Growth Hormone and Sertoli Cells

Growth hormone (GH) exists in two forms produced by alternative splicing. The most common form includes all 5 exons translated to a 191 amino acid protein. The second form is derived from a smaller mRNA that omits part of exon III, resulting in a slightly shorter protein.

GH is produced by somatotrophs, specialized cells in the anterior pituitary gland. It is released in pulsatile bursts throughout the day, with higher release occurring at night during deep sleep.

Factors influencing GH release include nutritional status, metabolic conditions, age-related sex steroids, adrenal glucocorticoids, thyroid hormones, renal function, and hepatic function.

The blood-testis barrier is formed by sustentacular cells in the testes. It prevents immune system cells from detecting newly formed antigens on sperm cells, protecting them from attack.

Sustentacular cells, also called nurse cells or Sertoli cells, provide support for sperm production. They form the blood-testis barrier, provide structural support, secrete fluid for sperm transport, nourish developing cells, produce hormones, and phagocytose damaged cells.

Leydig cells, also called interstitial cells, are located between seminiferous tubules in the testes. They produce and secrete androgens, primarily testosterone, into the surrounding interstitial fluid.

Sperm travels from the seminiferous tubules to the straight tubules (tubuli recti), then to the rete testis, and finally to the efferent ductules.

Ejaculation is the process of expelling semen from the penis. It involves coordinated contractions of muscles in the reproductive system and is controlled by the nervous system.

Male reproductive glands that contribute about 60% of the volume of semen. They produce a fluid rich in fructose and prostaglandins, which nourish and help propel sperm.

A walnut-sized gland that produces a milky, alkaline fluid that helps neutralize the acidity of the vagina and contributes about 20-30% of semen volume.

Two pea-sized glands that secrete a clear, viscous fluid that lubricates the urethra and neutralizes any residual acidic urine.

A coiled tube attached to the testis where sperm mature and are stored.

Cells in the testes that nourish and support developing sperm cells. They form the blood-testis barrier, protecting sperm from the immune system.

Cells in the testes that produce testosterone, the primary male sex hormone.

A hormone produced by the pituitary gland that stimulates sperm production in the testes. It also regulates the growth and development of Sertoli cells.

A hormone produced by the pituitary gland that stimulates testosterone production by Leydig cells.

A group of neurons in the hypothalamus that produce and release gonadotropin-releasing hormone (GnRH) in a pulsatile manner, controlling the release of FSH and LH from the pituitary gland.

A negative feedback mechanism where rising testosterone levels inhibit the production and release of GnRH and LH, ultimately reducing testosterone production.

Sex Hormone-Binding Globulin (SHBG) is a protein produced in the liver that binds to testosterone with high affinity and specificity. This binding acts as a 'gatekeeper', controlling the amount of free testosterone available for action.

While both SHBG and Albumin bind to testosterone, SHBG has a much higher affinity and specificity for testosterone. Albumin binds with lower affinity & less specifically, carrying a larger but less tightly bound amount of testosterone.

Testosterone exerts its effects through two main pathways: 1) Direct action via the androgen receptor, and 2) Conversion to other hormones, such as estradiol and DHT, through enzymatic reactions.

Testosterone is converted to Dihydrotestosterone (DHT) by the enzyme 5α-reductase. This conversion is irreversible, meaning DHT cannot be converted back to testosterone.

Aromatase is an enzyme that converts testosterone to estradiol, a type of estrogen. Aromatase is found in various tissues including Leydig cells, adipose tissue, brain, and bone.

The androgen receptor has three functional domains: 1) Transcriptional regulation, 2) DNA binding, and 3) Ligand binding. Both testosterone and DHT bind to the same androgen receptor.

Testosterone plays a crucial role in male development, including: sexual differentiation, libido, pubertal larynx growth (deeper voice), muscle growth, red blood cell production, and stimulation of spermatogenesis.

Although both testosterone and DHT activate the androgen receptor, DHT is the most potent activator. DHT has a longer half-life than testosterone, and plays a more crucial role in promoting prostate development, male external genitalia, and facial hair growth.

Genetic sex refers to the chromosomal makeup determined at fertilization, either XX (female) or XY (male). Gonadal sex refers to the development of the gonads (ovaries or testes) based on the specific genes on the Y chromosome.

The SRY gene, located on the Y chromosome, produces the testis determining factor (TDF). TDF initiates the development of testes by converting bipotential cells into Sertoli cells. Without the SRY gene, ovarian development occurs instead.

Testosterone (T) and Insulin-like peptide 3 (INSL3) are the two primary hormones that regulate the descent of the testes from the abdomen into the scrotum.

The release of gonadotropin-releasing hormone (GnRH) from the hypothalamus increases in a pulsatile manner during puberty, leading to the activation of the Hypothalamic-Pituitary-Gonadal (HPG) axis.

Testosterone is responsible for the development of secondary sexual characteristics in males, such as facial hair growth, deepening of the voice, and increased muscle mass.

Testosterone has two main functions: anabolic effects, such as increasing muscle mass and bone density, and behavioral effects, such as influencing libido.

Spermatogenesis is the continuous process of germ cell differentiation that produces spermatozoa (sperm) in the seminiferous tubules of the testes.

FSH stimulates spermatogenesis, the process of sperm production, while LH stimulates the production of testosterone by Leydig cells, which is also essential for spermatogenesis.

The three stages of spermatogenesis are spermatocytogenesis, spermiogenesis, and spermiation. Spermatocytogenesis involves cell division and growth, spermiogenesis involves maturation of spermatids into sperm, and spermiation is the release of mature sperm into the tubule lumen.

The blood-testis barrier is formed by Sertoli cells in the seminiferous tubules to protect developing sperm cells from the immune system. This barrier prevents the immune system from attacking the sperm cells, which contain antigens that are foreign to the body.

The epididymis is a coiled tube attached to the testis where sperm undergo maturation, a process that takes about two weeks. It also stores sperm until ejaculation and removes defective sperm cells.

Sperm travels from the efferent ductules to the ductus epididymis (head, body, and tail), then to the ductus deferens, ejaculatory duct, and finally through the urethra.

The ductus deferens, also known as the vas deferens, is a muscular tube that transports sperm from the epididymis to the ejaculatory duct. It can also store sperm for months, reabsorbing unused sperm.

The urethra is divided into four parts: intramural, prostatic, membranous, and spongy. It transports both urine and semen.

The penis consists of three cylindrical bodies: two corpora cavernosa and one corpus spongiosum. The corpora cavernosa fill with blood during erection, while the corpus spongiosum surrounds the urethra, ensuring its patency during ejaculation.

Seminal vesicles are accessory glands that produce a thick, yellowish fluid rich in nutrients and prostaglandins, which contributes to semen volume and helps propel sperm.

The prostate gland produces a milky fluid that neutralizes vaginal acidity, contributes to semen volume, and also helps with sperm motility.

Bulbourethral glands, also called Cowper's glands, secrete a viscous, alkaline fluid that lubricates the urethra and neutralizes any residual urine.

Semen is a whitish, sticky mixture that comprises sperm cells and fluids secreted from various accessory glands. It provides nutrients, lubrication, and a buffer to aid sperm survival and transport.

The penis receives its blood supply from the internal iliac arteries, mainly via the internal pudendal arteries, to facilitate erection and blood flow through the corpora cavernosa.