Ground substance in the extracellular matrix (ECM) and cell-matrix interactions. A. The main components of ECM: fibers and ground substance. The principal components of ground substance are glycosaminoglycans (GAGs), proteoglycans, and adhesive glycoproteins. B. GAG structure. GAGs are heteropolysaccharides produced from repeating disaccharide units, one of which is N-acetylated. Shown here is the most abundant GAG, hyaluronic acid, a single molecule that can consist of thousands of disaccharides (e.g., D-glucuronic acid, N-acetyl-D-glucosamine) and occupy a large volume in relation to its molecular weight. C. Proteoglycan structure. Most GAGs are shorter than hyaluronic acid; they are not found free but are covalently attached to one member of a set of core proteins, forming a proteoglycan. The GAGs in proteoglycans are usually sulfated and highly negatively charged, attracting cations (e.g., sodium) and binding water. D. Activated integrin dimers attach cells to the ECM. Shown is a schematic model of how the extracellular portions of activated integrin complexes in a focal adhesion complex on a fibroblast cell are attached to the adhesive glycoprotein, fibronectin. Maintenance of the activated state requires interactions between integrins and the actin cytoskeleton. E. Inactive integrins detach from the ECM. When cytoskeleton binding is lost, the integrins undergo a conformational change that greatly decreases the extracellular binding affinity, and they detach from adhesive glycoproteins. Also indicated in this figure are additional interactions of fibronectin with hyaluronic acid and collagen, suggesting the complex networks that can be formed in ECM. Source: CONNECTIVE TISSUE PROPER, The Big Picture: Histology Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: https://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/2058/ashhist_ch2_f005.png&sec=155783901&BookID=2058&ChapterSecID=155783845&imagename= Accessed: October 01, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved