Reproductive Systems: Structure, Function, and Hormonal Regulation

Reproductive Systems Overview

Introduction

The reproductive system is essential for sexual reproduction, involving the production, storage, and transport of gametes (sperm and ova). It also supports fertilization, implantation, and the development of offspring. Both male and female systems have specialized organs and hormonal controls.

• Sexual Reproduction: Male sperm (23 chromosomes) and female egg (23 chromosomes) combine to form a zygote (46 chromosomes).

• Zygote: A fertilized egg.

• Embryo: The stage after the egg is in the uterus and cell division begins.

• Fetus: Defined as the developing human from the 9th week after conception.

Gonads and Gametes

• Gonads: Primary sex organs that produce gametes and sex hormones.

• Female Gonad: Ovary; produces ova, estrogen, and progesterone.

• Male Gonad: Testis; produces sperm and androgens (e.g., testosterone).

Sex Differentiation

Genetic and Hormonal Basis

Sex differentiation is determined by genetic and hormonal factors, primarily the presence or absence of the SRY gene on the Y chromosome.

• SRY Gene: Triggers development of testes and testosterone production, leading to male sex organ development.

• Without SRY: Fetal sex organs remain female.

Common Intersex Conditions

• Intersex: Individuals born with sex characteristics that do not fit typical binary definitions.

Condition	Genetic Basis	Main Features
Non-classical Congenital Adrenal Hyperplasia	Excess testosterone	Masculinization of XX individuals
Klinefelter Syndrome	47, XXY	Extra X chromosome, underdeveloped male characteristics
Turner Syndrome	45, X	Females lack a second sex chromosome, ovarian development issues
Androgen Insensitivity Syndrome	XY, insensitive to androgens	Female or ambiguous sex characteristics

Female Reproductive System

Anatomy

• Ovaries: Produce ova and secrete hormones (estrogen, progesterone, relaxin).

• Fallopian (Uterine) Tubes: Transport ova from ovaries to uterus; site of fertilization.

• Uterus: Site of implantation and growth of embryo/fetus; consists of fundus, body, cervix.

• Vagina: Muscular canal for birth, menstruation, and intercourse.

• External Genitalia: Includes mons pubis, clitoris, labia minora, labia majora.

• Pelvic Floor: Muscles supporting pelvic organs; weakness can lead to prolapse or incontinence.

• Mammary Glands: Modified sweat glands in the breast, involved in milk production and secretion.

Ovarian Follicles and Ovulation

• Primordial Follicles: Immature ova; ~5 million before birth, ~300,000 at birth, ~400 mature during reproductive years.

• Follicle-Stimulating Hormone (FSH): Stimulates follicle growth.

• Ovulation: Expulsion of oocyte from ovary, typically on day 14 of a 28-day cycle, triggered by Luteinizing Hormone (LH).

• Corpus Luteum: Degenerated follicle post-ovulation; secretes estrogen and progesterone.

Uterus and Endometrium

• Layers of Uterus:

  • Perimetrium: Outer serous membrane.

  • Myometrium: Middle layer of smooth muscle; contracts during labor.

  • Endometrium: Inner lining; site of implantation, sheds during menstruation if no fertilization.

• Endometrium Structure:

  • Stratum Functionalis: Thickens and sheds during menstrual cycle.

  • Stratum Basalis: Replaces functionalis after shedding; not shed during menstruation.

• Blood Supply: Internal iliac arteries → uterine arteries → radial arteries in myometrium → straight and spiral arterioles.

Vagina

• Structure: 10 cm (~4 inches) long, smooth muscle tube lined with stratified squamous epithelium.

• Location: Posterior to bladder/urethra, anterior to rectum/anal canal.

• Function: Passageway for birth, menstruation, and intercourse.

External Genitalia

• Mons Pubis: Fat pad over pubic bone.

• Clitoris: Contains sensory neurons and erectile tissue; essential for orgasm.

• Labia Minora: Highly vascularized, contains sensory neurons and erectile tissue.

• Labia Majora: Skin, fat, connective and erectile tissue.

Mammary Glands

• Function: Milk production and secretion (lactation).

• Hormonal Control:

  • Prolactin: Stimulates lactation.

  • Oxytocin: Stimulates milk secretion.

  • Nursing: Stimulates further prolactin secretion.

• Features: Nipple, areola, suspensory ligaments, lactiferous ducts.

Female Reproductive Cycle

Phases of the Cycle

• Menstrual Phase (Days 1-5):

  • Primordial follicles begin to mature (FSH).

  • Top layer of endometrium is shed, causing bleeding.

• Proliferation Phase (Days 5-14):

  • Mature follicle releases ova (LH triggers ovulation).

  • Endometrium thickens (estrogen).

• Postovulatory Phase (Days 15-28):

  • Degenerating follicle releases progesterone and estrogen.

  • Endometrium fully thickens and is maintained (estrogen and progesterone).

Hormonal Regulation

• FSH (Follicle Stimulating Hormone): Stimulates follicle development.

• LH (Luteinizing Hormone): Causes ovulation.

• Estrogen: Thickens and maintains endometrium; stimulates development of endometrial glands and blood vessels.

• Progesterone: Maintains endometrium; stimulates development of endometrial glands and blood vessels.

Male Reproductive System

Anatomy

• Testes: Produce sperm and testosterone.

• Scrotum: Sac of skin holding and protecting testes; maintains temperature for sperm production.

• Epididymis: Stores and matures sperm.

• Vas/Ductus Deferens: Transports sperm from epididymis to ejaculatory duct.

• Ejaculatory Duct and Urethra: Deliver sperm to penis for ejaculation.

• Accessory Glands:

  • Seminal Vesicle: Produces substances to help sperm survive.

  • Prostate Gland: Ejaculatory duct and urethra run through it; adds fluid to semen.

  • Bulbourethral (Cowper's) Glands: Secrete alkaline fluid to neutralize urine acidity and lubricate urethra.

• Penis: Contains erectile tissue; fills with blood during arousal, causing erection.

Scrotum and Testes

• Scrotum: External sac; maintains temperature 2°C cooler than body for optimal sperm production.

• Testes:

  • Sperm production (spermatogenesis) occurs in seminiferous tubules.

  • Testosterone production occurs in interstitial (Leydig) cells.

  • Attached to body via spermatic cord (contains nerves, blood vessels, lymphatics, vas deferens).

Pathways of Gametes

Sperm Pathway

• Sperm produced in seminiferous tubules of testes.

• Stored and matured in epididymis.

• Transported via vas/ductus deferens.

• Passes through ejaculatory duct and urethra for ejaculation.

Egg Pathway

• Ova released from ovary during ovulation.

• Transported via fallopian/uterine tube (site of fertilization).

• If fertilized, travels to uterus for implantation.

Summary Table: Key Hormones and Functions

Hormone	Source	Main Function
FSH	Pituitary	Stimulates follicle development
LH	Pituitary	Triggers ovulation
Estrogen	Ovary	Thickens and maintains endometrium
Progesterone	Ovary	Maintains endometrium, supports pregnancy
Testosterone	Testis	Promotes sperm maturation, secondary male characteristics
Prolactin	Pituitary	Stimulates lactation
Oxytocin	Pituitary	Stimulates milk secretion

Key Equations

• Chromosome Number in Zygote: $23 ext{ (sperm)} + 23 ext{ (egg)} = 46 ext{ (zygote)}$

Additional info:

• Some anatomical and physiological details are inferred for completeness and clarity.

• Hormonal regulation and cycle phases are expanded for academic context.