Physiology of the Male Reproductive System

Male Sexual Response

Physiological changes occur to male genitalia during sexual arousal.

Learning Objective

Outline the process of male sexual response

Key Takeaways

Key Points

  • Male sexual response is demonstrated by vasodilation and blood engorgement of the penis, leading to an erection.
  • The testes rise and grow larger and warmer as blood pressure rises.
  • The muscles of the pelvic floor, the vesicles, and the prostrate contract, injecting sperm into the urethra of the penis and resulting in the onset of orgasm.
  • Ejaculation continues with orgasm.
  • Following orgasm, there is a gradual loss of erection and a feeling of relaxation known as the refractory period.
  • Cognitive factors involving visual stimuli and high levels of activity in the amygdala and hypothalamus contribute to sexual arousal and sexual response in males.

Key Terms

  • erection: The physiological process by which a penis becomes engorged with blood.
  • sexual arousal: Changes that occur during or in anticipation of sexual activity.
  • refractory period: The period after excitation, during which a membrane recovers its polarization and is not able to respond to a second stimulus.
  • sex flush: Increased blood flow leading to reddening of the skin in response to sexual arousal or orgasm.
  • tumescence: The normal engorgement of the erectile tissue with blood.
  • genitalia: Sex organs.

The erect penis is commonly correlated with male sexual arousal.

Physiological Effects of Arousal

Physical and/or psychological stimulation leads to vasodilation and subsequent increased blood flow into the three spongy areas that run along the length of the penis (the two corpora cavernosa and the corpus spongiosum). The penis grows enlarged and firm, the skin of the scrotum is pulled tighter, and the testes are pulled up against the body.

As sexual arousal and stimulation continues, the glans of the erect penis will swell wider. As the genitals become further engorged with blood, their color deepens and the testes can grow up to 50% larger. As the testes continue to rise, a feeling of warmth may develop around them and the perineum. With further sexual stimulation, the heart rate increases, blood pressure rises, and breathing becomes more rapid. The increase in blood flow in the genitals and other regions may lead, in some men, to a sex flush.

The muscles of the pelvic floor, the ductus deferens (between the testes and the prostate), the seminal vesicles, and the prostate gland may begin to contract in a way that forces sperm and semen into the urethra inside the penis. This is the onset of orgasm and once this has started, the man likely will continue to ejaculate and orgasm fully, with or without further stimulation. If sexual stimulation stops before orgasm, the physical effects of the stimulation, including the vasocongestion, will subside in a short time. Repeated or prolonged stimulation without orgasm and ejaculation can lead to discomfort in the testes that is sometimes called “blue balls.”

Nonarousal-Related Erection

The relationship between erection and arousal is not one-to-one. Some men older than age 40 report that they do not always have an erection when sexually aroused. A male erection can occur during sleep (nocturnal penile tumescence) without conscious sexual arousal or due to mechanical stimulation (e.g. rubbing against a bed sheet) alone. A young man or one with a strong sexual drive may experience enough sexual arousal for an erection with a passing thought or just the sight of a passerby. Once erect, his penis may gain enough stimulation from contact with the inside of his clothing to maintain the erection for more time.

Post-Orgasm Response

After orgasm and ejaculation, a refractory period usually ensues, characterized by loss of erection, a decline in any sex flush, decreased interest in sex, and a feeling of relaxation associated with the action of the neurohormones oxytocin and prolactin. The intensity and duration of the refractory period can be very short in a highly aroused young man in a highly-arousing situation, perhaps without even a noticeable loss of erection. It can be as long as a few hours or days in mid-life and older men.

Hormonal Responses

Several hormones affect sexual arousal, including testosterone, cortisol, and estradiol. However, the specific roles of these hormones are not clear. Testosterone is the most commonly-studied hormone involved with sexuality, and it plays a key role in sexual arousal in males, with strong effects on central arousal mechanisms.

Sperm

Sperm are the male “seeds,” germ cells, or gametes.

