The Female Gonad in Two Species of Microplana (Platyhelminthes, Tricladida, Rhynchodemidae): Ultrastructural and Cytochemical Investigations

The female gonad of the land planarians Microplana scharffi and Microplana terrestris consists of two small germaria located ventrally in the anterior third of the body and of two ventro-lateral rows of oblong vitelline follicles distributed between the intestinal pouches. Both these structures are enveloped by a tunica composed of an outer extracellular lamina and an inner sheath of accessory cells. Oocyte maturation is characterized by the appearance of chromatoid bodies and the development of endoplasmic reticulum and Golgi complexes. These organelles appear to be correlated with the production of egg granules with a fenestrated/granular content of medium electron density, about 4-5 mum in diameter, which remain dispersed in the ooplasm of mature oocytes. On the basis of cytochemical tests showing their glycoprotein composition, and their localization in mature oocytes, these egg granules have been interpreted as yolk. In the vitelline follicles, vitellocytes show the typical features of secretory cells with well-developed rough endoplasmic reticulum and Golgi complexes involved in the production of eggshell globules and yolk. The eggshell globules, which appear to arise from repeated coalescences of two types of Golgi-derived vesicles, contain polyphenols and, when completely mature, they measure about 1-1,2 mum in diameter and show a meandering/concentric content pattern as is typical of the situation observed in most Proseriata and Tricladida. Mature vitellocytes also contain a large amount of glycogen and lipids as further reserve material. On the basis of the ultrastructural features of the female gonad and in relation to the current literature the two species of rhynchodemids investigated appear to be closely related to the freshwater planarians belonging to the family Dugesiidae.