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INTRODUCTION

Various substances may form deposits in the skin. They may be classified as endogenous and exogenous. Endogenous substances are the principal topic of this chapter. They include substances that are normal constituents of the skin but are abnormally deposited (eg, increased deposition of mucin) and substances that are not normal constituents of the skin (eg, amyloid). Exogenous pigmentary deposits, such as tattoos, drug-related pigmentation and heavy metals, and the special category of endogenous deposition, such as melanin depositions, are discussed in Chap. 15.

Abnormal deposition of endogenous substances exhibits particular histochemical staining properties. However, different substances may show similar staining by the hematoxylin and eosin (H&E) stain (Fig. 16-1). These deposits with similar staining properties are often grouped together and were once considered to be the same chemical substances. Examples are mucin, hyalin, and colloid. When a dermatopathologist examines H&E-stained tissue sections, he or she notices the color characteristics and common texture of a substance in 1 of these 3 groups. The dermatopathologist then focuses on the subset of conditions that manifests these particular characteristics. Special stains, immunostains, electron microscopy, and clinical information provide further aid in the differential diagnosis. In many instances, clinical knowledge is essential in the final synthesis of all data to reach a reasonable conclusion. This process is always at work in the experienced pathologist’s mind. An algorithmic approach to differential diagnosis is presented in Fig. 16-1. Tables 16-1, 16-2, 16-3, 16-4, 16-5, 16-6 will assist in this analytical process.

TABLE 16-1The Cutaneous Mucinoses

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