Learning Objective

Describe the anatomy and function of sperm

Key Takeaways

Key Points

  • Sperm fertilize the oocyte, donate the paternal chromatin, and provide the centrosome that maintains the zygote’s microtubule system.
  • Sperm have three parts: a head, which holds the chromatin, a midpiece filled with mitochondria to provide energy, and a flagellum or tail to move the sperm from the vagina to the oocyte.
  • Sperm with one tail, such as human sperm, are referred to as spermatozoa.
  • Sperm quality and quantity decrease with age.

Key Terms

  • anisogamy: The form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form.
  • spermatozoa: A motile sperm cell or moving form of the haploid cell that is the male gamete.
  • acrosome: A caplike structure over the anterior half of the sperm’s head.
  • ATP: An acronym for adenosine triphosphate, which transports chemical energy within cells for metabolism.
  • oogamy: A form of anisogamy (heterogamy) in which the female gamete (oocyte) is significantly larger than the male gamete (sperm) and is non-motile. The male gametes are highly motile and compete for the fertilization of the immotile oocyte.

Example

The term sperm is derived from the Greek word for seed and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there are marked differences in the size of the gametes, with the smaller termed the “male” or sperm cells. Sperm cells cannot divide and have a limited lifespan. After fusion with egg cells during fertilization, a new organism forms, beginning as a totipotent zygote. The human sperm cell is haploid so that its 23 chromosomes can join the 23 chromosomes of the female egg to form a diploid cell. During fertilization, the sperm provides the following three essential parts to the oocyte:

  1. A signalling or activating factor that causes the metabolically dormant oocyte to activate
  2. The haploid paternal genome
  3. The centrosome, which is responsible for maintaining the microtubule system

Sperm Anatomy

 

Closeup of Mammalian Fertilization

Closeup of Mammalian Fertilization: Micrograph of a sperm poised to enter an ovum

Sperm develop in the testes and consist of a head, a midpiece, and a tail. The head contains the nucleus with densely coiled chromatin fibers, surrounded anteriorly by an acrosome that contains enzymes for penetrating the female egg. The midpiece has a central filamentous core with many mitochondria spiraled around it.

Sperm Physiology and Function

In animals, most of the energy (ATP) for sperm motility is derived from the metabolism of fructose carried in the seminal fluid. This takes place in the mitochondria located in the sperm’s midpiece. This energy is used for the journey through the female cervix, uterus, and uterine tubes.

Motile sperm cells typically move via flagella and require a water medium in order to swim toward the egg for fertilization.These cells cannot swim backwards due to the nature of their propulsion. The uniflagellated sperm cells (with one flagellum) of animals are referred to as spermatozoa.

This sperm diagram indicates the acrosome, plasma membrane, nucleus, centriole, mitochondria, terminal disc, axial filament, head, midpiece, tail, endpiece, periacrosomal space, cell membrane, acrosome, nuclear vacuoles, nucleus, nuclear envelope, subacrosomal space, outer acrosome membrane, ecuatorial segment, postacrosomal region, postacrosomal sheet, posterior ring, connecting piece, redundant nuclear envelope, mitochondrial sheath, outer dense fibers, central pair, and axoneme.

Human Sperm: Detailed and labeled diagram of a human spermatozoa

 

Fertility Factors

Sperm quantity and quality are the main parameters in semen quality, a measure of the ability of semen to accomplish fertilization. The genetic quality of sperm, as well as its volume and motility, all typically decrease with age.

Spermatogenesis

Male gametes (sperm cells) are haploid cells produced via spermatogenesis.

