Introduction

Fertilization is a crucial step in animal development that initiates the processes that lead to development. The events that occur during fertilization happen in the cytoplasm and involve the release of calcium ions. Calcium ions play a vital role in activating the egg’s metabolism and initiating development by releasing inhibitors from maternally stored messages, allowing these mRNAs to be translated, and releasing the inhibition of nuclear division. The release of calcium ions varies from species to species and has been found to be critical for activating development throughout the animal kingdom.

Release of Intracellular Calcium Ions

The release of calcium ions from the egg can occur in two ways, either through a soluble factor from the sperm or through the binding of the sperm to receptors on the egg cell surface. In sea urchins, the latter is likely to occur.

Inositol 1,4,5-Trisphosphate (IP3) – The Releaser of Calcium

Inositol 1,4,5-trisphosphate (IP3) is the primary agent responsible for releasing calcium ions from intracellular storage throughout the animal kingdom. IP3 is formed by the splitting of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate by the enzyme phospholipase C (PLC), which also yields diacylglycerol (DAG). IP3 releases calcium ions into the cytoplasm by opening the calcium channels of the endoplasmic reticulum, and DAG activates protein kinase C, which raises the pH of the egg.

The IP3 pathway initiates the liberation of calcium ions and the alkalinization of the egg, both of which are involved in the initiation of development. In sea urchin eggs, IP3 is formed immediately at the site of sperm entry and can be detected within seconds of sperm-egg attachment. IP3-responsive calcium channels have been found in the egg endoplasmic reticulum, and the IP3 formed at the site of sperm entry is thought to bind to these IP3 receptors, releasing calcium ions locally.

Phospholipase C – The Generator of IP3

Phospholipase C (PLC) is required for generating IP3 and activating the IP3 pathway. The activation of PLC is dependent on the presence of a stimulus that triggers its activation, such as the binding of the sperm to receptors on the egg cell surface.

Conclusion

The activation of egg metabolism in sea urchins occurs through the release of calcium ions, which are released by the primary agent inositol 1,4,5-trisphosphate (IP3). IP3 is generated by the enzyme phospholipase C and is formed immediately at the site of sperm entry. The liberation of calcium ions and the alkalinization of the egg are involved in the initiation of development and are crucial for the processes that lead to development.