| Summary: | The phagocytic system of the periwinkle and the in vivo and in vitro response towards foreign particles was studied by light and electron microscopy, using carmine, yeast (S, cereviciae), human erythrocytes, bacteria (E. coli) and viruses (lambda bacteriophage) as test-particles. The involvement of humoral factors was studied by haemagglutination/haemagglutination-inhibition experiments. It was found that: 1) Injected markers are cleared by diapedesis through the epithelium covering the head, the mantle cavity and the foot (exluding the sole of the foot). Emboli formed in the circulation are cleared relatively quickly by circulating haemocytes; haemocytes containing the marker may persist for longer periods in adjacent tissues but eventually find their way to diapedesis sites through circulatory routes. Clearance of carmine is not completed during the observation time (64 days), whereas yeast appears to elicit a much quicker response. Intracellular digestion may enhance the removal of injected markers in the case of digestible materials. 2) Periwinkle haemocytes form a homogenous population with respect to morphology (as determined by light and electron microscopy) and function (all haemocytes appear to be capable of migrational and phagocytic responses towards injected markers). The haemocytes are avidly phagocytic when challenged in vivo or in vitro with vertebrate RBC or yeast; around 90% of the haemocytes contain one or more particles in 30 min, and the average content of each phagocyte is 2.2 particles. Additional uptake apparently proceeds at a steady state which is considered to depend upon the rate of intracellular digestion or membrane synthesis. The correlation of in vitro to in vivo results is discussed, and lower values in the latter case are attributed to diapedesis. 3) The connective tissue of the periwinkle is composed of ground substance with fibres resembling collagen, and 5 types of cells, i.e. pore cells, calcium cells, supportive cells, granular cells and tissue-associated haemocytes. Among these, phagocytosis of injected markers, i.e. human RBC, bacteria (E. coli) and virus particles (lambda bacteriophage) is restricted to haemocytes, which by their morphology cannot be classified as a separate subset, and so it appears that the periwinkle does not contain fixed or tissue resident phagocytes. Reports of fixed phagocytes in other invertebrates are discussed, as well as functional aspects of the pore cell.4) The haemolymph of the periwinkle contains a factor which agglutinates several types of vertebrate erythrocyte in addition to bacteria (E. coli) and yeast (S. cerevisiae). The active part of this molecule is a protein, and so it classifies as a lectin (Goldstein et al 1980). Agglutination of H-RBC was inhibited most strongly by L-xylose, but moderate inhibition was obtained with other pentoses (D-ribose, D-xylose, a-L-fucose). The lectin appears to function as a defence molecule, since (1) a moderate increase is found in circulating levels upon antigenic challenge (human erythrocytes), and (2) particles which are agglutinated by the lectin are phagocytosed more avidly than particles which fail to agglutinate. The baseline level of the lectin is quite variable between individuals, and is not affected by age or sex; this polymorphism is believed to be genetically determined, although to some extent it may also be modified by environmental factors (starvation, antigenic challenge).
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