Effects of follicle-stimulating hormone, bovine somototrophin and okadaic acid on cumulus expansion and nuclear maturation of Blue fox (Alopex lagopus) oocytes in vitro

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Abstract
The meiotic competence and meiosis resumption of Blue fox (Alopexlagopus) oocytes from anoestrous animals were followed. Oocytem in diameter did not achieve germinal vesicle breakdown (GFBD) by 72 h of culture, which indicates their meiotic incompetence. Oocytes larger than 100 µm in diameter underwent GVBD after 48 h of culture (27%) and reached metaphase II (MII) after 72 and 96 h (20% and 27%) in control medium. Both bST and OA accelerated resumption of meiosis (bST: 55% GVBD and 42% MII after 48 h; OA: 66% GVBD after 18 h). In contrast, FSH significantly reduced meiosis resumption (only 3% GVBD and MII after 72 h) and induced changes in the shape of cumulus granulosa (CG) cells and F-actin assembly typical for cumulus expansion. However, the innermost layers of CG cells (corona radiata) remained connected with the oocyte via gap junctions until the end of culture. Cumuli of oocytes cultured in control, bST-supplemented or OA-supplemented medium did not expand (changes in cell shape and F-actin redistribution did not occur). Moreover, especially in media with bST and OA an increased detachment and rapid disconnection of their gap junctions with the oocyte were observed. These results suggest that under invitro conditions FSH stimulates expansion of the CG cells and the attached membrana granulosa cells but in contrast it secures heterologous gap junctions between cytoplasmic processes of the corona radiata cells and oolemma during 3 days of culture. Thus, in agreement with the invivo situation in which Canidae oocytes are ovulated in the GV stage, the cumulus, mainly corona radiata cells, controls resumption of meiosis in Blue fox oocytes under invitro conditions also.

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