Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 20

The evolution of higher organisms required that single cells first formed multicellular associations. Once this was accomplished individual cells or groups of cells could then specialize for specific functions. Ultimately the evolution of tissues and organs was possible. It is likely that this all began with the first cell adhesion molecules that allowed two cells to stick together for reproduction. Over time, new adhesion molecules evolved and the link between them and the intracellular environment began to appear ultimately leading to the formation of fully-fledged adhesion junctions found in human tissues. This theoretical model for the evolution of cell adhesion junctions is summarized in the following figure (Figure 3.1).

Figure 3.1. A theoretical model for the evolution of cell adhesion junctions.

In its simplest form, cell adhesion involves two identical molecules: homotypic cell adhesion. Binding between two different cell adhesion molecules is called heterotypic cell adhesion. These two simplest associations, as well as some others discussed in the next chapter, led to the next step in cell adhesion mechanisms: clustering of cell adhesion molecules to form more complex adhesion structures. Today, these are seen as the highly organized adhesion junctions that consist of cell adhesion molecules as well as accessory and adaptor proteins that allow other interactions including links to the intracellular cytoskeleton and the extracellular matrix.

In this chapter, we will examine the major junctions that are found in human tissues and some of the diseases that are related to their structure and function. Later, we'll look in detail at some of the cell adhesion molecules that are present in these junctions, some of which also work independently.