Describe the process of fertilization in humans.

Process of Fertilization in Humans

Fertilization in humans is the fusion of a haploid male gamete (sperm) and a haploid female gamete (secondary oocyte) to form a diploid zygote. It occurs in the ampulla of the fallopian tube and involves several coordinated steps:

1. Ovulation and Sperm Transport

• The secondary oocyte (arrested in metaphase II) is released from the ovary during ovulation (around day 14 of the menstrual cycle).
• Sperm deposited in the vagina undergo capacitation (biochemical changes) in the female reproductive tract and swim upward through the cervix, uterus, and into the fallopian tube (surviving 24–48 hours).

2. Acrosomal Reaction and Penetration of the Zona Pellucida

• A single sperm binds to the zona pellucida (glycoprotein layer) of the oocyte via ZP3 receptors.
• The sperm undergoes the acrosomal reaction, releasing acrosin (an enzyme) that digests the zona pellucida, allowing the sperm to penetrate.
• Only one sperm successfully enters the oocyte.

3. Fusion of Gametes and Cortical Reaction

• The sperm’s plasma membrane fuses with the oocyte’s plasma membrane, triggering the cortical reaction.
• The oocyte releases cortical granules, which harden the zona pellucida (zona reaction), preventing polyspermy (entry of multiple sperm).
• The sperm’s nucleus (male pronucleus) and the oocyte’s nucleus (female pronucleus) form.

4. Formation of the Zygote

• The male and female pronuclei replicate their DNA and align at the metaphase plate.
• Their nuclear membranes break down, and chromosomes combine to form a diploid zygote (2n = 46 chromosomes).
• The zygote undergoes first cleavage as it moves toward the uterus for implantation.

Key Hormonal Regulation

• LH surge triggers ovulation.
• Progesterone (from the corpus luteum) prepares the endometrium for implantation.

This process ensures genetic diversity and the continuation of the human life cycle.