Comprehensive Study Guide: The Human Reproductive System, Fertilisation, and Development

Gametes and Reproductive Systems

• Definition of Gametes: Gametes are the scientific terms used to describe sex cells.     * Male Gamete: Known as the sperm.     * Female Gamete: Known as the egg (ovum).
• Male Reproductive System Structures and Functions:     * Bladder: Stores urine before excretion (not directly involved in gamete production but part of the local anatomy).     * Testes: The specific location where sperm cells are produced.     * Sperm Duct: Responsible for carrying sperm from the testes to the penis.     * Urethra: A tube that transports both urine and semen out of the penis; part of both the urinary and reproductive systems.     * Penis: The organ inserted into the vagina to deposit sperm during sexual intercourse.
• Female Reproductive System Structures and Functions:     * Ovary: The site of egg production.     * Oviduct (Fallopian Tube): Carries eggs from the ovaries to the womb/uterus using microscopic hair-like structures called cilia. This is also the specific site where fertilisation occurs.     * Uterus (Womb): Features a thick, spongy lining composed of blood where the baby (embryo/foetus) develops.     * Vagina: The passage where the penis is inserted during intercourse.     * Cervix: A ring of muscle located at the bottom of the vagina (connecting it to the uterus).

Fertilisation and Early Development

• Fertilisation Process: Fertilisation occurs when the nucleus of an egg fuses with the nucleus of a sperm.
• Conception: This is the moment of fertilisation, signifying the "beginning" of a new life.
• Genetic Information:     * DNA is held within the nuclei of the gametes.     * At the point of fertilisation, this genetic information joins together.
• Chromosomal Composition:     * Sperm (Male Gamete): Contains $23$ chromosomes.     * Egg (Female Gamete/Ovum): Contains $23$ chromosomes.     * Zygote: Formed after fertilisation, containing $46$ chromosomes arranged in $23$ pairs.     * Embryo: Also contains $46$ chromosomes in $23$ pairs.
• Cell Division and Growth:     * Following fertilisation, the zygote begins to divide to produce a large ball of cells.     * This ball of cells eventually develops into an embryo.     * Numeracy Data on Division: In the early stages, a zygote may divide every $30\,\text{minutes}$. The growth progression of cells over time is as follows:         * At $0\,\text{minutes}$: $1$ cell (the zygote).         * At $30\,\text{minutes}$: $2$ cells.         * The division continues exponentially until it forms an embryo of over $1000$ cells.

Chronology of Embryonic and Foetal Development

• Developmental Stages and Sizes:     * $4$ weeks: The embryo is approximately $1/4\,\text{inch}$ long.     * $6$ weeks: The embryo is approximately $1/2\,\text{inch}$ long.     * $8$ weeks: This is the biological threshold where the embryo is officially reclassified as a foetus.     * $12$ weeks: The foetus is approximately $3\,\text{inches}$ long.     * $14$ weeks: The foetus is approximately $6\,\text{inches}$ long.     * $20$ weeks: The foetus is approximately $8\,\text{inches}$ long.     * $24$ weeks: The foetus is approximately $14\,\text{inches}$ long.     * $32$ weeks: The foetus is approximately $16\,\text{inches}$ long.     * $40$ weeks (Birth): The baby is born, typically measuring between $19\text{-}21\,\text{inches}$ long.
• Comparative Embryology: There are notable similarities in the early embryonic development of different animal species, including fish, chickens, pigs, and humans.

Structures in Pregnancy

• Uterus: The organ where the baby grows throughout pregnancy.
• The Placenta: A critical interface where the mother's blood and the baby's blood come into close proximity (without mixing directly).     * Function: Oxygen and food (nutrients) are passed from the mother to the baby. Waste products, such as carbon dioxide, are passed from the baby to the mother.
• The Umbilical Cord: The physical connection that joins the placenta to the baby at the site that will become the belly button.     * Function: Attaches the baby to the placenta.
• The Amniotic Sac: A protective sac surrounding the foetus.     * Amniotic Fluid: The fluid contained within the sac.     * Function: Protects the baby from external knocks and bumps.
• Trimesters: Pregnancy is divided into three stages: the First, Second, and Third trimesters.

