Chapter 26: The Reproductive System – Structure, Function, and Physiology

Chapter 26: The Reproductive System

Introduction: The Reproductive System

The reproductive system is essential for the production of offspring and the continuation of species. It consists of primary sex organs (gonads) and accessory reproductive organs. The gonads (testes in males, ovaries in females) produce gametes and secrete sex hormones, while accessory organs support gamete transport, fertilization, and development.

• Gonads: Primary sex organs; produce gametes (sperm and ova) and secrete hormones (testosterone, estrogens).

• Accessory reproductive organs: Ducts, glands, and external genitalia that support reproductive function.

26.1 Overview of the Reproductive System and Meiosis

Chromosome Sets and Ploidy

Chromosome number is described by the term "-ploid." Most human cells are diploid (2n), containing two sets of chromosomes. Gametes are haploid (n), containing one set of chromosomes, produced by meiosis.

• Diploid (2n): Two sets of chromosomes (somatic cells).

• Haploid (n): One set of chromosomes (gametes: sperm and ova).

Mitosis vs. Meiosis

Mitosis and meiosis are two types of cell division. Mitosis produces genetically identical diploid cells for growth and repair. Meiosis produces genetically unique haploid gametes for sexual reproduction.

Feature	Mitosis	Meiosis
Location	Somatic cells	Germ cells (testes, ovaries)
Number of divisions	1	2
Daughter cells	2 diploid, identical	4 haploid, variable

Stages of Meiosis

Meiosis consists of two consecutive divisions: Meiosis I and Meiosis II. It results in four haploid cells, each genetically distinct due to crossing over and independent assortment.

• Meiosis I: Homologous chromosomes separate (reduction division).

• Meiosis II: Sister chromatids separate, similar to mitosis.

26.2 Anatomy of the Male Reproductive System

Structure and Function

The male reproductive system includes the testes, ducts, accessory glands, and the penis. The testes produce sperm and hormones, while ducts transport sperm and glands add secretions to semen.

• Testes: Site of sperm production and hormone secretion.

• Ducts: Epididymis (sperm maturation), ductus deferens (sperm transport), ejaculatory ducts, and urethra (shared with urinary system).

• Accessory glands: Seminal vesicles, prostate gland, bulbourethral glands (add fluids to semen).

• Penis: Organ of copulation; delivers sperm to female tract.

Testes and Seminiferous Tubules

The testes are divided into lobules containing seminiferous tubules, the site of spermatogenesis. Interstitial (Leydig) cells secrete testosterone, while sustentacular (Sertoli) cells support sperm development.

• Tunica vaginalis: Outer covering of the testis.

• Tunica albuginea: Fibrous capsule dividing testis into lobules.

• Seminiferous tubules: Site of sperm production.

Male Duct System and Penis

Sperm travel from the seminiferous tubules through the epididymis, ductus deferens, ejaculatory ducts, and urethra. The penis contains erectile tissues and the urethra for semen and urine passage.

• Epididymis: Sperm maturation and storage.

• Ductus deferens: Transports sperm to urethra.

• Penis: Contains corpora cavernosa and corpus spongiosum.

Accessory Glands

Accessory glands produce seminal fluid, which nourishes and protects sperm.

• Seminal vesicles: Secrete fructose-rich, alkaline fluid.

• Prostate gland: Secretes citrate, PSA, and antimicrobial chemicals.

• Bulbourethral glands: Secrete mucus and alkaline fluid.

26.3 Physiology of the Male Reproductive System

Spermatogenesis

Spermatogenesis is the process of sperm production in the seminiferous tubules. It involves mitosis, meiosis, and spermiogenesis (maturation of spermatids to spermatozoa).

• Spermatogonia: Diploid stem cells; divide by mitosis.

• Primary spermatocytes: Undergo meiosis I to form secondary spermatocytes (haploid).

• Secondary spermatocytes: Undergo meiosis II to form spermatids.

• Spermiogenesis: Spermatids mature into spermatozoa.

Hormonal Regulation

The hypothalamic-pituitary-gonadal axis regulates spermatogenesis and testosterone production.

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

• FSH (Follicle-stimulating hormone): Stimulates spermatogenesis.

• LH (Luteinizing hormone): Stimulates testosterone secretion by Leydig cells.

26.4 Anatomy of the Female Reproductive System

Structure and Function

The female reproductive system includes the ovaries, uterine tubes, uterus, vagina, external genitalia, and mammary glands. The ovaries produce ova and hormones, while other structures support fertilization, gestation, and childbirth.

• Ovaries: Site of oogenesis and hormone production.

• Uterine tubes (fallopian tubes): Transport ova; site of fertilization.

• Uterus: Site of implantation, fetal development, and labor.

• Vagina: Receives sperm, serves as birth canal.

• External genitalia (vulva): Protects internal organs.

• Mammary glands: Produce milk for newborns.

Uterine Tubes and Uterus

The uterine tubes transport ova and are the typical site of fertilization. The uterus supports menstruation, implantation, and fetal development.

• Fimbriae: Finger-like projections that help capture the ovulated oocyte.

• Uterine wall layers: Endometrium (inner), myometrium (muscular), perimetrium (outer).

Vagina, Vulva, and Mammary Glands

The vagina serves as the passage for sperm, menstrual flow, and childbirth. The vulva includes external genitalia, and the mammary glands are responsible for lactation.

• Vulva: Includes mons pubis, labia majora/minora, clitoris.

• Mammary glands: Modified sweat glands for milk production.

26.5 Physiology of the Female Reproductive System

Oogenesis

Oogenesis is the process of ovum formation in the ovaries. It begins before birth, pauses, and resumes at puberty, producing one mature ovum per cycle.

• Oogonia: Diploid stem cells; most degenerate before birth.

• Primary oocytes: Arrested in prophase I until puberty.

• Secondary oocyte: Released at ovulation; completes meiosis II only if fertilized.

Ovarian and Uterine Cycles

The female reproductive cycle consists of the ovarian cycle (follicle development and ovulation) and the uterine cycle (endometrial changes). Hormonal regulation synchronizes these cycles.

• FSH: Stimulates follicle development and estrogen secretion.

• LH: Triggers ovulation and corpus luteum formation.

• Estrogen: Promotes development of female structures.

• Progesterone: Prepares endometrium for implantation.

Phase	Ovarian Cycle	Uterine Cycle
1	Follicular (days 1-13)	Menstrual/Proliferative
2	Ovulation (day 14)	---
3	Luteal (days 15-28)	Secretory

26.6 Methods of Birth Control

Birth control methods prevent pregnancy by various mechanisms, including blocking sperm, inhibiting ovulation, or preventing implantation.

• Barrier methods: Condoms, diaphragms.

• Hormonal methods: Oral contraceptives, injections, vaginal rings.

• Sterilization: Vasectomy, tubal ligation.

• IUDs: Prevent implantation.

• Chemical methods: Spermicides.

• Behavioral methods: Rhythm, sympto-thermal methods.

26.7 Sexually Transmitted Infections (STIs)

STIs are caused by bacteria, parasites, or viruses and can disrupt reproductive health. Common examples include chlamydia, gonorrhea, syphilis, trichomoniasis (bacterial/parasitic), and HPV, genital herpes (viral).

• Bacterial/Parasitic STIs: Cause inflammation, discharge, and tissue damage.

• Viral STIs: Invade host cells and may cause chronic infection or cancer.

Summary Table: Spermatogenesis vs. Oogenesis

Feature	Spermatogenesis	Oogenesis
Onset	Puberty	Before birth
Process	Continuous	Cyclic
Duration	Throughout life	Until menopause
Gametes produced	Millions/day	1/month