Reproductive Physiology of the Male



Reproductive Physiology of the Male



Key Points


Functional anatomy


1. The male reproductive system consists of many individual organs acting in concert to produce spermatozoa and deliver them to the female’s reproductive tract.


2. Normal spermatogenesis requires maintenance of uniform testicular temperature 2° to 6° C lower than core body temperature.


3. Emission is the release of spermatozoa and accessory gland fluids into the pelvic urethra, whereas ejaculation is the forceful expulsion of semen from the urethra.


Spermatogenesis


1. Spermatogenesis is a lengthy orchestrated process in which diploid stem cells divide by mitosis to maintain their own numbers and cyclically produce progeny that undergo meiotic division and differentiation into haploid germ cells.


2. Testicular size can predict daily sperm production.


Hypothalamic-pituitary-testicular axis


1. The reproductive system of the male is regulated by the hypothalamus, which is hormonally linked to the anterior pituitary and testes by luteinizing hormone and follicle-stimulating hormone.


Puberty


1. Puberty is not synonymous with sexual maturity.


2. Puberty results from a continuous process of endocrine changes that are initiated shortly after birth.


Anabolic steroids


1. Anabolic steroids are androgen derivatives that exert negative feedback on the hypothalamic-pituitary-testicular axis.



Functional Anatomy


The Male Reproductive System Consists of Many Individual Organs Acting in Concert to Produce Spermatozoa and Deliver Them to the Female’s Reproductive Tract


The male reproductive system is made up of a number of individual organs acting in concert to produce spermatozoa and deliver them to the reproductive tract of the female. This concerted effort involves both the neuroendocrine (hypothalamus and anterior pituitary glands) and the genital system. The genital organs consist of two testes, each suspended within the scrotum by a spermatic cord and external cremaster muscle; two epididymides; two deferent ducts; accessory sex glands; and the penis. The accessory sex glands include paired ampullae, paired seminal vesicles (vesicular glands), a prostate gland, and paired bulbourethral glands (Cowper glands). The presence of individual accessory glands, the testicular orientation, the type of penis, and the site of semen deposition in the female are dependent on the species (Table 40-1).




Normal Spermatogenesis Requires Maintenance of Uniform Testicular Temperature 2° to 6° C Lower Than Core Body Temperature


Normal spermatogenesis in most mammals is dependent upon maintenance of uniform testicular temperature between 2° to 6° C lower than core body temperature. Elevated testicular temperature reduces the numbers of live, normal spermatozoa. Thermoregulation of the testes in domestic animals is maintained by the pendulous scrotal sac, the testicular vasculature, the dartos and cremaster muscles, and the scrotal skin. A pendulous scrotum facilitates thermoregulation by using several mechanisms such as conduction, convection, and evaporation. The internal spermatic artery of many mammals is highly convoluted and in farm animals the coiling is so extensive that the artery forms a vascular cone on the dorsal pole of the testis. The testicular vascular cone is composed of a venous pampiniform plexus network surrounding the highly coiled testicular artery. This ramification reduces the mean blood pressure and permits transfer of heat from the testicular arteries (high temperature) to the veins (low temperature) by a counter-current heat-exchange system. This mechanism of heat exchange is possible because the spermatic artery is extensively coiled and in close proximity to the venous pampiniform plexus. In addition, there are periarterial veins and arteriovenous shunts that facilitate heat transfer as well as the transfer of hormones such as testosterone from the veins to the arteries. The dartos and cremaster muscles can increase or reduce the exposure surface area of the scrotum and move the testes closer to or farther from the abdomen, depending on their state of contracture. The scrotal skin is usually thin, generally lacks subcutaneous fat, has relatively little hair or wool, and contains numerous sebaceous and sweat glands. The blood and lymphatic system in the scrotal skin is very extensive, with blood vessels near the skin surface, facilitating radiation of heat. In hot environments the blood flow in the scrotal skin increases and the evaporation per unit area of scrotal skin is greater than the evaporation from the general body surface. In the scrotum, the number and volume of the sweat glands per unit skin surface is greater than other body regions. In addition, the scrotal skin has thermoreceptors that trigger a local and a systemic response in the presence of an increase in local temperature. Locally the blood flow and scrotal sweating will increase. The systemic response will increase the number of breaths per minute (polypnea).


As previously mentioned, in domestic mammals, normal testicular function, especially normal spermatogenesis, is temperature dependent and requires an environment that is lower than core body temperature. Therefore, in normal domestic males, the testes are located outside the abdominal cavity, in the scrotum. Failure of one or both of the testes to descend into the scrotum is known as cryptorchidism. Although the cryptorchid testis is still capable of producing androgens, it is incapable of producing normal spermatozoa. Consequently, a bilaterally cryptorchid male would be sterile. The cryptorchid testis is more prone to torsion of the spermatic cord and 10 times more likely to be neoplastic. Cryptorchidism appears to be genetic, although the exact mechanism is not completely understood and may vary among species. It is most common in boars, dogs, and stallions and least common in bulls, rams, and bucks. Descent of the testes into the scrotum normally occurs in domestic animals during the following time periods:



For the majority of domestic species, passage of the testes through the internal rings by 2 weeks after birth is necessary for a final scrotal position to occur. Many animals may have testes in the inguinal region at birth, and the testes may remain there for weeks or months before descending into the scrotum. In the dog, testicular descent is uncommon after 14 weeks of age and does not occur after 6 months of age. In the stallion, although it is considered abnormal, descent of inguinally retained testes has been known to occur as late as 2 to 3 years of age.


