The main source of circulating estrogen levels is the thecal and granulosa cells of the ovarian follicles, working together via the two-cell, two-gonadotropin model.​

Option Analysis

Option (A) thecal and granulosa cells: Correct. Theca cells produce androgens (like androstenedione) under LH stimulation, which diffuse to granulosa cells. Granulosa cells, induced by FSH, express aromatase to convert these androgens into estradiol, the primary circulating estrogen in premenopausal females.​

Option (B) gonadotrophs: Incorrect. Gonadotrophs in the anterior pituitary secrete FSH and LH, which regulate ovarian estrogen production but do not synthesize estrogen themselves.​

Option (C) endometrial epithelia: Incorrect. Endometrial cells respond to estrogen (proliferating during the menstrual cycle) but do not produce significant circulating estrogen; they mainly express receptors for local effects.​

Option (D) Leydig cells: Incorrect. Leydig cells in testes produce testosterone (and minor local estrogen via aromatase), contributing negligibly to female circulating levels.​

Introduction to Circulatory Levels of Estrogen

Circulatory levels of estrogen, primarily estradiol, regulate female reproductive cycles, bone health, and more in premenopausal women. Ovaries produce over 90% of circulating estrogen via specialized cells, essential knowledge for CSIR NET Life Sciences.​

The Two-Cell, Two-Gonadotropin Model

Theca interna cells respond to LH by synthesizing androgens from cholesterol. These androgens cross the basal lamina to granulosa cells, where FSH induces aromatase (CYP19) for conversion to estradiol, entering circulation. This synergy maximizes estrogen output during follicular development.​

• LH binds theca G-protein receptors, raising cAMP for androgen production.​

• FSH upregulates granulosa aromatase, preventing androgen buildup.​

• Post-ovulation, corpus luteum sustains lower levels.​

Why Not Other Sources?

Gonadotrophs release FSH/LH but lack steroidogenic enzymes. Endometrial epithelia proliferate under estrogen influence without exporting it systemically. Leydig cells focus on male androgens, with minimal female relevance. Postmenopause, adipose tissue contributes via aromatase, but ovaries dominate reproductively active phases.​

Exam Relevance for CSIR NET

This MCQ tests ovarian endocrinology basics. Option (A) aligns with standard texts; distractors confuse local vs. circulating roles.​