Male Reproductive Physiology and the Process of Fertilization

Anatomy of the Male Reproductive System

The male reproductive system can be viewed in a sagittal cross-section which reveals the relationship between internal and external structures.
Ureters: These tubes carry urine from both kidneys down to the urinary bladder.
Urinary Bladder and Trigone:
- The bladder stores urine.
- The trigone is a specific area of the urinary bladder that can be manually triggered to release urine in individuals suffering from paralysis.
Glandular Structures:
- Seminal Glands: Produce a significant portion of the fluid that makes up semen.
- Prostate Glands: Glands located below the bladder that contribute to seminal fluid.
- Ejaculatory Duct: Features two small openings through which semen enters the urethra.
- Bulbourethral Gland: Small glands located below the prostate that secrete a lubricating fluid.
The Penis:
- Crus: The root of the penis where it attaches to the external part of the body.
- Corpus Spongiosum: A mass of erectile tissue that surrounds the urethra.
- Corpora Cavernosa: Two masses of erectile tissue that make up the bulk of the penis.
- Spongy Urethra: The portion of the urethra that passes through the corpus spongiosum.
- Foreskin (Prepuce): A layer of thin tissue that covers the glans of the penis.
- Glans Penis: The expanded distal end of the penis.
- External Urethral Opening: The point where urine and semen exit the body.
Erectile Tissue Characteristics:
- Located in the body of the penis.
- Comprised of a dense network of elastic fibers.
- Becomes engorged with blood during an erection.
- Contains deep arteries that supply the tissue.
- During erection, the penis stiffens and elevates to an upright position to facilitate entry into the vagina.

Meiosis and Gamete Production

Fertilization requires the union of two cells (gametes).
In humans, each gamete contributes half of the required chromosomes ( $23$ ) to produce a zygote with a full set of $46$ chromosomes.
The Process of Meiosis:
- Humans produce gametes through meiosis.
- Most body cells are diploid, meaning they contain two sets of chromosomes ( $46$ total).
- One set is maternal (shown in red) and one set is paternal (shown in blue).
- Homologous Chromosomes: Matched pairs consisting of one maternal and one paternal chromosome.
Stages of Meiosis I:
- Interphase: Chromosomes are duplicated, resulting in two identical copies called sister chromatids.
- Molecular Structure: Each sister chromatid consists of DNA wound around histone proteins, coiling into a tight helical fiber.
- Early Prophase I: A spindle forms, and duplicated centrosomes migrate toward opposite poles.
- Condensation and Synapsis: Chromosomes condense and homologous chromosomes stick together in pairs.
- Recombination (Crossing Over): Segments of non-sister chromatids trade places, resulting in the recombination of maternal and paternal genetic material.
- Attachment: The nuclear envelope breaks down, and microtubules from opposite poles attach to each chromosome of the homologous pair.
- Metaphase I: Chromosome pairs are positioned in the middle of the cell.
- Anaphase I: Homologous chromosomes separate and move toward opposite poles, though each chromosome still consists of two sister chromatids.
- Result: The cell started with $46$ chromosomes, but each daughter cell now has only $23$ chromosomes.
Stages of Meiosis II:
- Microtubules from opposite poles attach to the chromosomes, which move to the center of the cell.
- Sister chromatids separate, becoming full-fledged chromosomes moving to opposite poles.
- Nuclear envelopes reform, and each daughter cell divides.
Conclusion of Meiosis:
- The process begins with a single diploid cell.
- It ends with four haploid cells, each containing a single set of chromosomes ( $23$ ).
- These haploid cells mature into gametes (sperm in males and oocytes in females).

Spermatogenesis and Hormonal Regulation

Spermatogenesis: The specific process of sperm production.
- Occurs in the testes within the walls of the seminiferous tubules.
- Nurse Cells (Sertoli Cells): Purple-colored cells within the tubule walls that support and help in sperm development.
- Timeline: Starts at puberty and continues until approximately age $70$ .
- Duration: It takes approximately $64$ days from the beginning to the end of a single sperm production cycle.
Sperm Anatomy:
- Head: Contains the nucleus and is capped with an acrosome (which contains enzymes).
- Middle Piece: Contains mitochondria for energy.
- Tail (Flagellum): Provides motility, allowing the sperm to swim through the vagina, cervix, and uterus into the uterine (fallopian) tubes.
Hormonal Control:
- Regulated by the hypothalamus, pituitary gland, and the testes.
- GnRH (Gonadotropin-Releasing Hormone): Released by the hypothalamus.
- FSH (Follicle-Stimulating Hormone): Stimulates sperm production.
- LH (Luteinizing Hormone): Stimulates the production of testosterone.
- Testosterone: The primary androgen in males.
  - It is a steroid hormone that circulates in the blood bound by one of two types of proteins.
  - It diffuses across target membranes and the hormone receptor binds to DNA in the nucleus.
  - Functions of Testosterone:
    - Stimulates spermatogenesis.
    - Maintains libido (sexual drive) and related male behaviors.
    - Stimulates bone and muscle growth.
    - Establishes secondary sex characteristics (hair growth, deepening of the voice).
    - Maintains accessory organs of the male reproductive system.

The Process of Fertilization

The Odds of Fertilization:
- Approximately $300,000,000$ sperm enter the vagina during intercourse.
- Millions die immediately due to the acidic environment or flow out of the vagina.
- Survival is aided by protective elements in the fluid surrounding the sperm.
The Journey through the Female Tract:
- The Cervix: Usually closed, but opens during ovulation. The cervical mucus thins to a watery consistency to allow passage.
- The Uterus: Muscular contractions help move the sperm toward the egg. However, the woman's immune system identifies sperm as foreign invaders and destroys thousands.
- The Fallopian Tubes: Half of the remaining sperm enter the empty tube, while half enter the tube containing the unfertilized egg. Only a few thousand sperm reach this stage.
- Against the Current: Cilia in the tube push the egg toward the uterus; sperm must swim against this motion.
Capacitation and Hyperactivity:
- Chemicals in the reproductive tract change the membranes of the sperm heads.
- Sperm become hyperactive, swimming harder and faster toward the destination.
Reaching the Egg:
- Only a few dozen of the original $300,000,000$ sperm reach the egg.
- Corona Radiata: The outer layer of cells covering the egg that sperm must push through.
- Zona Pellucida: The outer layer of the egg itself. Sperm reach specialized receptors here.
- Acrosomal Reaction: Sperm release digestive enzymes from the acrosome to burrow through the zona pellucida.
Fusion and Block to Polyspermy:
- The first sperm to make contact with the egg cell membrane fuses with it.
- The egg pulls the sperm inside.
- This fusion triggers the release of chemicals that push other sperm away and create an impenetrable fertilization membrane.
- The zona pellucida hardens, trapping any other sperm inside it.
Formation of the Zygote:
- Male Pronucleus: The male genetic material spreads out and forms a new membrane containing $23$ chromosomes.
- Female Pronucleus: The female genetic material finishes dividing upon fusion, resulting in a nucleus with $23$ chromosomes.
- Microtubules: Spider-web-like threads pull the two pronuclei toward each other.
- Amphimixis: The two sets of chromosomes join, completing fertilization.
- This single cell is now the zygote, possessing a unique genetic code ( $46$ chromosomes) that determines gender, eye color, and other traits.
Implantation:
- Cilia in the fallopian tube sweep the zygote toward the uterus for implantation in the rich uterine lining.