Human Reproductive Systems, Development, and Genetics

Reproductive Principles and the Human Biological System

• The survival of any species depends on reproduction, and the human species reproduces sexually through the joining of male and female sex cells to produce a zygote.

• The gonads are the primary sex organs that produce gametes.

• Secondary sex organs store gametes, facilitate fertilization, and support the developing baby.

The Male Reproductive System

• The male gonads consist of the testes (testicles), which are supported in a skin-covered pouch called the scrotum.

• The scrotum is divided into two sacs, each containing one testis.

• Sperm production requires a temperature approximately $2\,^{\circ}C$ lower than normal body temperature. Smooth muscle fibres in the scrotum contract or relax to move the testes closer to or further from the body for temperature regulation.

Internal Structure of the Testes

• Each testis is approximately $4.5\,\text{cm}$ long, $2.5\,\text{cm}$ wide, and $3\,\text{cm}$ thick.

• Testes contain between $200$ and $300$ lobules filled with seminiferous tubules.

• Seminiferous tubules are lined with cells that produce spermatozoa (sperm).

• Interstitial cells, located between the tubules, secrete testosterone.

Accessory Organs and Duct System

• Epididymis: A highly folded tubule (estimated at $5\text{--}6\,\text{m}$ if unraveled) on the rear surface of each testis; it stores sperm for up to a month while they finish maturing.

• Vas deferens (Sperm duct): Carries sperm from the epididymis into the abdominal cavity, looping behind the bladder to join the urethra.

• Urethra: A duct transporting both urine and semen out of the body through the penis.

• Penis: The organ used for transferring sperm to the female vagina. It contains erectile tissue with sponge-like spaces that fill with blood during arousal, causing enlargement and stiffness.

Seminal Fluid (Semen)

• Semen is a mixture of sperm and secretions from three glands:

  • Seminal vesicles: Pair of glands (approx. $5\,\text{cm}$ long) behind the bladder. They secrete a thick fluid rich in sugars (about $60\%$ of semen volume).

  • Prostate gland: Surrounds the urethra below the bladder. It secretes a thin, milky, alkaline fluid that activates sperm.

  • Bulbo-urethral glands (Cowper's glands): Small, pea-sized glands below the prostate. They secrete clear mucus as a lubricant before emission.

The Female Reproductive System

• The female gonads are the ovaries, located in the abdominal cavity and supported by ligaments.

• Each ovary is almond-shaped and approximately $3\,\text{cm}$ in length.

Internal Structure and Follicles

• The ovary consists of connective tissue (stroma) surrounded by a layer of germ cells.

• Each germ cell is enclosed in a follicle. As a follicle matures, it moves to the surface and ruptures to release an egg (ovulation).

Duct System and Uterus

• Uterine tubes (Fallopian tubes): Carry the egg from the ovary to the uterus. They feature fimbriae (finger-like projections) to guide the egg, and the lining contains cilia and smooth muscle to move the egg forward.

• Uterus (Womb): A hollow, pear-shaped organ. The wall consists of smooth muscle and a soft mucous membrane lining called the endometrium. It protects and nourishes the foetus.

• Cervix: The neck of the uterus that protrudes into the vagina.

• Vagina: A muscular canal (approx. $10\,\text{cm}$ long) that receives the penis and forms the birth canal.

• Vulva: External genitalia including the labia majora (fleshy folds with glands), labia minora (smaller skin folds), and the clitoris (highly sensitive erectile tissue).

Gametogenesis

• Gametogenesis: The process of gamete development from meiosis to mature cells.

Spermatogenesis

• Occurs in the seminiferous tubules of the testes.

• Spermatogonia (sperm mother cells): Diploid cells ($46$ chromosomes) that divide by mitosis to provide a continuous source of cells.

• Primary spermatocytes: Daughter cells that grow and undergo the first meiotic division.

• Secondary spermatocytes: Haploid cells ($23$ chromosomes) produced by the first division.

• Spermatids: Four haploid cells produced by the second meiotic division from one primary spermatocyte.

• Spermatozoa (Sperm): Mature sperm roughly $0.06\,\text{mm}$ long. They consist of:

  • Head: Contains nuclear material and an acrosome (fluid-filled vesicle with enzymes).

  • Neck: Connects head and middle piece.

  • Middle piece: Contains mitochondria for energy (respiration).

  • Tail (Flagellum): Provides propulsion.

• The entire process takes approximately $72$ days.

Oogenesis

• Occurs in the ovaries.

• Millions of diploid oogonia develop before birth. Some grow to become primary oocytes ($2n$) and begin prophase I but stop.

• At birth, they are enclosed in a primary follicle.

• After puberty, a follicle matures and the primary oocyte completes meiosis I, producing a large secondary oocyte ($n$) and a small first polar body.

• The secondary oocyte stops at metaphase II. Ovulation occurs.

• If fertilized, the second meiotic division completes, producing an ovum (mature egg) and a second polar body.

• One primary oocyte produces only one ovum.

