Compare and contrast fertilization in the mouse and the sea urchin – how they similar and how do they differ (and why?). Think about the structures, events and results. You might

want to do this by breaking down the steps. Also address the key role of Ca2+.

To compare and contrast fertilization in the mouse and the sea urchin, we need to examine the structures, events, and results involved in the process. By breaking down the steps, we can understand how they are similar and different, as well as the key role of Ca2+ (calcium) in fertilization.

1. Structures:
a. Mouse: In the mouse, fertilization occurs internally within the female reproductive tract. The female has a complex reproductive system comprising of ovaries, fallopian tubes, uterus, and cervix. The male has testes that produce sperm.
b. Sea urchin: In sea urchins, fertilization occurs externally in seawater. The female has gonads where the eggs are produced. The male ejects sperm into the water.

2. Events:
a. Mouse: Fertilization in mice involves several distinct events. After mating, sperm cells swim through the cervix, up the uterus, and into the fallopian tubes. There, the sperm must penetrate the protective barriers surrounding the egg, including the corona radiata and the zona pellucida. Upon successful penetration, the sperm fuses with the egg, leading to the formation of a zygote.
b. Sea urchin: In sea urchins, fertilization begins when the male releases sperm into the water. The sperm, aided by chemical signals, swim towards the eggs. Upon reaching the eggs, the sperm release enzymes that digest the jelly coat surrounding the eggs. This allows the sperm to bind to the vitelline membrane and subsequently penetrate it. The fusion of the sperm and egg forms a fertilized egg, known as a zygote.

3. Results:
a. Mouse: After fertilization, the mouse zygote undergoes a series of cell divisions and becomes a blastocyst. It then implants into the uterine wall, where it develops into an embryo and subsequently a fetus.
b. Sea urchin: Following fertilization, the sea urchin zygote undergoes division to form a hollow ball of cells called a blastula. The blastula then develops into a gastrula, which eventually gives rise to the fully formed sea urchin.

4. Key role of Ca2+:
Ca2+ plays a crucial role in both mouse and sea urchin fertilization processes. In sea urchins, an influx of Ca2+ into the egg triggers the cortical granule exocytosis, which results in the formation of a fertilization envelope. This envelope hardens and prevents additional sperm from binding to the egg. In mice, Ca2+ release is required for the completion of the meiotic division in the egg. It also leads to zona reaction, which involves the modification of the zona pellucida to prevent polyspermy.

In summary, while the general concept of fertilization is the same in both mice and sea urchins, they have distinct structures, events, and results. Mouse fertilization occurs internally, while sea urchin fertilization occurs externally. The events involved in sperm-egg interaction also differ. However, both processes require the release of Ca2+ and rely on it for various critical steps in the fertilization process.