Cortically restricted production of IP ₃ leads to propagation of the fertilization Ca ²⁺ wave along the cell surface in a model of the Xenopus egg

Abstract

The fertilization Ca 2+ wave in Xenopus laevis is a single, large wave of elevated free cytosolic Ca 2+ concentration that emanates from the point of sperm-egg fusion and traverses the entire diameter of the egg. This phenomenon appears to involve an increase in inositol-1,4,5-trisphosphate (IP 3) resulting from interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca 2+ release machinery. We have proposed models based on a static elevated distribution of IP 3, and dynamic [IP 3], however, these models have suggested that the fertilization wave passes through the center of the egg. Complementing these earlier models, we propose a more detailed model of the fertilization Ca 2+ wave in Xenopus eggs to explore the hypothesis that IP 3 is produced only at or near the plasma membrane. In this case, we find that the wave propagates primarily through the cortex of the egg, and that Ca 2+-induced production of IP 3 at the plasma membrane allows IP 3 to propagate in advance of the wave. Our model includes Ca 2+-dependent production of IP 3 at the plasma membrane and IP 3 degradation. Simulations in 1 dimension and axi-symmetric 3 dimensions illustrate the basic features of the wave. © 2004 Elsevier Ltd. All rights reserved.