Page 3 of 6

AN77.1-6 | Gametogenesis and fertilization — Part 2

Fertilisation: Stages, Mechanisms, and Consequences

Fertilisation (AN77.4) is the process by which two haploid gametes (spermatozoon and secondary oocyte) fuse to form a diploid zygote. In humans, fertilisation normally occurs in the ampulla of the fallopian tube (the widest part, closest to the ovary), usually within 12-24 hours of ovulation. The oocyte remains viable for approximately 24 hours after ovulation, while spermatozoa can survive in the female reproductive tract for up to 5 days (though their fertilising capacity is optimal for 1-2 days). This defines the fertile window of the menstrual cycle as approximately 6 days: the 5 days before ovulation plus the day of ovulation.

Figure: Fertilisation: Stages, Mechanisms, and Consequences

Figure: Fertilisation: Stages, Mechanisms, and Consequences

Before fertilisation can occur, spermatozoa must undergo capacitation — a series of physiological changes that occur during their passage through the female reproductive tract (primarily in the uterus and fallopian tube, over 6-8 hours). Capacitation involves: removal of glycoprotein coating and seminal plasma proteins from the sperm surface, cholesterol efflux from the sperm membrane (increasing membrane fluidity and permeability), influx of calcium ions, and hypermotility (vigorous, whip-like flagellar movement). Only capacitated spermatozoa can undergo the acrosome reaction and penetrate the oocyte. Of the approximately 200-300 million spermatozoa deposited in the vagina during ejaculation, only about 200-300 reach the ampulla — the vast majority are lost during transit.

The stages of fertilisation are:

1. Penetration of the corona radiata: The corona radiata is the outer layer of cumulus cells (granulosa cells) surrounding the ovulated oocyte. Spermatozoa penetrate this layer through a combination of sperm motility (the hyperactivated flagellar beating of capacitated sperm) and the action of hyaluronidase released from the sperm surface (which digests the hyaluronic acid matrix between cumulus cells).

2. Penetration of the zona pellucida (acrosome reaction): The zona pellucida is a thick glycoprotein layer surrounding the oocyte, composed of four glycoproteins: ZP1, ZP2, ZP3, and ZP4. ZP3 acts as the primary sperm receptor, binding to specific receptors on the sperm head and triggering the acrosome reaction — the exocytosis of the acrosomal contents (including acrosin, a serine protease, and other hydrolytic enzymes) that digest a pathway through the zona pellucida. The sperm penetrates the zona at an oblique angle, propelled by its hypermotile flagellar movement.

3. Fusion of sperm and oocyte membranes: After passing through the zona pellucida, the spermatozoon enters the perivitelline space and its cell membrane fuses with the oocyte plasma membrane (oolemma). This fusion occurs at the equatorial segment of the sperm head. The sperm head, midpiece, and tail enter the oocyte cytoplasm. The sperm mitochondria (in the midpiece) are tagged with ubiquitin and subsequently destroyed by the oocyte — this ensures maternal inheritance of mitochondrial DNA.

4. Cortical reaction and zona reaction (block to polyspermy): The entry of the spermatozoon triggers a wave of calcium release from the oocyte's endoplasmic reticulum (the calcium oscillation), which in turn triggers the cortical reaction — exocytosis of cortical granules (lysosomes lining the oocyte cortex) into the perivitelline space. The enzymes released from cortical granules modify the zona pellucida glycoproteins (particularly cleaving ZP2 and modifying ZP3), hardening the zona and destroying sperm receptors. This zona reaction constitutes the slow block to polyspermy, preventing additional spermatozoa from binding to and penetrating the zona pellucida. Polyspermy (fertilisation by more than one sperm) is lethal because it produces a polyploid zygote (triploidy) that cannot develop normally.

5. Completion of meiosis II and formation of pronuclei: The calcium oscillation also triggers the secondary oocyte to complete meiosis II, producing the definitive ovum and the second polar body. The female chromosomes decondense and a nuclear membrane forms around them, creating the female pronucleus. Simultaneously, the sperm head decondenses (protamines are replaced by histones), enlarges, and forms the male pronucleus. Both pronuclei replicate their DNA.

6. Syngamy (fusion of pronuclei): The male and female pronuclei migrate toward the centre of the cell, their nuclear membranes break down, and the chromosomes intermingle on the first mitotic spindle. This is the moment of true genetic union — the formation of the zygote (2n = 46 chromosomes). The first mitotic division (cleavage) follows immediately.

Consequences of fertilisation include: (1) restoration of the diploid chromosome number (46 chromosomes); (2) determination of chromosomal sex (XX female if the oocyte is fertilised by an X-bearing spermatozoon, XY male if by a Y-bearing spermatozoon); (3) initiation of cleavage (the zygote begins the rapid mitotic divisions that will lead to blastocyst formation); (4) activation of the oocyte (metabolic activation — increased protein synthesis, increased oxygen consumption, resumption of the cell cycle).

