Demonstration of Lysosomal Enzymes in Hemocytes of Mercenaria mercenaria (Mollusca: Bivalvia)

Abstract

Seven hydrolytic enzymes (acid phosphatase, beta-glucuronidase, beta-Nacetylglucosaminidase, acid-beta-galactosidase, and A-, B-, and C-esterases were demonstrated by cytochemical techniques in hemocytes of Mercenaria mercenaria prior to and following in vitro exposure to the dinoflagellate Isochrysis galbana. All of the hydrolases were localized in blunt granules; acid phosphatase, beta-glucuronidase, and the esterases also were found in dot-like cytoplasmic granules. Acid-beta-galactosidase, beta-N-acetylglucosaminidase, and A-, B-, and C-esterases were observed within the phagosomes, while acid phosphatase and beta-glucuronidase were present only in the granules adjacent to these organelles. Following phagocytosis of algae, elevated levels of enzyme activity were demonstrated. The significant role played by hemocytes in the defense mechanisms of bivalve molluscs has been well documented and reviewed recently by Cheng (1983a,b). One of the important functions that the hemocytes perform is the phagocytosis and degradation of foreign materials (Bayne et al., 1979; Moore & Eble, 1977; Moore & Gelder, 1983; Moore & Lowe, 1977); hence, hemocyte recognition and subsequent processing of these foreign materials have received increasing attention. Membrane receptors have been reported for molluscan hemocytes (Reinisch et al., 1983; Yoshino et al., 1979) and detailed studies have helped to elucidate the role of various organelles, particularly lysosomes, in the internal handling of the phagocytosed material (Bayne et al., 1979; Cheng, 1983a; Fries & Tripp, 1980; Hawkins & Howse, 1982; Huffman & Tripp, 1982; Moore, 1976; Moore & Eble, 1977; Moore & Gelder, 1983). Acid phosphatase and other lysosomal enzymes have been reported in hemocytes from Crassostrea virginica and Mercenaria mercenaria (see Cheng & Rodrick, 1975), while, more recently, Huffman & Tripp (1982) extended the range of enzymes demonstrated in hemocytes of Mya arenaria. Blunt granules, the dominant cytoplasmic granules in hemocytes of Mercenaria, have been shown to sequester neutral red and acridine orange selectively (Moore & Gelder, 1983). Yoshino & Cheng (1976) ultrastructurally localized acid phosphatase in vesicles comparable to the blunt granules normally seen with light microscopy. The present investigation was designed to identify and localize selected lysosomal enzymes in hemocytes of M. mercenaria prior to and following 1 We acknowledge the partial support of this work by NSF Grant Supplement PCM 8442943. 2 Present address: Department of Biology, Lasell Junior College, Newton, Massachusetts 02166,