Egg capsules of a parasitic turbellarian flatworm: ultrastructure of hatching sutures

Egg capsules of Syndisyrinx franciscanus, an intestinal parasite of sea urchins (Strongylocentrotus spp.), consist of a bulb, which contains the embryos, and a stalk-like filament. The wall of the bulb is about 12 microns thick and is composed of sclerotized proteins. The end of the bulb opposite the attachment of the filament bears a reticulum of hatching sutures. Transmission electron microscopy discloses that hatching sutures traverse the entire thickness of the capsule wall. The inner 9-10 microns of sutures are a uniform 20 nm in width and contain a trilaminar cementum. The outer 2-3 microns of sutures are 15 nm to more than 500 nm in width and contain an electron-lucent cementum. The latter may contain an irregular, median, electron-dense layer or, more commonly, electron-dense granules. The outside of some capsules is partially covered by a thin, electron-dense material. A previous study showed that sutures in intact capsules of Syndisyrinx franciscanus are not affected by host digestive fluids, but are severely weakened immediately prior to hatching owing to activities of the embryos. The hypothesis that the embryos secrete a hatching enzyme is supported by findings that sutures of intact capsules are not affected by externally applied trypsin, but become weakened when capsules are cut open and then incubated in trypsin. Scanning electron microscopy reveals that the outer parts of sutures often remain intact after hatching. We hypothesize that the ability of sutures to resist enzymatic attack from the outside, but not the inside, results from differences in the chemical properties of the cementums in outer and inner parts of sutures.