Folliculogenic factors in photoregressed ovaries: Differences in mRNA expression in early compared to late follicle development

Abstract

The early stages of ovarian folliculogenesis generally progress independent of gonadotropins, whereas later stages require signaling initiated by FSH. In Siberian hamsters, cycles of folliculogenesis are mediated by changes in photoperiod which depress the hypothalamic pituitary gonadal axis. Reduced gonadotropins lead to decreases in mature follicle development and ovulation; however, early stages of folliculogenesis have not been explored in regressed ovaries. We hypothesized that intraovarian factors that contribute predominantly to later stages of folliculogenesis would react to changes in photoperiod, whereas factors contributing to earlier stages would not change. To probe if the early stages of folliculogenesis continue in the photoinhibited ovary while late stages decline, we measured the mRNA abundance of factors that interact with FSH signaling (Fshr, Igf1, Cox2) and factors that can function independently of FSH (c-Kit, Kitl, Foxo3, Figla, Nobox, Sohlh1, Lhx8). While plasma FSH, antral follicles, and corpora lutea numbers declined with exposure to inhibitory photoperiod, the numbers of primordial, primary, and secondary follicles did not change. Expression of factors that interact with FSH signaling changed with changes in photoperiod; however, expression of factors that do not interact with FSH were not significantly altered. These results suggest that the photoinhibited ovary is not completely quiescent, as factors important for follicle selection and early follicle growth are still expressed in regressed ovaries. Instead, the lack of gonadotropin support that characterizes the non-breeding season appears to inhibit only final stages of folliculogenesis in Siberian hamsters.

Keywords: Follicle stimulating hormone; Folliculogenesis; Ovary; Seasonal reproduction.

Figure 1

A) Mean plasma FSH concentrations (mIU/ml) B) mean plasma 17-β estradiol concentrations (pg/ml) C) mean paired ovarian mass (mg) D) mean paired uterine mass (mg) from females exposed to long photoperiods (LD), short photoperiods (SD), and 2 weeks, 4 weeks, and 8 weeks post transfer from SD to LD photoperiod (PT2, PT4, PT8). Data are presented as mean ± SEM, columns with different letters indicate significant differences between groups (p < 0.05).

Figure 2

A) Representative ovarian cross sections stained with hematoxylin and eosin. A) long day control ovaries, B) short day regressed ovaries, C) post transfer from SD to LD for two, D) four, or E) eight weeks, representing recrudescing ovaries. Primordial follicles (PR); primary follicles (P), secondary follicles (S), antral follicles (A); corpus luteum (CL); hypertrophied granulosa cells (H).

Figure 3

Follicle/structure counts presented as average number per section for A) primordial B) primary C) secondary D) tertiary E) corpora lutea and F) hypertrophied granulosa cells. Counts were made across serially sectioned ovaries from females exposed to long photoperiods (LD), short photoperiods (SD), and 2 weeks, 4 weeks, and 8 weeks post transfer from SD to LD photoperiod (PT2, PT4, PT8). Data are presented as mean ± SEM, columns with different letters indicate significant differences between groups (p < 0.05).

Figure 4

Ovarian mRNA expression of genes involved with FSH signaling A) follicle stimulating hormone receptor (Fshr), B) Insulin like growth factor 1 (Igf1) and C) Cyclooxygenase-2 (Cox2) from Siberian hamsters exposed to long day (LD) stimulatory photoperiod, short day (SD) inhibitory photoperiod, and two, four, or eight weeks of long photoperiod following short day exposure (PT2, PT4, PT8). Graphical results are presented as mean ± SEM, relative to Gapdh, and columns with different letters are significantly different (p < 0.05).

Figure 5

Ovarian mRNA expression of transcription factors not dependent on FSH-signaling: A) Newborn ovary homeobox gene (Nobox), B) Spermatogenesis and oogenesis basic helix–loop–helix 1(Sohlh1), C) LIM-homeobox transcription factor 8 (Lhx8) and D) factor in the germ-line alpha (Figla) from Siberian hamsters exposed to long day (LD) stimulatory photoperiod, short day (SD) inhibitory photoperiod, and two, four, or eight weeks of long photoperiod following short day exposure (PT2, PT4, PT8). Graphical results are presented as mean ± SEM, relative to Gapdh, and columns with different letters are significantly different (p < 0.05).

Figure 6

Ovarian mRNA expression of growth factors not dependent on FSH-signaling: A) c-Kit (Kit), B) Kit ligand (Kitl), and C) Forkhead box O3 (Foxo3) from Siberian hamsters exposed to long day (LD) stimulatory photoperiod, short day (SD) inhibitory photoperiod, and two, four, or eight weeks of long photoperiod following short day exposure (PT2, PT4, PT8). Graphical results are presented as mean ± SEM, relative to Gapdh, and columns with different letters are significantly different (p < 0.05).