This tangential section, which runs parallel to RER membrane surfaces, demonstrates several enlarged cisterns of RER #1, #2 of an actively synthesizing fibroblast. Both ribosomes of the cistern surfaces and ribosomes of the cytosol are forming clusters or polysomes. The extracellular matrix components (collagen, molecules of the ground substance, and elastic fibers) are not accumulated in secretory granules before their secretion by fibroblasts; this means that synthesis and secretion are of constitutive type. Although, intensive protein synthesis leads to a slower polypeptide chain processing (post-translational modification) compared to the time of production. It may cause temporary storing of immature secretory products inside the RER cisterns. On the bottom left a mitochondrion is found.