Comprehensive Study Guide: The Male Reproductive System

The Two Sexes and Sexual Determination

Sexual Reproduction and Gametes * Sexual reproduction involves the combination of genes from two parents to form a fertilized egg, known as a zygote. * Gametes are the sex cells involved in this process. * The male gamete is the sperm, which is characterized by its motility. * The female gamete is the egg or ovum, which contains most of the nutrients required for the developing embryo. * In mammals, the female provides additional support through the shelter of the uterus and the placenta.
Sex Determination * The parent producing sperm is considered male and possesses a $Y$ chromosome. * The parent producing eggs is considered female. * Human cells contain $23$ pairs of chromosomes: $22$ pairs of autosomes and $1$ pair of sex chromosomes. * Sex chromosomes are $XY$ in males and $XX$ in females. * Males produce sperm where $50\%$ carry the $Y$ chromosome and $50\%$ carry the $X$ chromosome. * All eggs produced by the mother carry the $X$ sex chromosome. * The sex of the child is strictly determined by the type of sperm ( $X$ or $Y$ ) that fertilizes the mother’s egg.

Reproductive System Overview

Primary Sex Organs * These are the organs responsible for producing gametes (testes in males, ovaries in females).
Secondary Sex Organs * These organs are essential for reproduction but do not produce gametes. * In males, these are involved in the storage, survival, and transportation of sperm. * In females, these are designed to receive sperm, support zygote development, and provide nourishment for the developing fetus.
Secondary Sex Characteristics * These features develop during puberty and serve to attract a mate. * Examples include: pubic, axillary, and facial hair; scent glands; body morphology; and a low-pitched voice in males.

Hormones and Sex Differentiation

Early Embryonic Development * Gonads begin to develop at $6$ weeks as gonadal ridges. * At this stage, the embryo is sexually indifferent and possesses two sets of ducts: * Mesonephric Ducts: These develop into the male reproductive system, while the paramesonephric ducts degenerate. * Paramesonephric Ducts: These develop into the female reproductive tract, while the mesonephric ducts degenerate.
SRY Gene (Sex-determining Region of Y Gene) * Found only in males, this gene codes for a protein that triggers the testes to secrete testosterone and Mullerian-inhibiting factor. * These hormones bind to receptors on target cells to determine which duct system degenerates. * Female development is the "default" path occurring due to the absence of male hormones, rather than the presence of estrogen (which is high during pregnancy anyway).
Androgen-Insensitivity Syndrome (AIS) * In this condition, the individual is genetically male ( $XY$ embryo) and produces testosterone. * However, the target cells lack receptors for testosterone, so the hormone cannot bind. * The result is the development of external female genitalia. * At puberty, breasts and feminine secondary sex characteristics develop. * The condition is usually detected due to a lack of menstruation; the individual has no ovaries, uterus, or vagina, but has testes located in the abdomen. * Abdominal testes present a high risk for testicular cancer and must be removed.

Development of External Genitalia and Testicular Descent

Chronology of Differentiation * All fetuses at $8$ weeks have the same three basic structures. * Sexual differentiation begins by the end of week $9$ . * Fetuses are distinctly male or female by the end of week $12$ .
Homologous Structures * The penis is homologous to the clitoris. * The scrotum is homologous to the labia majora.
Descent of the Testes * Testes begin development near the kidneys. * The gubernaculum, a cordlike structure containing muscle, extends from the gonad to the abdominopelvic floor. * The gubernaculum shortens, guiding the testes through the abdominal wall via the inguinal canal into the scrotum. * Descent begins between weeks $6-10$ and is typically finished by week $28$ . * The location outside the pelvic cavity is essential for maintaining the low temperatures required for sperm production.
Cryptorchidism * This is a condition where one or both testes fail to descend prior to birth. * It occurs in approximately $3\%$ of males. * Often, the testes descend on their own within the first year; if not, it is treated with hormone therapy or surgery.

