Evaluation of numbers of microscopic and macroscopic follicles in cattle selected for twinning

We hypothesized that the number of microscopic follicles present in the ova ries of cattle selected for twin births (Twinner) would be greater than in the ovaries of contemporary Controls. Ovaries were collected from seven Con trol and seven Twinner cows at slaughter. The number of Small (1 to 3.9 mm) , Medium (4 to 7.9), and Large (> 8 mm) surface follicles was counted and o ne ovary was fixed for histological evaluation. Fifty to sixty consecutive 6-mu m slices were taken from a piece of cortical tissue, approximately 1 c m x 1 cm in area, located between the surface follicles. Microscopic follic les were classified as primordial (oocyte surrounded by a single layer of s quamous pregranulosa cells), primary (oocyte surrounded by a single layer o f one or more cuboidal granulosa cells), secondary (oocyte surrounded by tw o or more layers of granulosa cells), or tertiary (oocyte surrounded by mul tiple layers of granulosa cells with initiation of antrum formation to II m m in diameter). The total number of follicles was counted in 200 fields (2 mm x 2 mm) per ovary. A field containing no follicles was classified as emp ty. There were significantly more secondary follicles in Twinner compared w ith Control ovaries (12.9 vs 6.3; P < .05). Twinners also tended to have mo re small surface follicles (35.4 vs 49.0; P < 0.1). We conclude that ovarie s of Control and Twinner cows do not differ in the number of primordial fol licles or in the number of follicles activated into the growing pool; howev er, Twinner cows are able to maintain more growing follicles at the seconda ry and subsequent stages of development.