12.3 Fertilisation

Why Million of sperms are required in an ejaculation

  • Despite the large number of sperm released (~200-300 million per ejaculate), only a few hundred reach the vicinity of the ovum.

  1. Hostile vaginal environment

    • The vaginal environment is acidic (pH ~3.5-4.5), which can kill many sperm

    • Seminal fluid (alkaline) initially buffers this acidity temporarily, but many sperm are inactivated or die within minutes

    • Motility is impaired in the acidic environment until liquefaction and pH stabilisation occurs

  2. Cervical mucus barrier

    • The cervical canal is lined with mucus, which changes in viscosity:

      • Before ovulation: Thick and impenetrable

      • Around ovulation (day 14): Oestrogen causes it to become thin and watery, allowing sperm to pass

    • Still only a fraction (thousands) of sperm manage to penetrate into the uterus

  3. Immune response

    • Sperm are recognised as foreign cells

    • The female immune system (neutrophils, macrophages) phagocytoses and destroys many sperm

    • Inflammatory response further reduces sperm viability

  4. Uterine and tubal navigation

    • Sperm must navigate 7-15 cm through the uterus and up the fallopian tube

    • Many become trapped in endometrial crypts, lost in the wrong tube, or immobilised

    • Some sperm rest in the isthmus of the tube, released later closer to ovulation in a regulated manner

  5. Only one fertilises the oocyte

    • Sperm are needed in large numbers to:

      • Provide enzymes (e.g. hyaluronidase, acrosin) that help digest the corona radiata and zona pellucida

      • Trigger the zona reaction after one sperm enters, preventing polyspermy

      • Enhance the chances that at least one will successfully bind and fuse with the oocyte membrane

Changes in the spermatozoon form ejaculation to fertilisation

  1. Ejaculation

    • Semen (sperm + seminal plasma) is ejaculated into the posterior vagina

    • Initially, semen forms a gel-like coagulum due to proteins from seminal vesicles

  2. Liquefaction (15-30 mins post ejaculation)

    • Prostatic enzymes (e.g. PSA) liquefy the coagulum, freeing sperm

    • Sperm become motile nut are not yet capable of fertilisation

  3. Capacitation (occurs in uterus and fallopian tubes)

    • Timeframe: ~6-8 hours

    • A process involving:

      • Removal of glycoproteins and cholesterol from sperm plasma membrane (mainly in the acrosomal region)

      • Increased membrane fluidity and permeability

      • Increased intracellular calcium and cAMP

      • Transition from standard to hyperactivated motility, allowing strong tail thrashing to penetrate barriers

    • Crucial for acrosome reactions to occur

  4. Acrosome reaction

    • Initiated upon contact with zona pellucida glycoprotein (ZP3)

    • Outer acrosomal membrane fuses with plasma membrane

    • Enzymes released:

      • Hyaluronidase disperses cells of corona radiata

      • Acrosin digests zona pellucida proteins

  5. Penetration of the zona pellucida and fusion

    • Once through the zona, the sperm contacts the oocyte membrane (oolemma)

    • Membrane fusion occurs via integrins and fertilin receptors

    • Sperm nucleus, centriole, and other cytoplasmic contents enter the oocyte

    • Triggers cortical granule exocytosis (cortical reaction), which hardens the zona pellucida, blocking polyspermy

The process of fertilisation

  1. Sperm penetration

    • Sperm penetrate the:

      1. Corona radiata — outer layer of granulosa cells

      2. Zona pellucida — glycoprotein matrix

    • To reach the oocyte membrane, capacitated sperm must first:

      • Swim through and physically disperse the cells of the corona radiata using hyperactivated motility and the enzyme hyaluronidase

      • Bind to specific glycogenproteins (mainly ZP3) in the zona pellucida, which triggers the acrosome reaction

    • During the acrosome reaction, the sperm’s outer acrosomal membrane fuses with its plasma membrane, releasing enzymes like acrosin, which digest a pathway through the zona pellucida

    • Once a sperm reaches the perivitelline space (the narrow area between the zoa pellucida and the oocyte’s plasma membrane), it binds to the oolemma (oocyte membrane) and fuses with it.

  2. Oocyte activation

    • Fusion of sperm and egg activates the oocyte. This causes a sharp rise in intracellular calcium levels, which triggers the following responses:

      • Completion of meiosis II: the secondary oocyte, which was arrested in metaphase II, completes meiosis. This produces one large haploid ovum (with twenty-three chromosomes) and a small, non-functional cell called the polar body

      • Cortical reaction: Cortical granules located beneath the oocyte membrane release their contents into the perivitelline space. These substances chemically modify the zona pellucida, hardening it and rendering it impenetrable to additional sperm. This is known as the block to polyspermy

  3. Pronucleus formation

    • Once inside the oocyte, the sperm’s nuclear material decondenses and becomes the male pronucleus.

    • Both pronuclei replicate their DNA in preparation for the first mitotic division. The sperm’s centriole also contributes to the mitotic spindle apparatus of the future embryo

  4. Syngamy (fusion of pronuclei)

    • The male and female pronuclei move toward each other through the oocyte cytoplasm. Eventually, the membranes around both pronuclei dissolve, and the maternal and paternal chromosomes come together on a shared metaphase plate

    • This merging of genetic material from both parents is called syngamy, marking the official formation of a diploid zygote with 46 chromosomes

  5. Zygote clevage and transport

  • The newly formed zygote immediately begins a series of mitotic divisions, starting with the first cleavage, producing two blastomeres

  • As it divides, the zygote continues to travel down the fallopian tube toward the uterus. Over the next few days, it passes through the morula stage (a solid ball of cells) and becomes a blastocyst by around day 5. The blastocyst is the structure that will eventually implant into the endometrial lining of the uterus.