Host-Parasite Interrelationships Between the Freshwater Pulmonate Biomphalaria Pfeifferi and the Trematode Schistosoma Mansoni

TLDR: The observations indicate that the digestive gland of B. pfeifferi is not important as a storage organ for reserve material, and the main functions of the digestive cell are absorption and endocytosis of predigested food material, followed by intracellular digestion.

Abstract: The influence of infection with S. mansoni on reproduction, growth, food consumption and survival in B. pfeifferi was studied experimentally (Ch. II). The prepatent period in infected snails was shown to last 22-24 days under the conditions prevailing in the relevant experiment. The number of eggs laid by infected snails as compared with controls was significantly reduced from 7-10 days postinfection onwards. After day 13 postinfection it was completely suppressed in the majority of the infected snails. The increase in shell diameter in infected specimens was larger than that of the controls between 7 and 18 days, that in weight between 11 and 21 days postinfection. Gigantism did not occur. There was no difference in mortality between infected and control snails prior to the 11th week after the beginning of the experiment. After that, the mortality among infected specimens showed a sharp rise. Uninfected and infected snails consumed similar amounts of food (dried lettuce) in the first two weeks of the experiment. After that, infected snails consumed less food than controls. The digestive gland of B. pfeifferi was studied with histological, histochemical and ultrastructural methods (Ch. III). The highly folded epithelium of the ducts connecting the digestive gland with the pyloric part of the stomach, consists of cilia and/or microvilli bearing columnar cells with interspersed mucous cells. In the epithelium of the digestive gland two main cell types are distinguished: the digestive cell and the secretory cell. In addition some mucous cells occur. It is concluded that the main functions of the digestive cell are absorption and endocytosis of predigested food material, followed by intracellular digestion. Indigestible residues are accumulated in a large vacuole, which eventually is excreted. The secretory cell produces and secretes proteinaceous substances, presumably digestive enzymes. It does not store calcium. The so-called excretory cells, which are in fact degenerating secretory cells, have vacuoles containing yellow globules. These yellow globules together with cell debris are finally released into the lumen of the digestive gland. The observations indicate that the digestive gland is not important as a storage organ for reserve material. The lobules of the digestive gland are covered by a thin connective tissue sheath, consisting of ground substance, collagen-like fibrils, smooth muscle cells and cells with the appearance of pigment cells. The connective tissue between the lobules of the digestive gland and the acini of the ovotestis, and between these organs and the mantle, consists mainly of vesicular connective tissue cells. These cells are supposed to play an important metabolic role as storage cells for glycogen. The tegument of the daughter sporocyst of S. mansoni (Ch. IV), consisting of an outer syncytial anucleate layer which is joined by cytoplasmic connections to nucleated cell bodies (tegumental cells), has the same basic architecture as that described for various stages of trematodes and cestodes. Well-developed mitochondria, GER cisternae and tubules, free ribosomes and polysomes, and lipid droplets occur throughout the tegument. The ultrastructural features suggest that the outer layer is involved in uptake of nutrients by absorption and endocytosis. The poorly developed musculature is loosely arranged in two layers beneath the outer tegumental layer. The nucleated portions of the muscle fibres are situated between the tegumental cells. The protonephridial system consists of flame cells with efferent tubules. The bundle of cilia, implanted in the flame cell, beats in the cilia chamber or barrel. The proximal part of the barrel consists of alternating rib-like extensions of the flame cell and of the first tubule cell, respectively. Ultrafiltration probably takes place in the ribbed part of the barrel. The development of cercariae from germinal cells is briefly described. The pathological effects of infection with S. mansoni on the digestive gland epithelium of B. pfeifferi (Ch. V) are limited. From about 12 weeks after infection slight changes occur, but they are difficult to distinguish from changes due to ageing of the cells. The effect of starvation on the digestive gland epithelium is much more marked than the adverse influence of infection. Compression of digestive gland lumina by daughter sporocysts was not observed. Changes in the connective tissue of the digestive gland due to the presence of daughter sporocysts were: development of muscle cells into cells with the appearance of pigment cells and/or degeneration of these muscle cells, and an increase in the number of amoebocytes, which become involved in the elimination of cell debris and in the formation of a loose layer over the parasites.