Ovaries of Tubificinae (Clitellata, Naididae) resemble ovary cords found in Hirudinea (Clitellata) |
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Authors: | Anna Z Urbisz Mariola Krodkiewska Piotr Świątek |
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Institution: | (1) Department of Animal Histology and Embryology, Silesian University, Bankowa 9, 40-007 Katowice, Poland;(2) Department of Hydrobiology, Silesian University, Bankowa 9, 40-007 Katowice, Poland |
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Abstract: | 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. |
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