FIGURE 6.13 • Collagen and elastic fbers. Photomicrograph of a mesentery spread stained with resorcin-fuchsin. The mesentery is very thin, and the microscope can be focused through the entire thickness of the tissue. The delicate threadlike branching strands are the elastic fibers (E). Collagen fibers (C) are also evident. They are much thicker; although they cross one another, they do not branch. 200.
hydrophobic and allows for random coiling of its fibers. This permits elastic fibers to “slide” over one another or to be stretched and then recoil to their original state. Elastin also contains desmosine and isodesmosine, two large amino acids unique to elastin, which are responsible for the covalent bonding of elastin molecules to one another. These covalent bonds link four elastin molecules into either desmosine or isodesmosine cross-links (Fig. 6.14). Elastin forms fibers of variable thicknesses, or lamellar layers (as in elastic arteries). Elastin is encoded by one of the largest genes in the human genome. The elastin gene consists of 28 kilobases, but less than 10% of the kilobases carry the sequence that encodes elastin.
 Fibrillin-1 (350 kilodaltons) is a glycoprotein that forms fine microfibrils measuring 10 to 12 nm in diameter. During the early stages of elastogenesis, fibrillin-1 microfibrils are used as substrates for the assembly of elastic fibers. The microfibrils are formed first; elastin material is then deposited on the surface of the microfibrils. Elastin-associated fbrillin microfbrils play a major role in organizing elastin into fibers. The absence of fibrillin
FIGURE 6.14 • Diagram of elastin molecules and their interaction. a. Elastin molecules are shown joined by covalent bonding between desmosine and isodesmosine (purple) to form a cross-linked network. Inset shows enlargement of the elastin molecule in its individual and random-coiled conformation with the covalent bond formed by desmosine. b. The effect of stretching is shown. When the force is withdrawn, the network reverts to the relaxed state as in panel a. (Modified with permission from Alberts B, et al. Essential Cell Biology, p. 153. Copyright 1997. Routledge, Inc., part of The Taylor & Francis Group.) microfibrils during elastogenesis results in the formation of elastin sheets or lamellae, as found in blood vessels. Abnormal expression of the fbrillin gene (FBN1) is linked to Marfan’s syndrome, a complex, autosomal dominant, connective tissue disorder. Immunoﬂuorescence of a skin biopsy specimen from a person with this syndrome shows an absence of elastin-associated fibrillin microfibrils. One of the consequences of the disease is abnormal elastic tissue.
With both the TEM and SEM, elastin appears as an amorphous structure of low electron density. In contrast, the fibrillin microfibrils are electron dense and are readily apparent even within the elastin matrix (Fig. 6.15). In mature fibers, the fibrillin microfibrils are located within the elastic fiber and at its periphery. The presence of microfibrils within the fiber is associated with the growth process; thus, as the fiber is formed and thickens, the microfibrils become entrapped within the newly deposited elastin.
 FOLDER 6.2 Clinical Correlation: Sun Exposure and Molecular Changes in Photoaged Skin