Introduction

This chapter complements the next pair which are concerned with the cellular basis of morphogenesis: it is not possible to appreciate how cells cooperate to build tissues without understanding how the structural molecules of the cells and their environment guide, constrain, facilitate and generate cell behaviour. Indeed, it might seem possible to discuss morphogenesis by concentrating almost solely on the events that take place at the molecular level, but this view is oversimple for several reasons: first, the molecular basis of many cell properties is not yet understood so that they can only be considered phenomenologically; second, some aspects of cell behaviour have a strong random or stochastic aspect and cannot easily be predicted from molecular information; finally, many morphogenetic events depend on macroscopic structures or forces exerted over large areas and, in these cases, analysis at the molecular level provides few insights. But it is important not to underestimate the importance of the events taking place at the molecular level during morphogenesis as their study has provided more than a background to understanding cell behaviour, it has explained many aspects of organogenesis.

The morphogenetically significant molecules fall into three geographically distinct categories: those in the extracellular matrix (ECM), those that are components of the cell membrane and those that comprise the intracellular cytoskeleton. The purpose of this chapter is to describe these molecules and to consider their functions, both directive and permissive.