Printed in Great Britain © The Company of Biologists Limited 1993 IMAGING CALCIUM WAVES IN EGGS AND EMBRYOS

Sea urchin eggs and those of most other deuterostomes are activated at fertilization by an increase in cytoplasmic free calcium concentration ([Ca2+]i) that triggers the onset of the embryonic cell division cycles. We can image the calcium wave using fluorescent calcium indicator dyes and confocal microscopy. There are two components to the [Ca2+]i increase at fertilization. The first is due to a rapid calcium influx caused by a calcium action potential; this leads to a small increase in [Ca2+]i just beneath the plasma membrane with spherical symmetry. After a latent period of some 15s, there is a second large and rapid increase in [Ca2+]i localized to the region of sperm–egg contact: during the latent period [Ca2+]i does not change but within 1s of the end of the latent period [Ca2+]i reaches 2mmoll21. The calcium wave then spreads across the egg with a velocity of 5mms21. Behind the advancing wavefront, the calcium concentration is uniformly high, even within the egg nucleus, though there are no indications that intranuclear calcium concentration differs from [Ca2+]i. 2+]i falls uniformly towards resting levels over the next 500s. In cases where there is an apparent inhomogeneity in [Ca2+]i in either the cortex or the nucleus, we find that the calcium indicator dye is inhomogeneously distributed. This appears to be due to uptake of the indicator dye (Fluo-3), probably into mitochondria. The artefact can be avoided by using a dextran-conjugated dye