Morphological and cytochemical characteristics of monocytes from inshore hagfish Eptatretus burgei

Monocytes were observed in the blood of inshore hagfish Eptatretus burgei. The monocytes were round or oval, high nucleus/cytoplasm ratio, and have several round or oval eosinophilic granules, which show dark red or blackish red color when stained with May-Grünwald (MG), Giemsa, and MG-Giemsa stain. The granules were positive for alkaline phosphatase, acid phosphatase, and α-naphthyl butyrate esterase. Also, the granules show metachromatic (reddish purple) with toluidine blue. However, ꞵ-glucuronidase, α-naphthyl acetate esterase, naphthol AS-D chloroacetate esterase and peroxidase were not detected in the monocytes, and negative for periodic acid Schiff reaction, alcian blue, Sudan black B, Sudan III, and oil red O. The monocytes engulfed many yeast particles (zymosan).