Male Reproductive System - Detailed Notes

The Male Reproductive System

The Two Sexes

• Sexual reproduction is biparental; offspring receive genes from two parents, resulting in genetic diversity. They are not genetically identical to either parent.

• Gametes (sex cells) are produced by each parent through meiosis, ensuring genetic variation.

• A zygote (fertilized egg) forms from the fusion of two gametes, combining both parents’ genes.

• Male and female gametes unite their genes to create a zygote, initiating the development of a new organism.

• Spermatozoon: the motile gamete produced by the male, carrying the Y chromosome, which determines maleness.

• Ovum: the female gamete containing nutrients essential for the developing embryo but lacking a Y chromosome.

• Mammals: the female provides a protected internal environment and prenatal nutrition, crucial for embryonic development.

• Male reproductive system: designed to produce sperm and deliver them into the female body via the penis.

• Female reproductive system: produces eggs, receives sperm, facilitates gamete union, supports fetal development, and nourishes offspring; utilizes the vagina.

Overview of the Reproductive System

• The reproductive system includes primary and secondary sex organs, each with distinct roles.

• Primary sex organs (gonads): testes in males and ovaries in females, responsible for producing gametes and sex hormones.

• Secondary sex organs: all other organs essential for reproduction, including ducts, glands, and the penis in males, and uterine tubes, the uterus, and the vagina in females.

  • Male: includes a system of ducts and glands that store and transport sperm; the penis delivers sperm cells into the female reproductive tract.

  • Female: the uterine tubes, uterus, and vagina receive sperm, support fertilization, and harbor the developing fetus.

• External genitalia: located in the perineum and are externally visible, except for accessory glands in the female perineum.

• Internal genitalia: primarily located within the pelvic cavity, except for the testes and associated ducts, which are housed in the scrotum.

The Male Perineum

• The male perineum is a diamond-shaped region located between the thighs, defined by the pubic symphysis, ischial tuberosities, and coccyx. It includes the urogenital triangle (containing the base of the penis and scrotum) and the anal triangle (containing the anus), which are divided by the perineal raphe.

Secondary Sex Characteristics

• Secondary sex characteristics: features that differentiate the sexes and play a role in mate attraction; these are not directly involved in reproduction.

• Develop at puberty under the influence of sex hormones.

• Both sexes: development of pubic and axillary hair, maturation of associated scent glands, and changes in voice pitch.

• Male: development of facial hair, increased body hair, and a more muscular physique.

• Female: characteristic distribution of body fat, breast enlargement, and smoother skin.

Androgen-Insensitivity Syndrome

• Androgen-Insensitivity Syndrome (AIS) occurs when a genetic male (XY chromosomes) is resistant to male hormones (androgens).

  • A girl shows typical pubertal changes but does not menstruate.

  • Presence of testes in the abdomen due to undescended testicles.

  • Karyotype reveals XY chromosomes, indicating genetic maleness.

  • Testes produce normal male levels of testosterone, but the body cannot respond to it.

  • Target cells lack functional androgen receptors.

  • External genitalia develop along female lines due to the lack of androgen signaling.

  • Absence of uterus or menstruation due to the lack of Müllerian duct development, which is normally inhibited by androgens.

Chromosomal Sex Determination

• Cells contain 23 pairs of chromosomes, totaling 46.

  • 22 pairs of autosomes: chromosomes not involved in sex determination.

  • 1 pair of sex chromosomes: determines genetic sex (XY in males, XX in females).

• Males produce sperm containing either an X or a Y chromosome in equal proportions.

• All eggs carry the X chromosome.

• The sex of the child is determined by the type of sperm that fertilizes the egg.

  • X-carrying sperm fertilizes the egg: results in a female (XX).

  • Y-carrying sperm fertilizes the egg: results in a male (XY).

Prenatal Hormones and Sexual Differentiation

• Initially, a fetus is sexually undifferentiated; early development is similar in both sexes.

• Gonads begin to develop at 5 to 6 weeks as gonadal ridges, precursors to either testes or ovaries.

• Two sets of ducts are located adjacent to each gonadal ridge: mesonephric (wolffian) ducts and paramesonephric (müllerian) ducts.

