Ultrastructural study of oogenesis in the acotylean Echinoplana celerrima, (Platyhelminthes, Polycladida)

Here, the ultrastructure of the female gonad of the marine polyclad Echinoplana celerrima was investigated using both light and electron microscopy. The female gonads are dispersed in the dorsal parenchyma between the digestive ramifications. Each ovary is separated from adjacent tissues by a homogeneous basal lamina and is characterized by the presence of a germinative zone, growing oocytes at the previtellogenic stage, and at the vitellogenic stage.The germinative zone is located at the dorsal part of the gonad and is characterized by the presence of undifferentiated young germ cells or oogonia having a large nucleus with a well-shaped nucleolus. Growing oocytes at the previtellogenic stage are characterized by the production and accumulation of eggshell granules. Indeed, the cytoplasm is gradually packed with cytoplasmic organelles and electron-dense inclusions that coalesce to form eggshell granules. When maturing, the latter become formed in their center by an electron-dense core surrounded by very electron-dense areas of variable shape. During the vitellogenesis stage, the oocytes undergo a notable increase in the cytoplasmic volume due to the accumulation of specific organelles such as free ribosomes and endoplasmic reticulum. These specific cytoplasmic organelles are probably involved in the production of vesicles containing a medium-dense and granular material. Repeated coalescence of these vesicles gives rise to the yolk globules. At this stage, the cytoplasm also displays some multivesicular bodies (MVBs). The origin of these MVBs and their function is discussed.