Anatomy of the Male Reproductive System

The Testes * The testes are two oval organs, roughly $4\,cm$ long and $2.5\,cm$ in diameter. * Tunica Vaginalis: A saclike extension of the peritoneum covering the testes anteriorly. * Tunica Albuginea: A white fibrous capsule. Septa divide the testis into compartments containing seminiferous tubules. * Seminiferous Tubules: The site of sperm production, lined with thick germinal epithelium consisting of germ cells. * Sustentacular Cells: Promote sperm development and form the blood-testis barrier via tight junctions, protecting sperm from the immune system. * Interstitial Cells: Located between tubules; they produce testosterone. * O2 Supply: Testicular arteries have low blood pressure, resulting in poor oxygen supply. Sperm develop large mitochondria to help them survive the hypoxic environment of the female reproductive tract.
The Scrotum * A pendulous pouch divided into two compartments by a median septum; the left testicle sits lower than the right. * The skin contains nerves, hair, and sebaceous glands. * Thermoregulation: Sperm production requires a temperature about $2^\circ C$ cooler than core body temperature. * Cremaster and Dartos Muscles: Contract in the cold to bring testes closer to the body; relax in heat to move them away. * Pampiniform Plexus: A network of veins that uses countercurrent heat exchange to cool arterial blood ( $37^\circ C$ ) down to approximately $35^\circ C$ as it enters the testis.
Spermatic Ducts * Efferent Ductules: Approximately $12$ tiny ciliated ducts that collect sperm and transport it to the epididymis. * Epididymis: An $18\,ft$ long coiled duct on the posterior testis; it is the site of sperm maturation and storage. It reabsorbs fluid and dead sperm. Stored sperm are viable for $40-60$ days. * Ductus (Vas) Deferens: A muscular tube $45\,cm$ long passing through the inguinal canal to the posterior bladder. It performs peristalsis during orgasm. * Ejaculatory Duct: A $2\,cm$ duct passing through the prostate to empty into the urethra.
Accessory Glands * Seminal Vesicles: Located posterior to the bladder; they empty into the ampulla. * Prostate Gland: Located below the bladder; surrounds the urethra and ejaculatory duct. * Bulbourethral (Cowper) Glands: Located near the bulb of the penis; empty into the penile urethra. They produce a lubricating fluid that neutralizes the acidity of residual urine.
The Penis * Consists of an internal root, a visible shaft ( $4\,in$ long when flaccid), and the glans. * Prepuce (Foreskin): Skin over the shaft that extends over the glans. Sebaceous glands here produce smegma (associated with Mycobacterium smegmatis). * Erectile Tissue: Three cylindrical bodies that fill with blood. * Corpus Spongiosum: Single body along the ventral side; encloses the penile urethra and ends in a dilated bulb ensheathed by the bulbospongiosus muscle. * Corpora Cavernosa: Paired bodies that diverge like a "Y"; each crus attaches to the pubic arch and is covered by the ischiocavernosus muscle.

Puberty, Hormones, and Aging

Definitions * Puberty: The period from the onset of gonadotropin secretion until the ability to reproduce sexually is attained (first ejaculation of viable sperm in males). * Adolescence: Extends from puberty until the person attains full adult height.
The Brain-Testicular Axis * The hypothalamus produces Gonadotropin-Releasing Hormone (GnRH). * GnRH stimulates gonadotrope cells in the anterior pituitary to secrete FSH and LH. * LH (Luteinizing Hormone): Stimulates interstitial cells to produce testosterone. * FSH (Follicle Stimulating Hormone): Stimulates sustentacular cells to secrete androgen-binding protein (ABP). * ABP interacts with testosterone to stimulate spermatogenesis. * Testosterone Effects: Stimulates enlargement of secondary sex organs (penis, testes, ducts, glands), muscle mass growth, erythropoiesis, and libido (sex drive). Sustains these functions throughout adulthood.
Aging and Climacteric * Testosterone peaks at $7\,mg/day$ at age $20$ and declines to $1/5$ of that by age $80$ . * After age $50$ , a rise in FSH and LH produces the male climacteric, which can cause mood changes and hot flashes. * Impotence (Erectile Dysfunction): Affects $20\%$ of men in their $60s$ and $50\%$ of men in their $80s$ . However, over $90\%$ of impotent men remain able to ejaculate.