Contraception: Anatomical and Physiological Principles

The NMC competency AN77.5 requires understanding the anatomical principles underlying contraception. Contraceptive methods work by interrupting one or more steps in the reproductive process: gametogenesis, ovulation, sperm transport, fertilisation, or implantation.

Figure: Contraception: Anatomical and Physiological Principles

1. Barrier methods prevent the physical meeting of spermatozoa and oocyte. Male condom — a sheath covering the penis that prevents spermatozoa from entering the vagina. Also protects against sexually transmitted infections (STIs), making it the only contraceptive with dual protection. Female condom — a polyurethane pouch inserted into the vagina. Diaphragm/cervical cap — dome-shaped devices placed over the cervix, used with spermicide. Anatomical basis: these methods exploit the fact that spermatozoa must traverse the cervical canal to reach the upper reproductive tract; blocking this passage prevents fertilisation.

2. Hormonal methods suppress ovulation and/or create hostile conditions for sperm transport and implantation. Combined oral contraceptive pills (COCPs) contain oestrogen and progestogen. Their primary mechanism is suppression of the LH surge (and to a lesser extent FSH), thereby preventing ovulation. Secondary mechanisms include: thickening of cervical mucus (impeding sperm penetration), thinning of the endometrium (reducing receptivity for implantation), and altered tubal motility. Progestogen-only pills (POPs) primarily thicken cervical mucus and alter the endometrium; ovulation suppression is inconsistent. Injectable progestogens (Depot medroxyprogesterone acetate/DMPA — marketed as Antara in India's family planning programme) suppress ovulation and alter cervical mucus. Subdermal implants (Implanon/Nexplanon — single etonogestrel rod) provide long-acting progestogen release for up to 3 years. Hormonal IUDs (levonorgestrel-releasing intrauterine system/LNG-IUS, e.g., Mirena) primarily act locally — thickening cervical mucus, thinning endometrium, and creating a hostile environment for sperm transport within the uterine cavity. Anatomical basis: hormonal methods exploit the HPG axis feedback mechanisms to suppress ovulation and the dependence of endometrial receptivity and cervical mucus quality on hormonal milieu.

3. Intrauterine devices (IUDs): The copper IUD (Cu-T 380A — the most widely used IUD globally and a mainstay of India's family planning programme) releases copper ions that are toxic to spermatozoa and create a sterile inflammatory reaction in the endometrium that prevents implantation. It can also be used as emergency contraception if inserted within 5 days of unprotected intercourse. Anatomical basis: the device sits within the uterine cavity, exploiting the confined space and its lining to create a hostile environment.

4. Surgical methods (permanent sterilisation): Tubectomy (female sterilisation) involves interruption of the fallopian tubes by ligation, excision, cauterisation, or application of clips/rings, preventing the oocyte from meeting spermatozoa. It is the most common contraceptive method in India (approximately 36% of married women of reproductive age). It can be performed laparoscopically, by mini-laparotomy (postpartum or interval), or via the newer hysteroscopic approach (Essure — though this has been discontinued in many countries). Vasectomy (male sterilisation) involves cutting and sealing the vas deferens (ductus deferens) bilaterally, preventing spermatozoa from reaching the ejaculate. It is simpler, safer, and less expensive than tubectomy, yet remains underutilised in India (only ~0.3% of couples rely on vasectomy — a reflection of gender disparity in contraceptive responsibility). Anatomical basis: these methods exploit the fact that gamete transport depends on the patency of specific anatomical conduits (fallopian tubes and vas deferens).

5. Natural/fertility awareness methods: Based on identifying the fertile window and abstaining from intercourse during that period. Methods include: calendar method (Rhythm method — calculating fertile days based on cycle length), basal body temperature method (detecting the 0.3-0.5°C rise in BBT after ovulation due to progesterone), cervical mucus method (Billings method — detecting the change from thick, opaque mucus to thin, clear, stretchable mucus around ovulation), and symptothermal method (combining BBT and cervical mucus observation). Anatomical basis: these methods exploit the limited viability of gametes (oocyte 24 hours, spermatozoa 3-5 days) and the hormonally driven changes in cervical mucus and body temperature.

6. Emergency contraception: Levonorgestrel (1.5 mg single dose within 72 hours of unprotected intercourse) primarily delays or inhibits ovulation. Ulipristal acetate (30 mg within 120 hours) is a progesterone receptor modulator that delays ovulation even after the LH surge has begun. Copper IUD (within 5 days) is the most effective emergency contraceptive. Mifepristone (low-dose) is used in some countries for emergency contraception.

