In routinely stained H&E preparations, reticular fibers cannot be identified positively. When visualized in the light microscope with special techniques, reticular fibers have a threadlike appearance. Because they contain a greater relative number of sugar groups than collagen fibers, reticular fibers are readily displayed by means of the periodic acid–Schiff (PAS) reaction. They are also revealed with special silver-staining procedures such as the Gomori and Wilder methods. After silver treatment, the fibers appear black; thus, they are said to be argyrophilic (Fig. 6.12). The thicker collagen fibers in such preparations are colored brown.
FIGURE 6.12 • Reticular fbers in the lymph node. Photo-micrograph of a lymph node silver preparation showing the connective tissue capsule at the top and a trabecula extending from it at the left. The reticular fibers (arrows) form an irregular anastomosing network. 650.
Reticular fibers are named for their arrangement in a mesh-like pattern or network.
In loose connective tissue, networks of reticular fbers are found at the boundary of connective tissue and epithelium, as well as surrounding adipocytes, small blood vessels, nerves, and muscle cells. They are also found in embryonic tissues. The prevalence of reticular fibers is an indicator of tissue maturity. They are prominent in the initial stages of wound healing and scar tissue formation, where they provide early mechanical strength to the newly synthesized ECM. As embryonic development or wound healing progresses, reticular fibers are gradually replaced by the stronger type I collagen fibers. Reticular fibers also function as a supporting stroma in hemopoietic and lymphatic tissues (but not in the thymus). In these tissues, a special cell type, the reticular cell, produces the collagen of the reticular fiber. This cell maintains a unique relationship to the fiber. It surrounds the fiber with its cytoplasm, thus isolating the fiber from other tissue components.
In most other locations, reticular fibers are produced by fibroblasts. Important exceptions to this general rule include the endoneurium of peripheral nerves, where Schwann cells secrete reticular fibers, tunica media of blood vessels, and muscularis of the alimentary canal, where smooth muscle cells secrete reticular and other collagen fibers.
Elastic fibers allow tissues to respond to stretch and  distension.
Elastic fbers are typically thinner than collagen fibers and are arranged in a branching pattern to form a three-dimensional network. The fibers are interwoven with collagen fibers to limit the distensibility of the tissue and prevent tearing from excessive stretching (Plate 6, page 196).
Elastic fibers stain with eosin but not well, so they cannot always be distinguished from collagen fibers in routine H&E preparations. Because elastic fibers become somewhat refractile with certain fixatives, they may be distinguished from collagen fibers in specimens stained with H&E when they display this characteristic. Elastic fibers can also be selectively stained with special dyes such as orcein or resorcin-fuchsin, as shown in Figure 6.13.
The elastic property of the elastin molecule is related to its unusual polypeptide backbone, which causes random coiling.
Elastic fibers are produced by many of the same cells that produce collagen and reticular fibers, particularly fibroblasts and smooth muscle cells. In contrast to collagen fibers, however, elastic fibers are composed of two structural components: A central core of elastin and a surrounding network of fbrillin microfbrils.  Elastin (72 kilodaltons) is a protein that, like collagen, is rich in proline and glycine. Unlike collagen, it is poor in hydroxyproline and completely lacks hydroxylysine. The random distribution of glycines makes the elastin molecule