PRINCIPLES OF TOXICOLOGY

8 Dermal and Ocular Toxicology: Toxic
Effects of the Skin and Eyes
DERMAL AND OCULAR TOXICOLOGY
WILLIAM F. SALMINEN and STEPHEN M. ROBERTS
The skin is the body’s first line of defense against external toxicant exposure. Normal skin is an
excellent barrier to many substances, but because of its enormous surface area (1.5–2.0 m 2 ), it can act
as a portal of entry for many diverse chemicals that come into contact with it, causing local and/or
systemic effects. Understanding the composition of the skin and factors that influence the migration
of chemicals across it are prerequisites to understanding the various manifestations of toxicant injury
of the skin. Ocular toxicity will also be touched on in this chapter since many aspects pertaining to
skin toxicity are relevant to ocular toxicity; the main difference is that the eye seldom serves as a
significant 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 toxicity
8.1 SKIN HISTOLOGY
The skin is composed of two layers: the outer epidermis and the underlying dermis. The two layers
are firmly associated and together form a barrier that ranges in thickness from 0.5–4 mm or more in
different parts of the body. The epidermis and dermis are separated by a basement membrane, which
has an undulating appearance. The uneven interface gives rise to dermal ridges and provides the basis
for the fingerprints used in personal identification since the patterns of ridges are unique for each
individual. Hair follicles, sebaceous glands, and eccrine (i.e., sweat) glands span the epidermis and are
embedded in the dermis. A third subcutaneous layer lays below the dermis and is composed mainly
of adipocytes. Even though this layer is not technically part of the skin it plays an integral role by
acting 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 majority
of the epidermis is composed of keratinocytes, which undergo keratinization, a process during which
they 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 their
aqueous environment, which is replaced by lipids. The superficial cells of the stratum corneum are
continuously 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 and
stratum spinosum) are responsible for the continual supply of new keratinocytes and initiation of the
keratinization process. The migration and differentiation of keratinocytes from the lower viable layers
Principles 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