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Effects of follicle size and electrolytes and glucose in maturation medium on nuclear maturation and developmental competence of bovine oocytes

Kobe Minori Public Corporation, Kobe 651-2204, Japan, ¹ YS New Technology Institute Inc., Tochigi 329-0512, Japan, ² National Institute of Livestock and Grassland Science, Tochigi 329-2793, Japan, ³ Prefectural Federation of Agricultural Mutual Aid Association, Hyogo 651-2272, Japan and ⁴ Graduate School of Science and Technology, Kobe 657-8501, Japan

Correspondence should be addressed to H Iwata, West Agricultural Office of Kobe City (Seishin-Bunka-Center), 1058 Jyunna Ikawadani-Chou Nishi-ku, Kobe City, 651-2124 Japan; E-mail: QYM04636{at}nifty.com

The concentrations of electrolytes (Na, K, Cl, Mg and Ca) and glucose in small follicle (SF) follicular fluid (SFF) and large follicle (LF) follicular fluid (LFF) from slaughterhouse-derived ovaries were studied. Oocytes were matured in medium based on synthetic oviductal fluid. The effects of various concentrations of electrolytes (Na, K, Ca and Mg) and glucose in the maturation medium on the progression of nuclear maturation and subsequent development were also studied. K in SFF was significantly greater than that in LFF. The Mg concentration in follicular fluid (FF) is 2.0–2.3 mM, which is greater than the concentration present in medium generally used for culture. The glucose concentration in FF is about 3.5–3.9 mM and rapidly decreases during the preservation of ovaries. LF oocytes resumed nuclear maturation and progressed to the M2 stage significantly faster than those collected from SF oocytes. In addition, more LF oocytes developed to blastocysts than did SF oocytes. Changing the Na/K ratio in the maturation medium from 16 to 24 did not affect either the progression of nuclear maturation or the rate of development. A low concentration of Mg (0.5 mM) combined with a low Ca concentration (0.5 mM) inhibited the rate of development, but did not affect the progression of nuclear maturation. On the other hand, increasing the Mg concentration to 2.0 mM from 0.5 mM hastened the progression of nuclear maturation and improved the rate of blastulation, irrespective of the Ca concentration. The progression of nuclear maturation was faster and the rate of development was greater with 5.56 mM glucose than with 1.5 mM glucose. The difference in time needed to progress to M2 among the experiment was about 2–3 h. Therefore, prolonging the maturation periods from 21 to 24 h did not change the rate of development. Our results show that the concentrations of Mg and glucose in the maturation medium and the follicle size enveloping the oocyte affect the progression of nuclear maturation and subsequent development. The time requirement for oocytes to reach M2 is strongly related to the developmental competence of the oocytes.

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