Fertility, Infertility, ART, Surrogacy, and the Sex Ratio

The NMC competency AN77.6 covers teratogenic influences (addressed in the Introduction to Embryology topic), fertility and sterility, surrogate motherhood, and the social significance of the sex ratio.

Figure: Fertility, Infertility, ART, Surrogacy, and the Sex Ratio

Fertility and infertility: Fertility refers to the ability to conceive and produce offspring. Fecundability is the probability of achieving a pregnancy in a single menstrual cycle (approximately 20-25% in healthy couples). Infertility is defined as the failure to conceive after 12 months of regular unprotected intercourse (6 months in women over 35). It is classified as primary (never conceived) or secondary (previously conceived but now unable to). Causes are distributed approximately equally: male factors 30-40% (including oligospermia, asthenospermia, azoospermia, varicocele, ejaculatory dysfunction), female factors 30-40% (including anovulation/PCOS — the most common cause, tubal disease, endometriosis, uterine factors), combined factors 10-15%, and unexplained 10-15%.

Assisted Reproductive Technologies (ART) have revolutionised infertility treatment since the birth of Louise Brown in 1978 (the first IVF baby). India's first IVF baby (Durga/Kanupriya Agarwal) was born in 1978, just 67 days after Louise Brown. Key ART procedures include:

In vitro fertilisation (IVF): The standard ART procedure involving: (1) controlled ovarian hyperstimulation (using gonadotropins to produce multiple follicles), (2) oocyte retrieval (transvaginal ultrasound-guided follicular aspiration), (3) in vitro fertilisation (mixing oocytes and prepared spermatozoa in culture medium), (4) embryo culture (monitoring development to 2-cell, 4-cell, 8-cell, morula, or blastocyst stage), and (5) embryo transfer (placing selected embryos into the uterus via a transcervical catheter). Success rates vary by age: approximately 40-45% per cycle in women under 35, declining to 10-15% in women over 40.

Intracytoplasmic sperm injection (ICSI): A single spermatozoon is directly injected into the oocyte cytoplasm using a micromanipulation system. This bypasses the normal processes of capacitation, acrosome reaction, and zona penetration, making it the treatment of choice for severe male factor infertility (severe oligospermia, obstructive and non-obstructive azoospermia with testicular sperm extraction).

Gamete intrafallopian transfer (GIFT): Oocytes and spermatozoa are placed directly into the fallopian tube, allowing fertilisation to occur in vivo. Rarely performed now.

Intrauterine insemination (IUI): Washed and concentrated spermatozoa are placed directly into the uterine cavity, bypassing the cervix. Used for mild male factor, cervical factor, and unexplained infertility.

Surrogacy involves a woman (surrogate mother) carrying a pregnancy for another individual or couple (intended parents). Traditional surrogacy uses the surrogate's own oocytes (she is the genetic mother). Gestational surrogacy uses embryos created from the intended parents' gametes (or donor gametes) — the surrogate has no genetic relationship to the child. India was a major destination for commercial surrogacy until the Surrogacy (Regulation) Act, 2021, which banned commercial surrogacy and permits only altruistic surrogacy (no payment beyond medical expenses) for Indian married couples who have been married for at least 5 years, with the surrogate being a close relative of the couple. Single parents, unmarried couples, same-sex couples, and foreign nationals are excluded. The Act aims to prevent exploitation of economically vulnerable women, though critics argue it restricts reproductive autonomy.

Sex ratio refers to the proportion of males to females in a population. The natural (primary) sex ratio at conception is approximately 105 males per 100 females (due to the slightly higher motility and fertilisation rate of Y-bearing spermatozoa). The secondary sex ratio at birth should be approximately 105:100, but in India it has been skewed by sex-selective practices — prenatal sex determination followed by sex-selective abortion of female foetuses. The Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act, 1994 (amended 2003) prohibits sex determination for non-medical purposes and regulates the use of prenatal diagnostic techniques. Despite this legislation, the child sex ratio (0-6 years) declined from 945 (1991 Census) to 914 (2011 Census), with extreme skewing in states like Haryana (834), Punjab (846), and Jammu & Kashmir (862). The 2021 NFHS-5 data shows some improvement, with the sex ratio at birth improving to approximately 929 females per 1,000 males nationally.

The consequences of a skewed sex ratio include: increased trafficking of women, bride-buying practices, polyandry, increased violence against women, and demographic imbalance. The Beti Bachao Beti Padhao campaign (launched 2015) aims to address declining child sex ratios through awareness, enforcement of PCPNDT Act, and improving education for girls. Medical professionals have a legal and ethical obligation to refuse sex determination requests for non-medical purposes — violation carries imprisonment of up to 5 years and a fine of up to ₹1,00,000 under the PCPNDT Act.