The Reproductive System

Introduction to Sexual Reproduction

Sexual reproduction is the biological process by which organisms produce offspring through the combination of genetic material from two parents. This process ensures genetic diversity and is fundamental to the continuation of species.

• Sex determination: The sperm determines the sex of the child. Genetic makeup: XX for female, XY for male.

• Gonads (testes/ovaries): Produce gametes (sperm or oocyte) and secrete sex hormones.

Male Reproductive System

Anatomy and Function

• Testes (gonads): Produce sperm and testosterone. Each testis contains seminiferous tubules, where sperm are produced. Normal sperm production requires a temperature about 2-3°C below core body temperature, regulated by the cremaster and dartos muscles.

• Epididymis: Site of sperm maturation and storage; reabsorbs degenerated sperm.

• Ductus (vas) deferens: Conveys sperm during sexual arousal via peristaltic contractions; also stores sperm.

• Male reproductive ducts: Include ejaculatory ducts (formed by the union of the duct from the seminal vesicle and ampulla of ductus deferens) and the urethra (shared by reproductive and urinary systems; subdivided into prostatic, membranous, and spongy urethra).

• Accessory glands: Produce fluids added to semen. Include seminal vesicles, prostate gland, and bulbourethral (Cowper’s) glands.

• Semen: Composed of sperm and glandular fluids; slightly alkaline; average ejaculate contains about a teaspoonful with 250-400 million sperm. Less than 100 million sperm per ejaculate is considered infertility.

Accessory Glands

• Seminal Vesicles: Pair of pouch-like organs at the base of the bladder; produce ~60% of semen. Secrete alkaline fluid (neutralizes acidity), fructose (energy for sperm), clotting proteins (help retain semen in vagina), and prostaglandins (enhance sperm motility and viability).

• Prostate Gland: Inferior to bladder; contributes ~25% to semen. Secretes citric acid (for ATP production), proteolytic enzymes, and seminalplasmin (antibiotic). Prostate issues can affect urination.

• Bulbourethral (Cowper’s) Glands: Two small glands inferior to prostate; secrete alkaline fluid and mucus to lubricate and protect sperm in the urethra.

Penis

• Contains the urethra, serving as a passageway for semen and urine.

• Composed of three cylindrical masses of erectile tissue.

• The glans penis is the terminal part, with the external urethral orifice; covered by prepuce (foreskin) in uncircumcised males.

• The root is the attached portion.

Erection

• Parasympathetic fibers release nitric oxide (NO), causing smooth muscle relaxation in arterioles and increased blood flow, resulting in erection.

Sperm Structure and Production

• About 300 million sperm are produced daily.

• Sperm have a nucleus with 23 chromosomes (haploid, n).

• Survival in the female reproductive tract is up to 48 hours.

• Structure: Head (contains DNA), acrosome (enzymes for egg penetration), midpiece (mitochondria for energy), tail (flagellum for movement).

Male Puberty and Hormonal Control

• Begins around age 10-14.

• FSH and LH are produced in larger amounts by the anterior pituitary, regulated by GnRH from the hypothalamus.

• LH stimulates Leydig cells to secrete testosterone (synthesized from cholesterol), which is converted to DHT by 5 alpha-reductase in external genitals and prostate.

• FSH stimulates spermatogenesis; FSH and testosterone act on Sertoli cells to produce androgen-binding protein (ABP), maintaining high testosterone concentration.

• Sertoli cells release inhibin, which inhibits FSH.

• Androgens (testosterone and DHT) stimulate development of male reproductive system, secondary sexual characteristics, sexual function, and anabolism (protein synthesis, muscle and bone mass).

Cell Division: Mitosis & Meiosis

Mitosis

Mitosis is the process of somatic cell division, producing two genetically identical diploid cells (2n) for growth and tissue repair.

• Human somatic cells: 23 pairs of chromosomes (46 total).

• Phases: Interphase (DNA replication), Prophase (chromatin condenses), Metaphase (chromosomes align), Anaphase (chromatids separate), Telophase (nuclear envelopes reform), Cytokinesis (cytoplasm divides).

Meiosis

Meiosis is a specialized cell division producing gametes (sperm/egg) with half the chromosome number (haploid, n). It introduces genetic variation through crossing over and independent assortment.

• Two successive divisions: Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate).

• Results in four genetically unique haploid cells.

• Fertilization restores diploid number (2n = 46).

