D3.1 Reproduction

D3.1 Reproduction

Overview

Reproduction is the biological process by which organisms produce offspring, ensuring the continuation of their species. There are two fundamental types:

1. Asexual reproduction — Offspring produced from a single parent; genetically identical (clones)

2. Sexual reproduction — Offspring produced from fusion of gametes from two parents; genetically unique

This topic covers human reproductive systems, the menstrual cycle, fertilisation, and methods of contraception.

Asexual vs Sexual Reproduction

Asexual Reproduction

Definition: Production of offspring from a single parent without fusion of gametes.

Characteristics:

• One parent only

• No gametes or fertilisation

• Offspring are genetically identical to parent (clones)

• Rapid reproduction

• No genetic variation (except through mutation)

• Involves mitosis only

Types of asexual reproduction:

Advantages:

• Fast and efficient

• Only one parent needed

• All individuals can reproduce

• Successful genotype preserved

• Good in stable environments

Disadvantages:

• No genetic variation

• Cannot adapt quickly to environmental changes

• Harmful mutations passed to all offspring

• Susceptible to disease (all genetically identical)

Sexual Reproduction

Definition: Production of offspring from fusion of haploid gametes from two parents.

Characteristics:

• Usually two parents

• Involves meiosis (gamete production)

• Fertilisation (fusion of gametes)

• Offspring genetically unique

• Combines genetic material from both parents

• Generates genetic variation

Advantages:

• Genetic variation in offspring

• Ability to adapt to changing environments

• Harmful mutations can be "diluted" or eliminated

• Combines beneficial alleles from both parents

Disadvantages:

• Slower than asexual reproduction

• Requires finding a mate

• Only 50% of population (females) produce offspring directly

• Energy cost of mating behaviours and gamete production

Comparison Summary

Human Male Reproductive System

Structure and Function

Semen Composition

Semen = Sperm + Seminal fluid

Normal semen parameters:

• Volume: 2–5 mL per ejaculation

• Sperm count: >15 million/mL

• Motility: >40% moving

• Normal morphology: >4%

Sperm Structure

Sperm (spermatozoa) are highly specialised cells for delivering genetic material to the egg.

Adaptations of sperm:

• Streamlined shape for swimming

• Haploid nucleus (half chromosome number)

• Acrosomal enzymes for egg penetration

• Many mitochondria for energy

• Flagellum for motility

• Minimal cytoplasm (reduces size)

Spermatogenesis

Spermatogenesis is the production of sperm in the seminiferous tubules.

Location: Seminiferous tubules in testes

Duration: ~74 days from start to mature sperm

Process:

Key features:

• Continuous from puberty throughout life

• Temperature sensitive (testes outside body at ~35°C)

• Produces millions of sperm daily

• Regulated by FSH and testosterone

Hormonal Control of Male Reproduction

Negative feedback:

• High testosterone inhibits GnRH and LH release

• High inhibin inhibits FSH release

• Maintains stable hormone levels

Human Female Reproductive System

Structure and Function

Egg (Ovum) Structure

Adaptations of egg:

• Large cell with abundant cytoplasm (nutrients for early embryo)

• Haploid nucleus

• Non-motile (transported by cilia and muscle contractions)

• Protective layers (zona pellucida, corona radiata)

Oogenesis

Oogenesis is the production of eggs (ova) in the ovaries.

Key differences from spermatogenesis:

Stages:

Polar bodies: Small cells with little cytoplasm; receive one set of chromosomes but degenerate; ensure egg retains most cytoplasm.

The Menstrual Cycle

The menstrual cycle is the monthly cycle of changes in the female reproductive system that prepares for pregnancy.

Average duration: 28 days (range: 21–35 days)

Phases of the Menstrual Cycle

1. Menstruation (Days 1–5)

• Endometrium breaks down and is shed

• Blood and tissue exit through vagina

• Low levels of oestrogen and progesterone

• FSH begins to rise

2. Follicular Phase (Days 1–13)

Ovarian events:

• FSH stimulates follicle development

• Several follicles begin to develop; usually one becomes dominant

• Developing follicles produce oestrogen

• Oestrogen levels rise

Uterine events:

• Endometrium regenerates and thickens (proliferative phase)

• Blood vessel growth in endometrium

• Oestrogen stimulates endometrial growth

3. Ovulation (Day 14)

• Surge in LH (triggered by high oestrogen)

• Mature follicle ruptures

• Secondary oocyte released from ovary

• Oocyte swept into oviduct by fimbriae

• Egg viable for ~24 hours

4. Luteal Phase (Days 15–28)

Ovarian events:

• Ruptured follicle becomes corpus luteum

• Corpus luteum produces progesterone (and some oestrogen)

• Progesterone levels high

Uterine events:

• Endometrium maintained and further developed (secretory phase)

• Glands secrete nutrients for potential embryo

• Blood supply increases

If fertilisation occurs:

