| Summary: | The blood cells of the common blue mussel <i>Mytilus edulis</i> have been characterized using a range of criteria including, electron microscopy, lectin-affinity and enzyme localization. Three distinct sub-populations of haemocyte have been identified; these can be characterized on the basis of ultrastructural morphology as small agranular cells, granular cells containing small granules and granular cells containing large granules. Lectin-binding studies have shown the small granules of the granular cells to be positive for <i>Helix pomatia</i> lectin (HPA) indicating the presence of N-acetyl-α-galactosaminyl residues within the granules. The larger granules were not positive for HPA but did bind wheat-germ agglutinin (WGA) which has an affinity for N-acetyl-β-glucosaminyl residues and N-acetyl-β-D-glucosamine oligomers. Furthermore the WGA-positive granules demonstrated a differential pattern of binding according to granule size, so that peripheral labelling was observed for granules of around 0.5 μm diameter whereas labelling occurred throughout the matrix for granules over 1 μm diameter. The lectin binding studies also demonstrated binding of both HPA and WGA as well as <i>Tetragonolobus purpureas</i> lectin( TPA) to the plasma membrane of a number of haemocytes; however the results did not demonstrate any correlation between surface lectin binding and cell type, as defined by the ultrastructural morphology. A range of hydrolytic enzymes was localized in association with the large granules of the granular cells, these included arylsulphatase, β-glucuronidase, elastase, lysozyme and cathepsin B, indicating that these granules constitute a form of lysosome. The small granules contained protease enzymes and in particular cathepsin G antibodies showed a high affinity for these granules.
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