Two6 64 Male Reproductive Physiology testosterone DHT

• Sperm transport from epididymis - 2-12 days 
• Sperm stored in cauda epididymis 
• Sperm maturation in epididymis. Epididymis function is regulated by testesterone and DTH, influenced by temperature

• More appropriately termed the “blood–seminiferous tubule barrier,” it has two components:
• - an anatomic or mechanical element and
• - functional elements.

The mechanical barrier is created, in part, by muscle-like myoid cells that surround seminiferous tubules. Regulation of molecular traffic also occurs at the level of capillary endothelial cells. However, the most important component of this barrier is the synaptic tight junctions between Sertoli cells that preclude the passage of large molecules and lymphocytes. These anatomic elements are necessary but not sufficient for maintaining the immunologic “sanctuary” status within the tubule, because they are not observed in other protected areas of the reproductive tract

Functional elements - to suppress the normal immune response. Several mechanisms likely work in concert to protect sperm from destruction. First, the types and number of lymphocytes are restricted in anatomically vulnerable regions in the germinal epithelium. There is also evidence to suggest that immunologic tolerance plays a role in the functional blood-testis barrier. The leading theory proposes that within the anatomically weaker areas (rete testis, efferent tubule, epididymis) of the barrier, there is a small, continuous leak of sperm antigens. This leak generates T-suppressor cells and immune tolerance, similar to desensitization protocols for common environmental allergens. However, with larger antigenic challenges, a true immune response results. Contributing to this tolerance, it is now clear that Sertoli cells express various mediators that act locally to generate an immunosuppressive environment within the testis.

The value of a blood-testis barrier is fully realized after puberty, because foreign “antigens” on postmeiotic germ cells exist only after spermarche. A testicular insult such as biopsy, torsion, or trauma will not induce antisperm antibodies if it occurs before puberty. After puberty, however, immunologic infertility is a known risk. Clinically, the blood-testis barrier may also limit chemotherapy access to cancer cells sequestered behind it and result in isolated cancer recurrence within the testis.

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• 1 basal lamina, 2 spermatogonia, 3 spermatocyte 1st order, 4 spermatocyte 2nd order, 5 spermatid, 6 mature spermatid, 7 Sertoli cell, 8 tight junction (blood testis barrier)

• Spermatogenesis involves
• (1) a proliferative phase as spermatogonia divide to replace their number (self-renewal) or differentiate into daughter cells that become mature gametes;
• (2) a meiotic phase when germ cells undergo a reduction division, resulting in haploid (half the normal DNA complement) spermatids; and
• (3) a spermiogenesis phase in which spermatids undergo a profound metamorphosis to become mature spermatozoa.

Spermatogenesis is a remarkably complex and specialized process of DNA reduction and germ cell metamorphosis. Requires approximately 64 days


Testis Stem Cell Migration - During early prenatal development, primordial germ cells migrate to the gonadal ridge and associate with Sertoli cells to form primitive testicular cords. These primitive germline stem cells are termed gonocytes after the gonad differentiates into a testis by forming seminiferous cords. They are called spermatogonia after migration to the periphery of the tubule. These early migrating germ cells have properties similar to embryonic stem cells and are likely the source of adult germ cell tumors. The failure of germ cells to migrate into the primitive testicle is also thought to be a cause of extragonadal germ cells tumors and adult infertility resulting from azoospermia with Sertoli cell–only testicular histology.

Testis Stem Cell Renewal. Spermatogonia within the testis stem cell niche are replenished in a process termed stem cell renewal.

Testis Stem Cell Proliferation - In the human, pale type A (Ap) spermatogonia in the basal stem cell niche of the seminiferous tubule divide at 16-day intervals to form B spermatogonia. B spermatogonia are committed to become spermatocytes.

Spermatogenesis begins with type B spermatogonia dividing mitotically to form primary spermatocytes within the adluminal compartment. Primary spermatocytes are the first germ cells to undergo meiosis. In this process, a meiotic division is followed by a typical mitotic reduction division, resulting in daughter cells with a haploid chromosome complement.

Spermiogenesis - During spermiogenesis, round Sa spermatids mature into spermatozoa. During this maturation sequence, cell division does not occur, but there are extensive changes to the spermatid nucleus and cytoplasm. These include the loss of cytoplasm, migration of cytoplasmic organelles, formation of the acrosome from the Golgi apparatus, formation of the flagellum from the centriole, nuclear compaction to about 10% of former size, and reorganization of mitochondria around the sperm midpiece.

Type Ad spermatogonia ("dark") - The population of spermatogonia is maintained by type Ad spermatogonia. These cells do not directly participate in producing sperm, instead serving to maintain the supply of stem cells for spermatogenesis. Each type Ad spermatogonium divides to produce another type Ad spermatogonium, which can further carry on spermatogenesis, and one type Ap spermatogonium, which differentiates further.

Type Ap spermatogonia ("pale") - repeatedly divide mitotically to produce identical cell clones linked by cytoplasmic bridges. The connections between cells allow development to be synchronised. When repeated division ceases, the cells differentiate into type B spermatogonia. This stage is referred to as the spermatogonial phase.

Sperm transport through the human epididymis has been calculated to take from 2 to 12 days.

Sperm transit time through the caput-corpus epididymis is roughly similar to the transit time through the cauda epididymis and is more likely related to daily testicular sperm production rather than a man's age or the frequency of ejaculation.

Because normal human testicular sperm are immotile as they enter the epididymis and remain relatively immotile within the caput, mechanisms other than sperm motility must exist to transport sperm through the epididymis. Initially, sperm are carried into the ductuli efferentes by rete testis fluid, and fluid flow is facilitated by fluid resorption by ductal epithelial cells mediated by the estrogen receptor. Motile cilia and myoid cell contractions within the ductuli efferentes also assist with sperm movement. Within the epididymis proper, the principal mechanism responsible for sperm transport is likely the spontaneous, rhythmic contraction of the contractile cells surrounding the epididymal duct.