The testis is the pivotal organ of the male reproductive system. It must be remembered, however, that all testicular functions are profoundly influenced by the neuroendocrine system. The testis is responsible for steroidogenesis, primarily the production of androgens, as well as the generation of haploid germ cells by spermatogenesis. These two functions occur in the Leydig cells and the seminiferous tubules, respectively.


Functionally, the testis is considered to have three compartments. The interstitial tissue compartment, containing the Leydig cells, surrounds the seminiferous tubules and bathes them with testosterone-rich fluid. The other two compartments reside within the seminiferous tubules. The basal compartment contains spermatogonia, which divide through mitosis, whereas the adluminal compartment represents a special environment where spermatocytes undergo meiosis and continue their meiotic divisions to differentiate into spermatids and finally into spermatozoa. Within the seminiferous tubules, the Sertoli cells, which provide support and nourishment to the developing germ cells, extend from the basal compartment into the adluminal compartment. Tight-junctional complexes between the Sertoli cells separate the basal and adluminal compartments and form the major component of the blood-testis barrier, which functionally prevents many compounds found in the blood and interstitial fluid from entering the adluminal compartment.


The seminiferous tubules empty their contents into the rete testis, which subsequently transports the spermatozoa and seminiferous tubular fluid into the epididymis. The epididymis is a single tortuous duct of considerable length (from 2 m in the cat to 80 m in the stallion) that is anatomically divided into three segments: head, or caput; body, or corpus; and tail, or cauda. The epididymis not only is a conduit for spermatozoa but also provides a special environment in which spermatozoa are concentrated, undergo maturation, and acquire fertilizing capacity. Spermatozoa that enter the caput from the rete testis are immotile and incapable of fertilization. Only after they undergo migration and maturation through the caput and corpus are both motility and the capacity for fertilization achieved. The cauda epididymis and the deferent duct, into which the cauda empties, serve as a storage depot for mature spermatozoa; together, these are known as the extragonadal sperm reserves. The spermatozoal transit time through the caput and corpus epididymis is not altered by ejaculation and is similar (2 to 5 days) for domestic species. Storage time in the cauda epididymis is more variable among species (3 to 13 days) and can be reduced by several days in sexually active males. Animals that rest sexually for 7 to 10 days have a maximum number of spermatozoa in the cauda epididymis, and this reserve is reduced by at least 25% with daily or every-other-day ejaculation.


The deferent ducts, or vasa deferentia, pass through the inguinal rings into the abdomen and connect the cauda epididymis with the pelvic urethra. In most species the terminal portion of the deferent ducts enlarges to form prominent ampullae such as those found in ruminants and the stallion. In other species the ampullae either are absent or are anatomically indistinct from the vasa deferentia. The ampullae serve as an additional storage depot for spermatozoa, and in some species, such as the bull, stallion, and dog, ampullary glands add to the ejaculate. Along with spermatozoa, ejaculated semen is composed primarily of accessory gland secretions that add volume, nutrients, buffers, and a number of other substances whose exact functions are unknown. The contribution to the ejaculate by each of the accessory glands varies with the species and is responsible for the variation in concentration, volume, and character between ejaculates. The seminal vesicles lie lateral to the ampullae near the neck of the bladder. In the bull, ram, and buck, these organs are firm and lobulated with a narrow lumen, whereas in the stallion and boar, they are more saclike. The dog and tom lack seminal vesicles but have relatively prominent prostate glands, especially the dog. The prostate gland is present in all domestic males and is intimately associated with the pelvic urethra, but it varies in size and appearance among species. The bulbourethral glands of the tom are almost as large as the prostate, but these glands are absent in the dog. In the stallion and bull, the bulbourethral glands are small, round to ovoid structures that lie adjacent to the pelvic urethra near the ischial arch, whereas those of the boar are large and cylindrical. The male llama/alpaca lacks seminal vesicles, and the bulbourethral and prostate glands are present.


The copulatory organ of the male is the penis. It is more or less cylindrical in all species and extends from the ischial arch to near the umbilicus on the ventral abdominal wall, except in the tom and the llama/alpaca, in which the penis points posteriorly in the relaxed state. The body of the penis is surrounded by a thick fibrous capsule (the tunica albuginea) that encloses numerous cavernous spaces (the corpus cavernosum penis) as well as the corpus spongiosum penis, which immediately surrounds the urethra. Erection is a psychosomatic event that involves mutually occurring actions of the vascular, neurological, and endocrine systems. Contraction of the ischiocavernosus muscle during erection results in occlusion of venous outflow. At the same time, the parasympathetically mediated relaxation of corpus cavernosum and corpus spongiosum results in these cavernous spaces becoming engorged with blood, and the penis becomes elongated and turgid.



Emission Is the Release of Spermatozoa and Accessory Gland Fluids into the Pelvic Urethra, Whereas Ejaculation Is the Forceful Expulsion of Semen from the Urethra


Emission is the release of spermatozoa and accessory gland fluids into the pelvic urethra as a result of sympathetically mediated thoracolumbar reflex contraction of the smooth muscle in the ductus deferens and accessory glands. Ejaculation is the forceful expulsion of semen from the urethra and is prompted by a parasympathetically mediated sacral reflex that induces rhythmic contractions of the bulbospongiosus, ischiocavernosus, and urethralis muscles. After ejaculation, a sacral sympathetically mediated increase in the smooth muscle tone of the cavernous spaces increases the outflow of blood, and contraction of the retractor penis muscle withdraws the penis into the prepuce. The seminal characteristics of the different species are listed in Table 40-2.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Reproductive Physiology of the Male

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