Interrelationships of vitelline and muscle cells within the vitelline follicles of the blood fluke Aporocotyle simplex (Digenea, Aporocotylidae) and morphological evidence for the modification of vitelline material for eggshell formation

This study of the fish blood fluke Aporocotyle simplex represents the first detailed transmission electron microscopical (TEM) investigation of the vitellarium of an aporocotylid digenean blood fluke. It revealed some unusual characteristics in the cytoarchitecture of the vitelline follicles and demonstrated modifications of the vitelline granules for eggshell formation. The vitelline follicles consist of vitellocytes at different developmental stages surrounded by sarcoplasmic processes of myocytes which occur throughout each follicle. Sites of intimate contact occur between the vitellocytes and the myocytes. Individual vitelline globules (0.1–0.2 μm in diameter) accumulate in quite small clusters of 10–20 and have a dense, heterogeneous matrix possessing central and peripheral regions with a greater density. Modifications of the vitelline globules take place within the clusters and are first apparent when the vitellocytes reach the lumen of the vitelline duct and vitelline reservoir. Globules within the clusters become confluent, and, when the vitellocytes reach the lumen of the oviduct and proximal ootype, these consolidated clusters contain a shapeless, loosely packed, dense material which is released from the vitellocytes by exocytosis. This investigation has provided morphological evidence for shell formation from modified vitelline globules in the form of a discontinuous, thin layer (~ 0.07 μm in thickness) of electron-dense shell material around the fertilized ovum and associated vitellocytes in the proximal ootype. The eggshell of intra-uterine eggs acquires an additional thin, heterogeneous outer layer, increasing its thickness to ~ 0.1 μm. The cytoarchitecture of the vitellarium, modifications of the vitelline globules within the clusters and the structure of the eggshell of A. simplex may prove to be of value in studies examining relationships between the three distinct lineages of aporocotylid digeneans.