In Chapter 2 we followed development up to the first nontrivial manifestation of multicellularity, the appearance of the blastula. To describe the mechanism of blastula formation we needed a model for cleavage, as well as for the collective behavior of a large number of cells in contact with one another. We based our model on physical parameters such as surface tension, cellular elasticity, viscosity, etc. and when possible related these quantities to experimentally known information such as the expression of particular genes. However, we have so far not dealt with the fundamental question concerning multicellularity: what holds the cells of a multicellular organism together?

As cells differentiate (see Chapter 3) they become biochemically and structurally specialized and capable of forming multicellular structures, with characteristic shapes, such as spheroidal blastulae, multilayered gastrulae, planar epithelia, hollow ducts or crypts. The appearance and function of these specialized structures reflect, among other things, differences in the ability of cells to adhere to each other and the distinct mechanisms by which they do so. During development certain cell populations need to bind, to varying extents, to some of their neighbors but not to others. In mature tissues the nature of the cell–cell adhesion contributes to their functionality: the manner in which two cells bind tightly to one another in the epithelial sheet lining the gut, for example, must be different from the looser attachment between the endothelial cells lining blood vessels.