Learning Objective

Describe the process of spermatogenesis

Key Takeaways

Key Points

  • Spermatogenesis begins with a diploid spermatogonium in the seminiferous tubules, which divides mitotically to produce two diploid primary spermatocytes.
  • The primary spermatocyte then undergoes meiosis I to produce two haploid secondary spermatocytes.
  • The haploid secondary spermatocytes undergo meiosis II to produce four haploid spermatids.
  • Each spermatid begins to grow a tail and a mitochondrial-filled midpiece, while the chromatin is tightly packaged into an acrosome at the head.
  • Maturation removes excess cellular material, turning spermatids into inactive, sterile spermatozoa that are transported via peristalis to the epididymus.
  • The spermatozoa gain motility in the epididymus, but do not use that ability until they are ejaculated into the vagina.
  • Spermatogenesis requires optimal environmental conditions.

Key Terms

  • spermatozoa: A motile sperm cell, or moving form of the haploid cell that is the male gamete.
  • spermatocyte: A male gametocyte, from which a spermatozoon develops.
  • axoneme: Cytoskeletal inner core structure of eukaryotic flagella.
  • spermatid: A haploid cell produced by meiosis of a spermatocyte that develops into a spermatozoon.
  • spermatogonium: Any of the undifferentiated cells in the male gonads that become spermatocytes.

Spermatogenesis is the process by which male primary sperm cells undergo meiosis and produce a number of cells calls spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes and each secondary spermatocyte into two spermatids or young spermatozoa. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, which in turn produce four spermatozoa.

Male Gametogenesis

Spermatozoa are the mature male gametes in many sexually reproducing organisms. Spermatogenesis is the male version of gametogenesis and results in the formation of spermatocytes possessing half the normal complement of genetic material.

In mammals, it occurs in the male testes and epididymis in a stepwise fashion that takes approximately 64 days.

Spermatogenesis, essential for sexual reproduction is highly dependent upon optimal conditions to occur correctly. DNA methylation and histone modification have been implicated in the regulation of this process. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of sperm produced with increase in age.

Steps in Spermatogenesis

Step 1: Spermatocytogenesis

 

Spermatocytogenesis

Spermatocytogenesis: Diagram of the steps of spermatocytogenesis, including type Ad spermatogonium, type Ap Spermatogonium, type B spermatogonium, primary spermatocyte, and secondary spermatocyte.

Mitotic division of a diploid spermatogonium that resides in the basal compartment of the seminiferous tubules, resulting in two diploid intermediate cells called primary spermatocytes.

Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubules, duplicates its DNA, and subsequently undergoes meiosis I to produce two haploid secondary spermatocytes.

Secondary spermatocytes later divide into haploid spermatids. During this division, random inclusion of either parental chromosome and chromosomal crossover both increase the genetic variability of the gamete.

Each cell division from a spermatogonium to a spermatid is incomplete; the cells remain connected to one another by bridges of cytoplasm to allow synchronous development. Not all spermatogonia divide to produce spermatocytes; otherwise, the supply would run out. Instead, certain types of spermatogonia divide to produce copies of themselves, thereby ensuring a constant supply of gametogonia to fuel spermatogenesis.

Step 2: Spermatidogenesis

The creation of spermatids from secondary spermatocytes. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen in histological preparations.

Step 3: Spermiogenesis

At this stage, each spermatid begins to grow a tail and develop a thickened midpiece where the mitochondria gather and form an axoneme. Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is packaged with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant tightly packed chromatin is transcriptionally inactive. The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosome. One of the centrioles of the cell elongates to become the tail of the sperm.

The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the Sertoli cells with the aid of peristaltic contraction. While in the epididymis, the spermatozoa gain motility and become capable of fertilization. However, transport of the mature spermatozoa through the remainder of the male reproductive system is achieved via muscle contraction rather than the spermatozoon’s recently acquired motility.

Physiology of Spermatogenesis

 

Seminiferous Tubule

Seminiferous Tubule: Micrograph showing seminiferous tubule with maturing sperm.

Maturation takes place under the influence of testosterone, which removes the remaining unnecessary cytoplasm and organelles. The excess cytoplasm, known as residual bodies, is phagocytosed by surrounding Sertoli cells in the testes. The resulting spermatozoa are now mature but lack motility, rendering them sterile. The mature spermatozoa are released from the protective Sertoli cells into the lumen of the seminiferous tubule in a process called spermiation.

