Chapter 26 – Male Reproductive System: Structure, Function, and Gametogenesis

Reproductive System

Overview and Function

The reproductive system is responsible for producing offspring and ensuring the continuation of a species. While not essential for individual survival, it is vital for the survival of the species.

• Primary Function: Production of gametes (sperm in males, eggs in females) and hormones.

• Survival: Not required for individual survival, but necessary for reproduction.

• Products: Gametes and sex hormones (e.g., testosterone in males).

Male Reproductive System

General Structure and Gamete Production

• Gametes Produced: Spermatozoa (sperm cells).

Scrotum

• Function: Houses and protects the testes; regulates temperature for optimal sperm production.

• Three Parts:

  1. Skin: Outermost layer; provides protection.

  2. Dartos Muscle: Smooth muscle; wrinkles the scrotal skin to reduce heat loss.

  3. Cremaster Muscle: Skeletal muscle; elevates or lowers the testes to maintain temperature.

Cremaster Muscle

• Function: Contracts to raise the testes closer to the body for warmth or relaxes to lower them away from body heat.

Testes

• Location: Within the scrotum.

• Function: Produce sperm and secrete testosterone.

• Analogous Organ: Ovaries in females.

• Associated Hormone: Testosterone.

• Parts:

  • Seminiferous Tubules: Site of sperm production (spermatogenesis).

  • Rete Testis & Efferent Ductules: Transport sperm from seminiferous tubules to the epididymis.

  • Epididymis: Stores and matures sperm.

Vas (Ductus) Deferens

• Location: Extends from the epididymis into the pelvic cavity.

• Function: Transports mature sperm to the urethra during ejaculation.

Penis

• Location: External genital organ.

• Functions:

  1. Delivers sperm into the female reproductive tract.

  2. Excretes urine from the body.

• Erectile Tissues: Allow for erection by filling with blood.

  • Corpora Cavernosa: Two columns of erectile tissue on the dorsal side.

  • Corpus Spongiosum: Single column surrounding the urethra on the ventral side.

• Head (Glans Penis): Distal end of the penis.

• Prepuce (Foreskin): Fold of skin covering the glans; protects the glans.

• Smegma: Sebaceous secretion under the prepuce; lubricates and protects the glans.

Accessory Glands

• 1. Seminal Vesicles: Produce seminal fluid rich in fructose to nourish sperm.

• 2. Prostate Gland: Secretes a slightly alkaline fluid to enhance sperm motility.

• 3. Bulbourethral (Cowper's) Glands: Secrete mucus for lubrication and neutralization of acidic urine in the urethra.

• Semen Composition: Sperm plus secretions from the seminal vesicles, prostate, and bulbourethral glands.

Gamete Production: Spermatogenesis

Spermatogenesis Overview

Spermatogenesis is the process by which sperm are produced in the seminiferous tubules of the testes.

• Gamete Made: Spermatozoon (sperm cell).

• Parts of the Sperm:

  • Acrosome: Contains enzymes to penetrate the egg.

  • Head: Contains the nucleus with genetic material.

  • Neck/Midpiece: Packed with mitochondria to provide energy for movement.

  • Tail (Flagellum): Propels the sperm forward.

• Location of Spermatogenesis: Seminiferous tubules.

• Support Cells:

  • 1. Interstitial (Leydig) Cells: Produce testosterone.

  • 2. Nurse (Sertoli) Cells: Support and nourish developing sperm; form the blood-testis barrier.

  • Spermiation: Release of mature sperm into the lumen of the seminiferous tubule.

  • Blood-Testis Barrier: Protects developing sperm from immune attack.

Three Processes of Spermatogenesis

• 1. Mitosis: Cell division producing identical diploid cells.

  • Purpose: Maintain stem cell population and produce primary spermatocytes.

  • Cell Cycle Review:

    • Interphase: Cell grows and DNA replicates.

    • Mitosis (PMAT): Prophase, Metaphase, Anaphase, Telophase – chromosomes separate.

