Hymenolepis diminuta: the mechanism of egg hatching

The fine structure and histochemistry of the envelopes surrounding the mature oncosphere of Hymenolepis diminuta have been investigated by transmission electron microscopy and light microscope observations of JB-4 embedded material. The fate of these envelopes during the hatching sequence in vitro has been followed by phase-contrast and scanning electron microscopy. Hatching is considered to comprise 4 stages. Stage 1 involves the mechanical breakage and removal of the shell and the outer cytoplasmic layer of the inner envelope; Stage 2 the activation of the oncosphere and the swelling of the gelatinous layer of the inner envelope; and Stage 3 the digestion and rupture of the embryophore. This is accomplished both by secretions from the penetration gland and by the action of external digestive enzymes, together with hook activity. Trypsin is more effective than amylase in digesting the embryophore. Stage 4 involves the enzymatic weakening of the gelatinous layer which helps the oncosphere to tear itself free with its hooks. Amylase is more effective than trypsin in attacking the gelatinous layer. On emergence from the gelatinous layer, the oncosphere is still enveloped by the 'oncospheral membrane', although this covering is soon lost. Once activated, the oncosphere is capable of completing the hatching sequence by itself, without the addition of enzymes. This process is a lengthy one, however, taking up to 2 h, and the expenditure of its glandular and energy reserves makes successful gut penetration by the oncosphere unlikely.