcium and magnesium and is inhibited by anions which sequester them such as phosphate, EDTA, citrate and oxalate. Medusa cells have been observed in samples from both rodents and humans and can be very prominent when eosinophilia accompanies allergy, parasitosis and malignancy.