Hormonal Regulation of Reproduction

• Endocrine glands release hormones into the extracellular fluid to be transported via the blood to target organs.

• Pituitary Gland: Regulates gonadal activity through gonadotropins:

  • Follicle-stimulating hormone (FSH): Stimulates follicle maturation in females and sperm production in the seminiferous tubules in males.

  • Luteinising hormone (LH): Promotes ovulation and corpus luteum formation in females; stimulates interstitial cells in males to secrete testosterone.

Other Reproductive Hormones

• Testosterone: Essential for sperm maturation and male secondary sexual characteristics.

• Oestrogens: Produced by follicles and the corpus luteum; develop female reproductive structures and secondary sexual characteristics.

• Progesterone: Produced by the corpus luteum; maintains the endometrium and prepares mammary glands.

• Prolactin: Maintains milk production.

• Oxytocin: Causes uterine contractions and milk let-down.

• Human chorionic gonadotropin (HCG): Produced by the placenta to maintain the corpus luteum during pregnancy.

Reproductive Cycles

The Ovarian Cycle

• Average length is $28$ days, with a range of $20\text{--}40$ days.

• Follicular Phase: FSH and LH stimulate follicle growth. A primary follicle develops into a secondary follicle as fluid accumulates, then into a mature (Graafian) follicle.

• Ovulation: Around Day $14$, a surge in LH causes the mature follicle to burst, releasing the oocyte.

• Luteal Phase: The ruptured follicle turns into the corpus luteum (yellow body). If fertilization doesn't occur, it degenerates into the corpus albicans (white body) after $8\text{--}10$ days.

The Menstrual Cycle

• Menstruation (Days 1--4): Shedding of the endometrium, accompanied by bleeding.

• Preovulation (Days 5--12): Endometrial repair and thickening; follicle development.

• Ovulation (Days 13--15): Release of the egg.

• Secretion (Days 16--20): Endometrial glands secrete watery mucus; development of the corpus luteum.

• Premenstruation (Days 21--28): Corpus luteum degenerates; endometrium deteriorates.

Fertilization and Early Development

The Fertilization Process

• Insemination: Deposition of sperm in the vagina.

• Sperm travel through the cervix and uterus to the uterine tubes within minutes, aided by muscular contractions.

• High sperm mortality means hundreds of millions are required, though only thousands reach the egg.

• Acrosomal Reaction: Enzymes in the sperm acrosome break down the corona radiata and zona pellucida.

• Once a sperm penetrates, a fertilization membrane forms to block others.

• The male pronucleus and female pronucleus fuse to form a diploid zygote.

Pre-Embryonic Sequence

• Cleavage: The zygote undergoes mitosis to form two, four, eight, and sixteen cells.

• Blastocyst: By Day $6$, a hollow ball of cells reaches the uterus. It contains an inner cell mass (embryoblast) of stem cells.

• Implantation: The blastocyst sinks into the endometrium to gain nourishment.

• Embryonic period: First two months of pregnancy.

• Foetal period: After the second month.

Stem Cells and Germ Layers

• Totipotent stem cells: Early embryo cells capable of forming an entire human and all membranes.

• Pluripotent stem cells: Inner cell mass cells capable of giving rise to foetal tissues but not placental tissue.

• Multipotent stem cells: Specified cells giving rise to specific functions (e.g., blood stem cells).

Primary Germ Layers

• Ectoderm: Outermost layer; forms skin, hair, glands, and the nervous system.

• Mesoderm: Middle layer; forms muscles, skeleton, heart, blood, and connective tissue.

• Endoderm: Innermost layer; forms linings of the digestive tract, lungs, and liver.

Embryonic Membranes and the Placenta

• Amnion: Secretes amniotic fluid to protect the embryo as a shock absorber and maintain temperature.

• Chorion: Becomes the foetal portion of the placenta.

• Placenta: An organ (surface area approx. $16\,\text{m}^2$) providing nutrition, excretion, respiration, and hormone production. Maternal and foetal blood do not mix; exchange occurs via diffusion across chorionic villi.

• Umbilical Cord: Contains two umbilical arteries (carrying blood to the placenta) and one umbilical vein (returning blood to the foetus).

Gestation and Birth (Parturition)

• Gestation: Typically $280$ days from the last menstrual period.

• Stage 1: Dilation of the cervix ($8\text{--}9$ hours for first child). Waves of contraction dilate the cervix to $10\,\text{cm}$.

• Stage 2: Stage of expulsion ($20$ minutes to $2$ hours). The baby is pushed through the birth canal (vagina).

• Stage 3: Placental stage (Afterbirth). Expulsion of the placenta and membranes approx. $5$ minutes after birth.

Post-Natal Circulatory Changes

• The foramen ovale (opening between atria), ductus arteriosus (lung bypass), and ductus venosus (liver bypass) must close as the baby begins to use its own lungs and liver.

• The first breath is triggered by high carbon dioxide levels and the shock of birth.

Maternal Health and Teratogens

• Diet: Requires increased energy (approx. $850\,\text{kJ/day}$ in the second half) and protein ($65\,\text{g/day}$). Folic acid prevents neural tube defects (Spina Bifida).

