Editorial [Hot Topic: The Extracellular Matrix in Health and Disease(Executive Editor: Johannes A. Eble)]

The connective tissue is exceptional in terms of its huge versatility and variability. An important characteristic of the connective tissue is the intercellular space. The mesenchymal cells of the connective tissue are spaced widely apart and form a huge extracellular space between them. This space is filled with a dense meshwork of mainly fibrillar extracellular matrix (ECM) molecules, which together with elastic fibers bestow on the connective tissue the ability to withstand tensile forces plastically and elastically. The fibrillar meshwork is embedded in an electron microscopically amorphous substance, a negatively charged ground substance which because of its swelling capacity is responsible for the tissue pressure. Proteoglycans, the major constitutents of this ground substance, are abundant in cartilage, whereas incompressible biominerals make up most of ground substance in bone, thus enabling them to serve as the nondeformable skeletal framework of our body. Yet, the recent decades have revealed that the tissue which because of its mechanical ability to shape the body, organs and tissues has been called the stromal compartment in contrast to the organ-specific parenchyme, can do much more than to function as mere ‘stuffing material’. One characteristic of ECM proteins is their ability to aggregate into supramolecular structures, such as fibers and twodimensional networks, in a highly ordered manner. In most cases, this formation of ECM aggregates is a self-organizing process, which additionally can be initiated and influenced by cells. In the first review, P. Yurchenco and B. Patton [1] describe the latest findings and models concerning the (self-) assembly of the basement membrane (BM), a two-dimensional sheet-like ECM structure, which is the borderline of the connective tissue to other tissues, such as epithelium, endothelium, muscle and neuronal tissue, as well as fat cells. Several deficiencies of the BM cause severe human diseases, partially due to misorganized BM formation. In addition to explaining the molecular mechanisms, P. Yurchenco and B. Patton describe therapeutic approaches and strategies to treat some of these diseases. A special emphasis is put on defects of muscle tissue and the nervous system. Although the connective tissue/stromal compartment of these tissues are generally regarded as less important for the physiological roles of these organs, the described disease examples show, that the stromal ECM and BM are indispensably required for their full functionality.