Male Reproductive System pt. 2

Bulbourethral Glands

Final addition to semen is made by two bulbourethral glands that release a thick salty fluid that lubricates the end of the urethra and vagina, and helps to clean urine residues from penile urethra.
This fluid is released after a male becomes sexually aroused, and shortly before the release of semen.

Leydig cells

Interstitial cells located between seminiferous tubules in the testes.

Produce 6 to 7 mg of testosterone a day.

Testosterone levels until puberty

In male embryos, testosterone is secreted by Leydig cells in the seventh week of development, with peak concentrations in the second trimester. The early release of testosterone leads to anatomical differentiation of male sexual organs.

In childhood, testosterone levels are low.
They increase during puberty.

Function of testosterone

Testicular steroidogenesis results in testosterone concentrations that are 100 times higher in testes than in circulation.
Maintaining normal levels promotes spermatogenesis, low level can lead to infertility.
Systemic circulation of testosterone plays a role in muscle development, bone growth, development of secondary sexual characteristics, and maintaining libido in males and females.

Testosterone in women

Ovaries and adrenal glands secrete small amounts of testosterone.
Most is converted into estradiol.

Control of testosterone (not blood)

gonadotropin-releasing hormone (GnRH) from hypothalamus stimulates endocrine release of hormones from pituitary gland.
Binding of GnRH to receptors on anterior pituitary gland stimulates release of LH and FSH.
FSH binds to Sertoli cells within seminiferous tubules to promote spermatogenesis.
FSH stimulates Sertoli cells to produce hormones called inhibins, which function to inhibit FSH release from pituitary, thus reducing testosterone secretion.
LH binds to receptors on Leydig cells in testes and upregulates production of testosterone.

Control of testosterone (blood)

Low blood concentrations of testosterone stimulate hypothalamic release of GnRH.
GnRH stimulates anterior pituitary to secrete LH into bloodstream.
In testis, LH binds to LH receptors on Leydig cells and stimulates release of testosterone.
When concentrations of testosterone in blood reach a threshold, testosterone itself will bind to androgen receptors on hypothalamus and anterior pituitary, inhibiting synthesis and secretion of GnRH and LH.
When blood concentrations of testosterone decline again, testosterone no longer interacts with the receptors and GnRH and LH are once again secreted.