Fertilisation
கருத்தரித்தல்
Fertilisation is not a random collision; it is a highly regulated biological checkpoint. Most students fail to distinguish between insemination and fertilisation. Remember: millions of sperm enter the vagina, but only one achieves syngamy. Precision is the difference between a rank and a repeat.
The idea
Fertilisation is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote. This event occurs exclusively in the ampullary region of the fallopian tube. Upon contact, the sperm induces changes in the zona pellucida layer of the ovum. These changes block polyspermy, ensuring that only one sperm fertilises the ovum.
Why it matters
It restores the diploid number of chromosomes and initiates the development of a new organism.
NEET lens
NEET focuses on the exact site of fertilisation; 'Ampullary-isthmic junction' is the precise answer. Do not confuse this with the site of implantation. Expect questions on the block to polyspermy and the role of the zona pellucida. Memorise the sequence: Insemination, Fertilisation, Implantation.
Clinical significance
Failure in the zona pellucida reaction leads to polyspermy, resulting in a non-viable zygote and early embryonic death. Ectopic pregnancy occurs if the zygote implants outside the uterus, often due to failure in transport mechanisms post-fertilisation. These are critical clinical failures of the reproductive process.
Introduction
Fertilization in humans is the fusion of a sperm (n) with a secondary oocyte (n) in the ampullary region of the fallopian tube, resulting in a diploid zygote (2n) and triggering completion of meiosis II in the oocyte.
Core concept
Location: ampullary-isthmic junction (ampulla) of the fallopian tube. Steps: (1) Capacitation: sperm acquire fertilizing capacity in female reproductive tract (~7 hours). (2) Acrosome reaction: sperm contacts zona pellucida → acrosome releases enzymes (acrosin, hyaluronidase) → penetrate zona. (3) Sperm-oocyte fusion: sperm plasma membrane fuses with oocyte → triggers cortical reaction → zona hardening (block to polyspermy). (4) Completion of meiosis II: secondary oocyte (arrested in metaphase II) completes meiosis → ovum (n) + second polar body. (5) Pronuclei formation: male pronucleus (n) + female pronucleus (n). (6) Syngamy: pronuclei fuse → zygote (2n). Cleavage begins as zygote moves to uterus.
Go deeper
Block to polyspermy (only one sperm fertilizes): (1) Fast block: membrane depolarization within seconds of first sperm fusion → prevents additional sperm fusion. (2) Slow block (cortical reaction): cortical granules release contents → zona pellucida hardens (zona reaction) → no more sperm can penetrate. Both blocks together ensure monospermy.
From Class 10
Fertilization is when a sperm fuses with an egg to form a zygote — the very first cell of a new individual. This happens in the fallopian tube in humans.