Reproduction, Lifespan, and IVF

Reproduction and Lifespan: Human Fertility and In Vitro Fertilization

Presenter: Prof Karl Swann

Total Fertility Rate (TFR) and Global Trends

Definition of Total Fertility Rate (TFR): The total fertility rate is a summary metric representing the total number of births a woman would be expected to have if she experienced the specific birth rates observed in women of each age group in one particular year throughout her entire childbearing lifespan.

Replacement Level:

• The replacement level fertility is established at a value of $2.1$.

Global Trends and Comparisons (1950–2023):

• Nigeria: Historically high fertility rates, ranging from $6$ to $7$ births per woman, showing a moderate decline but remaining significantly above the global average.
• World Average: Has seen a steady decline from approximately $5$ births in $1950$ toward the replacement level of $2.1$.
• Europe (UN Data): Currently sits below the replacement level, trending between $1$ and $2$ births per woman.
• South Korea: Exhibits one of the lowest fertility rates globally, significantly below the replacement level and lower than the European average as of $2023$.

Individual Female Fertility and Age-Specific Changes

Fertility Decline with Maternal Age:

• There is a sharp decline in the likelihood of pregnancy as women age.
• Data indicates that female fertility is highest between the ages of $20$ and $30$.
• A significant drop-off occurs after age $35$, with the likelihood of pregnancy approaching near-zero levels as a woman reaches $50$.

Aneuploidy in Human Eggs:

• Aneuploidy (the presence of an abnormal number of chromosomes) in oocytes is a primary driver of age-related fertility decline.
• Maternal Age < 20: Low incidence of aneuploidy (< 20\%).
• Maternal Age 20–32: Moderate incidence.
• Maternal Age \ge 33: Sharp increase in aneuploidy, exceeding $60\%$ in women over the age of $35$ to $40$.

IVF Data and Egg Quality

Data from the Human Fertilisation and Embryology Authority (HFEA) highlights the impact of oocyte quality on birth rates per embryo transferred:

• Patient Eggs: The birth rate per embryo transferred drops precipitously with age, from approximately $25-30\%$ in the early $20s$ to less than $5\%$ by age $45$.
• Donor Eggs: When eggs from younger donors are used, the birth rate remains relatively stable (between $25\%$ and $30\%$) regardless of the recipient mother's age. This confirms that the decline in fertility is primarily due to oocyte quality rather than uterine environment.

Female Germ Cell Development and Folliculogenesis

Germ Cell Depletion Timeline:

• 5-6 Months Post-Conception: Germ cell numbers peak at approximately $7.0 \times 10^6$ (7 million).
• Birth: Numbers drop to approximately $1.0 \times 10^6$ to $2.0 \times 10^6$.
• Age 10: Numbers decline further to approximately $0.3 \times 10^6$ to $0.6 \times 10^6$.
• Age 50: Germ cell supply is essentially exhausted ($\approx 0$).

Follicular Anatomy and Stages:

1. Primordial Follicle: The resting stage.
2. Maturing Follicle: Identified by the development of follicular cells and the Zona Pellucida.
3. Vesicular or Graafian Follicle: A mature follicle containing:     - Follicular Antrum: Fluid-filled cavity.     - Theca Interna and Externa: Outer layers.     - Primary Oocyte: Progressing through meiosis.     - Secondary Oocyte: Formed after division, arrested in Meiotic Metaphase II (MII).

Male Fertility and Sperm Transport

Declining Sperm Counts:

• Research (Sharpe, 2012; Hum Reprod Update, 2023) indicates a trend of declining sperm counts in the male population.

Sperm Journey in the Female Tract:

• Initial Entry: > 100 \times 10^6 (100 million) sperm deposited.
• Fallopian Tubes: $\approx 1,000$ sperm reach the tubes.
• Around the Oocyte: Only $\approx 10$ sperm successfully reach the immediate vicinity of the egg.
• Timing: Fertilization typically occurs between a few hours and $6$ days after copulation.

Sperm Function: Capacitation and the Acrosome Reaction

Capacitation:

• Occurs during incubation in the female tract (modeled in media containing bicarbonate).
• Characterized by an increase in cAMP and Ca^{2+} in the principal piece of the sperm tail.
• Results in Hyperactivation, marked by increased intracellular $pH$ and intensified motility.

Acrosome Reaction:

• Involves a large $Ca^{2+}$ increase in the sperm head.
• The reaction occurs before the sperm reaches the Zona Pellucida.
• Potentially stimulated by Progesterone.

The Role of CatSper Channels:

• $Ca^{2+}$ influx is mediated by CatSper channels.
• These channels are activated by Progesterone and an increased pH.
• Progesterone has been shown to stimulate specific $Ca^{2+}$ oscillations in human spermatozoa (Kelly et al., 2018).