• Embryo implants in endometrium (~day 20–23)

• Embryo produces hCG (human chorionic gonadotropin)

• hCG maintains corpus luteum

• Progesterone continues to support pregnancy

If no fertilisation:

• Corpus luteum degenerates (~day 24)

• Progesterone and oestrogen levels fall

• Endometrium cannot be maintained

• Menstruation begins (day 1 of next cycle)

Hormonal Control of the Menstrual Cycle

Feedback Mechanisms

Negative feedback (most of cycle):

• Low/moderate oestrogen inhibits FSH and LH

• Progesterone inhibits GnRH, FSH, and LH

• Prevents multiple ovulations

Positive feedback (before ovulation):

• High oestrogen (at end of follicular phase) stimulates LH surge

• LH surge triggers ovulation

• Only time positive feedback occurs in the cycle

Graphical Representation

Hormone                                          
Levels   │                                       
         │           ╱╲ LH surge                
         │          ╱  ╲                        
         │    ╱────╱    ╲                       
         │   ╱ Oestrogen ╲    ╱───────╲         
         │  ╱              ╲  ╱Progesterone      
         │ ╱                ╲╱          ╲       
         │╱                              ╲      
         └────────────────────────────────────→
          1    7    14    21    28  Days
              │      │      │
         Menstruation │    Luteal phase
              Follicular  Ovulation
              phase

Endometrium │                    ┌────────────┐
Thickness   │          ┌────────┤            │
            │    ┌─────┤        │            │
            │────┤     │        │            └──
            └────────────────────────────────────→
             1    7    14    21    28  Days

Fertilisation

Fertilisation is the fusion of a sperm nucleus with an egg nucleus to form a diploid zygote.

Location

• Usually occurs in the ampulla (widest part of the oviduct)

• Within 24 hours of ovulation

Process of Fertilisation

1. Sperm Capacitation

• Sperm undergo changes in female reproductive tract

• Takes 6–8 hours

• Membrane changes allow acrosome reaction

• Increases sperm motility

2. Acrosome Reaction

1. Sperm reaches corona radiata

2. Sperm binds to zona pellucida (ZP3 glycoprotein)

3. Acrosomal membrane fuses with sperm membrane

4. Acrosomal enzymes released (hyaluronidase, acrosin)

5. Enzymes digest pathway through corona radiata and zona pellucida

3. Sperm-Egg Fusion

1. Sperm membrane fuses with egg membrane

2. Sperm nucleus enters egg cytoplasm

3. Sperm tail and mitochondria left outside (usually)

4. Egg is "activated"

4. Cortical Reaction (Block to Polyspermy)

1. Egg activation triggers calcium wave

2. Cortical granules release contents

3. Zona pellucida hardens (zona reaction)

4. Prevents additional sperm from entering

5. Polyspermy (multiple sperm fertilising egg) is lethal

5. Completion of Meiosis II

1. Egg completes meiosis II (was arrested at metaphase II)

2. Second polar body expelled

3. Female pronucleus forms

6. Pronucleus Formation and Fusion

1. Sperm nucleus decondenses → male pronucleus

2. Male and female pronuclei approach each other

3. Nuclear membranes break down

4. Chromosomes mix on first mitotic spindle

5. Zygote formed (diploid, 2n = 46)

Summary of Fertilisation Events

Early Embryonic Development

From Zygote to Implantation

Implantation

1. Blastocyst hatches from zona pellucida

2. Trophoblast cells contact endometrium

3. Trophoblast secretes enzymes

4. Blastocyst burrows into endometrium

5. Trophoblast forms connections with maternal blood supply

6. Will develop into placenta

Role of hCG

Human chorionic gonadotropin (hCG):

• Produced by trophoblast/developing placenta

• Maintains corpus luteum

• Corpus luteum continues producing progesterone

• Prevents menstruation

• Detected in pregnancy tests

• Levels peak at ~10 weeks, then placenta takes over hormone production

Contraception

Contraception refers to methods used to prevent pregnancy.

Methods of Contraception

1. Barrier Methods

Prevent sperm from reaching egg

Advantages:

• Protect against STIs (condoms)

• No hormones

• Used only when needed

Disadvantages:

• Must be used correctly each time

• May interrupt spontaneity

• Can break or slip

2. Hormonal Methods

Prevent ovulation and/or alter uterine lining

Mechanism of hormonal contraception:

1. Suppress FSH and LH → prevent follicle development and ovulation

2. Thicken cervical mucus → prevent sperm entry

3. Thin endometrium → prevent implantation

Advantages:

• Highly effective

• Some have non-contraceptive benefits (regulate periods, reduce acne)

• Long-acting options available

Disadvantages:

• Side effects (mood changes, weight gain, headaches)

• No protection against STIs

• Requires prescription

• Some have daily compliance requirement

3. Intrauterine Devices (IUDs)

Copper IUD mechanism:

