Changes in Messenger Ribonucleic Acid Encoding Luteinizing Hormone Receptor, Cytochrome P450-Side Chain Cleavage, and Aromatase are Associated with Recruitment and Selection of Bovine Ovarian Follicles1

Abstract

The objective of this study was to investigate changes in expression of mRNAs encoding FSH receptor (FSHr), LH receptor (LHr), cytochrome P450 side-chain cleavage (P450(scc)), cytochrome P450 17alpha-hydroxylase (P450(c17)), and cytochrome P450 aromatase (P450(arom)) during recruitment and selection of bovine ovarian follicles. Dairy heifers (4-5 per group) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, or 96 h after initiation of the first follicular wave following estrus as determined by ultrasonography (Time 0 = initiation of follicular wave; mean +/- SEM = 42.0 +/- 2.6 h after estrus). Expression of mRNAs encoding FSHr, LHr, P450(scc), P450(c17), and P450(arom) was detected by in situ hybridization and quantified by image analysis. Antral follicles were classified as healthy or atretic. Healthy follicles expressed higher (p < 0.01) amounts of mRNAs for gonadotropin receptors and steroidogenic enzymes than did atretic follicles, and expression of LHr, FSHr, P450(scc), P450(c17), and P450(arom) increased (p < 0.01) with follicular size and stage of the follicular wave. Expression of mRNAs for P450(scc), P450(arom), and LHr was time- and size-dependent during recruitment and selection. During recruitment, expression of mRNAs for P450(scc) and P450(arom) was first detected in granulosa cells of 16 of 21 of the follicles 4-6 mm in diameter at 12 h. At 24 and 36 h, almost all follicles 6-9 mm in diameter, but not those 4-5 mm in diameter, expressed both P450(scc) and P450(arom) mRNA in the granulosa cells. At 48 h and thereafter, P450(scc) and P450(arom) mRNA were expressed predominantly in one healthy large follicle per cow with a few exceptions. Expression of LHr mRNA was first detected in granulosa cells at 36 h and was always found in granulosa cells of one follicle > or = 8 mm per cow with exception of one cow at 36 h (no expression) and another two cows, one each at 36 and at 84 h (expression in 2 follicles). In addition, LHr mRNA expression in the granulosa cell layer was limited to follicles that also expressed mRNAs for P450(scc) and P450(arom) in the granulosa cells. In summary, follicular recruitment in cattle was associated with expression of P450(scc) and P450(arom) mRNA within granulosa cells, and the process of follicular selection was associated with initiation of LHr mRNA expression in granulosa cells.

Appearance and number of follicles and change in the concentration of serum FSH in female bovine fetuses

“…Bao et al (1997) reported that FSH receptors are present on granulosa cells in some primary follicles in cows; therefore, primary follicles may be able of respond to a low concentration of FSH. The appearance of primary follicles at day 91 was not associated with an increase in oestradiol (in the ovary or the serum content).…”

“…The appearance of secondary follicles at day 120 was associated with an increase in serum FSH. Bao et al (1997) reported the presence of FSH receptors on the granulosa cells in secondary and antral follicles. In the present study, the increase in the concentration of FSH at day 120 coincided with the development of primary follicles to secondary follicles.…”

“…It has been reported that FSH affects the development of secondary follicles to dominant follicles in the ovaries of adult mammals (Erickson and Danforth, 1995). FSH stimulates the activation of aromatase and accelerates the production of oestrogen (Bao et al, 1997). The proliferation of granulosa cells in the follicle is enhanced by an autocrine action of oestrogen, which is stimulated by FSH (Peters and McNatty, 1980).…”

Parturition to resumption of ovarian cyclicity: comparative aspects of beef and dairy cows

“…Follicle selection occurs in the face of declining FSH concentrations and is the process by which the growing cohort of follicles is reduced to the ovulatory quota for the species (in cattle it is generally one; Sunderland et al, 1994). The selected follicle survives in an environment of reduced FSH owing to the development of LH receptors in the granulosa cells (Xu et al, 1995;Bao et al 1997) and increased intrafollicular bioavailable IGF-I (Austin et al, 2001;Canty et al, 2006). The increased bioavailable IGF-I is achieved by reduced IGF-I-binding proteins (IGFBP) because of increased IGFBP protease activity.…”

Follicular dynamic and ovulation in cattle – a review

“…In heifers, receptors for LH appeared in the granulosa cells of the future dominant follicle about 8 h before the beginning of deviation. In addition, LH-receptor mRNA expression in the granulosa cell layer was limited to follicles that also expressed mRNAs for P450scc and P450arom in the granulosa cells (BAO et al, 1997). The LH stimulates the production of estradiol and insulin-like growth factor-1.…”