Another reason for eggs

Roman soldiers used them for protein1. In Mexico, men steal them from endangered sea turtles for their supposed effects on virility2. Bird eggs and roe, the ripe ovaries of fish, have a rich balance of proteins, fats and minerals – nutritionally, almost everything a predator needs. The whole point of these things is to feed something for an extended period of time. It’s no wonder eggs are so delicious.

The applications go beyond adding energy to our diets and structure to baked foods. Laying hens also contribute to medicine. Fertilized chicken eggs are used to grow viruses for mass production of vaccines. In 2007, scientists figured out how to genetically engineer hens to incorporate certain cancer-fighting proteins right into their egg whites, in a more efficient way to manufacture drugs that has been dubbed “pharming3.

This morning, enthusiasts have yet another reason to celebrate, since a new study suggests that bird eggs might hold even more promise for medical research.

It has to do with migration, but not the kind you’re used to hearing about with birds. Cellular migration refers to the movement of cells within an organism during growth or embryonic development. For a long time, biologists studying this behaviour focused on the movement of single cells in isolation. In the last decade, however, the focus shifted to cells moving in a large, cohesive group. This collective migration is a fundamental part of gastrulation and neural crest development – two of the necessary steps for turning a blob of cells into a fully formed embryo during development (watch a time lapse video of this process in zebrafish).

Collective cell movement, or epithelial migration, occurs on a grand scale during bird embryo development. Every fertilized egg contains a tiny blastula, the hollow ball of cells that will eventually become a fetus. Early on, the cells of outer blastoderm layer of the ball start to expand across the vitelline membrane that surrounds the egg yolk, in a process known as epiboly. Eventually, the expanding sheet of cells envelops the entire yolk – a requirement for the yolk sustain the embryo during its transformation from a ball of cells into a viable chick.

Bird embryo and yolk

A chicken embryo grows while attached to its yolk, because of epiboly. Modified from drawing by D.G. Mackean.

This around-the-yolk migration happens rapidly, within days. From the perspective of a single cell, it’s a feat that bioengineer Evan Zamir likens to “an ant walking across the earth”4. And we still don’t know exactly how birds do it, with their humongous yolks; so far, most research on epithelial migration has involved other organisms.

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