Spermatogenesis

Process Overview

Spermatogenesis is the process of sperm formation, occurring in the seminiferous tubules of the testes and taking about 65-75 days.

• Spermatogonia (2n): Stem cells divide by mitosis; some differentiate into primary spermatocytes.

• Primary spermatocytes (2n): Undergo meiosis I to form two secondary spermatocytes (n).

• Secondary spermatocytes (n): Undergo meiosis II to form four spermatids (n).

• Spermiogenesis: Spermatids mature into spermatozoa (sperm).

Spermatogenesis Sequence

Seminiferous Tubule Cells

• Spermatogenic cells: Form sperm.

• Sertoli (sustentacular) cells: Support and nourish developing sperm, form blood-testis barrier, secrete inhibin, regulate effects of testosterone and FSH.

• Interstitial (Leydig) cells: Secrete testosterone.

Female Reproductive System

Anatomy and Function

• Vagina: Fibromuscular canal from exterior to uterine cervix; acidic environment; muscularis with two layers of smooth muscle.

• Hymen: Partially closes vaginal orifice.

• Uterus: Fundus, body, isthmus, cervix; supported by ligaments. Three layers: perimetrium (outer), myometrium (muscular), endometrium (inner, with stratum functionalis and basalis).

• Blood supply: Uterine, arcuate, and radial arteries; straight arterioles supply stratum basalis, spiral arteries supply stratum functionalis.

• Cervical mucus: Produced by cervix; composition changes during cycle to facilitate or inhibit sperm passage.

• Uterine (fallopian) tubes: Transport oocytes/zygotes; infundibulum with fimbriae, ampulla (widest part), isthmus (joins uterus); peristaltic contractions move oocyte.

• Ovaries: Paired glands homologous to testes; produce gametes (secondary oocytes/eggs) and hormones (progesterone, estrogens, inhibin, relaxin).

Oogenesis and Follicular Development

• Oogenesis: Formation of gametes in ovary; begins before birth. Primordial germ cells become oogonia (2n), which develop into primary oocytes (arrested in meiosis I until puberty).

• At birth: 200,000–2,000,000 primary oocytes; ~40,000 at puberty; ~400 mature in a lifetime.

• Each month, FSH and LH stimulate development of several follicles; usually only one reaches ovulation.

• Follicular development: Primordial → primary → secondary → mature (Graafian) follicle. Ovulation releases secondary oocyte; if fertilized, completes meiosis II to form ovum and second polar body.

Female Reproductive Cycle

The female reproductive cycle includes the ovarian and uterine (menstrual) cycles, regulated by hormonal changes. Typical duration is 28 days.

• Menstrual phase (days 1–5): Shedding of stratum functionalis due to declining estrogen and progesterone.

• Preovulatory (proliferative) phase (days 6–13): Follicle growth; endometrium rebuilds under estrogen influence.

• Ovulation (day 14): Release of secondary oocyte triggered by LH surge.

• Postovulatory (luteal/secretory) phase (days 15–28): Corpus luteum forms, secretes progesterone and estrogens; prepares endometrium for implantation. If no fertilization, corpus luteum degenerates, hormone levels fall, and menstruation begins.

Hormonal Regulation of the Female Reproductive Cycle

• Gonadotropin-releasing hormone (GnRH): From hypothalamus; stimulates FSH and LH release from anterior pituitary.

• FSH: Initiates follicular growth; stimulates follicles to secrete estrogens.

• LH: Stimulates further follicle development, estrogen production, triggers ovulation, and promotes corpus luteum formation.

• Estrogens: Promote development of reproductive structures, secondary sex characteristics, protein synthesis, lower blood cholesterol, inhibit GnRH, LH, and FSH.

• Progesterone: Prepares and maintains endometrium, mammary glands; inhibits GnRH and LH.

• Relaxin: Relaxes uterus, increases flexibility of pubic symphysis, dilates cervix at end of pregnancy.

• Inhibin: Inhibits FSH and LH secretion.

Summary Table: Key Hormones and Their Functions

Key Equations and Definitions

• Diploid (2n): Two sets of chromosomes (46 in humans).

• Haploid (n): One set of chromosomes (23 in humans).

• Fertilization equation:

• Negative feedback: Testosterone and inhibin suppress FSH and LH secretion to regulate spermatogenesis and hormone levels.

Additional info: Some details, such as the exact temperature for sperm production (about 2-3°C below core body temperature), and the average number of sperm per ejaculate, were inferred from standard academic sources.