Which ovarian hormone primarily inhibits FSH secretion from the anterior pituitary?

Inhibin is a peptide hormone secreted by granulosa cells in the ovary and by Sertoli cells in the testis. It selectively suppresses follicle-stimulating hormone (FSH) secretion from the anterior pituitary without significantly affecting luteinizing hormone (LH). Inhibin levels are highest in the luteal phase, helping prevent excessive follicular recruitment. Normal serum inhibin B levels in reproductive-age females are about 45–120 pg/mL during the follicular phase.

Rationale for correct answer

C. Inhibin directly inhibits FSH release by acting on the anterior pituitary gonadotrophs, providing negative feedback to regulate follicular development and prevent multiple dominant follicles in a cycle.

Rationale for incorrect answers

A. Estrogen can exert both negative and positive feedback on gonadotropin release, but its negative feedback affects both LH and FSH; selective inhibition of FSH is mainly by inhibin.

B. Progesterone primarily inhibits GnRH pulse frequency and has stronger effects on LH suppression; it is not the main selective inhibitor of FSH.

D. Relaxin is produced by the corpus luteum and placenta, mainly to soften pelvic ligaments and prepare the cervix for labor, with no significant role in FSH suppression.

Take home points

• Inhibin is the main selective suppressor of FSH.
• Granulosa cells secrete inhibin in females; Sertoli cells do so in males.
• Estrogen and progesterone provide broader gonadotropin feedback, not selective FSH inhibition.
• Inhibin levels are highest in the luteal phase.

Oogenesis is the process of haploid gamete formation in females, beginning before birth and completed only if fertilization occurs. Primary oocytes are arrested in prophase I from fetal life until ovulation. At ovulation, meiosis I completes, producing the secondary oocyte, which arrests in metaphase II until sperm penetration triggers completion of meiosis II. Normal female gametes have 23 chromosomes (n).

Rationale for correct answer

B. The secondary oocyte is arrested in metaphase II due to cytostatic factor, maintaining meiotic arrest until fertilization. Sperm entry activates calcium-dependent pathways that resume meiosis II, producing the ovum and second polar body.

Rationale for incorrect answers

A. Prophase I arrest occurs in primary oocytes from fetal life until puberty; this is before formation of the secondary oocyte.

C. Anaphase II is the stage where sister chromatids separate during completion of meiosis II; it occurs only after fertilization starts, not before.

D. Telophase I follows anaphase I when primary oocytes complete meiosis I; this produces the secondary oocyte but is not the arrest point before fertilization.