Fertilisation defect

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sperm meets the egg in the fallopian tube. This meeting is not random. Complex chemical changes and signals are essential for sperm-egg communication. The steps of fertilisation are as follows:

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1. The process starts while the sperm resides in the fallopian tube prior to ovulation. During this time it undergoes changes called capacitation (increased motility and capacity to fertilise an egg), which is essential for fertilisation.

2. Sperm gains the full fertilising capability while passing through the outer cells surrounding the egg, called cumulus. 

3. The chemical changes that happen in the outer wall of the sperm head (acrosomal reaction) are essential for the sperm to bind to the egg. This reaction is to some extent influenced by the outer wall of the egg (zona pellucida). Sperm and egg fusion is also guided by various proteins. 

4. Once the sperm binds to the egg, the DNA is injected in the egg. The outer wall of the egg becomes harder and impermeable to other sperm, thereby preventing multiple sperms from entering the egg (zona reaction).

5. The fusion of sperm and egg is followed by egg activation due to an increase in the calcium in the egg.

 

Subtle defects in oocyte and sperm can lead to defective fertilisation:

 

• Sperm defects like abnormal acrosomes can result in poor or no zona pellucida binding. The sperm fails to attach to the egg leading to no fertilisation.

• Sperm DNA integrity has been proposed to be a pre-requisite for normal fertilisation. Even a normal semen sample may include sperm with altered genetic material. High levels of sperm DNA fragmentation can lead to reduced fertilisation and increased miscarriage rate. 

• Failed egg activation can lead to failed fertilisation even if the sperm DNA has been injected into the egg.