Sexual Reproduction and The Male Reproductive System: Structure, Function, and Development

Sexual Reproduction and Development

Fundamental Biological Distinctions

Sexual reproduction involves the combination of genetic material from two parents to produce offspring. The most basic biological distinction between male and female is the type of gamete produced and the presence or absence of the Y chromosome.

• Gametes: Sex cells (sperm in males, eggs in females) that combine to form a zygote (fertilized egg).

• Male: Produces motile sperm; presence of Y chromosome.

• Female: Produces nutrient-rich eggs; absence of Y chromosome.

• In mammals: Females provide a sheltered internal environment and prenatal nutrition for the embryo.

Overview of the Reproductive System

Primary and Secondary Sex Organs

The reproductive system is organized into primary and secondary sex organs, each with distinct roles in reproduction.

• Primary sex organs (gonads): Produce gametes (testes in males, ovaries in females).

• Secondary sex organs: Necessary for reproduction but do not produce gametes.

  • Male: Ducts, glands, penis (delivers sperm).

  • Female: Uterine tubes, uterus, vagina (receive sperm, harbor fetus).

Secondary Sex Characteristics

Secondary sex characteristics are features that distinguish the sexes and play a role in mate attraction, developing at puberty.

• Both sexes: Pubic and axillary hair, scent glands, pitch of voice.

• Male: Facial hair, visible hair on torso/limbs, muscular physique.

• Female: Body fat distribution, breast enlargement, relatively hairless skin.

Chromosomal Sex Determination

Human Karyotype and Sex Chromosomes

Sex is determined by the combination of sex chromosomes inherited from the parents.

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes, 1 pair of sex chromosomes (XX for females, XY for males).

• Males produce sperm carrying either X or Y chromosomes; all eggs carry X chromosomes.

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

  • X-carrying sperm → female (XX)

  • Y-carrying sperm → male (XY)

Prenatal Hormones and Sexual Differentiation

Developmental Pathways

Sexual differentiation is regulated by genetic and hormonal factors during embryonic development.

• Fetus is sexually undifferentiated for the first ~7 weeks.

• Gonads develop at 5-6 weeks as gonadal ridges.

• Two sets of ducts:

  • Mesonephric (Wolffian) ducts: Develop into male reproductive system.

  • Paramesonephric (Müllerian) ducts: Develop into female reproductive tract.

• SRY gene (on Y chromosome): Codes for testes-determining factor (TDF), initiating testes development.

• Testes secrete testosterone (stimulates male duct development) and Müllerian-inhibiting factor (MIF) (causes degeneration of female ducts).

Hormonal Control of Sex

Key Hormones

Sexual development and differentiation are controlled by androgens, estrogens, and progestins.

• Androgens (e.g., testosterone): Development of male reproductive organs and secondary sexual characteristics; more prevalent in males.

• Estrogens (e.g., estradiol): Pubertal development of female traits; also involved in some aspects of masculinization.

• Progestins (e.g., progesterone): Support female reproductive traits.

• Female development occurs in the absence of androgen hormones.

Development of the External Genitalia

Embryonic Structures and Homology

External genitalia develop from common embryonic structures and differentiate under hormonal influence.

• Genital tubercle: Becomes the head (glans) of the penis or clitoris.

• Urogenital folds: Enclose urethra in males (forming penis) or form labia minora in females.

• Labioscrotal folds: Become scrotum in males or labia majora in females.

• Genitalia are undifferentiated until ~week 9; by week 12, male or female genitalia are distinctly formed.

• Homologous structures:

  • Penis ↔ Clitoris

  • Scrotum ↔ Labia majora

Descent of the Gonads

Migration and Positioning

Gonads initially develop high in the abdomen and migrate to their final positions.

• Gubernaculum: Connective tissue cord guiding migration of gonads.

• Testes: Descend through the inguinal canal around 7 months; position is crucial for spermatogenesis.

• Ovaries: Descend to the inferior brim of the lesser pelvis; gubernaculum becomes supporting ligaments.

Male Reproductive Anatomy

Structure and Function

The male reproductive system is specialized for the production, maturation, and delivery of sperm.

• Testes: Produce sperm and testosterone.

• Epididymis: Sperm maturation and storage.

• Ductus (Vas) deferens: Transports sperm.

• Accessory glands:

  • Seminal vesicles: Secrete fluid rich in fructose (energy for sperm).

  • Prostate gland: Secretes milky fluid aiding sperm motility.

  • Bulbourethral glands: Secrete lubricating mucus.

• Penis: Copulatory organ for sperm delivery.

• Scrotum: Houses testes; regulates temperature.

