Introduction

Regulation of the cell division cycle is an essential process by which the cell monitors its growth and differentiation. Maintaining the proper controls on these cellular processes is essential not only during embryonic development but also throughout the lifetime of an animal. During embryonic development, in a temporally and topographically distinct manner, a wide variety of cells exhibit the capacity to become quiescent, to proliferate, and to irreversibly withdraw from the cell cycle and undergo terminal differentiation. Thus, both the entry of a cell into the cell cycle from a state of quiescence and the exit of the cell from active cycling must be precisely regulated if normal cell growth and differentiation are to be maintained. Furthermore, these two distinct types of cell cycle regulation must be coordinated with the regulation of differentiation. Over the past decades, much has been learned about the mechanisms that control cell cycle progression in vitro, primarily as it relates to cancer. Only in recent years has an understanding of how the cell cycle is controlled in vivo in normal development begun to emerge.

The ocular lens has served as a model system for unraveling the roles of cell cycle regulatory genes in a developmental context. A relatively simple tissue with a well-described blueprint of cell division and morphogenesis, the lens has been ideal for studying the coordination of both cell growth and differentiation in vivo.