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

Abstract

We hypothesized that the number of microscopic follicles present in the ovaries of cattle selected for twin births (Twinner) would be greater than in the ovaries of contemporary Controls. Ovaries were collected from seven Control 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 one ovary was fixed for histological evaluation. Fifty to sixty consecutive 6-microm slices were taken from a piece of cortical tissue, approximately 1 cm x 1 cm in area, located between the surface follicles. Microscopic follicles were classified as primordial (oocyte surrounded by a single layer of squamous pregranulosa cells), primary (oocyte surrounded by a single layer of one or more cuboidal granulosa cells), secondary (oocyte surrounded by two or more layers of granulosa cells), or tertiary (oocyte surrounded by multiple layers of granulosa cells with initiation of antrum formation to < or = 1 mm 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 empty. There were significantly more secondary follicles in Twinner compared with Control ovaries (12.9 vs 6.3; P < .05). Twinners also tended to have more small surface follicles (35.4 vs 49.0; P < 0.1). We conclude that ovaries of Control and Twinner cows do not differ in the number of primordial follicles or in the number of follicles activated into the growing pool; however, Twinner cows are able to maintain more growing follicles at the secondary and subsequent stages of development.

Numbers of Antral Follicles During Follicular Waves in Cattle: Evidence for High Variation Among Animals, Very High Repeatability in Individuals, and an Inverse Association with Serum Follicle-Stimulating Hormone Concentrations1

“…Indirect evidence indicates that the number of follicles during follicular waves probably reflects the size of the ovarian reserve in humans [32,51] and cattle [1,3,52]. Thus, the cattle with high serum FSH concentrations and relatively low numbers of follicles during waves in the present study also may have had relatively smaller ovarian reserves.…”

“…In support of this possibility, serum FSH is higher in older women [53] and cows [54] compared to their younger counterparts. Factors that regulate variation in antral follicle numbers during waves may include not only size of the ovarian reserve, differential responsiveness of antral follicles to FSH [55], differential secretion of hormones and growth factors that alter FSH responsiveness (e.g., LH [56]), and genetic mechanisms [52,55] but also growth factors that regulate recruitment and growth of preantral follicles, such as antimullerian hormone [57][58][59], KIT ligand [60,61], INHBA [62], and growth differentiation factor 9 [60,63,64]. Moreover, our findings imply that estradiol and INHA may not be the only feedback regulators of FSH secretion during follicular waves.…”

Oocyte development in cattle: physiological and genetic aspects

“…Compared with ovaries from unselected cattle, ovaries from the twinner population have more microscopic secondary follicles (Cushman et al, 2000) and more small (≥ 5 mm), medium (5.1 to 9.9 mm) and large (≥ 1.0 mm) surface follicles (Echternkamp et al, 2004). In twinner cattle, the heritability of ovulation rate is 0.35 and there is a high genetic correlation (0.75) between ovulation rate and twinning rate (Gregory et al, 1997); therefore, one can theoretically use routine ultrasound evaluation to select females that produce more follicles, more oocytes and more embryos in assisted reproduction programs (Figure 5;De Roover et al, 2005).…”

Increased abundance of aromatase and follicle stimulating hormone receptor mRNA and decreased insulin-like growth factor-2 receptor mRNA in small ovarian follicles of cattle selected for twin births1,2

“…Long-term genetic selection of cattle (Twinner) for the production of twin ovulations and fraternal twin births, using an index of ovulation and twinning rate (Echternkamp et al, 1990a;Gregory et al, 1990;Van Vleck et al, 1991), has enhanced ovarian follicular development and increased the incidence of twin births. Ovaries of Twinner females had a 2-fold greater density of secondary preantral follicles (Cushman et al, 2000), 50% more small (≤5 mm) and medium (6 to 12 mm) antral follicles (Echternkamp et al, 2004), and >70% frequency of twin or multiple ovulations (Echternkamp et al, 2007(Echternkamp et al, , 2009) compared with females not selected for twins (Control). Both IGF-1 and -2 are important promoters of FSH-mediated development of antral follicles, selection of pre-ovulatory follicles, and corpora lutea (CL) formation (Spicer and Echternkamp, 1995).…”