Spermatogenesis is highly sensitive to fluctuations in the environment, particularly hormones and temperature. Seminiferous epithelium is sensitive to elevated temperature in humans and is adversely affected by temperatures as high as normal body temperature. Consequently, the testes are located outside the body in a sack of skin called the scrotum. The optimal temperature is maintained at 2 °C below body temperature in human males. This is achieved by regulation of blood flow and positioning towards and away from the heat of the body by the cremaster muscle and the dartos smooth muscle in the scrotum. Dietary deficiencies (such as vitamins B, E, and A), anabolic steroids, metals (cadmium and lead), x-ray exposure, dioxin, alcohol, and infectious diseases will also adversely affect the rate of spermatogenesis.

Spermatozoon: Diagram of parts of a spermatozoon, including the acrosome, plasma membrane, nucleus, centriole, mitochondria, terminal disc, axial filament, tail, endpiece, midpiece, and head.

Semen

Semen is a fluid produced by the seminal vesicles.

Learning Objective

Describe semen

Key Takeaways

Key Points

  • Seminal fluid mixes with fluids produced by the prostate and bulbourethral glands.
  • The seminal fluid provides nutrition and protection for sperm during its journey through the female reproductive tract.
  • Semen initially coagulates in the vagina, then liquefies to allow the sperm to move.

Key Terms

  • seminal vesicle: One of two simple tubular glands located behind the male urinary bladder, responsible for the production of about sixty percent of the fluid that ultimately becomes semen.
  • seminal fluid: Semen is a fluid that helps in promoting the survival of spermatozoa and provides a medium through which they can move.
  • semen: The fluid produced in male reproductive organs of an animal that contains the reproductive cells.

Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads (sexual glands) and can fertilize female ova. In humans, seminal fluid contains several components besides spermatozoa, including enzymes (proteolytic and others) and fructose. These elements promote the survival of spermatozoa and provide a medium for motility. Semen is produced and originates from the seminal vesicles, located in the pelvis. The process that results in the discharge of semen is called ejaculation.

Semen Production and Secretion

During the process of ejaculation, sperm pass through the ejaculatory ducts and mix with fluids from the seminal vesicle, the prostate, and the bulbourethral glands to form semen. The seminal vesicles produce a yellowish viscous fluid rich in fructose, amino acids, and other substances that make up about 70% of human semen. The prostatic secretion, influenced by dihydrotestosterone, is a whitish (sometimes clear), thin fluid containing proteolytic enzymes, citric acid, acid phosphatase, and lipids. The bulbourethral glands secrete a clear fluid to lubricate the lumen of the urethra.

Sperm Protection and Transport

Sertoli cells, which nurture and support developing spermatocytes, secrete a fluid into seminiferous tubules that helps transport sperm to the genital ducts. The ductuli efferentes possess cuboidal cells with microvilli and lysosomal granules that modify the semen by reabsorbing some fluid. Once the semen enters the ductus epididymis, the principal cells (which contain pinocytotic vessels indicating fluid reabsorption) secrete glycerophosphocholine, which most likely inhibits premature capacitation.

The seminal plasma provides a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal environment of the vagina is a hostile one for sperm cells, as it is acidic (from the native microflora producing lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile environment. Basic amines such as putrescine, spermine, spermidine, and cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment of the vaginal canal and protect DNA inside the sperm from acidic denaturation.

Characteristics of Ejaculate

According to the World Health Organization, normal human semen has a volume of 2 ml or greater, pH of 7.2 to 8.0, sperm concentration of 20×106 spermatozoa/ml or more, sperm count of 40×106 spermatozoa per ejaculate or more, and motility of 50% or more within 60 minutes of ejaculation. After ejaculation, the latter part of the semen coagulates immediately, forming globules. After about 15–30 minutes, a prostate-specific antigen present in the semen causes the decoagulation of the seminal coagulum. It is postulated that the initial clotting helps keep the semen in the vagina, while liquefaction frees the sperm to make their journey to the ova.

