ULTRASTRUCTURAL AND CYTOCHEMICAL STUDY OF THE VITELLARIA IN THE MARINE TRICLAD SABUSSOWIA-DIOICA (PLATYHELMINTHES, TRICLADIDA, MARICOLA)

The development of vitelline cells (or vitellocytes) within the vitell aria of the marine triclad Sabussowia dioica has been studied by menas of ultrastructural and cytochemical investigations. Maturing vitelloc ytes have a small nucleus with some patched chromatin and a well-devel oped nucleolus; the abundant cytoplasm contains free ribosomes, mitoch ondria, RER and Golgi complex, along with several types of inclusions: egg-shell globules, yolk globules, lipid droplets and glycogen. Vitel locytes are associated with acessory cells which externally surround t he vitellarium to form a kind of lamina or tunica and internally envel ope vitellocytes by their long, flat cytoplasmic projections. The egg- chell globules appear to arise from repeated coalescence of small Golg i derived vesicles and at the onset of formation show a multigranular pattern. Later on, after vesicle fusion, they reach a diameter of 2.5- 2.7 mu-m and show a peculiar pattern with an electron-dense content, r eacting positively to the test for polyphenols, which is interrupted b y some small spots or narrow channels consisting of proteins. Yolk glo bules are membrane-bound inclusions of about 2-2.5 mu-m in diameter co ntaining a loose, granular material of medium electron-density which i s completely digested by pronase and does not react with the Thiery te st. Numerous lipid droplets, usually surrounded by glycogen granules, are also present. A comparative analysis of the egg-shell globule stru cture in the marine triclads studied to date suggest that the content of this type of inclusions in S-dioica (family Cercyridae) has an inte rmediate pattern with respect to those previously described in Cercyra hastata (family Cercyridae) and Procerodes lobata (family Procerodida e).