How Does a Frog Egg Solve Geometry Problems?

One of the big challenges in biology is to understand how cells are physically organized by molecules, which are much smaller than the cell. This challenge is epitomized by frog eggs, which are enormous compared to most cells. After fertilization, frog eggs cleave in the middle, and then cleave again at right angles, on their way to becoming embryos. The question of how these cleavage planes are accurately positioned has interested biologists for 200 years. We have studied this problem using microscopy and biochemistry in frog and fish eggs, and in cell free extracts made from frog eggs. The answers lie in the behavior of starburst-like arrays of microtubules called asters, and in how they grow and interact inside the egg.