The Mechanism of Fertilization

Sperm-Egg Interaction:

• The sperm must penetrate the Zona Pellucida (ZP) and the plasma membrane of the egg.
• Sperm-Egg Fusion (Mammals):     - Involves the Inner Acrosomal Membrane (iam) and the Equatorial Segment (eq) of the sperm head.     - Occurs in the Perivitelline Space (pvs).     - Followed by the release of Cortical Granules (cg) to prevent polyspermy.

Infertility: Factors and Prevalence

Definition: Infertility is defined as the failure to conceive after $1$ or $2$ years of regular unprotected intercourse. It affects > 10\% of couples.

Female Factors (Totaling 49.4% in specific study data):

• Ovulation Problems: $19.8\%$
• Tubal Factors: $10.1\%$
• Uterine Factors: $8.3\%$
• Combined factors (Ovulation/Tubal/Uteral): Various percentages ranging from $2.2\%$ to $7.7\%$.

Male Factors (Totaling 57.5% in specific study data):

• Abnormal Sperm: $13.8\%$
• Varicocele: $6.6\%$
• Post-infectious factors: $3.7\%$
• Congenital factors: $2.3\%$
• Endocrine defects: $2.2\%$
• Sexual factors: $8.4\%$
• Other factors: $6.3\%$

In Vitro Fertilization (IVF) and Assisted Reproduction

History and Statistics:

• 1978: First IVF baby, Louise Brown, born through the work of Bob Edwards, Jean Purdy, and Patrick Steptoe.
• Current Scale: $\approx 4 \times 10^6$ (4 million) IVF treatments performed annually, resulting in $\approx 1 \times 10^6$ (1 million) live births.
• Prevalence: Accounts for $2\%$ of births in the UK and up to $10\%$ in other regions.

The IVF Process:

1. Super Ovulation: Using hormones like LH (Luteinizing Hormone) and FSH (Follicle Stimulating Hormone) to produce $\approx 10$ oocytes per cycle.
2. Sperm Selection: Methods include the Swim-up technique in culture media or Density gradient centrifugation.
3. Fertilization: Requires $\approx 10 \times 10^6$ sperm for standard IVF. The process takes up to $16$ hours.

Oocyte State During Fertilization:

• The human oocyte is ovulated and arrested at Meiotic Metaphase II.
• Upon fertilization, it completes meiosis, releasing the 2nd polar body.
• Formation of the female pronucleus and male pronucleus follows during interphase.

The Physiology of Egg Activation: Calcium Signaling and PLC Zeta

Calcium Oscillations:

• Fertilization triggers a series of $Ca^{2+}$ oscillations (spikes) in the egg cytoplasm.
• In humans, these spikes can reach concentrations of $1500$ to $2250\,nM$.

Phospholipase C zeta (PLC\zeta):

• A sperm-specific 70 kDa protein identified in all mammals.
• Mechanism: It acts on InsP3 (Inositol trisphosphate) pathways to trigger the release of $Ca^{2+}$ from internal stores.
• Localization: Antibody staining shows PLC\zeta is localized in the Inner Acrosomal Membrane (iam) and the Equatorial Segment (eq) of the sperm head in species like mouse and pig.

Intracytoplasmic Sperm Injection (ICSI)

• Definition: A procedure where a single sperm is injected directly into the egg.
• Prevalence: Accounts for $\approx 70\%$ of all IVF treatments.
• Efficacy: High spikes in $Ca^{2+}$ ($\ge 10$ spikes) are associated with successful activation.

Comparative Assisted Reproductive Technologies (ART)

• IUI (Intrauterine Insemination): Sperm is introduced into the uterus, bypassing the cervix.
• IVF: Fertilization occurs in a dish; sperm must penetrate the cumulus cells and zona pellucida.
• ICSI: Bypasses all physiological barriers (cumulus, ZP, oolemma) by direct injection.
• Sperm Retrieval: Used for surgical collection of sperm when not present in the ejaculate.

Embryo Transfer and Adjunct Technologies

Embryo Transfer Risks:

• Multiple Births: IVF/ICSI increases the risk of twins ($5-25\%$) and triplets ($3\%$) compared to natural rates ($2\%$\ for twins).
• Strategy: Often $1$ or $2$ embryos are transferred; rigorous embryo selection is vital to balance success rates and multiple-birth risks.

Adjunct Technologies:

1. Time-lapse Imaging: Used for non-invasive embryo selection based on developmental kinetics.
2. Embryo Cryopreservation: Utilizes vitrification (ultra-rapid freezing) for storage.
3. Pre-implantation Screening: Testing for aneuploidy to ensure chromosomal normalcy before transfer.

Learning Objectives Summary

1. Understand the underlying causes of low fertility in the human population.
2. Explain the physiological mechanisms of fertilization based on observations from IVF.
3. Evaluate the benefits and associated risks of various IVF technologies.