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In the journey through the cell suggested by C. de Duve (A guide tour of the living cell. Scientific American books, vol. 2, 1984) a molecular-sized cytonaut traveling toward a cell, before reaching the plasma membrane, would first need to go through a jungle of stems, branches, rain forest vines, and lianas. In tissues, this messy tangle is the extracellular matrix. The extracellular matrix is a scaffold of proteins and carbohydrates located around the cells that is synthesized by the cells themselves. Some authors suggest that this definition only applies to the insoluble components of the extracellular matrix.

The extracellular matrix was invented by multicellular organisms. It was necessary to keep cells together by adhesion, and therefore tissues appeared. During evolution, extracellular matrix got many other functions, not only adhesion, such as being responsible for the mechanical properties of most tissues (both in plant and animals), maintaining cell morphology, allowing cell communication, stablishing pathways for cell migration, modulating cell differentiation and physiology, maintaining growth factors in some places, and many others. In tissues, features like resistance, hardness, elasticity, hydration or optical properties, depend on the extracellular matrix features. The amount, composition and organization of the extracellular matrix is different in different tissues (Figure 1). There are tissues, such as epithelia and nervous tissue, containing a low amount of extracellular matrix, while in other tissues, such as the connective proper, cartilage, and bone, the extracellular matrix occupies most of the volume and is essential for their functions. The molecular components of the extracellular matrix change from one tissue to another and are continually renewed. Synthesizing and removing the extracellular matrix is a permanent work that cells do.

Extracellular matrix
Figure 1. In this figure, several types of extracellular matrices are shown. They are stained with different histological dyes. Asterisks indicate extracellular matrix. A) Hyaline cartilage. B) Compact bone matrix. C) Dense regular connective tissue (tendon). D) Gelatinous connective tissue of the umbilical cord. E) Cell wall of the vascular system cells of a plant stem. F) Epithelial cells. Note that there is almost no extracellular matrix between these cells. G) Electron microscopy image of the the nervous tissue where extracellular matrix is very scarce between plasma membranes of different neurons.

Cells communicate with the extracellular matrix through cell membrane molecules, mostly integrins, which are transmembrane adhesion proteins that recognize and bind to extracellular matrix molecules.

In plant tissues, the cell wall can be regarded as a very different extracellular matrix, although not everybody agrees. It has very different properties compared to animal tissues. The cell wall provides rigidity to the cell, plant tissues and to the whole plant body. It is nearly an impermeable barrier and protects against pathogens and mechanical damage, among other functions.

Extracellular matrix
Figure 2. Drawing of the main molecules of the extracellular matrix of connective tissue.

The main molecules that form the extracellular matrix of animals are: structural proteins such as collagen and elastin, glycosaminoglycans, proteoglycans and glycoproteins. In plants, the cell wall is made up of carbohydrates (mainly cellulose) and glycoproteins (Figures 2 and 3). All of them are in aqueous solution, along with other smaller molecules, as well as ions. It is the amount, proportion and type of each type ofthese molecules what make the differences between extracellular matrices.

Extracellular matrix
Figure 3. Scanning electron microscopy of the intestine submucosa showing large collagen fibers.
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