Ultrastructural Studies on Kronborgia (Platyhelminthes, Fecampiidae) - the Differentiated Vitellocyte of Kronborgia-Isopodicola Blair and Williams

Abstract

In the adult parasitic female form of K. isopodicola, the vitellocyte cytoplasm contains yolk platelets, lipid bodies, glycogen, myelin figures and mitochondria. The platelets consist of an outer granular zone and an opaque core. All platelets are surrounded by several membranes. Many are cloven into segments by fissures containing membranes. Frequently, small peripheral fragments of the granular zone are pared away, and the core undergoes fragmentation by the same process. Spherical dense bodies are found in the cytoplasm. During the cocoon phase, the platelets are often intricately fragmented, and many pieces are paracrystalline. In the newly deposited egg, many platelets comprise only core segments, which are typically paracrystalline, frequently polygonal, and enveloped by multiple membranes. Spherical dense bodies are not encountered at this stage. The platelets are unlike the 'yolk globules' of Digenea, but are similar to vitelline platelets described for polyclads. In morphology and mode of utilisation they bear some resemblance to yolk granules of Amphibia. The membranes are interpreted as isolating membranes of cellular autophagy. Glycogen synthesis is related to autophagic events involved in yolk degradation. The spherical bodies probably represent eggshell granules; complex shell granules, characteristic of other platyhelminths, were not observed in K. isopodicola.