The Male Reproductive System: Structure, Function, and Regulation

Essentials of Sexual Reproduction

Overview of Sexual Reproduction

Sexual reproduction involves the combination of genetic material from two parents, resulting in offspring that are genetically unique. This process relies on specialized reproductive cells called gametes—the egg from the female and the sperm from the male—which fuse to form a zygote. The organs that produce gametes are called gonads: the ovaries in females and the testes in males. In addition to gamete production, gonads secrete hormones that regulate reproductive and secondary sexual characteristics. Secondary sex organs, such as the penis, uterus, and mammary glands, are also essential for successful reproduction.

• Gametes: Egg (female), Sperm (male)

• Gonads: Ovaries (female), Testes (male)

• Secondary Sex Organs: Structures that support reproduction but do not produce gametes

Male Reproductive System

General Function and Anatomy

The primary function of the male reproductive system is the production and delivery of sperm. The system begins with the testes and ends with the penis, encompassing a series of ducts and accessory glands that facilitate sperm maturation, storage, and transport.

• Testes: Site of sperm and hormone production

• Epididymis: Sperm maturation and storage

• Vas deferens: Sperm transport

• Accessory glands: Seminal vesicles, prostate, bulbourethral glands

• Penis: Organ for semen deposition

Testes

The testes are the primary male gonads, responsible for producing sperm and sex hormones, primarily testosterone. Each testis contains numerous seminiferous tubules, where immature sperm are generated. Between these tubules are interstitial (Leydig) cells, which secrete testosterone. Sustentacular (nurse) cells within the tubules provide support, nutrients, and protection for developing sperm, and secrete inhibin to regulate sperm production.

• Spermatogenesis: Occurs in seminiferous tubules

• Leydig cells: Produce testosterone

• Nurse cells: Support and regulate sperm development

Epididymis and Vas Deferens

The epididymis is a coiled duct on the posterior aspect of the testis where sperm mature over approximately 20 days. Mature sperm are stored here and remain viable for 40–60 days. If not ejaculated, they are reabsorbed. The epididymis continues as the vas deferens, a muscular tube that transports sperm during ejaculation and merges with the seminal vesicle to form the ejaculatory duct.

• Sperm maturation: Occurs in the epididymis

• Vasectomy: Surgical cutting of the vas deferens for sterilization

Scrotum and Temperature Regulation

The scrotum houses the testes, epididymis, and spermatic cord. Sperm production requires a temperature approximately 2°C below core body temperature. The scrotum regulates testicular temperature through the cremaster muscle and dartos fascia, which contract or relax in response to temperature changes. The pampiniform plexus of veins helps cool arterial blood before it reaches the testes.

• Cold: Muscles contract, bringing testes closer to the body

• Heat: Muscles relax, suspending testes further from the body

Accessory Glands

Seminal Vesicles

The seminal vesicles are paired glands that secrete a fluid forming about 60% of semen volume. Their secretions contain nutrients and substances that enhance sperm motility.

Prostate Gland

The prostate gland surrounds the prostatic urethra and contributes about 30% of semen volume. Its secretions include ions and enzymes that aid sperm motility and liquefy semen after ejaculation.

Bulbourethral Glands

The bulbourethral glands are located at the root of the penis and secrete a clear, slippery fluid during sexual arousal. This fluid lubricates the urethra and neutralizes residual urine acidity, protecting sperm.

Penis

The penis is the organ responsible for depositing semen in the vagina. It consists of three erectile tissues: one corpus spongiosum and two corpora cavernosa, which fill with blood during erection. The prepuce (foreskin) covers the glans and is highly sensitive.

• Erectile tissues: Corpus spongiosum, corpora cavernosa

• Prepuce: Foreskin, highly innervated

Puberty and Hormonal Regulation

Puberty and Adolescence

Male reproductive function remains dormant until puberty, typically beginning around ages 10–12. Puberty is initiated by increased secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). LH stimulates Leydig cells to produce testosterone, while FSH increases the sensitivity of seminiferous tubules to testosterone, promoting spermatogenesis.

• GnRH: Stimulates pituitary to release FSH and LH

• FSH: Promotes spermatogenesis

• LH: Stimulates testosterone production

Effects of Testosterone

Testosterone is the principal male sex hormone responsible for the development of primary and secondary sexual characteristics. It promotes the growth of sex organs, sperm production, increased muscle and bone mass, deepening of the voice, and the development of body hair and libido.

• Primary effects: Growth of sex organs, sperm production

• Secondary effects: Body hair, muscle mass, voice changes, libido

Spermatogenesis

Stages of Sperm Development

Spermatogenesis is the process by which sperm are produced in the seminiferous tubules. It begins at puberty and continues throughout life. The process involves several stages:

1. Spermatogonia: Stem cells that divide by mitosis; one remains a stem cell, the other becomes a primary spermatocyte.

2. Primary spermatocyte: Undergoes meiosis I to produce two secondary spermatocytes.

3. Secondary spermatocyte: Each undergoes meiosis II to produce two spermatids (total of four).

4. Spermatids: Differentiate into mature sperm through spermiogenesis, developing a flagellum and shedding excess cytoplasm.

Mature sperm are composed of a head (containing the nucleus and acrosome), a midpiece (rich in mitochondria for ATP production), and a tail (flagellum for motility).

Semen Composition

Semen is the fluid expelled during ejaculation, consisting of sperm and secretions from the prostate and seminal vesicles. A typical ejaculation contains 2–5 mL of semen, with a sperm count of 50–120 million/mL. Counts below 25 million/mL are associated with infertility.

• Prostatic fluid: Contains ions and enzymes for sperm motility and semen liquefaction

• Seminal vesicle fluid: Provides nutrients and prostaglandins to enhance sperm motility and stimulate uterine contractions

Regulation of Spermatogenesis

Hormonal Feedback

Spermatogenesis is regulated by a feedback loop involving inhibin (produced by nurse cells), which inhibits FSH release and thus reduces the response to testosterone. LH and testosterone levels remain unaffected by inhibin.

Andropause and Anabolic Steroids

Andropause

Andropause refers to the age-related decline in testosterone secretion, typically after age 50. As testosterone levels fall, FSH and LH levels rise. Most men experience little to no symptoms, but some may have hot flashes, mood changes, or other effects similar to menopause. Sperm production usually continues throughout life.

Anabolic Steroids

Anabolic steroids are synthetic derivatives of testosterone used medically to induce puberty or treat muscle-wasting diseases, but are often abused for performance enhancement. Side effects include acne, mood swings, decreased sperm count, testicular atrophy, and increased risk of cardiovascular disease. Steroids can cause testicular shrinkage by disrupting natural testosterone production, leading to reduced stimulation of the testes.

Sample Questions and Answers

Sample Multiple Choice Questions

• Which of the following is not a way the body balances acids and bases? Answer: B. Lymphatic buffers

• Which of the following allows the collecting duct to concentrate urine? Answer: D. The osmolarity of the tissue around it is higher in the medulla than cortex.

• If you are dehydrated, your ADH levels __________ which causes your urine volume to __________, and your urine become __________ concentrated. Answer: A. Increase; decrease; more

• Normal urine from a healthy person can be expected to contain all of the following except __________. Answer: B. Glucose

• Anabolic steroids are known to cause shrinking of the testicles. What is the best explanation for this? Answer: B. Disrupt the production of testosterone in the testicles.