The egg is stocked with nutrients and informational molecules that power and direct the early stages of development. Fertilization activates the egg and stimulates rearrangement of the cytoplasm, setting up the body axes and organizing positional information that will control determination and differentiation.
learning outcomes
You should be able to:
Compare the contributions of the sperm and the egg to the zygote.
Explain how sperm entry converts frog egg symmetry from radial to bilateral.
Explain how the distribution of β-catenin in the egg cytoplasm is altered following fertilization and why this alteration is important.
Apart from its nucleus, what is the major contribution of the sperm to subsequent cell signaling in the embryo?
The sperm contributes the centriole to the embryo, and the centriole is the origin of the microtubules of the primary cilia, which serve signaling functions.
How is the gray crescent generated in the fertilized frog egg?
The unfertilized frog egg has an animal (upper) and a vegetal (lower) hemisphere. The outer cytoplasm of the animal hemisphere is pigmented, but the vegetal hemisphere cytoplasm is not. The sperm binds to the animal hemisphere and stimulates rotation of the outer (cortical) cytoplasm toward the site of sperm entry. This rotation creates a band of more lightly pigmented cytoplasm opposite the site of sperm entry.
How does β-catenin become more concentrated on the dorsal side of the amphibian embryo?
In the unfertilized frog egg, β-catenin and its degrading enzyme GSK-
The uneven distribution of informational molecules in the cytoplasm of the fertilized egg is essential to later events in development. In the next section you will see how these informational molecules end up in different cells of the developing embryo.