The ovary of the gonochoristic marine triclad Sabussowia dioica: ultrastructural and cytochemical investigations

The ovary of the gonochoristic marine triclad Sabussowia dioica was examine d by transmission electron microscopy in order to detect the principal char acteristics of the developing female germ cells and their associated cells. Some unusual features discovered were as follows: (1) The presence of rese rve material inclusions randomly distributed in the ooplasm, despite the fa ct that S. dioica is a neoophoran platyhelminth provided with separate vite lline cells that are specifically appointed to the production and accumulat ion of yolk. The inclusions consist of aggregates of a fibrogranular materi al of medium electron-density enveloped by a double-layered smooth membrane . Both the genesis and composition of these inclusions represent novelties. They derive from coalescence of small fibrogranular aggregates transferred from the nucleus in association with vesicles and cisternae of the nuclear envelope (annulate lamellae) or through pores with similar materials synth esized by free polysomes and the endoplasmic reticulum (ER). These inclusio ns most likely consist of ribonucleoproteins and their significance and lik ely role are discussed. (2) The production through a double (auto-heterosyn thetic) mechanism of small granules with a heterogeneous structure and a gl ycoproteic composition, which at late stages of oocyte maturation become pr incipally distributed in a monolayer in the cortical ooplasm. (3) The prese nce of a cellular gonadal wall and of accessory cells that surround develop ing oocytes and play an active role in transferring newly synthesized subst ances to them. The functional significance of these ultrastructural charact eristics of the female gonad of S. dioica are discussed in comparison with those previously described in hermaphroditic triclads and other platyhelmin ths. (C) 1999 Elsevier Science Ltd. All rights reserved.