The paired ovaries of E. albidus are like a bunch of grapes and are composed of clearly separated units, syncytial germ cysts (clusters), which are surrounded by a thin layer of somatic cells. Each cyst maintains the connection with the ovary by an extended stalk that is composed of somatic cells. The spatial architecture of the germ-line cysts found in E. albidus is the same as in other clitellate annelids that have been studied to date. As a rule, germ cells are located at the cyst periphery and each has only one ring canal that connects it to the common and centrally located cytoplasmic mass, the cytophore. Here we present data about the F-actin and microtubular cytoskeleton and some molecular components of the germ-line cysts. We show that the ring canals have an inner rim that is enriched with microfilaments and proteins that contain phosphotyrosine. The microtubules form a loose network in the cytoplasm of the oocyte and nurse cells; moreover, some of them pass through the ring canals to the cytophore. Numerous microtubules are also located in the somatic cells. The germ-line cysts in E. albidus ovaries consist of 16 cells, which is the lowest known number of interconnected germ cells within clitellate annelids. During oogenesis, the fate of interconnected germ cells differentiates and only one cell develops as the future egg, while the other 15 become nurse cells. This differentiation means ovary meroism. The nurse cells gather cell organelles and storage material that then pass through the ring canals and cytophore moving towards the growing oocyte. At the end of oogenesis, the vitellogenic oocyte surrounds the siblings' cells together with the cytophore and engulfs their remnants into the ooplasm. No morphological or molecular markers of the apoptosis of the nurse cells were found. Moreover, the nurse cells did not undergo polyploidisation. The measured DNA level was 4C, which indicates that these cells are not highly-specialised.
The ultrastructure of the ovaries and oogenesis was studied in three species of three genera of Tubificinae. The paired ovaries are small, conically shaped structures, connected to the intersegmental septum between segments X and XI by their narrow end. The ovaries are composed of syncytial cysts of germ cells interconnected by stable cytoplasmic bridges (ring canals) and surrounded by follicular cells. The architecture of the germ-line cysts is exactly the same as in all clitellate annelids studied to date, i.e. each cell in a cyst has only one ring canal connecting it to the central, anuclear cytoplasmic mass, the cytophore. The ovaries found in all of the species studied seem to be meroistic, i.e. the ultimate fate of germ cells within a cyst is different, and the majority of cells withdraw from meiosis and become nurse cells; the rest continue meiosis, gather macromolecules, cell organelles and storage material, and become oocytes. The ovaries are polarized; their narrow end contains mitotically dividing oogonia and germ cells entering the meiosis prophase; whereas within the middle and basal parts, nurse cells, a prominent cytophore and growing oocytes occur. During late previtellogenesis/early vitellogenesis, the oocytes detach from the cytophore and float in the coelom; they are usually enveloped by the peritoneal epithelium and associated with blood vessels. Generally, the organization of ovaries in all of the Tubificinae species studied resembles the polarized ovary cords found within the ovisacs of some Euhirudinea. The organization of ovaries and the course of oogenesis between the genera studied and other clitellate annelids are compared. Finally, it is suggested that germ-line cysts formation and the meroistic mode of oogenesis may be a primary character for all Clitellata.
Recent studies on the ovary organization and oogenesis in Tubificinae have revealed that their ovaries are small polarized structures that are composed of germ cells in subsequent stages of oogenesis that are associated with somatic cells. In syncytial cysts, as a rule, each germ cell is connected to the central cytoplasmic mass, the cytophore, via only one stable intercellular bridge (ring canal). In this paper we present detailed data about the composition of germ-line cysts in Tubifex tubifex with special emphasis on the occurrence and distribution of the cytoskeletal elements. Using fixed material and live cell imaging techniques, we found that the entire ovary of T. tubifex is composed of only one, huge multicellular germ-line cyst, which may contain up to 2,600 cells. Its architecture is broadly similar to the cysts that are found in other clitellate annelids, i.e. a common, anuclear cytoplasmic mass in the center of the cyst and germ cells that are connected to it via intercellular bridges. The cytophore in the T. tubifex cyst extends along the long axis of the ovary in the form of elongated and branched cytoplasmic strands. Rhodamine-coupled phalloidin staining revealed that the prominent strands of actin filaments occur inside the cytophore. Similar to the cytophore, F-actin strands are branched and they are especially well developed in the middle and outermost parts of the ovary. Microfilaments are also present in the ring canals that connect the germ cells with the cytophore in the narrow end of the ovary. Using TubulinTracker, we found that the microtubules form a prominent network of loosely and evenly distributed tubules inside the cytophore as well as in every germ cell. The well-developed cytoskeletal elements in T. tubifex ovary seem to ensure the integrity of such a huge germ-line cyst of complex (germ cells - ring canals - cytophore) organization. A comparison between the cysts that are described here and other well-known female germ-line cysts is also made.
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