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INTRODUCTION

The major function of the skin is to serve as a barrier to various exogenous substances, chemicals, and microbes. Collagen, ground substance (dermal mucin), and elastic fibers are the 3 major components of the dermis and are crucial to maintaining the integrity and elasticity of the skin. Collagen is the most abundant of the 3 components of the dermis (80% by dry weight), consisting primarily of type I collagen in the reticular dermis, type III collagen in the papillary and adventitial dermis, and type IV collagen forming basement membranes for epithelial structures and blood vessels. Over 20 distinct collagens are now recognized.1,2 The amino acid sequence and composition determines the individual collagen type. Collagens are composed of 3 polypeptides, or alpha chains, that assume a stabilizing triple-helical conformation depending on glycine residues located at every third position of the polypeptides. Hydroxyproline (synthesized from proline) is necessary for triple-helix stability at physiologic temperatures. Fibroblasts initially secrete collagen in a soluble form (procollagen), which is then cleaved to insoluble fibers (collagen) that are crosslinked by oxidation of lysyl and hydroxylysine residues. It is this crosslinking that gives collagen its tensile strength. Whereas type I collagen provides tensile strength to the skin, type III collagen participates in anchoring the epidermis and adnexae to the dermis.

Like many tissues within the body, the dermis and its collagen are constantly remodeled by a complex process of collagen synthesis and degradation by ubiquitous collagenases.3 Various substances are known to interfere with collagen synthesis, in particular corticosteroids, which affect several steps in the collagen biosynthetic pathway.4 Other conditions resulting in decreased levels of reducing agents, such as ascorbic acid and oxygen, which are required for hydroxylation of prolyl residues, lead to deficient collagen crosslinking, resulting in scurvy (poor wound healing, skin fragility, easy bruising, and bleeding) and sometimes stasis ulcers.5

Alterations of collagen, which often exist in concert with deficient elastin and ground substance, are present in a diverse group of diseases manifesting in some individuals as sclerosis, including scleroderma and a number of diseases that produce changes histologically indistinguishable from scleroderma (ie, sclerodermoid disorders such as chronic graft-versus-host disease). Sclerosis is characterized by abundant collagen deposition with replacement/displacement of elastic fibers and relative lack of dermal cellularity (ie, fibroblasts and dermal dendritic cells). In other conditions, such as those preceding scar and keloid formation, collagen is produced in greater quantities than normal by increased numbers of fibroblasts, referred to as fibrosis. In other conditions, such as aplasia cutis congenita and focal dermal hypoplasia, collagen is deficient, resulting in dermal atrophy. Rarely, cutaneous diseases may result from the loss of connective tissue fibers to the skin surface (eg, “perforating” diseases); other conditions are characterized by collagen defects at the molecular level (eg, Ehlers-Danlos syndrome, osteogenesis imperfecta). Since the last edition of this textbook, there has been an explosion of knowledge pertaining to the molecular basis ...

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