  • In males, mesonephric (wolffian) ducts develop into the male reproductive tract under the influence of testosterone; paramesonephric ducts degenerate due to Müllerian-inhibiting factor (MIF).

  • In females, paramesonephric (müllerian) ducts develop into the female reproductive tract in the absence of androgens; mesonephric ducts degenerate.

• SRY gene (sex-determining region of Y chromosome) is present only in males and essential for testes development.

  • SRY codes for testes-determining factor (TDF), a protein that initiates the differentiation of the gonads into testes.

• Testes begin to secrete testosterone at 8 to 9 weeks, driving the development of male characteristics.

  • Testosterone stimulates the mesonephric ducts to develop into the epididymis, vas deferens, and seminal vesicles.

  • Simultaneously, the testes secrete Müllerian-inhibiting factor (MIF), causing the paramesonephric ducts to degenerate, preventing the development of a uterus and fallopian tubes.

• In the absence of a Y chromosome and, consequently, TDF, the cortex of the embryonic gonads develops into ovaries, and the embryo develops female characteristics.

• Estrogen levels are high during pregnancy due to maternal hormone production, but female development primarily results from the absence of androgen hormones rather than the presence of estrogen.

Development of the External Genitalia

• The external genitalia develop similarly in both sexes during early development.

• Genital tubercle differentiates into the glans of the penis in males or the glans clitoris in females.

• Urogenital folds fuse in males to enclose the urethra and form the penis or develop into the labia minora in females.

• Labioscrotal folds develop into the scrotum in males or the labia majora in females.

• By week 12, the external genitalia are distinctly formed, allowing for sex determination via ultrasound.

• Male and female organs that develop from the same embryonic structure are homologous, reflecting their common developmental origin.

  • The penis is homologous to the clitoris.

  • The scrotum is homologous to the labia majora.

Male Reproductive Anatomy

• The male reproductive system consists of the scrotum, testes, epididymis, ductus deferens, seminal vesicles, prostate gland, bulbourethral glands, urethra, and penis, each playing a critical role in sperm production, storage, and delivery.

The Scrotum

• The scrotum and penis constitute the external genitalia of the male, situated in the perineum.

• Occupy the perineum: a diamond-shaped area located between the thighs, bordered by the pubic symphysis, ischial tuberosities, and coccyx.

• Scrotum: a pouch of skin, muscle, and fibrous connective tissue that houses the testes, providing a protective and temperature-regulated environment.

• Typically, the left testicle descends lower than the right to prevent compression against each other, optimizing temperature regulation.

• The skin of the scrotum contains sebaceous glands, sparse hair, and rich sensory innervation, and exhibits darker pigmentation.

• Internally, the median septum divides the scrotum into right and left compartments, each housing a testis.

• Perineal raphe: a medial seam on the scrotum surface that indicates the location of the median septum; it extends anteriorly along the ventral side of the penis and posteriorly to the anus.

• The spermatic cord is a bundle of fibrous connective tissue that contains the ductus deferens, blood and lymphatic vessels, and the testicular nerve, extending through the inguinal canal into the pelvic cavity.

• External inguinal ring: the inferior entrance to the inguinal canal.

• Internal inguinal ring: the superior exit from the inguinal canal into the pelvic cavity.

• The testes reside in the scrotum because sperm production requires a cooler temperature than the core body temperature of 37°C; optimal sperm production occurs at around 35°C.

Temperature Regulation of Testes

• Cremaster muscle: consists of strips of the internal abdominal oblique muscle that enmesh the spermatic cord.

  • In response to cold temperatures, the cremaster muscle contracts, pulling the testes upward toward the body for warmth.

  • In warm temperatures, it relaxes, allowing the testes to descend further from the body to dissipate heat.

• Dartos muscle: a subcutaneous layer of smooth muscle within the scrotum.

  • When cold, the dartos muscle contracts, wrinkling the scrotum and reducing its surface area to conserve heat, while also holding the testes against the warm body.

• Pampiniform plexus: a complex network of veins from the testes that surrounds the testicular artery within the spermatic cord.

  • This network acts as a countercurrent heat exchanger, cooling arterial blood before it reaches the testes, which is essential for spermatogenesis.

