Caption: Muscle myoblast (primitive embryonic muscle cell) secreting extracellular matrix (ECM). A myoblast is a type of stem cell that occurs in muscles. Skeletal muscle fibers are made when myoblasts fuse together. As a result muscle fibers have multiple nuclei. The extracellular matrix (ECM) is a complex structural tissue component surrounding and supporting cells in mammalian tissues. The ECM is often referred to as the connective tissue. The ECM is composed of 3 major classes of biomolecules: 1) Structural proteins - collagen and elastin, 2) Specialized proteins - such as: fibrillin, fibronectin, and laminin and 3) Proteoglycans - complex proteins and long chains of repeating disaccharide units called glycosaminoglycans (GAGs, such as: hyaluronic acid, chondroitin sulfate, heparin, heparan sulfate). Collagens are the most abundant proteins found in the animal kingdom. It is the major protein comprising the ECM. There are at least 12 types of collagen. Collagens are predominantly formed by fibroblasts but epithelial cells also synthesize these proteins. Fibronectins attach cells to a variety of extracellular matrices. Fibronectin attaches cells to all matrices except type IV that involves laminin as the adhesive molecule. Laminin anchors cell surfaces to the basal lamina. All basal laminae contain a common set of proteins and GAGs. These are type IV collagen, heparan sulfate proteoglycans, entactin and laminin. The basal lamina is often referred to as the type IV matrix. Many cells bind to components of the extracellular matrix. The cell to ECM adhesion is regulated by specific cell surface cellular adhesion molecules (CAM) known as integrins. The integrins transmit mechanical stimuli from the ECM to the cytoskeleton.
Magnification*: x1,200
Type: SEM
Copyright 1999 Dennis Kunkel Microscopy, Inc.
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