Introduction

The morphogenesis of epithelial tissue is the major event in early development; later, it is responsible for many of the structures in vertebrate embryos and for the complete external form of invertebrates; the topic is thus central to understanding how structure emerges during development. Epithelial cells may easily be recognised in sectioned material because they tend to associate in polarised, monolayer sheets. These are found in a range of forms that includes bounding membranes of tissues and a wide variety of tubes and vesicles. Indeed, to a very great extent, epithelia define the early embryo, with the role of the mesenchyme being merely to fill the spaces between them, and a convincing argument could be made that the most important problem in morphogenesis is to explain how epithelial sheets come to form such a diverse set of structures. In this chapter, we will examine many of these epithelial structures and discuss some of the cellular and molecular mechanisms underlying their morphogenesis in vertebrate and invertebrate embryogenesis.

Before we explore the morphogenetic roles of epithelia, it is worth reviewing briefly some of their important properties. Epithelial cells have a characteristic morphology which is independent of the large-scale organisation of the sheet in which they are located. In almost all cases, the constituent cells of an epithelial sheet make strong side-to-side adhesions to their neighbours. On their basal surface is a lamina made of extracellular matrix macromolecules to which subjacent mesenchyme adheres, while the superior surface, in sharp contrast, usually faces a vesicle, a tubule or the outside world; they thus display a characteristic polarity.