Spermatogenesis and Semen

Spermatogenesis Process * Type A spermatogonium (mitosis) $\rightarrow$ Type B spermatogonium $\rightarrow$ (passes blood-testis barrier) $\rightarrow$ Primary spermatocyte $\rightarrow$ (Meiosis I) $\rightarrow$ Secondary spermatocytes $\rightarrow$ (Meiosis II) $\rightarrow$ Spermatids $\rightarrow$ (Spermiogenesis) $\rightarrow$ Spermatozoa.
The Spermatozoon Anatomy * Head: Pear-shaped, $4$ to $5$ microns long. Contains the nucleus (haploid chromosomes), acrosome (enzymes to penetrate the egg), and basal body. * Tail: Divided into the midpiece (contains mitochondria for ATP/flagellar movement), principal piece (axoneme with fibers), and endpiece (narrow tip).
Semen (Seminal Fluid) * Typical volume: $2-5\,mL$ per orgasm. * Composition: $60\%$ seminal vesicle fluid, $30\%$ prostate fluid, $10\%$ sperm. * Sperm count: Normal is $50-120 \text{ million/mL}$ . Infertility is associated with counts below $25 \text{ million/mL}$ . * Chemical Components: * Fructose: Provides energy for motility. * Prostaglandins: Stimulate female peristaltic contractions to move sperm. * Seminogelin: A sticky fibrin-like protein. * Serine Protease: Liquefies the sticky semen $20-30$ minutes after ejaculation. * Spermine: A base that stabilizes sperm pH at $7.2$ to $7.6$ .

Male Sexual Response

Circulation and Innervation * Arteries: Deep artery supplies corpora cavernosa (dilation causes erection); dorsal artery provides normal supply. * Nerves: Abundance of tactile, pressure, and temperature receptors. The dorsal nerve of the penis and internal pudendal nerves lead to the sacral spinal cord (integrating center).
Stages of Response * Excitement: Characterized by vasocongestion (genital engorgement), myotonia (muscle tone), and increased heart rate/BP. Parasympathetic signaling triggers Nitrous Oxide (NO) secretion, dilating deep arteries. * Plateau: Respiratory and cardiovascular rates are maintained; erection is maintained. * Orgasm and Ejaculation: A $15$ -second reaction. * Emission: Sympathetic nervous system propels sperm through ducts as secretions are added. * Expulsion: Semen in the urethra triggers muscular contractions. * Resolution: Sympathetic signals constrict arteries, reducing blood flow (detumescence). * Refractory Period: Lasts $10$ minutes to a few hours, during which another erection is impossible.
Viagra Mechanism * Sexual stimulation leads to NO release, activating guanylate cyclase to produce cGMP. * cGMP causes vasodilation and erection. * Viagra acts as a phosphodiesterase (PDE5) inhibitor, preventing the degradation of cGMP and thereby prolonging the erection.

Sexually Transmitted Diseases (STDs)

Bacterial STDs * Gonorrhea (Neisseria gonorrhoeae): Painful urination and pus discharge. $20\%$ of women are asymptomatic. Can lead to infertility via uterine tube scarring. Incubation: $2-5$ days. * Syphilis (Treponema pallidum): * Primary Stage: Chancre formation (hard, painless lesion). * Secondary Stage: Rashes/eruptions, highly contagious, lasts up to $5$ years. * Tertiary Stage: Permanent damage to cardiovascular and nervous systems. * Chlamydia (Chlamydia trachomatis): Most prevalent in the US ( $3-5$ million cases/year). Often asymptomatic in females; painful urination in males. Linked to cervical cancer risk. Lymphogranuloma venereum is a severe form with groin buboes.
Viral STDs * Genital Herpes: Caused primarily by HSV-2 (some HSV-1). Virus becomes latent in nerves. Symptoms include blisters on genitals. Treated with acyclovir; there is no cure. * Genital Warts (HPV): At least $50\%$ of sexually active people acquire it. Symptoms range from small bumps to large growths. Incubation: $3-4$ months. Some types cause cervical cancer; a vaccine is available.
Protozoan STDs * Trichomoniasis (Trichomonas vaginalis): Causes foul-smelling, yellow-green discharge in females; usually asymptomatic in males. Most common curable STD in women.