Ultrastructural studies on the daughter sporocysts of Meiogymnophallus minutus (Digenea: Gymnophallidae)

Abstract

The daughter sporocyst body wall is a syncytial microvillous tegument resting on a basement lamina, followed by circular and longitudinal muscle fibres and five kinds of subtegumentary cells, namely flame, myoblast, germinal, secretory and digestive cells. The last two have cytoplasmic bridges connecting them to the tegument and their principal function is probably nutrition. Secretory cells probably synthesize hydrolytic enzymes which pass via the tegument into the host's haemolymph for exogenous digestion of nutrients. Simple nutrients are probably absorbed over the entire surface of the microvilli whereas undigested or partly digested nutrients are absorbed by micropinocytosis at their base. Some nutrients may be digested in the tegument but most pass into the digestive cells which undergo a cycle including synthetic, absorptive, digestive, storage, autophagic and resting phases. Nutrients are supplied to developing embryos by extensions of digestive cells which line the brood chamber. Residual bodies are shed into the host's haemolymph, or into the brood chamber. In ageing sporocysts, microvilli and later the tegument completely degenerate and subtegumental cells undergo extensive autolysis, finally, the body wall ruptures, releasing the remaining fully formed cercariae.