FSH and LH Transport to the Ovaries: Hypothalamic-Pituitary-Gonadal Axis Dynamics

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are gonadotropins synthesized in the anterior pituitary and delivered to the ovaries via the systemic circulation. Their release is pulsatile, governed by the hypothalamic-pituitary-gonadal (HPG) axis.

Stepwise Pathway

1. Hypothalamic pulse generator: GnRH neurons in the arcuate/preoptic area release GnRH into the hypophyseal portal circulation every 60–120 min (follicular) to 240 min (luteal).
2. Pituitary gonadotrophs: GnRH binds Gq-coupled receptors → IP₃/DAG → Ca²⁺/PKC → FSH/LH synthesis and exocytosis. Pulse frequency/amplitude differentially regulates FSH vs LH (low freq favors FSH; high freq favors LH).
3. Systemic transport: FSH (glycoprotein, ~30 kDa, half-life 3–4 hrs) and LH (~28 kDa, half-life 20–60 min) enter general circulation via pituitary venous drainage.
4. Ovarian delivery: Ovarian artery (branch of aorta) → capillary network in theca and granulosa cells. High vascular permeability ensures hormone access.
5. Receptor binding:
   • FSH receptor (FSHR) on granulosa cells → cAMP/PKA → follicular growth, aromatase (estradiol), inhibin B.
   • LH receptor (LHR) on theca cells → androstenedione → substrate for granulosa aromatase; on granulosa (late follicular) → luteinization, progesterone, ovulation.
6. Feedback: Estradiol (negative then positive feedback), inhibin B (selective FSH suppression), progesterone (LH suppression).

Clinical Relevance

• GnRH agonists/antagonists (IVF): Suppress or control pituitary surge.
• PCOS: Elevated LH:FSH ratio (>2:1) drives theca androgen excess.
• Hypogonadotropic hypogonadism: Low GnRH → low FSH/LH → anovulation.
• Menopause: Loss of negative feedback → elevated FSH > LH.

The HPG axis exemplifies a tightly regulated endocrine cascade where pulsatile gonadotropin delivery orchestrates follicular selection, ovulation, and steroidogenesis.