The activation process in a variety of deuterostome and protostome eggs is accompanied by cytosolic calcium transients that usually take the form of either a single or multiple propagating waves. Through the injection of aequorin (a calcium- specific luminescent reporter) and the use of a Photon Imaging Microscope (PIM), I report that the eggs of zebrafish (Danio rerio) are no exception, in that they generate a single calcium activation wave that traverses the egg at a velocity of around 9μm/sec. There appears, however, to be no difference between the calcium-mediated activation response of eggs with regards to the presence or absence of sperm in the spawning medium. This suggests that these eggs are normally activated when they come in contact with their spawning medium, and then subse...[ Read more ]

The activation process in a variety of deuterostome and protostome eggs is accompanied by cytosolic calcium transients that usually take the form of either a single or multiple propagating waves. Through the injection of aequorin (a calcium- specific luminescent reporter) and the use of a Photon Imaging Microscope (PIM), I report that the eggs of zebrafish (Danio rerio) are no exception, in that they generate a single calcium activation wave that traverses the egg at a velocity of around 9μm/sec. There appears, however, to be no difference between the calcium-mediated activation response of eggs with regards to the presence or absence of sperm in the spawning medium. This suggests that these eggs are normally activated when they come in contact with their spawning medium, and then subsequently fertilized. The aspermic wave is initiated at the animal pole in the region of the micropyle, appears to propagate mainly through the yolk-free egg cortex, then terminates at the vegetal pole. As neither sperm nor external calcium are required for the initiation (or propagation) of the activation wave, this suggests that an alternative wave-trigger must be involved. In zebrafish eggs the propagating calcium wave is not followed by a wave of cortical granule exocytosis as is the case in medaka (Oryzias Zatipes) eggs. What is observed, however, is a near-synchronous exocytosis of cortical granules all over the egg surface some 30-60 seconds after the initiation of the calcium wave. Medaka eggs, however, contain a single layer of cortical granules located just below the plasma membrane, whereas the distribution of cortical granules in zebrafish eggs is more complex. The cortex of the animal hemisphere contains multiple layers of small cortical granules which increase in size (with an accompanying decrease in the number of layers) in an animal to vegetal pole direction. Thus, the differences in zebrafish and medaka egg morphology might account for the differences in the relationship between the calcium wave and cortical granule exocytosis during activation.