Testosterone: Regulation (Nursing)

00:01 So now let's look at the process of spermatogenesis and how it is regulated and controlled.

00:09 Production of our gametes and sex hormones is regulated by sequence of hormonal events that start at the hypothalamus in the brain and the anterior pituitary gland in the brain and go down to the testes.

00:25 We refer to this as the Hypothalamic-pituitary-gonadal axis or the HPG axis.

00:35 This is going to involve the interaction of hormones, Gonadotropin-releasing hormone, Follicle-stimulating hormone, Luteinizing hormone, Testosterone, and Inhibin.

00:52 So let's look at the sequence of regulatory events in the HPG axis.

00:59 So first, we start at the hypothalamus.

01:03 The hypothalamus is going to release the gonadotropin-releasing hormone.

01:10 Gonadotropin-releasing hormone is transported to the Anterior pituitary where it binds to anterior pituitary, gonadotropic cells.

01:21 This causes these cells to release follicle-stimulating hormone and luteinizing hormone.

01:31 Follicle-stimulating hormone is what's going to stimulate the process of spermatogenesis indirectly by stimulating the sustenocytes in the seminiferous tubule to release another protein known as androgen-binding protein or ABP.

01:51 ABP is going to keep the concentration of testosterone high near this spermatogenic cells thus promoting spermatogenesis.

02:04 The luteinizing hormone is going to bind to interstitial endocrine cells called Leydig cells prodding them to secrete testosterone.

02:15 The rising testosterone levels are what actually triggers to spermatogenesis.

02:22 Testosterone entering the blood is going to stimulate the sex organs maturation, as well as development and maintenance of secondary sex characteristics and also affect libido.

02:36 But what goes up must come down, and in order to maintain homeostasis, rising testosterone levels feedback on the hypothalamus, which then inhibits the gonadotropin-releasing hormone and the pituitary to inhibit the gonadotropin release.

02:57 Also, the Sertoli cells, or sustenocytes are going to release another hormone called inhibin.

03:04 And this is going to happen when the sperm count is really high.

03:08 This inhibin also inhibits gonadotropin-releasing hormone and follicle-stimulating hormone release.

03:16 So to recap.

03:18 The hypothalamus is going to release gonadotropin-releasing hormone to the anterior pituitary.

03:26 In the anterior pituitary gland, we're going to release two hormones: Follicle-stimulating hormone and luteinizing hormone.

03:35 Follicle-stimulating hormone is going to stimulate the sustenocytes or Sortoli cells to produce an androgen-binding protein or ABP.

03:46 ABP is going to sustain the testosterone levels and the testosterone is going to trigger spermatogenesis.

03:55 The luteinizing hormone or is going to stimulate the interstitial endocrine cells or Leydig cells.

04:02 In the interstitium surrounding the seminiferous tubules.

04:07 It stimulates these cells to secrete testosterone, also triggering spermatogenesis.

04:14 High levels of testosterone circulate back to the hypothalamus and provide negative feedback, causing the hypothalamus to stop releasing the gonadotropin-releasing hormone.

04:28 This occurs until the testosterone levels drop below a certain point, and then we start the process again.

04:36 Locally, the sustenocytes in response to high sperm numbers will trigger the release of inhibin.

04:44 Inhibin also stops the release of gonadotropin-releasing hormone as well as the release of follicle-stimulating hormone from the hypothalamus and the anterior pituitary.

04:59 The amount of testosterone and sperm produced by the testes reflects a balance among the three interacting sets of hormones.

05:09 This balance takes about three years to achieve, after which testosterone and sperm production are pretty stable for the rest of the life.

05:20 Without gonadotropin-releasing hormone and gonadotropin follicle-stimulating hormone and luteinizing hormone, the testes would atrophy, and the sperm and testosterone production would cease.

05:33 Before birth in utero, a male has testosterone levels that are 2/3 that of adult.

05:41 So they're pretty high.

05:43 And soon after birth, these levels are going to recede and they're going to remain low through childhood until puberty commences.

05:55 So if we compare the plasma testosterone levels over time, what we find is that in utero, male hormones are high as they are needed for the development of male reproductive structures during the development of the fetus.

06:12 These levels will fall at birth and remain low during early childhood.

06:18 At puberty, the testosterone levels begin to rise again.

06:22 And as the relationship between the hormones and the hypothalamus pituitary, gonadal axis or HPG axis, figure out their balance, eventually, you will begin to have mature sperm being in the semen.