Background on Cell Growth and the Cell Cycle
Cell growth and the cell cycle are distinct processes. In most proliferating cell types, a consistent cell size is achieved because cell growth and division are balanced. But these two processes can be completely uncoupled, as in the growth of the frog oocyte, an enormous single cell. Upon fertilization, the zygote undergoes the inverse process, rapidly dividing into thousands of cells without any cell growth. Clearly this process cannot continue indefinitely: at some point embryonic cells must begin growing in order to keep dividing. But it is not clear when, where or how cells begin growing during frog development.
The growth of embryonic cells is driven by yolk consumption in frogs, as well as in the great majority of metazoan species. As a major maternal investment and a final arbiter of the size and complexity of the embryo, the yolk content of eggs has been an important impetus in the evolution of development. Despite this central role, yolk consumption is very poorly understood at the molecular level.