Harmful Agents and Environmental Factors

• Placental Permeability: Certain harmful substances are capable of crossing the placenta and damaging the developing baby.
• Identified Harmful Substances:     * Alcohol.     * Nicotine.     * German measles virus.     * Certain medications/medicines.
• Specific Impacts of Alcohol and Smoking:     * Smoking: Correlated with lower birth weights.     * Foetal Alcohol Syndrome (FAS): A result of alcohol consumption during pregnancy with specific long-term symptoms.
• Numerical Data: Smoking and Birth Weight (Sample of $10,000$ births over $5$ years):     * $0$ cigarettes per day: $3.4\,kg$ average birth weight.     * $5$ cigarettes per day: $3.3\,kg$ average birth weight.     * $10$ cigarettes per day: $3.2\,kg$ average birth weight.     * $15$ cigarettes per day: $3.1\,kg$ average birth weight.     * $20$ cigarettes per day: $2.9\,kg$ average birth weight.     * $25$ cigarettes per day: $2.8\,kg$ average birth weight.     * $30$ cigarettes per day: $2.7\,kg$ average birth weight.     * Trend: As the number of cigarettes smoked per day increases, the average birth weight of the baby decrease linearly.

Multiple Births and Twin Formation

• Identical Twins (Monozygotic):     * Formation: One egg is fertilised by one sperm. The resulting zygote/ball of cells then divides to form two separate embryos.     * Characteristics: They are the same sex and look identical because they share the same genetic information (DNA).     * Prevalence: Occurs in approximately $1$ in $250$ births. Identical twins account for about $33\%$ of all twins born.
• Non-identical Twins (Dizygotic):     * Formation: Two separate eggs are released at the same time, and each is fertilised by a different sperm.     * Characteristics: They look different and are genetically only as similar as standard siblings.     * Influence: The chance of having non-identical twins is influenced by the mother's family history.
• Statistics and Trends in Multiple Births:     * Multiple births are increasing due to the use of fertility drugs and older maternal ages.     * Maternal Age: Between $1996$ and $2006$, there was a $182\%$ increase in multiple births in women aged $35$ and over.     * Fertility Drugs: Natural conception results in twins in about $1.25\%$ of births, compared to $25\%$ when fertility drugs are used.
• Ultrasound Identification: Indicators of identical twins on a scan include the babies sharing a single sac with no separating membrane or sharing one placenta. However, a DNA test is the only definitive confirmation.

Infertility and Assisted Reproduction

• Infertility Definition: The inability of a couple to conceive a baby naturally.
• Identified Causes of Infertility:     * Female Infertility: Blocked oviducts, STDs, obesity, age, cancer, alcohol/cigarettes.     * Male Infertility: Low sperm count, varicocele, STDs, obesity, cancer, alcohol/cigarettes.     * General: For $25\%$ of couples, the cause is unidentifiable. Preventable factors include obesity, eating disorders, and alcohol abuse.
• IVF (In Vitro Fertilisation):     * Process: An egg is removed from the woman's ovaries and fertilised with sperm in a laboratory environment. The resulting embryo is then returned to the woman's uterus to grow.     * History: Louise Brown was the world's first "test-tube baby," born via IVF on $25th\,July\,1978$.

Questions & Discussion

• Starter Diagram Description: The diagram shows a sperm and an egg joining during fertilisation to form a zygote ($46$ chromosomes), which then becomes an embryo.
• Word Grid Exercise: Key terms include Sperm, Vagina, Twins, Egg, Oviduct, Cervix, Uterus, Nucleus, Ovaries, Teste, DNA, and Double Helix.
• Placenta and Cord Discussion:     * Q: What is the name of the cord connecting mother to baby? A: The umbilical cord.     * Q: What substances pass to the baby? A: Oxygen and food/nutrients.
• Structure Matching:     * 1. Protects baby: Amniotic sac.     * 2. Attaches baby to the placenta: Umbilical Cord.     * 3. Oxygen/food to baby, waste to mother: Placenta.
• Twin and Infertility Starter:     * Q: How are identical and non-identical twins different? A: Identical twins come from one egg/sperm and are genetically the same; non-identical come from two eggs/sperm and are like siblings.     * Q: Why might couples be unable to conceive? A: Blocked tubes, low sperm count, health issues, or lifestyle factors.