Superovulatory response of one ovary is related to the micro- and macroscopic population of follicles in the contralateral ovary of the cow

Abstract

We hypothesized that the ovulatory response of one ovary to FSH would be related positively to the size of the primordial and growing pools of follicles in the other ovary. Nonlactating cows (n = 26) were unilaterally ovariectomized and 2 days later were superovulated. The superovulatory response was classified as low (< 5 corpora lutea [CL]), Medium (5-14 CL), or High (> 14 CL). Surface follicles on the ovary removed before superovulation were classified as small (1-3 mm), medium (3-7 mm), or large (> 7 mm), and the ovary was then fixed and serially sectioned. Follicles ≤ 1 mm in diameter in 388 ± 38 fields (2 x 2 mm) per cow were classified as primordial, primary, secondary, or tertiary. By classification, suboptimal ovaries contained < 100 follicles ≤ 1 mm and optimal ovaries contained > 250 follicles ≤ 1 mm. Number of CL was correlated positively with total number of primordial, tertiary, and medium surface follicles. Number of empty fields (2 x 2-mm fields containing no follicles) was correlated negatively with superovulatory response and number of primordial follicles. Number of CL was related to number of tertiary follicles in a positive linear manner and the number of medium follicles in a positive quadratic manner (r2 = 0.66). Numbers of primordial, tertiary, small surface follicles, medium surface follicles, and total surface follicles were lower (p ≤ 0.06) in the Low superovulatory response group than in the Medium or High group. Suboptimal ovaries had fewer small surface follicles and fewer CL than Optimal ovaries (p < 0.05). We conclude that superovulatory response in cattle is related positively to the pools of primordial and growing follicles in the bovine ovary.