Ultrastructural and cytochemical aspects of the female gonad of Geoplana burmeisteri (Platyhelminthes, Tricladida, Terricola)

Abstract

The ultrastructure of the female gonad of the land planarian Geoplana burmeisteri was investigated by means of electron microscopy and cytochemical techniques. It consists of two small germaria located ventral to the intestine and of two irregular, lateral rows of vitelline follicles, both enveloped by a tunica composed of an extracellular lamina and an inner sheath of accessory cells. Accessory cell projections completely surround developing oocytes and vitellocytes. The main feature of oocyte maturation is the appearance of chromatoid bodies and the development of the rough endoplasmic reticulum (RER) and Golgi complexes. These organelles appear to be correlated with the production of egg inclusions of medium electron density, about 1.5-1.8 microm in diameter, which remain scattered in the ooplasm of mature oocytes. On the basis of cytochemical tests demonstrating their glycoprotein composition, these inclusions were interpreted as residual yolk globules. Vitellocytes are typical secretory cells with well-developed RER and Golgi complexes that are mainly involved in the production of yolk globules and eggshell globules, respectively. Eggshell globules appear to arise from repeated coalescence of small Golgi-derived vesicles and, at an intermediate stage of maturation, show a multigranular pattern. Later, after vesicle fusion, they reach a diameter of 1.3-1.6 microm when completely mature and show a meandering/concentric pattern, as is typical of the situation seen in most Proseriata and Tricladida. The content of yolk globules is completely digested by pronase, while the content of eggshell globules is unaffected. Mature vitellocytes contain, in addition, a large quantity of glycogen and lipid droplets as further reserve material. On the basis of the ultrastructural characteristics of the female gonad described above and in relation to the current literature, we conclude that G. burmeisteri appears to be more closely related to the freshwater triclads, in particular to members of the Dugesiidae, than to the marine triclads.

The female gonad in two species of Microplana (Platyhelminthes, Tricladida, Rhynchodemidae): Ultrastructural and cytochemical investigations

“…In the vitelline follicles, accessory cells encircling vitellocytes show ultrastructural features similar to those found in G. burmeisteri (Falleni et al, 2006). The presence of lysosome-like bodies and glycogen in the cytoplasm of these cells together with cytoplasmic protrusions from vitellocytes facing the accessory cells seem to suggest a trophic role also for the accessory cells in the vitellarium.…”

“…Among rhabdithophoran flatworms accessory cells surrounding germ cells throughout oogenesis have also been reported in some Proseriata (SopottEhelers, 1994(SopottEhelers, , 1995, Tricladida (Gremigni and Nigro, 1983;Tekaya et al, 1999;Falleni et al, 2006) and Rhabdocoela (Falleni et al, 2002). A different situation has been found in the rhabdocoelan Typhloplanida and Temnocephalida (Falleni and Lucchesi, 1992;Falleni et al, 1998Falleni et al, , 2005 where accessory cells are only located at the periphery of the germarium.…”

“…In addition, the accessory cell cytoplasm of M. scharffi contains a large quantity of glycogen, as is the case in G. burmeisteri (Falleni et al, 2006). As previously speculated this could represent a further reserve material available for the maturing oocytes and possibly for the embryo during the early stages of development preceding the incorporation of vitelline cells.…”

“…The only extensive ultrastructural investigation published so far is on Geoplana burmeisteri belonging to the Geoplanidae (Falleni et al, 2006). This study evidenced that this species is more closely related to the freshwater triclads rather than to marine triclads.…”

“…to play a trophic role in transferring low molecular weight precursors from the surrounding tissue to the developing oocytes besides having a protective and supporting function (Nigro and Gremigni, 1987;Falleni and Gremigni, 1992;Falleni et al, 2002Falleni et al, , 2006. In addition, the accessory cell cytoplasm of M. scharffi contains a large quantity of glycogen, as is the case in G. burmeisteri (Falleni et al, 2006).…”