A freeze fracture study of the developing tegumental outer membrane of Schistosoma mansoni

The freeze fracture technique has been used to quantify changes in the integral components of the double outer membrane of Schistosoma mansoni during the 6-week period of development within the mouse. The intramembraneous particle (IMP) density on the P1 face begins to rise within 6 h of host penetration, reaches a maximum at day 4 and then falls rapidly after day 9, so that it is at a low level between 3 and 6 weeks. The E1 face IMP density follows the same course as that of the P1 face except that maximum particle density is recorded on day 1 and the counts begin to fall on day 5. The IMP density on the P2 face remains at a consistently low level throughout development. The E2 face IMP density rises gradually to a peak at day 4, when the parasites have migrated to the lungs, and remains thereafter at a similar level, so that by 6 weeks the E2 face has a higher IMP density than the other three fracture faces. The E2 face IMP show a marked increase in size on day 4. Morphological studies indicate that a different type of inclusion body makes a transient appearance in the tegument of the lung worms, and immunocytochemical techniques show the lung worms to be nonimmunogenic. It is suggested, therefore, that the E2 face IMP may represent complexes of parasite antigens and acquired host antigens. The tegumental membranes of cultured specimens have also been examined by freeze fracturing and the IMP densities compared with those obtained from in vivo parasites; the cultured schistosomula have a lower E2 face particle density than the in vivo specimens.