M6(5) HPG Axis, Testosterone and Sperm Production Levels

HPG axis

The HPG axis regulates the production of male gametes and sex hormones

HPG axis steps

1. Hypothalamus releases GnRH which acts on the anterior pituitary (via hypophyseal portal veins)

2. Anterior pituitary gonadotropic cells releases FSH and LH

3. FSH causes sustenocytes to release ABP

4. ABP keeps local concentration of testosterone high, facilitating spermatogenesis

5. LH stimulates interstitial endocrine cells to secrete testo

Plasma Testosterone and Sperm Production Levels versus Age

1. Prenatal

2. Early Infancy

3. Childhood

4. Puberty

5. Adult Hormone Balance

6. Adult State

Prenatal

Gonadotropin (FSH + LH) + testosterone levels high (~2/3 adult levels)

Male development depends on testosterone

• Fetal testes produce testosterone

• Drives development of:

  • male reproductive ducts

  • external genitalia

Early Infancy

Shortly after birth → brief hormone (i.e. testosterone) rise (necessary to develop reproductive organs)

• Then GnRH, LH, FSH, testosterone all drop

• These hormone levels stay very low throughout childhood

Puberty

Hypothalamus becomes less sensitive to negative feedback

• It now requires higher testosterone levels to shut off GnRH

👉 Result:

• ↑ GnRH → ↑ LH & FSH → ↑ testosterone

Adult Hormone Balance

Once puberty stabilizes (~3 years):

👉 A feedback loop is established:

• GnRH (hypothalamus) → stimulates

• LH & FSH (pituitary) → stimulate

• Testes:

  • LH → Interstitial Endocrine Cells → testosterone

  • FSH → Sertoli → sperm production

Adult State

• Hormone levels become stable

• Leads to:

  • consistent testosterone levels

  • continuous sperm production

👉 Maintained until late life (gradual decline)