p27Kip1 Negatively Regulates the Activation of Murine Primordial Oocytes

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Abstract

In mice, small oocytes (primordial oocytes) are enclosed within flattened granulosa cells to form primordial follicles around birth. A small number of primordial oocytes enter the growth phase, whereas others are quiescent. The mechanism regulating this selection of primordial oocytes is not well understood. The objective of the present study was to understand the role of p27Kip1, which regulates cell cycle progression in somatic cells, in the growth initiation of primordial oocytes in neonatal mice. We studied the localization of p27Kip1 in 0-, 3-, 5-, 7- and 21-day-old mouse ovaries by immunohistochemistry. Ovaries from 3-day-old mice were treated with p27Kip1 siRNAs (small interfering RNAs), and knockdown of p27Kip1 was determined by immunohistochemistry and Western blotting. Ovaries treated with siRNAs were organ-cultured for 6 days, and oocyte growth was estimated histologically. Expression of p27Kip1 was undetectable in the primordial oocytes of newborn mice. In the 3-day-old ovaries (n=3), p27Kip1 was demonstrated in the nucleus of 36 ± 6% primordial oocytes. The percentage of p27Kip1-positive primordial oocytes increased to 72 ± 8 (n=3), 85 ± 7 (n=3) and 93 ± 5 (n=3) in the 5-, 7- and 21-day-old mouse ovaries, respectively. After knockdown of the p27Kip1 protein by siRNAs, a higher proportion of oocytes entered the growth phase in cultured ovaries than those in the control. These results suggest that p27Kip1 negatively regulates primordial oocyte growth and that knockdown of p27Kip1 leads primordial oocytes to enter the growth phase in vitro.<br>

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