Table 1.
The summary of germ-line cyst organization in different groups of animals in conjunction with the cytoskeleton.
Fig 1.
Schematic illustration of the spatial organization of the T. tubifex ovary.
The ovary is conically shaped with its narrow end attached to the intersegmental septum (is) via a thin ligament (li) and is polarized along its long axis. Germ cells at subsequent stages of oogenesis occur in three zones (I, II, III): zone I contains oogonia (oo), undifferentiated germ cells (cystocytes) (gc) occur in zone II and germ cells that are morphologically differentiated into growing oocytes (o) and smaller nurse cells (nc) occur in zone III. Oocytes grow on only one side of the ovary and gradually protrude into the body cavity. The entire ovary is composed of only one, huge germ-line cyst. Each germ cell in a cyst (ovary) is connected to the central anuclear core (cytophore) via one stable intercellular bridge (ring canal – ellipse). The cytophore (cy) is poorly developed in zones I and II, whereas it is prominent and branched in zone III. In addition to the germ cells, somatic cells (sc) occur – they are localized inside the ovary and on its surface and form a thin ovary envelope.
Fig 2.
General organization of an ovarian cyst in T. tubifex.
(A) The ovary of Tubifex tubifex is conically shaped and polarized. Three zones can be distinguished (I, II and III). Each zone contains germ cells at subsequent stages of oogenesis: oogonia in zone I, undifferentiated cells in zone II and nurse cells and oocytes in zone III. In the third zone, several oocytes (o) grow successively on only one side of the ovary and protrude into the body cavity. An extensive cytophore (cy) is marked in this zone. Whole-mounted preparation, Nomarski interference contrast. (B) and (C) sections through zone III, in which two morphologically distinct germ cell categories occur – nurse cells (nc) and oocytes (o). The branched cytophore (cy) occupies the central position in the germ-line cyst. Arrowheads mark the cytophore regions that are enriched with cytoskeletal elements (see Figs 2A and 3). Arrows point to the somatic cells that accompany the germ cells inside the ovary and form the outer ovary envelope; nu – oocyte nucleus. B longitudinal section, fluorescence microscopy, Histocryl semi-thin section double stained with DAPI and PI; C cross section, light microscopy, Epon semi-thin section stained with methylene blue.
Fig 3.
The cytophore organization in an ovarian cyst of T. tubifex.
The entire ovary was stained with DAPI for the visualization of nuclei (A) and rhodamine-conjugated phalloidin to detect F-actin (B); A merged view (C). Maximum-intensity projections of the Z stacks that cover the entire ovary indicate that the entire ovary is composed of one multicellular germ-line cyst. A branched cytophore (stars) extends along the long axis of the ovary in the cyst; the cytophore is thin and inconspicuous in zones I and II, while it is more voluminous and branched in zone III; gc – germ cells, o – oocytes, nc – nurse cells. Confocal microscopy.
Fig 4.
F-actin localization in a T. tubifex ovarian cyst.
The entire ovary double stained with DAPI and rhodamine-conjugated phalloidin. Maximum-intensity projections of the Z stacks that cover the interior of the ovary. (A and B) whole ovaries, (C) detail of area between zones II and III. Long bundles of actin filaments occur (stars) in the branched cytophore (cy). F-actin is also localized in the cortical cytoplasm (arrows) of nurse cells (nc) and oocytes (o). Confocal microscopy.
Fig 5.
Organization of F-actin in an ovarian cyst of T. tubifex.
(A and B) The initial cytophore with thin F-actin bundles occurs at the narrow end of the ovary (zones I and II) (stars). The cytophore with its F-actin bundles reaches every germ cell (gc). The ring canals that connect the germ cells to the cytophore have rims that are enriched with F-actin (arrows). The F-actin also occurs in the cortical cytoplasm of the germ cells (arrowheads). Fluorescence microscopy, whole-mounted preparation stained with rhodamine conjugated phalloidin. C-E Fragment of zone II double stained with DAPI (C) and rhodamine-phalloidin (D); merged view (E). The cytophore has thin bundles of F-actin (stars) that reach each germ cell (gc). The ring canals rims are enriched with F-actin (arrows) and cortical F-actin can be observed in the germ cells (arrowheads). Fluorescence microscopy.
Fig 6.
Localization of microtubules in a T. tubifex ovarian cyst.
Zone III. Live cells stained with Hoechst 33342 (A) and Tubulin Tracker Green (B); (C-H) merged. (A-D) Microtubules form a network that fills the entire cytoplasm of the cytophore (cy); note microtubules in the ring canals that connect the germ cells (gc) with the cytophore (arrowheads) and the bundles of microtubules in the cytophore branches (stars in D). (E-F) A prominent network of microtubules (arrows) occurs in the cytoplasm of the germ cells (gc) and in the vitellogenic oocytes (o); nu – nucleus. (G) Mitotic spindles (arrowheads) of dividing germ cells were observed in zone III; stars indicate metaphase plates. (H) Long bundles of microtubules (arrowheads) were also detected in the cytoplasmic projections of the somatic cells (arrows point to the nuclei of these cells); gc – germ cells, o – oocytes. Fluorescence microscopy.
Table 2.