Mammalian Fertilization: Sensing Mechanism Near the Oocyte

Table of Contents

1. Sensing mechanism: Hyperactivation

Definition: Hyperactivation is a critical phase of the sensing mechanism during sperm migration, occurring toward the end of capacitation.
Characteristics:
- Increased swimming velocity and force.
- Asynchronous flagellar beating with higher bending.
- Detachment of sperm from oviduct epithelial cells.
- Only capacitated sperm exhibit hyperactivation.
- Enables sperm to respond differently to fluid currents.
- Capacitated sperm rotate around their long axis, aiding detachment.
- Hyaluronidase enzyme helps digest the cumulus cell matrix.

Definition: Thermotaxis is the ability of capacitated sperm to sense temperature differences.
Thermal Gradient: A 2ºC gradient exists between the oviduct isthmus and the warmer ampullary region.
Behavior:
- Sperm preferentially swim from cooler to warmer sites.
- Sensitivity to temperature differences as small as 0.014ºC over a millimeter.

Definition: Chemotaxis involves the attraction of capacitated sperm toward the egg.
Source of Attraction:
- Oocyte and cumulus cells secrete molecules during late sperm migration.
- These compounds attract sperm specifically when they are capacitated.
Potential Chemotactic Factors:
- Progesterone:
  - Produced by cumulus cells.
  - Binds to sperm receptors, activating Ca2+ channels in the tail.
  - Induces sperm hyperactivity.
- CRISP1 (in mice):
  - Secreted by cumulus cells.
  - Attracts sperm through CatSper channels.
- Human Cumulus Factors:
  - Form a gradient, guiding sperm movement through the cumulus.
  - Active only when sperm are capable of fertilizing the egg.
Intracellular pH Increase: Chemotaxis occurs after sperm intracellular pH rises during capacitation.
Significance: Ensures sperm reach the egg at the right time for successful fertilization.

Mammalian sperm employ rheotaxis, thermotaxis, and chemotaxis to navigate toward the egg, ensuring precise timing and successful fertilization.