PRINCIPLES OF TOXICOLOGY - Biology East Borneo

8 Dermal and Ocular Toxicology: ToxicEffects of the Skin and EyesDERMAL AND OCULAR TOXICOLOGYWILLIAM F. SALMINEN and STEPHEN M. ROBERTSThe skin is the body’s first line of defense against external toxicant exposure. Normal skin is anexcellent barrier to many substances, but because of its enormous surface area (1.5–2.0 m 2 ), it can actas a portal of entry for many diverse chemicals that come into contact with it, causing local and/orsystemic effects. Understanding the composition of the skin and factors that influence the migrationof chemicals across it are prerequisites to understanding the various manifestations of toxicant injuryof the skin. Ocular toxicity will also be touched on in this chapter since many aspects pertaining toskin toxicity are relevant to ocular toxicity; the main difference is that the eye seldom serves as asignificant portal of entry because of its small surface area. In this chapter you will learn about the• Composition of the skin• Ability of the skin to defend against toxicants• Types of skin maladies• Commonly used tests to determine skin disorders• Composition of the eye and exposures pertaining to ocular toxicity8.1 SKIN HISTOLOGYThe skin is composed of two layers: the outer epidermis and the underlying dermis. The two layersare firmly associated and together form a barrier that ranges in thickness from 0.5–4 mm or more indifferent parts of the body. The epidermis and dermis are separated by a basement membrane, whichhas an undulating appearance. The uneven interface gives rise to dermal ridges and provides the basisfor the fingerprints used in personal identification since the patterns of ridges are unique for eachindividual. Hair follicles, sebaceous glands, and eccrine (i.e., sweat) glands span the epidermis and areembedded in the dermis. A third subcutaneous layer lays below the dermis and is composed mainlyof adipocytes. Even though this layer is not technically part of the skin it plays an integral role byacting as a heat insulator and shock absorber. (See Figure 8.1.)The epidermis is composed of several layers of cells—some living and some dead. The majorityof the epidermis is composed of keratinocytes, which undergo keratinization, a process during whichthey migrate upward from the lowest layers of the epidermis and accumulate large amounts of keratin(80 percent once fully mature and nonviable). By the time they reach the outer layer of the epidermis,the stratum corneum, the cells are no longer viable. They have become flattened and have lost theiraqueous environment, which is replaced by lipids. The superficial cells of the stratum corneum arecontinuously lost and must be replaced by new cells migrating from the lower layers of the epidermis.The lowest layers of the epidermis immediately adjacent to the dermis (stratum germinativum andstratum spinosum) are responsible for the continual supply of new keratinocytes and initiation of thekeratinization process. The migration and differentiation of keratinocytes from the lower viable layersPrinciples of Toxicology: Environmental and Industrial Applications, Second Edition, Edited by Phillip L. Williams,Robert C. James, and Stephen M. Roberts.ISBN 0-471-29321-0 © 2000 John Wiley & Sons, Inc.157