  • Without the pampiniform plexus, warm arterial blood would overheat the testes, impairing sperm production.

  • By the time arterial blood reaches the testis, it is 1.5° to 2.5°C cooler than the body's core temperature.

The Testes

• Testes (testicles): paired endocrine and exocrine glands that produce sex hormones (testosterone) and sperm.

• Each testis is oval and slightly flattened, measuring approximately 4 cm in length and 2.5 cm in diameter; covered anteriorly and laterally by the tunica vaginalis.

• Tunica albuginea: a white fibrous capsule that covers the testes, providing structural support.

• Connective tissue septa extend inward from the tunica albuginea, dividing each testis into 250 to 300 wedge-shaped lobules.

• Seminiferous tubules: the site of sperm production within the testes; each lobule contains one to three seminiferous tubules.

• Each tubule is lined with a thick germinal epithelium consisting of germ cells (developing sperm) and sustentacular cells (Sertoli cells).

• Sustentacular (Sertoli) cells: located between germ cells, providing structural and metabolic support; they protect germ cells, facilitate their development, and supply nutrients, waste removal, and growth factors.

• Interstitial (Leydig) cells: situated between the tubules, responsible for producing testosterone, essential for male sexual development and function.

The Spermatic Ducts

• The spermatic ducts transport sperm from the testis to the urethra, facilitating ejaculation.

  • Efferent ductules: approximately 12 small, ciliated ducts that collect sperm from the rete testes and transport it to the epididymis.

  • Duct of the epididymis (head, body, and tail): the site of sperm maturation and storage, where sperm remain fertile for 40 to 60 days; a single, highly coiled duct approximately 6 m long, adhering to the posterior of the testis; sperm mature and gain motility as they travel through the duct; if not ejaculated, they disintegrate and are reabsorbed by the epididymis.

  • Ductus (vas) deferens: a muscular tube approximately 45 cm long, extending from the scrotum through the inguinal canal to the posterior surface of the bladder; it widens behind the bladder, forming the terminal ampulla; the duct unites with the duct of the seminal vesicle; the thick wall of smooth muscle is heavily innervated by sympathetic nerve fibers, facilitating peristaltic contractions during ejaculation.

  • Ejaculatory duct: a short, 2 cm duct formed by the union of the ductus deferens and the seminal vesicle; it passes through the prostate gland and empties into the urethra, delivering sperm and seminal fluids.

• The male urethra (18 cm long) is shared by the reproductive and urinary systems, serving as the conduit for both urine and semen.

  • The urethra is divided into three regions: the prostatic urethra (within the prostate gland), the membranous urethra (short segment passing through the pelvic floor), and the spongy (penile) urethra (running through the penis).

The Accessory Glands

• Three sets of accessory glands in the male reproductive system contribute fluids to semen: seminal vesicles, prostate gland, and bulbourethral glands.

  • Seminal vesicles: a pair of glands located posterior to the bladder that empty into the ejaculatory duct, contributing approximately 60% of semen volume; their secretions are rich in fructose to nourish the sperm and prostaglandins to promote uterine contractions.

  • Prostate gland: surrounds the urethra and ejaculatory duct just inferior to the bladder; composed of 30 to 50 compound tubuloacinar glands that empty through about 20 pores in the prostatic urethra; its thin, milky secretion constitutes approximately 30% of semen and contains enzymes and nutrients that support sperm viability.

  • Bulbourethral (Cowper) glands: located near the bulb of the penis; during sexual arousal, they produce a clear, slippery fluid that lubricates the head of the penis in preparation for intercourse and neutralizes the acidity of residual urine in the urethra, protecting the sperm.

Semen

• Semen (seminal fluid): the fluid expelled during orgasm, containing sperm and secretions from the testes, seminal vesicles, prostate gland, and bulbourethral glands.

• Typically, 2 to 5 mL of fluid are expelled during ejaculation, consisting of 60% seminal vesicle fluid, 30% prostatic fluid, and 10% sperm and spermatic duct secretions.

• Normal sperm count ranges from 50 to 120 million/mL; counts below 20 to 25 million/mL are indicative of infertility.

