From zygote to lens: Emergence of the lens epithelium

Abstract

At the dawn of the twentieth century, Hans Spemann first discovered embryonic induction when he demonstrated that lenses failed to form in the frog, Rana fusca, following the destruction of the optic vesicle. Since that time, lens induction remains at the forefront of investigations of embryonic development. However, unlike in the early days of vertebrate experimental embryology, advanced genetic tools and techniques make it possible to paint a detailed molecular picture of how the lens comes into being. It is this picture, although not complete, that emerges in the following pages. Drawing from investigations using a number of different model systems and experimental approaches, this work traces the journey from fertilized zygote to the differentiation of the lens epithelium. These transitions from zygote to inner cell mass to epiblast to ectoderm to neural plate border to preplacodal ectoderm to lens placode to lens vesicle to lens epithelium occur through sequential paracrine signals that induce specific transcription factors leading to the production of proteins characterizing each stage. Understanding the paracrine signals that take place during normal embryonic development informs much of the current revolution fueled by the conversion of embryonic stem cells or induced pluripotent stem cells into differentiated cell types in vitro.