Cytoskeletal sheets appear as universal components of mammalian eggs

The eggs of two mammalian species have been shown to contain novel cytoskeletal elements, referred to as cytoskeletal sheets, which undergo stage-specific changes in spatial organization at three key developmental transitions, fertilization, compaction, and blastocyst formation. If cytoskeletal sheets have an integral role in these developmental transitions, the sheets should be present in the eggs of other mammals as well. We examined the eggs of four additional species to determine if sheets were present. Our results indicate that sheets were present and they can be categorized into two classes based on their surface appearance. Cytoskeletal sheets in eggs of hamsters and rats have a smooth surface appearance, while eggs from humans, cows, pigs, and mice have a fibrous surface appearance. In addition, we observed that species-specific variations exist in the width of the sheets and in the density of the sheets (i.e., number per micron 2) in the eggs. These species-specific variations may relate to the role of the sheets during early development.