• Seminal vesicles contribute a viscous, yellowish fluid containing fructose and other carbohydrates, citrate, prostaglandins, and a protein called proseminogelin, providing energy and promoting fertilization.

• The prostate produces a thin, milky white fluid containing calcium, citrate, phosphate ions, a clotting enzyme (proseminogelin → seminogelin), and a protein-hydrolyzing enzyme called serine protease (PSA, prostate-specific antigen), which contribute to semen coagulation and liquefaction.

• The stickiness of semen facilitates fertilization by ensuring that sperm remain in the female reproductive tract.

  • The clotting enzyme from the prostate activates proseminogelin, converting it to seminogelin, a sticky fibrin-like protein.

  • Seminogelin entangles the sperm and adheres to the inner walls of the vagina and cervix, preventing semen from draining back into the vagina and promoting the uptake of sperm-laden clots into the uterus.

  • 20 to 30 minutes after ejaculation, serine protease (PSA) from prostatic fluid breaks down seminogelin, liquefying the semen and releasing the sperm to facilitate their motility.

• Sperm motility requires an elevated pH and an energy source, both provided by seminal fluids.

  • Prostatic fluid buffers the acidity of the vagina, raising the pH from approximately 3.5 to 7.5, creating a more favorable environment for sperm survival.

  • Seminal vesicles provide fructose and other sugars that are metabolized by sperm mitochondria to produce ATP, fueling movement up the vagina and uterus.

  • Prostaglandins in semen may thin the mucus of the cervical canal and stimulate contractions in the uterus and uterine tubes, aiding the spread of semen.

Endocrine Control of Puberty

• As the hypothalamus matures, it begins to produce gonadotropin-releasing hormone (GnRH), initiating puberty.

• GnRH stimulates cells in the anterior pituitary (gonadotropes) to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

  • Follicle-stimulating hormone (FSH) stimulates sustentacular cells in the testes to secrete androgen-binding protein (ABP), which binds testosterone and maintains it in the seminiferous tubule lumen, promoting spermatogenesis.

  • Luteinizing hormone (LH), also known as interstitial cell–stimulating hormone (ICSH), stimulates interstitial cells in the testes to produce testosterone.

• Adulthood: hormonal regulation maintains male reproductive function and secondary sex characteristics.

  • Testosterone sustains the male reproductive tract, sperm production, and libido.

  • Inhibin, secreted by sustentacular cells, provides negative feedback to the pituitary, suppressing FSH output and reducing sperm production without affecting LH and testosterone secretion.

Hormonal Relationships

• GnRH from the hypothalamus stimulates the anterior pituitary to secrete FSH and LH, initiating the hormonal cascade that regulates male reproductive function.

• FSH stimulates sustentacular cells to secrete androgen-binding protein (ABP), which is essential for maintaining high levels of testosterone in the seminiferous tubules.

• LH stimulates interstitial cells to secrete testosterone (androgen), the primary male sex hormone responsible for the development and maintenance of male characteristics.

• In the presence of ABP, testosterone stimulates spermatogenesis, the production of sperm in the seminiferous tubules.

• Testosterone also stimulates libido and the development of secondary sex organs and characteristics, contributing to male sexual function and appearance.

• Testosterone exerts negative feedback effects that reduce GnRH secretion from the hypothalamus and decrease the pituitary's sensitivity to GnRH, maintaining stable testosterone levels.

• Sustentacular cells secrete inhibin, which selectively inhibits FSH secretion, reducing sperm production without affecting testosterone secretion, providing a mechanism for regulating sperm count.

Sperm and Semen

• Spermatogenesis: the process of sperm production in the seminiferous tubules, involving cell division, remodeling, and genetic recombination.

• Involves three principal events:

  • Division and remodeling of large germ cells into small, motile sperm cells with flagella, optimizing their ability to reach and fertilize an egg.

  • Reduction of chromosome number by one-half through meiosis, ensuring that the resulting sperm cells are haploid.

  • Shuffling of genes so that each chromosome contains new combinations of genetic information, promoting genetic variation in the offspring.

• Meiosis plays a critical role by recombining genes and reducing chromosome number, resulting in four haploid daughter cells that will differentiate into sperm.

