(27.8) Regulation of Male Reproductive System

Define Hypothalamic-Pituitary-Gonagal Axis

Refers to the relationship between the structures that regulate the production of gametes and sex hormones

Discuss Hormonal Regulation of Testicular Function.

• Hypothalamus releases gonadotropin-releasing hormone (GnRH), which controls the release of the anterior pituitary hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in males

• FSH indirectly stimulates spermatogenesis by stimulating the sustentocytes to release androgen-binding protein, which keeps testosterone in the vicinity of the spermatogenic cells high

• LH stimulates the interstitial endocrine cells to produce testosterone

• Locally, testosterone entering blood acts as a final trigger for:

  • Spermatogenesis

  • Stimulates sex organ maturation

  • Development/maintenance of secondary sex characterisc

  • Libido

• Rising levels of testosterone inhibit hypothalamic release of GnRH and act directly on the anterior pituitary gland to inhibit gonadotropin release

• Inhibin is produced by the sustentocytes and released when sperm count is high, inhibits GnRH and FSH release

Relationship between Testosterone and Sperm Production

Amount of testosterone and sperm produced by testes reflects balance among three interacting sets of hormones

1. Balance takes 3 years to achieve, after which testosterone and serpm production are fairly table throughout life

2. Without GnRH and gonadotopins, testes atrophy, and sperm and testosterone production ceases

T/F: Testosterone remain same before and after birth

• Before birth → male infant has testosterone levels 2/3 of adult

• After birth → levels recede and remain low through childhood until puberty

Physiological effects of Testosterone on male reproductive anatomy.

Testosterone is synthesized from cholesterol and exerts its effects by activating specific genes, causing specific proteins to be synthesized

1. In some cells, testosterone must be converted to another hormone: dihydrotestosterone in the prostate or estradiol in some neurons of the brain

2. Targets accessory organs, initiating spermatogenesis, and acts on ducts, glands, and the penis, causing them to grow and assume adult size and function

3. Induces male secondary sex characteristics

   • Appearance of pubic, axillary, and facial hair

   • Deepening of the voice

   • Thickening of the skin and increase in oil production

   • Increase in bone and skeletal muscle size and mass

4. Increases basal metabolic rate

5. Masculinizes the brain

6. Basis of sex drive (libido)

7. Continues to exert effect into adulthood

T/F: Adrenal glands in males also produce other androgens

→ TRUE

• Although adrenal glands also produce androgens in small amounts, production is insufficient to maintain normal testosterone-mediated functions

Effect and Treatment of Testosterone Deficiency

• EFFECT

  • Leads to atrophy of accessory organs

  • Semen volume declines

  • Erection/ejaculation are impaired

• TREATMENT

  • Testosterone replacement

Which of the following reduces circulating blood levels of follicle-stimulating hormone (FSH) when the sperm count is high?

→ inhibin

• Sustentocytes respond to elevated sperm counts by releasing inhibin, a hormone that suppresses release of FSH

What is the testicular target for follicle-stimulating hormone (FSH)?

→ Sustentocytes (Sertoli cells)

• Sustentocytes in the walls of the seminiferous tubules respond to FSH by releasing androgen-binding protein, which maintains high testosterone levels near developing sperm cells

Which hormone promotes the formation of male secondary sex characteristics such as the appearance of pubic, axillary, and facial hair; enhanced hair growth on the chest; and a deepening voice?

→ Testosterone