ABIO303 - Fertilization & Polyspermy Prevention

What is chemotaxis in Sea Urchins

- Sperm are attracted toward eggs of their species by chemoattraction.

- Regulated by soluble molecules secreted by the egg

What is Sperm-activating peptides (SAPs)

- Small chemotactic peptides found in the jelly of sea urchin

- Guides sea urchin sperm toward the egg

- Species specific

- Ex: Resact

What is Resact?

- Sperm activating peptide

- Causes dramatic and immediate increase mitochondrial respiration and sperm motility.

What signaling pathway do sperm-activating peptides trigger?

- The cGMP (cyclic guanosine monophosphate) pathway, which opens ion channels → Ca²⁺ influx → changes in flagellar beating → directed swimming toward the egg.

What triggers the acrosomal reaction in sea urchin sperm?

- The head of the sperm (acrosome) making contact with the egg jelly coat.

What happens during the sea urchin acrosomal reaction?

- The acrosome membrane fuses with the sperm plasma membrane → release of enzymes → formation of the acrosomal process (actin-based).

How does the acrosomal reaction differ in vertebrates?

What are the main ECM layers in vertebrates?

- A thick extracellular envelope matrix called the zona pellucida (ZP)

What are the accessory cells that surround the mammalian egg?

- nurturing cells called the Cumulus

Where does the sperm bind to in sea urchin ECM?

- The sperm binds to the vitelline envelope (or layer)

What are the main protective egg layers in sea urchins?

- The jelly coat, the vitelline envelope, and the fertilization envelope

What triggers Ca²⁺ release during egg activation?

- Sperm-delivered PLCζ (phospholipase C zeta).

What is the pathway leading to Ca²⁺ release?

- PLC → IP₃ → binds IP₃ receptors on ER → Ca²⁺ release.

What does Ca²⁺ release cause?

Cortical granule exocytosis → blocks polyspermy and resumes meiosis.

What is the fast block to polyspermy?

- Membrane potential shift — sperm entry causes egg membrane depolarization, preventing other sperm fusion (occurs within seconds).

What is the slow block to polyspermy?

- Cortical granule reaction → enzymes modify the vitelline layer/zona pellucida (e.g., ZP2 cleavage) → no additional sperm can bind

What is Bindin?

- A sperm surface protein on the acrosomal process that binds to egg receptors.

Why is Bindin important?

- It ensures species-specific recognition between sperm and egg in sea urchins.

What protein powers sperm flagellar movement?

- Dynein, an ATPase motor protein.

How does dynein cause movement?

- It generates sliding between microtubule doublets in the flagellum → produces bending motion.

What are Izumo and Juno?

- Izumo1 = sperm membrane protein

- Juno = egg membrane receptor.

What is the role of Izumo and Juno?

- They mediate sperm-egg membrane fusion in mammals.

- After fusion, Juno is shed from the egg surface, preventing further sperm binding.

At what stage is the mammalian egg fertilized?

- At metaphase II of meiosis — fertilization triggers completion of meiosis and extrusion of the second polar body.

At what stage is the sea urchin egg fertilized?

- Before completion of meiosis — no polar bodies yet when sperm enters.