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Inside the Follicle: Granulosa and Theca

An egg never works alone. It sits inside a follicle wrapped by two cell layers that pass a molecule back and forth to make estrogen. Meet the elegant two-cell, two-gonadotropin system.

The follicle is a tiny factory

An egg in the ovary is never bare. It lives inside an ovarian follicle — the egg at the center, surrounded by support cells, the whole thing sitting in fluid. The follicle is the ovary's working unit: it nurtures the egg, makes the hormones, and (if all goes well) eventually releases the egg. Two cell layers do the hormone work, and learning their division of labor unlocks the rest of this track.

The inner ring is the granulosa cells, hugging the egg. The outer ring is the theca cells, forming the follicle's wall. Each layer listens to a different pituitary hormone, and each does half of a single chemical reaction — which is why neither can make estrogen alone.

The two-cell, two-gonadotropin model

Here is the central idea. The theca cells respond to LH from the pituitary and turn cholesterol into androgens. But theca cells lack aromatase, so they can go no further. The androgens diffuse inward to the granulosa cells, which respond to FSH. FSH switches on aromatase in the granulosa cells, and aromatase converts those androgens into estradiol. Two cells, two hormones, one product. Remember the slogan from guide 1 — estrogen is made from androgens — and this picture clicks into place.

THE TWO-CELL, TWO-GONADOTROPIN MODEL

  Pituitary                       Pituitary
     |                               |
    LH                              FSH
     v                               v
  THECA CELL  ---androgens-->  GRANULOSA CELL
  (outer)                       (inner)
  cholesterol                   aromatase ON
     |                             |
     v                             v
  androgens  - - - diffuse - - >  ESTRADIOL

  Theca makes the raw material (androgens).
  Granulosa finishes the job (-> estradiol).
  Neither can do it alone.
LH drives theca androgen production; FSH drives granulosa aromatase, which finishes estradiol.

Granulosa cells do more than aromatase

Granulosa cells are also the ovary's reporters. As a follicle grows, its granulosa cells secrete two messenger proteins that talk back to the brain: inhibin, which selectively dials down FSH (more on this in guide 4), and anti-Müllerian hormone (AMH), made by small early follicles. Because AMH reflects how many young follicles remain, clinicians use a blood AMH level as a rough gauge of a woman's remaining egg supply — her ovarian reserve.