Ca2+, fertilization and egg activation

Published on June 11, 2012 Updated on September 18, 2020   50 min

A selection of talks on Reproduction & Development

I am Karl Swann from Cardiff University. In this lecture, I'm going to talk about the role of calcium in the activation of development at fertilization.
At fertilization, the egg is transformed into a developing embryo. In most species, the sperm triggers this change and the events brought about by the sperm are referred to as activation events. The sperm activates the egg to develop into an embryo. One of the most dramatic examples of the change seen at fertilization is in the sea urchin egg. When the sperm fertilizes the egg within a minute, you can see the formation of a fertilization envelope around the egg that you can see in the right-hand image. In this lecture, I shall introduce some examples of events that have been studied in sea urchin eggs. However, for most of the lecture, I shall talk about what we know about fertilization in mouse egg. The mouse egg is a useful model for mammalian fertilization in general. In fact, we now know more about the molecular mechanisms at fertilization in the mouse than in any other species.
This is an image of a mouse egg that has been ovulated and is ready to be fertilized. It's the section taken both through the egg and the surrounding cumulus cells, which are released at the same time as the oocyte from the ovary. The DNA in this image is stained in red. You can see the nuclei of the surrounding cumulus cells. Within the oocyte in the center, you can see the DNA is condensed and it's present in a metaphase plate. That's because the oocyte is arrested at the second meiotic division at metaphase, you can see this labeled with the yellow arrow. What you can also see with another yellow arrow is the product of the first meiotic division, which is present as a first polar body. This is a set of chromosomes that's been extruded from the first meiotic division. The mouse egg, therefore, strictly speaking, should always be referred to as an oocyte because it hasn't completed meiosis, but it's often referred to an egg in the literature, and I shall generally refer to it as an egg in this lecture.