After penetration into the mammalian host, theSchistosoma mansoni cercariae transforms intoschistosomula. This immature larval form of theparasite migrates via the circulatory system of thehost through the lungs, reaching the hepatic portalsystem where it matures and couples. A number ofmorphologic, behavioral and metabolic changesthat take place during the development of theparastie have been well characterized. However,the understanding of the molecular mechanismsthat regulate these changes at the gene level arestill limited. In this paper we describe attempts wehave undertaken to better understand some of thesemechanisms.During penetration through the skin of themammalian host the cercaria looses its tail and theglycocalix that surrounds its body. The tegumentof the newly transformed schistosomula undergoesfurther modifications. Observations made by trans-mission electron microscopy indicate that the tril-aminate tegumental membrane of the schistomulumbecomes gradually heptalaminate (D Hockley 1973Int J Parasitol 3: 13-25). As observed by scan-ning electron microscopy, soon after penetrationthe ventral sucker is retracted into the body, whereit remains until the parasites arrive at the portalsystem (J Crabtree 1980 Parasitology 81: 553-564). After two days a regression of the spines inthe mid-body and a change in the spination of themouth are observed. By the third day, the centralportion of the body is almost totally devoid ofspines. The protruding tegumentary ridges on theapical region of the body and the raised basal bulbsare no longer visible. The tegument at this stagebecomes pitted. At about day 14 after penetrationthe spines have redeveloped. The annular ridgesof the pitted tegument have become interconnectedby longitudinal septae. An enlargement of themouth is observed and the oral sucker becomesmore apparent. At day 18 the ventral sucker hasincreased in size and protrudes from the body sur-face.After penetration, the body of the schisto-somulum starts to contract and extend rhythmically(Crabtree loc. cit. ). These movements proceed untilthe worm reaches the hepatic portal system, andhave been associated with the facilitation of wormmigration through the capillaries in the lung andsystemic organs. The reduction in movement isaccompanied by a decrease in maximum length.It has also been postulated that the changes thatoccur in body shape prevent the escape of theschistosomulum from the hepatic portal system,trapping the parasites in this site.Several metabolical and physiological changesare detected after transformation of the parasitefrom cercariae to schistosomula. It has been dem-onstrated that transferrin and platelet derivedgrowth factor stimulate in vitro growth and devel-opment of schistosomula (L Clemens 1989 JParasitol 75 : 417-421). Hormones have also beendemonstrated to play an important role in parasitematuration. In this context, it has been shown thatthe production of ecdysteroids present in the ma-turing worm peak at day 6 after transformation (GTorpier 1982 Parasitology 84: 123-130). Seroto-nin has been shown to not affect the activity levelsof adenylate cyclase in cercariae, in contrast to thedeveloping parasite and adult worms where an in-creased level of the enzyme is observed in the pres-ence of serotonin (M Kasschau 1982 Nature 296:
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Oliveira et al. (1998) studied this question.