Morphology and histochemistry of a PAS-positive granular cell in the gills of the gilthead seabream, Sparus aurata L.

A periodic acid-Schiff-positive granular cell (PAS-G cell) was investigated by light and electron microscopy in the gills of the gilthead seabream (Sparus aurata L.). Despite the use of several fixatives, it was only possible to demonstrate metachromasia after fixing these cells with a solution of 10% formol and 5% acetic acid in methanol. Using this fixative, their granules stained purple with toluidine blue or thionin. In semithin resin sections, the granules also stained purple with toluidine blue-borax. Ultrastructurally, these cells possessed 2 types of homogeneous electron-dense granules and characteristic long thin surface processes resembling a string of beads. These PAS-G cell share morphological features with basophils and mast cells, and were metachromatic. They were observed in sites similar to those for mast cells in mammals.