An isoform of ferritin as a component of protein yolk platelets in Schistosoma mansoni.

Schistosoma mansoni possesses two isoforms of the iron storage protein ferritin, Fer1 and Fer2. At the mRNA level as well as at the protein level, Fer1 is much more abundant than Fer2; females contain an about 15-fold excess of Fer1 compared with males. In contrast, nearly equal amounts of Fer2 occur in both sexes. By electron microscopy we identified ferritin as a component of electron dense membrane-bound bodies in cells of the vitellarium. The mode of formation of these inclusions (as inferred from electron microscopy) and the abundance of phospholipid multilayered membranes suggest that these bodies are of a lysosomal nature. Here we interpret these ferritin-containing inclusions as protein yolk platelets. To date, most of the literature does not contain any hints of the existence of protein yolk in trematodes. The possible function of ferritin in embryonic development is discussed.