Mechanisms of initiation and propagation of the calcium wave during fertilization in deuterostomes

Abstract

In deuterostome species, after fertilization, the egg undergoes a transient increase in free cytoplasmic calcium. This increase begins at the sperm entry point and propagates across the egg towards the opposite pole. This phenomenon is referred to as the calcium wave. Conflicting theories have arisen about the mechanism of wave initiation by sperm. Some results favor Ca2+ channel transfer from the sperm to the egg, others favor signal transduction via a sperm receptor linked to G-protein and second messenger production pathway. Other results have raised the possibility of the existence of a diffusible sperm factor delivered into the egg during gamete fusion. These models are reviewed and criticized. The existence and nature of a sperm receptor and the potential involvement of tyrosine kinase is discussed. The propagation of the wave relies on a positive feedback mechanism. Recent evidence favors a role for calcium itself in Calcium- Induced Calcium Release processes which also involve Inositol(1,4,5)- trisphosphate. The actual activation of inositol phosphate turnover and the resulting increase in Ins(1,3,4,5)P4 concentration may be involved in refilling internal calcium stores during and after the calcium wave. Ultimately the biological significance of the wave pattern, expressed in many early fertilization events, is questioned.