Ultrastructural features of oogenesis in a free-living marine Platyhelminth, Vorticeros luteum.
The female gonad of the prolecithophoran turbellarian Voticeros luteum has a heterocellular organization as is typical in the neoophoran Platyhelminthes. It is composed of an ovarian and a vitellarian area that are not well separated from one another and from the surrounding tissues by a tunica. Each oocyte during maturation becomes completely surrounded by a single accessory cell whose function has been hypothesized on the basis of the present investigation. The main feature in oocyte differentiation appears to be the synthesis and accumulation of small electron-dense inclusions; these are about 1-2 microm in size, are elaborated by the Golgi apparatus and have a content which reacts partially to the cytochemical test for polyphenols. The genesis, composition and cortical localization of the inclusions suggest that they could be homologous to the egg-shell granules present in the oocytes of 'archoophoran' species. A comparative analysis and discussion of the oocyte inclusions observed to date in other turbellarians is undertaken and the hypothetical use of these structures as characters providing some new insights in clarifying the phylogenetic relationships among free-living Platyhelminthes is finally discussed.
Nigro
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Istituto di Zoologia e Anatomia Comparata, Universita' di Pisa, Pisa, Italy.
Gremigni
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