Meiosis

• Two forms of cell division: mitosis and meiosis, each with distinct functions.

  • Mitosis: a body cell duplicates its DNA and then divides to produce two genetically identical daughter cells, essential for growth, repair, and asexual reproduction.

  • Basis for division of a single-cell fertilized egg, growth of an embryo, all postnatal growth, and tissue repair.

  • Consists of four stages: prophase, metaphase, anaphase, telophase.

  • Meiosis: a specialized form of cell division that produces four genetically distinct gametes (haploid cells), each with only half the DNA of the diploid body cells, ensuring genetic diversity in sexual reproduction.

  • Combining male and female gametes with half the genetic material produces an embryo with the same number of chromosomes as each of the parents.

  • Meiosis is sometimes called reduction division because it reduces the chromosome number.

• Meiosis involves two successive cell divisions (meiosis I and meiosis II), following one round of DNA replication, resulting in four haploid cells.

• Meiosis I: prophase I, metaphase I, anaphase I, and telophase I. DNA is duplicated before this begins.

  • Prophase I: homologous chromosomes pair up side by side, forming a tetrad, allowing for genetic exchange.

  • Crossing-over: the exchange of genetic material between homologous chromosomes, creating new combinations of genes.

  • After meiosis I, each cell has 23 chromosomes, but each chromosome is double-stranded due to DNA replication.

• Meiosis II: prophase II, metaphase II, anaphase II, and telophase II; similar to mitosis but involves haploid cells.

  • Each double-stranded chromosome divides into two chromatids, which separate and are distributed to daughter cells.

  • At the end of meiosis II, each cell contains 23 single-stranded chromosomes, resulting in four haploid gametes.

Spermatogenesis

• Primordial germ cells originate in the yolk sac of the embryo, then migrate to and colonize the gonadal ridges, differentiating into spermatogonia, the stem cells of spermatogenesis.

• Puberty triggers spermatogenesis, leading to the continuous production of sperm in the seminiferous tubules.

• Spermatogonia reside along the periphery of the seminiferous tubules and divide by mitosis to maintain a population of stem cells and produce cells committed to becoming sperm.

  • One daughter cell of each division remains in tubule wall as a stem cell, known as a type A spermatogonium, ensuring a continuous supply of spermatogonia.

  • The other daughter cell migrates away from the wall and differentiates into a type B spermatogonium, which is committed to sperm production.

• The type B spermatogonium enlarges and becomes a primary spermatocyte, poised to undergo meiosis.

  • Sustentacular cells form a blood–testis barrier (BTB) that protects the primary spermatocyte from the body’s immune system, preventing autoimmune destruction of developing sperm cells.

  • The primary spermatocyte undergoes meiosis I, yielding two equal-sized, haploid, genetically unique secondary spermatocytes.

  • Each secondary spermatocyte then undergoes meiosis II, dividing into two spermatids, resulting in a total of four spermatids for each spermatogonium.

  • Spermiogenesis: the transformation of spermatids into spermatozoa, involving the remodeling of cells, shedding excess cytoplasm, and developing tails.

• Spermiogenesis: the final stage of sperm development, involving major structural changes as spermatids differentiate into spermatozoa, acquiring the features necessary for fertilization.

The Spermatozoon

• The spermatozoon consists of two main parts: the head and the tail, each with specialized structures for fertilization.

  • The head is pear-shaped, measuring 4 to 5 μm long, and contains the nucleus, acrosome, and basal body of the tail flagellum.

  • The nucleus contains a haploid set of chromosomes, carrying the male's genetic contribution to the offspring.

  • The acrosome is an enzyme-containing cap over the apical half of the nucleus that contains enzymes, such as hyaluronidase and acrosin, that facilitate penetration of the egg's outer layers.

  • The basal body is an indentation in the basal end of the nucleus where the flagellum attaches, anchoring the tail to the head.

• The tail is divided into three regions: the midpiece, principal piece, and endpiece, each contributing to sperm motility.

  • The midpiece contains mitochondria arranged around the axoneme of the flagella, generating ATP to power flagellar movement and sperm motility.

  • The principal piece constitutes most of the tail