The Scrotum and Temperature Regulation

Mechanisms of Temperature Control

Sperm production requires a temperature lower than core body temperature. The scrotum employs three mechanisms to regulate testicular temperature.

• Cremaster muscle: Contracts in cold, drawing testes upward; relaxes in warmth.

• Dartos muscle: Contracts in cold, wrinkling scrotum and reducing heat loss.

• Pampiniform plexus: Network of veins acting as a countercurrent heat exchanger, cooling arterial blood before it reaches the testes.

Pathway of a Sperm

Journey from Formation to Ejaculation

Sperm travel through a series of ducts and structures from their formation to ejaculation.

1. Seminiferous tubule: Sperm development.

2. Rete testis: Sperm partially mature.

3. Efferent ductules: Move sperm via cilia.

4. Head/Body of Epididymis: Sperm mature (~20 days).

5. Tail of Epididymis: Sperm stored (40-60 days); reabsorbed if not used.

6. Ductus (Vas) deferens: Transports sperm.

7. Ampulla: End unites with seminal vesicle.

8. Ejaculatory duct: Last of spermatic ducts.

9. Urethra: Sperm actively swimming; expelled during ejaculation.

Accessory Glands and Their Functions

Gland	Secretion	Function
Seminal vesicles	Fructose-rich fluid	Energy source for sperm; forms 60% of semen
Prostate gland	Milky secretion	Enhances sperm motility; forms 30% of semen
Bulbourethral glands	Mucus	Lubricates urethra; neutralizes acidity

Sperm and Semen

Spermatogenesis and Meiosis

Spermatogenesis is the process of sperm production, involving meiosis to ensure genetic diversity and proper chromosome number.

• Remodeling: Large germ cells become small, motile sperm with flagella.

• Reduction: Chromosome number reduced by half (from diploid 2n=46 to haploid n=23).

• Shuffling: Crossing-over during meiosis creates genetic variation.

• Four sperm cells produced from one germ cell by meiosis.

Meiosis I: Chromosomes replicate, homologous chromosomes pair (tetrads), crossing-over occurs, homologs separate, reduction division yields haploid cells. Meiosis II: Similar to mitosis, but chromosomes are not replicated; results in haploid daughter cells.

Summary equation:

$2n \rightarrow n$

Where $2n$ is the diploid chromosome number and $n$ is the haploid number after meiosis.

Composition and Function of Semen

• Semen: Fluid expelled during orgasm; 2-5 mL per ejaculation.

• Composition: 60% seminal vesicle fluid, 30% prostatic fluid, 10% sperm and duct secretions.

• Normal sperm count: 50-120 million/mL; below 20-25 million/mL indicates infertility.

• Stickiness: Clotting enzyme activates proseminogelin, forming seminogelin to entangle sperm and keep semen in vagina.

• Serine protease liquefies semen after 20-30 minutes, activating sperm.

• Prostaglandins: Thin cervical mucus, stimulate uterine contractions.

• Requirements for motility:

  • Elevated pH: Prostatic fluid buffers vaginal acidity (pH 3.5 to 7.5).

  • Energy source: Fructose from seminal vesicles.

Male Sexual Response

Phases and Neural Control

Male sexual response is divided into four phases, each regulated by blood flow and neural input.

• Excitement: Vasocongestion, myotonia, increased heart rate and blood pressure; erection due to parasympathetic nitric oxide release.

• Plateau: Sustained high physiological variables.

• Orgasm: Intense reaction, semen discharge, elevated cardiovascular responses.

• Ejaculation: Two stages—emission (sperm propelled by peristalsis), expulsion (muscular contractions expel semen).

• Resolution: Return to baseline; refractory period follows.

Blood and Nerve Supply to the Penis

• Internal pudendal artery: Divides into dorsal and deep arteries.

• Dorsal artery: Supplies skin, fascia, corpus spongiosum.

• Deep artery: Supplies corpus cavernosa; dilation causes erection.

• Innervation: Dorsal nerve and internal pudendal nerves; both autonomic and somatic fibers.

• Sympathetic: Erection in response to sensory input and sexual thoughts.

• Parasympathetic: Erection in response to direct stimulation.

Summary Table: Phases of Male Sexual Response

Phase	Main Features
Excitement	Vasocongestion, erection, increased heart rate
Plateau	Sustained high physiological variables
Orgasm	Semen discharge, peak cardiovascular response
Ejaculation	Emission and expulsion of semen
Resolution	Return to baseline, refractory period

Additional info: These notes expand on the original slides by providing definitions, explanations, and context for key terms and processes, ensuring a comprehensive understanding suitable for college-level Anatomy & Physiology students.