Intracellular digestion and cellular defecation in a monogenean, Diclidophora merlangi

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Abstract
The ultrastructural and cytochemical changes accompanying intracellular digestion and cellular defecation in the haematin cell of Diclidophora merlangi have been described. Blood proteins of the host-fish are sequestered by endocytosis and degraded within an interconnecting network of channels that form an integral, but changing, part of the cell. The digestive enzymes involved originate in the granular endoplasmic reticulum and are packaged in the Golgi apparatus and transferred to the channels in Golgi vesicles. The rate of haemoglobin absorption and the activity of the Golgi, as judged by vesicle counts and staining intensities for thiamine pyrophosphatase activity, are stimulated by the introduction of host protein into the gut lumen. The haematin residues of digestion are extruded periodically into the lumen by exocytosis involving membrane fusion. The process is a continuous one and, in worms starved of food, can result in the complete evacuation of pigment from the cell. It is suggested that a lysosomal system operates in the digestive cycle of the haematin cell.