The Schistosome Egg in Transit
- 1. Department of Microbiology, Tumor and Cell Biuology, Karolinska Institutet, Sweden
Abstract
The Schistosoma mansoni egg develops in the female worm ooctype where the oocyte and surrounding vitelline granules become enclosed by a protective shell. The schisto some egg is then transported through the uterus to emerge from the genital pore of the intravascular worm. It is not known how eggs successfully enter the perivascular space and how they transit the intestinal wall into the stools. That this route is far from fail-safe is evident from the fact that eggs only have a fifty-fifty chance to make it from their intravascular location into the environment. The fate of “failed” eggs, which are swept by the blood stream to form end up in the liver of infected individuals, can be studied by injecting schistosome eggs in an experimental thrombosis model into the mouse caecal vein. Perioval coagulation and repair in the liver was demonstrated by immune histology for fibrin and fibronectin. Trapped eggs caused transient fibrin deposition, suggesting a fibrinolytic process. This was confirmed by the demonstration of perioval plasmin in an infected monkey. Fibronectin deposition indicated an early repair process with extracellular matrix formation. That a similar sequence of events may occur in the intestinal wall was suggested by presence of perioval fibrin and fibronectin. However, successful transit of eggs may depend on several mechanisms. Passage through the vascular endothelium is a decisive moment determining the future of the egg and possibly the kind of host response elicited during the journey through the tissues. Our observations on in vitro oviposition suggest that the female schistosome ejects the egg through the vascular endothelium. Thus extravasation of “successful” eggs seems to occur by a mechanism distinct from that of failed eggs. This does not exclude the possibility that tissue degradation associated with fibrinolysis may be involved in successful transit of eggs into the excretions.