Haemocytes in Cerastoderma edule (Mollusca, Bivalvia): distinct cell types engage in different responses to sheep erythrocytes

Abstract

Three types of circulating haemocytes were identified in the haemolymph of the cockle Cerastoderma edule with light microscopy (LM), and electron microscopy (EM) and enzyme cytochemistry. Type I cells are large spreading haemocytes, with a ruffled surface, eccentric nucleus and distinct cytoplasmic organelles. These cells are esterase- and acid phosphatase- positive. Type II cells are spherical, with a smooth surface and central nucleus surrounded by a cytoplasm devoid of organelles. They are negative for esterase and acid phosphatase reactions. Type III cells, which have not been described before in molluscs, have a round shape, a small eccentric nucleus and a large vacuole filled with granular material (TEM) occupying the majority of the cytoplasm. These cells are peroxidase positive and morphologically resemble the serous cells of bivalves or the signet ring cells of urochordates. The cellular response of Cerastoderma edule to the intracoelomic injection of sheep red blood cells (SRBC) was studied. The cell types involved in E-rosette formation and phagocytosis were documented by LM, TEM and SEM. In light microscopy and ultrastructural analysis the E-rosette forming cell was identified as Type II cells. The Type I cells phagocytosed SRBC, autologous cells and remnants thereof. Red blood cells with disrupted membranes and different electron densities were observed not only inside but also in close vicinity of phagocytic cells, which may suggest that the cells produce a cytolytic factor. The function of Type III cells remains obscure.