• Copper is toxic to sperm

• Creates inflammatory response in uterus

• Prevents fertilisation and implantation

Advantages:

• Long-lasting

• No daily action required

• Copper IUD is hormone-free

Disadvantages:

• Insertion procedure required

• May cause heavier periods (copper)

• No STI protection

4. Natural Methods

Disadvantages:

• Require high motivation and discipline

• Less reliable than other methods

• No STI protection

5. Permanent Methods (Sterilisation)

Advantages:

• Permanent and highly effective

• No ongoing action required

Disadvantages:

• Difficult/impossible to reverse

• Surgical procedure required

• No STI protection

6. Emergency Contraception

Comparison of Contraceptive Methods

*Sterilisation reversal is difficult and not always successful

In Vitro Fertilisation (IVF)

IVF is an assisted reproductive technology where fertilisation occurs outside the body.

Indications for IVF

• Blocked or damaged oviducts

• Low sperm count or motility

• Endometriosis

• Unexplained infertility

• Genetic testing requirements

• Same-sex couples or single parents

IVF Process

1. Ovarian Stimulation

• FSH injections stimulate multiple follicle development

• GnRH agonists prevent premature ovulation

• Monitoring by ultrasound and blood tests

2. Egg Retrieval

• hCG injection triggers final egg maturation

• Eggs collected ~36 hours later

• Ultrasound-guided needle aspiration through vagina

• Usually 10–15 eggs retrieved

3. Sperm Collection and Preparation

• Sperm sample collected

• Washed and concentrated

• Best quality sperm selected

4. Fertilisation

Standard IVF:

• Eggs and sperm incubated together

• Sperm naturally fertilise eggs

ICSI (Intracytoplasmic Sperm Injection):

• Single sperm injected directly into egg

• Used for severe male factor infertility

5. Embryo Culture

• Fertilised eggs cultured in incubator

• Monitored for development

• Typically cultured to blastocyst stage (day 5)

6. Embryo Transfer

• One or two embryos transferred to uterus

• Via catheter through cervix

• Remaining embryos may be frozen

7. Luteal Support and Pregnancy Test

• Progesterone supplements support implantation

• Pregnancy test ~2 weeks after transfer

Success Rates

• Varies by age and cause of infertility

• Average ~30–40% per cycle for women under 35

• Decreases with maternal age

• May require multiple cycles

Ethical Considerations

Practical Investigations

Examining Reproductive Cells

Microscopy of sperm:

• Observe structure (head, midpiece, tail)

• Assess motility

• Count concentration

Microscopy of eggs (model organisms):

• Sea urchin or frog eggs commonly used

• Observe zona pellucida, corona radiata

Hormone Level Analysis

Interpreting hormone graphs:

• Identify phases of menstrual cycle

• Explain hormonal interactions

• Predict events (ovulation, menstruation)

Common Exam Questions

Typical Question Types

1. Compare sexual and asexual reproduction (4 marks)

   • Sexual: two parents; meiosis; fertilisation; genetic variation

   • Asexual: one parent; mitosis; no fertilisation; clones

   • Sexual slower but more adaptable

   • Asexual faster but no variation

2. Describe the hormonal control of the menstrual cycle (6 marks)

   • FSH stimulates follicle development

   • Follicles produce oestrogen

   • Oestrogen stimulates endometrial growth

   • High oestrogen triggers LH surge

   • LH surge causes ovulation

   • Corpus luteum produces progesterone

   • Progesterone maintains endometrium

   • Negative feedback controls hormone levels

3. Explain the events of fertilisation (5 marks)

   • Sperm undergoes capacitation

   • Acrosome reaction releases enzymes

   • Sperm penetrates zona pellucida

   • Membranes fuse; sperm enters egg

   • Cortical reaction prevents polyspermy

   • Meiosis II completed

   • Pronuclei fuse to form diploid zygote

4. Compare the structure of sperm and egg (4 marks)

   • Sperm: small, streamlined, flagellum for motility

   • Egg: large, non-motile, abundant cytoplasm

   • Both haploid

   • Sperm has acrosome; egg has zona pellucida

5. Explain how different contraceptive methods prevent pregnancy (5 marks)

   • Barrier: physically prevent sperm reaching egg

   • Hormonal: prevent ovulation; thicken cervical mucus

   • IUD: toxic to sperm (copper); thin endometrium (hormonal)

   • Natural: avoid intercourse during fertile period

   • Sterilisation: prevent gamete transport

6. Describe the stages of oogenesis (4 marks)

   • Oogonia divide by mitosis (before birth)

   • Primary oocytes begin meiosis I (arrested in prophase I)

   • At ovulation: meiosis I completed → secondary oocyte + polar body

   • At fertilisation: meiosis II completed → ovum + polar body

Key Terminology Glossary

Summary Comparison Tables

Male vs Female Gametes

Hormones of Reproduction

Menstrual Cycle Phases