Semen quality is a measure of the ability of semen to accomplish fertilization and thus a measure of a man’s fertility. Semen can be preserved for long-term storage by cryopreservation. For human sperm, the longest reported successful storage with this method is 21 years.

Hormonal Regulation of the Male Reproductive System

The male reproductive system is regulated by the production, stimulation, and feedback of specific hormones.

Learning Objective

Differentiate among the hormones of the male reproductive system

Key Takeaways

Key Points

  • GnRH is made in the hypothalamus and travels to the pituitary where it stimulates FSH and LH secretion.
  • FSH is necessary for sperm maturation.
  • LH binds to Leydig cells to stimulate testosterone secretion and androgen production.
  • Testosterone stimulates sex drive.
  • Inhibin acts as negative feedback to slow the release of FSH and GnRH.

Key Terms

  • GnRH: Gonadotropin-releasing hormone is a trophic peptide hormone responsible for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary.
  • LH: Luteinizing hormone is produced by the anterior pituitary gland and in males causes the synthesis and secretion of testosterone and androgen.
  • FSH: Follicle-stimulating hormone stimulates both the production of androgen-binding protein by Sertoli cells and the formation of the blood-testis barrier.

Hormonal control of spermatogenesis varies among species. In humans, the mechanisms are not completely understood. However, it is known that initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland, and Leydig cells. If the pituitary gland is removed, spermatogenesis can still be initiated by follicle-stimulating hormone (FSH) and testosterone.

The Role of Hormones in Male Reproduction

Studies from rodent models suggest that gonadotropin hormones (both LH and FSH) support the process of spermatogenesis by suppressing the proapoptotic signals and thus promoting spermatogenic cell survival. The Sertoli cells themselves mediate parts of spermatogenesis through hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product, testosterone.

Gonadotropin-Releasing Hormone

Gonadotropin-releasing hormone (GnRH) is mainly made in the preoptic area of the hypothalamus before traveling to the pituitary gland. There it stimulates the synthesis and secretion of the gonadotropins, FSH and luteinizing hormone (LH).

Follicle-Stimulating Hormone

Follicle-stimulating hormone (FSH)  is released by the anterior pituitary gland. Its presence in males is necessary for the maturation of spermatozoa. Follicle-stimulating hormone stimulates both the production of androgen-binding protein by Sertoli cells and the formation of the blood-testis barrier. Increasing the levels of FSH increases the production of spermatozoa by preventing the apoptosis of type A spermatogonia.

Luteinizing Hormone

Luteinizing hormone (LH) is released by the anterior pituitary gland. In the testes, LH binds to receptors on Leydig cells, which stimulates the synthesis and secretion of testosterone.

Testosterone

Testosterone is made in the interstitial cells of the testes. It stimulates the sex drive and is associated with aggression. Androgen-binding protein is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis, which can be 20-50 times higher than the concentration found in blood. The sequestering of testosterone in the testes is initiated by FSH, and only testosterone is required to maintain spermatogenesis.

Inhibin

Inhibin is secreted by the Sertoli cells and acts to decrease the levels of FSH. The hormone is released into the circulation when the sperm count is too high.

Hormonal Influence

Hormonal Influence: This flowchart details the steps involved in hormonal control of male reproduction.

This diagram depicts the hormonal regulation of male reproduction, including the following steps: hypothalamus, GnHR secretion, anterior pituitary, FSH and LH secretion, negative feedback, Leydig cells, Sertoli cells, testosterone secretion, inhibit secretion, spermatogenesis, various target tissues, maintenance of accessory reproductive organs and secondary sex characteristics, sex drive, protein synthesis in skeletal muscle, and bone growth in adolescents.