If a fertilized egg is to develop into a complex multicellular organism, it must divide many times, and the cells produced from those divisions must stick together. In addition, they must retain a specific spatial relationship with one another in order for the developing organism to function. Chapter 10 introduced the molecular mechanisms that hold multicellular organisms together. We begin by reviewing those mechanisms briefly before placing them in evolutionary context.

In animal development, proliferating cells commonly become organized into sheets of cells called epithelia that line the inside and cover the outside of the body. Within epithelia, adjacent cells adhere to each other by means of transmembrane proteins, especially cadherins, which form molecular attachments between cells. Epithelial cells also secrete an extracellular matrix made of proteins and glycoproteins, and these cells attach themselves to this matrix by means of other transmembrane proteins called integrins. Cadherins, integrins, and additional transmembrane proteins thus provide the molecular mechanisms for adhesion in animals.

Cadherins and integrins do not occur in complex multicellular organisms other than animals, but all such organisms require some means of keeping their cells stuck together. How do they do it? Plants also synthesize cell adhesion molecules that bind cells into tissues, but, in this case, the molecules are polysaccharides called pectins (pectins are what make fruit jelly jell). Molecules that control adhesion between adjacent cells are less well known in other complex multicellular organisms, but the general picture is clear. Without adhesion there can be no complex multicellularity, and different groups of eukaryotes have evolved distinct types of molecular “glue” for this purpose.