    • Cytokinesis: Division of cytoplasm, forming two cells.

  • Start: Spermatogonia (46 chromosomes).

  • End: Primary spermatocytes (46 chromosomes).

• 2. Meiosis: Two consecutive cell divisions producing haploid cells.

  • Purpose: Reduce chromosome number by half and increase genetic diversity.

  • Meiosis I: Homologous chromosomes separate.

    • Prophase I: Chromosomes condense, tetrads form (homologous pairs), crossing over occurs for genetic variation.

    • Tetrad: Pair of homologous chromosomes; allows crossing over.

    • Result: Two secondary spermatocytes (23 chromosomes each, duplicated).

  • Meiosis II: Sister chromatids separate.

    • Result: Four spermatids (23 chromosomes each, unduplicated).

    • Genetic Information: Each spermatid has half the genetic material of the original cell.

  • Cell Types:

    • Primary Spermatocyte: Diploid, enters meiosis I.

    • Secondary Spermatocytes: Haploid, after meiosis I.

    • Spermatids: Haploid, after meiosis II.

    • Sperm: Mature, motile gamete.

• 3. Spermiogenesis: Maturation of spermatids into spermatozoa.

  • Definition: Transformation of round spermatids into elongated, motile sperm.

  • Products: Four spermatozoa per original spermatogonium.

  • Genetic Information: Each sperm has half the genetic material of the original cell.

Capacitation

• Definition: Functional maturation of sperm in the female reproductive tract, enabling fertilization.

• Steps:

  1. Mixing with secretions from the seminal vesicles and prostate gland.

  2. Exposure to conditions in the female reproductive tract (e.g., cervical mucus).

Male Sexual Response

Phases of Sexual Response

• 1. Arousal:

  • Triggered by erotic stimuli.

  • Parasympathetic nervous system activation.

  • Results in erection due to increased blood flow to erectile tissues.

• 2. Emission:

  • Sympathetic nervous system activation.

  • Movement of sperm and glandular secretions into the urethra.

  • Results in formation of semen in the urethra.

• 3. Ejaculation:

  • Rhythmic contractions expel semen from the urethra.

  • Results in release of semen from the body.

Hormonal Control of Male Sexual Function

Hormonal Regulation

• Hypothalamus: Secretes gonadotropin-releasing hormone (GnRH).

• Anterior Pituitary: Releases two key hormones:

  1. Luteinizing Hormone (LH):

     • Target: Interstitial (Leydig) cells of the testes.

     • Stimulates: Production of testosterone.

     • Testosterone Effects: Promotes spermatogenesis, secondary sex characteristics, libido.

  2. Follicle-Stimulating Hormone (FSH):

     • Target: Nurse (Sertoli) cells.

     • Stimulates: Spermatogenesis and production of androgen-binding protein (ABP).

     • Also Stimulates: Inhibin production, which provides negative feedback to the pituitary.

     • Feedback Type: Negative feedback.

Summary Table: Key Structures and Functions

Structure	Function
Scrotum	Protects testes, regulates temperature
Testes	Produce sperm and testosterone
Epididymis	Stores and matures sperm
Vas Deferens	Transports sperm to urethra
Seminal Vesicles	Produce fructose-rich seminal fluid
Prostate Gland	Secretes alkaline fluid for sperm motility
Bulbourethral Glands	Secrete mucus for lubrication
Penis	Delivers sperm, excretes urine

Key Equations and Chromosome Numbers

• Chromosome Number in Spermatogenesis:

  • Start (Spermatogonium): $46$ chromosomes (diploid)

  • After Mitosis (Primary Spermatocyte): $46$ chromosomes (diploid)

  • After Meiosis I (Secondary Spermatocytes): $23$ chromosomes (haploid, duplicated)

  • After Meiosis II (Spermatids): $23$ chromosomes (haploid, unduplicated)

Additional info: Some details, such as the specific page numbers referenced, were not included as the original file did not provide the full textbook context. Academic context and definitions were expanded for clarity and completeness.