3. Fertilization

3.1 Overview

  • Human fertilization is the process where a sperm fuses with an egg to form a single cell called a zygote.
  • Occurs in ampulla region of fallopian tube .
  • Requires capacitation of sperm.

3.2 Steps in Fertilization

  1. Sperm binding to zona pellucida (ZP3 receptor)
  2. Acrosome reaction → release of enzymes
  3. Penetration of zona pellucida

◦       Once a sperm penetrates the zona pellucida, it binds to and fuses with the egg's plasma membrane, also known as the oolemma.

◦       The sperm's nucleus, carrying its genetic material, enters the egg's cytoplasm.

◦       When the sperm enters the oocyte, Ca2+ is released from the endoplasmic reticulum.

◦       The Ca2+ has several effects:

1.     Prevents other sperm from entering the oocyte

2.     Activates the oocyte to finish meiosis to become a haploid ovum

4. Fusion of sperm and oocyte membranes

5. Cortical reaction → block to polyspermy

◦       prevents other sperm from entering the egg after one has successfully fused.

6. Completion of meiosis II by oocyte

◦       The sperm's head swells to form the male pronucleus, while the egg completes its second meiotic division to form the female pronucleus.

7. Formation of zygote (46 chromosomes)

◦       The male and female pronuclei move together and fuse, combining their genetic material to form a single diploid cell called a zygote.

◦       This marks the completion of fertilization

3.3 Clinical Correlation

  • Polyspermy → non-viable embryos.
  • Assisted reproduction (IVF) mimics natural steps in vitro.