Ovarian morphology and follicular systems in yaks of different ages

Forty-five female yaks of different ages with known reproductive histories were used to determine the morphology and follicular systems of their ovaries. The histological structure of the ovaries in each age group was found to be similar and resembled that of cattle and buffalo. The atresia of primordial follicles took one form (oocyte atresia); that of growing follicles could be divided into two stages (early and late); and that of Graafian follicular were classified as early, definite and late stage. The details of the several types of atresia are described herein. The average total numbers of primordial follicles per ovary pair in 1-month-old calves, 1-year-old heifers, 2-year-old heifers and 7-10-year-old cows were 53,500 +/- 6300, 32,870 +/- 4500, 22,850 +/- 2800 and 9500 +/- 1200, respectively; those of growing follicles were 210 +/- 76, 815 +/- 95, 895 +/- 142 and 445 +/- 88, respectively; and those of Graafian follicles were 36.5 +/- 14.2, 41.7 +/- 12.3, 37.8 +/- 9.8 and 42.5 +/- 14.5, respectively. The percentage of atretic primordial follicles per ovary pair in 1-month-old calves, 1-year-old heifers, 2-year-old heifers and 7 to 10 year-old cows were 51.6, 55.5, 56.7 and 47.4%, respectively. The average total numbers of atretic growing follicles per ovary pair were 119.5 +/- 21.5, 605.5 +/- 74.3, 721.6 +/- 78.5 and 275.8 +/- 66.3, respectively; those of atretic Graafian follicles were 22.1 +/- 5.6, 21.2 +/- 7.6, 21.5 +/- 4.7 and 25.3 +/- 6.7, respectively.