11.6Cellular Adhesion and Tissue Architecture
In multicellular organisms, in which tissues and organs are formed by accumulation of many cells, intercellular adhesion and adhesion between cells and ECM
play an important role in the architecture of these tissues and organs. Epithelial tissue is a representative example of cell adhesion playing a critical role in tissue architecture. Epithelial tissue is the most fundamental tissue that comprises the body of an animal, and almost all organs in our body are formed based on epithelial tissue. This tissue, by enclosing the entire body of animals, plays a role in separating the internal environment of an individual from the external environment. This is similar to the cell membrane in cells. For this reason, the epithelial tissue plays various roles including restricting permeation of substances, structurally resisting external forces, selective exchange of substances between the internal environment of the body and the external environment, and receipt of information from the external environment.
Adhesion of Epithelial Cells
The sides of each epithelial cell form gap junctions as well as a junctional complex composed of a desmosome, tight junction, and adherens junction to allow the cells to bind to each other (Fig. 11-11A). Thus, cell adhesion help cells tightly bind to each other, binding so tightly that ions are unable to pass through the space between the cells. This also allows information to be exchanged between the cells. The transmembrane proteins found in the cell membrane are responsible for forming these junctions.
Adhesion between Cells and ECM
The cell membrane has receptors that specifically bind to ECM. The transmembrane protein called integrin is a well-known receptor for ECM (Fig. 11-11B). Integrin is composed of dimeric functional units of two types (α-subunits and β-subunits) and selectively binds to ECM by recognizing the RGD sequence comprising arginine-glycine-asparagine. There are many types of these two subunits, and this diversity reflects the types of integrin in tissues and the diversity of adhering partners. Furthermore, integrin not only binds cells to ECM but also plays a role in transferring information into the cell.
The transmembrane protein cadherin is a representative example of adhesion molecules involved in intercellular adhesion (Fig. 11-11C). The adherens junction that helps in the adherence of epithelial cells to one another is a result of cadherin. Some types of cadherins also act on the desmosome. The cadherin molecule is differs depending on the tissue, and it has a special property wherein the same type of molecule selectively binds to itself. In other words, cadherin is an adhesion molecule that binds the same types of cells to each other. This molecule, like integrin, is involved not only in cell adhesion but also in information exchange between cells.
Fig. 11-11 Epithelial tissue architecture
(A) Epithelial cells adhere to each other. Furthermore, at the basal surface, they adhere to the basal lamina of ECM. The electron microscope photograph shows adhesion between epithelial cells of the small intestine. (B) Integrin is an adhesion molecule that plays a central role in forming a bond between a cell and ECM. The transmembrane protein integrin plays a role in transducing information from outside the cell into the cell. FAK stands for focal adhesion kinase. (C) Cadherin, which binds same types of cells together, also transducts information from outside the cell.
Fig. 11-12 Cell polarity
Epithelial cells have a directionality with their top side facing the outside and their basal side facing the inside of the body, where transport of materials occurs between the outside and inside. Organelles and the cytoskeleton are functionally arranged according to this transport directionality.
The epithelial tissues covering surfaces in animal bodies create a division between two entirely different environments (environment internal and external to the body). They help in the exchange of substances and information between these two environments. There is a clear directionality to the structure and function of epithelial cells (Fig. 11-12). This orientation refers to the apical side, which is the side facing the external environment, and the basal side, which is the side facing the internal environment of the body. This type of cell directionality is called cell polarity. In epithelial cells, which exhibit polarity, the organelles and cytoskeleton are functionally arranged along the orientation of the transport of substances that have important functions in the cells. In addition to epithelial cells, many other types of cells exhibit polarity, and these polarities are closely connected to cell functions.