After migrating through the caput and corpus epididymis, sperm are retained in the cauda epididymis for varying lengths of time, depending on the frequency of sexual activity.

Of total sperm in epididymis, approximately half are stored in the caudal region.

Spermatozoa stored in the cauda epididymis, unlike testicular sperm, are capable of progressive motility and are able to fertilize eggs.

Sperm can remain viable for several weeks after vas deferens ligation. However, it is also clear that sperm fertility measured in vivo diminishes when sperm are maintained in the epididymis for prolonged periods of time.

• Fate of unejaculated epididymal sperm
• - lost through spontaneous seminal discharge
• - lost in urine
• - epididymal reabsorption
• - Phagocytosis of spermatozoa by macrophages within the epididymal lumen after ligation of the vas deferens

• Sperm Motility
• - Sperm gain the capacity for motility with migration through the epididymis. This is observed as a change in the pattern of motility and as an increase in the proportion of sperm exhibiting “mature” motility patterns.
• - Spermatozoa are able to develop motility based on contact time with the proximal epididymal epithelium.

• Sperm fertility maturation - 
• - Testicular sperm are incapable of fertilizing eggs unless injected into them with micromanipulation.
• - Ability of sperm to fertilize eggs is acquired gradually as the sperm pass through the epididymis.
• - Sperm from the proximal epididymis are able to bind to zona-free eggs, only sperm from the cauda epididymis can actually penetrate eggs. Thus sperm fertility maturation is, for the most part, achieved at the level of the late corpus or early cauda epididymis.

• Sperm Biochemical Changes
• - Epididymal sperm transit induces a net negative surface membrane charge, and sperm membrane sulfhydryl
• groups oxidize to disulfide bonds, improving sperm structural rigidity necessary for progressive motility and egg penetration.
• - Other post-testicular modifications of sperm membranes
• include changes in sperm lectin-binding properties, phospholipid and lipid content, glycoprotein composition, immunoreactivity, and iodination characteristics. Overall, these membrane modifications during epididymal passage may enhance sperm adherence to the egg zona pellucida.

Human vas deferens exhibits spontaneous motility. It also has the capacity to respond when stretched. Finally, fluid within the vas deferens can be propelled into the urethra by strong peristaltic contractions elicited either by electrical stimulation of the hypogastric nerve or by adrenergic neurotransmitters.  This suggests that immediately before emission, with sympathetic stimulation, sperm is rapidly transported from the distal epididymis through the vas deferens to the ejaculatory duct. This rapid transport is consistent with the vas deferens having the highest muscle-to-lumen ratio (approximately 10 : 1) of any hollow viscus in the body.

After sexual stimulation, however, the vas deferens contents are propelled proximally toward the epididymis because the distal vas deferens contracts with greater amplitude, frequency, and duration than the proximal segment.

• Testosterone is synthesized in the Leydig cells of the testes from pregnenolone by a series of reversible reactions; however, once testosterone is reduced by 5α- reductase into DHT or to estrogens by aromatase, the process is irreversible.
• In other words, whereas testosterone can be converted into DHT and into estrogens, estrogens and DHT cannot be converted into testosterone.

• Half life - 10-20 minutes 
• Production rate - 6-7 mg/day 
• Testosterone in plasma - 300-1000 ng/dl  (FDA definition of normal range)
• DHT in plasma  - 60ng/dl 
• DHT is major form of androgen in prostate, five fold higher than testosterone
• The metabolic end product is 17-ketosteroids

Dehydroepidandrosterone (DHEA) - all of the DHEA in plasma is of adrenal cortex origin, <1% of total testosterone in plasma is derived from DHEA 

Androstenedione - cannot be converted directly to DHT. An important role for androstenedione in the male may be its peripheral conversion to estrogens through the aromatase  reaction 

DHEA-Sulphate

Under normal conditions, the adrenals do not support significant growth of prostatic tissue.

The 17-ketosteroids are breakdown products of androgens.

From Testosterone and Androstenedione

• Testis - Testosterone
• Adrenal - DHEA, Androstenedione
• Prostate - formation of DHT within prostate 
• Liver - metabolism into 17 ketosteroids 
• Peripheral conversion to estrogens -

• 2% - Free, only free testosterone is bioavailable 
• 98% - Bound 

• Bound to 
• - Albumin - 40%
• - SHBG - 57%
• - Corticosteroid binding globulin

Serum albumin has a relatively low affinity for testosterone, but, given its abundance, it has a high capacity. In contrast, SHBG has a high affinity for binding steroids, but the protein is present in relatively low concentrations.

Administration of testosterone decreases SHBG levels in the plasma, whereas estrogen therapy stimulates SHBG levels.  Therefore, administration of small amounts of estrogen increases the total concentration of SHBG, and this effectively increases the binding of testosterone and thus lowers the free testosterone plasma concentration.

Prostate level of androgens and Androgen receptors increase with age, though the peripheral level of androgen is decreasing.

Prostate, unlike other androgen-dependent organs, maintains its ability to respond to androgens throughout life.  Despite high circulating levels of androgen, the adult penis loses its ability for androgen-dependent growth. If the penis maintained high levels of ARs throughout life, presumably the organ would grow until the time of death. In contrast, AR levels in the prostate remain high throughout aging

In the brain, skeletal muscle, and seminiferous epithelium, testosterone directly stimulates androgen-dependent processes. In the prostate, DHT is the principal androgen. DTH is more potent than testosterone. 

• 90% DHT - from testis 
• 10% DHT - from adrenal