• Teratogens: Agents causing physical defects.

  • Alcohol: Causes Foetal Alcohol Syndrome (FAS) (stunted growth, intellectual disability).

  • Smoking: Linked to lower birth weight and Sudden Infant Death Syndrome (SIDS).

  • Thalidomide: A drug from the 1950s causing limb malformations if taken between Days $28$ and $42$.

  • Rubella: Viral infection; if contracted during the first $10$ weeks, risk of deafness/blindness is $90\%$.

Contraception and Birth Control

• Natural Methods: Periodic abstinence (Rhythm, Temperature, Mucus methods), coitus interruptus (withdrawal), and lactational amenorrhoea (breastfeeding suppression of ovulation).

• Mechanical Barriers: Condom (prevents STIs), diaphragm/cervical cap (rubber caps used with spermicide), female condom.

• Chemical: Spermicides (immobilize sperm).

• Hormonal:

  • Combined pill (oestrogen + progesterone).

  • Mini pill (progestogen only).

  • Implants (Implanon NXT; lasts $3$ years).

  • Injections (Depo-Provera; lasts $12$ weeks).

• Intrauterine Devices (IUDs): Plastic/copper devices in the uterus (Mirena provides protection for $5$ years; copper IUDs up to $10$ years).

• Emergency Contraception: Morning-after pill (effective up to $72$ hours).

• Sterilization: Vasectomy (cutting vas deferens) and Tubal Ligation (cutting/clamping uterine tubes).

Sexually Transmitted Infections (STIs)

• Chlamydia: Caused by Chlamydia trachomatis. Often asymptomatic (silent infection); can lead to pelvic inflammatory disease (PID) and infertility.

• Gonorrhoea: Caused by Neisseria gonorrhoeae. Causes pain/discharge in males; often asymptomatic in females; can cause blindness in newborns.

• Syphilis: Caused by Treponema pallidum. Stages: Primary (chancre sores), Secondary (skin rash), Latent (hidden), and Tertiary (heart/brain damage).

• Genital Herpes: Caused by Herpes simplex virus (HSV-1 and HSV-2). Recurrent blisters; no cure.

• Genital Warts: Caused by Human Papillomavirus (HPV). Linked to cervical cancer.

• HIV/AIDS: Human Immunodeficiency Virus (retrovirus) destroys T-lymphocytes. Transmitted via blood, semen, and breast milk. Managed with antiretroviral drugs.

Assisted Reproductive Technology (ART)

• Infertility: Inability to conceive after one year of unprotected sex. Affects $1$ in $6$ Australian couples.

• Ovulation Induction: Use of FSH or Clomiphene to stimulate follicles.

• Artificial Insemination (IUI): Concentrated sperm released into the uterus via catheter.

• Gamete Intrafallopian Transfer (GIFT): Sperm and egg mixed and injected directly into uterine tubes.

• In Vitro Fertilisation (IVF): Fertilization occurs in a lab; embryos are transferred to the uterus $2\text{--}6$ days later.

• Intracytoplasmic Sperm Injection (ICSI): Single sperm injected directly into an egg.

• Surrogacy: Another woman carries the baby for a couple.

Foetal Diagnosis and Genetic Analysis

• Ultrasound: High-frequency sound waves create an image of the foetus.

• Amniocentesis: Administered between weeks $16\text{--}20$. Drawing amniotic fluid for genetic testing. Risk of miscarriage is small.

• Chorionic Villus Sampling (CVS): Foetal cells taken from the chorion at $9\text{--}19$ weeks. Higher risk of miscarriage ($2\%$).

• Foetal Monitoring: Recording heart rate during labour via ECG.

• Fetoscopy: Direct visual examination via fibre-optic scope.

• Biochemical analysis: Testing for marker proteins like Alpha-fetoprotein (AFP) for spina bifida or screening newborns for Phenylketonuria (PKU).

Genetics and Inheritance

• Gregor Mendel: Identified that traits are controlled by genes (factors) appearing in pairs.

• Alleles: Alternative forms of a gene. Dominant alleles mask recessive ones.

• Genotype: Genetic make-up (e.g., $AA$, $Aa$, $aa$).

• Phenotype: Physical appearance.

• Homozygous: Having two of the same allele.

• Heterozygous: Having two different alleles.

• Punnett Squares: Used to calculate probability of genotypes/phenotypes.

• Pedigrees: Family trees using squares (males) and circles (females). Shaded shapes have the trait.

• Sex-linked inheritance: Traits carried on the X chromosome (e.g., Res-green colour blindness, Haemophilia). Males are hemizygous ($X^hY$) and show the trait more often.

• Co-dominance: Both alleles expressed (e.g., MN blood types).

• Multiple Alleles: More than two alternatives (e.g., ABO blood system with alleles $I^A$, $I^B$, and $i$).

• DNA Profiling: Uses gel electrophoresis to create a unique banding pattern for identification, paternity testing, or heritage tracking.