Correlative and Dynamic Imaging of the Hatching Biology of Schistosoma japonicum from Eggs Prepared by High Pressure Freezing

Abstract

BackgroundSchistosome eggs must traverse tissues of the intestine or bladder to escape the human host and further the life cycle. Escape from host tissues is facilitated by secretion of immuno-reactive molecules by eggs and the formation of an intense strong granulomatous response by the host which acts to exclude the egg into gut or bladder lumens. Schistosome eggs hatch on contact with freshwater, but the mechanisms of activation and hatching are poorly understood. In view of the lack of knowledge of the behaviour of egg hatching in schistosomes, we undertook a detailed dynamic and correlative study of the hatching biology of Schistosoma japonicum. Methodology/Principal FindingsHatching eggs of S. japonicum were studied using correlative light and electron microscopy (EM). The hatching behaviour was recorded by video microscopy. EM preparative methods incorporating high pressure freezing and cryo-substitution were used to investigate ultrastructural features of the miracidium and extra-embryonic envelopes in pre-activated and activated eggs, and immediately after eggshell rupture. Lectin cytochemistry was performed on egg tissues to investigate subcellular location of specific carbohydrate groups.Conclusions/SignificanceThe hatching of S. japonicum eggs is a striking phenomenon, whereby the larva is liberated explosively while still encapsulated within its sub-shell envelopes. The major alterations that occur in the egg during activation are scission of the outer envelope-eggshell boundary, autolysis of the cellular inner envelope, and likely hydration of abundant complex and simple polysaccharides in the lacunal space between the miracidial larva and surrounding envelopes. These observations on hatching provide insight into the dynamic activity of the eggs and the biology of schistosomes within the host.

Ultrastructural studies on egg envelopes surrounding the miracidia of Mediogonimus jourdanei Mas-Coma et Rocamora, 1978 (Digenea, Microphalloidea, Prosthogonimidae)

“…The ultrastructure of egg envelopes of M. jourdanei resembles to great extent that described earlier in different species of schistosomes of medical and veterinary importance (Świderski et al 1980;Świderski 1984, 1985, 1986, 1988, 1993, 1994Jones et al 2008). An important difference is a very early degeneration of the outer envelope in eggs of M. jourdanei, which disappears very rapidly, leaving only a very thin, frequently discontinuous layer or small debris of cytoplasmic remnants.…”

“…Similarly, as described by Jones et al (2008) for S. japonicum, in M. jourdanei eggs, small pockets of low electrondense granular material of the inner envelope penetrate the very electron-dense material of the outer envelope at the border between these two layers.…”

Schistosomiasis in the People's Republic of China: the Era of the Three Gorges Dam

“…The life cycle is completed when the eggs hatch. The contained miracidia are liberated explosively, while still encapsulated within their subshell envelopes (103), and in turn, these infect receptive amphibious Oncomelania hupensis freshwater snails. The miracidium forms a sporocyst at the site of penetration, and this produces daughter sporocysts that migrate to the snail hepatopancreas and asexually produce larval cercariae for daily release into the surrounding water.…”

Molecular interactions of proteins associated with surface membrane biogenesis and renewal in the tegument of Schistosoma mansoni

“…This involved the combination of the preparative methods of high pressure freezing (HPF) for electron microscopy and cryosubstitution in uranyl acetate solutions. This method follows ultrastructural investigations characterising eggs of S. japonicum (Jones et al, 2008), and the human liver fluke Opisthorchis viverrini and I-TEM studies of the taeniid tapeworms Taenia ovis (Jabbar et al, 2010b). This method, applied here to whole adult S. mansoni worms, allows for superior membrane preservation and successful localisation of a variety of proteins.…”

“…Here, we have demonstrated that I-TEM can be successfully performed using a combination of cryoprepatation methods and fixation in uranyl acetate. Use of cryopreservation methods for ultrastructural studies of parasitic helminths has only previously been applied to investigations of eggs, such as those of schistosomes and other fluke species and, taeniid cestodes (Jabbar et al, 2010a;Jabbar et al, 2010b;Jones et al, 2008; Świderski et al, 2010). This report constitutes the first description of its utilisation in adult schistosomes.…”