ABSTRACT
The relationship between the structure of human skin and its barrier properties has
been the subject of extensive research over the past half century (1,2). There can be
little doubt that the stratum corneum, the skin’s outermost layer, plays an important
role in this respect by providing protection against the ingress of environmental
materials and controlling the egress of water. Investigations by many researchers
have further refined our understanding of the precise nature of the diffusion-
controlling barrier with consensus on the major role of the intercellular lipid
lamellae of the stratum corneum (3). There are many excellent and recent reviews
covering the structure of skin (4,5), the nature of the epidermis and its many
protective functions (6-8), and permeation across the skin (9,10). Although it has
been experimentally well established that it is the chemical morphology of the
stratum corneum that controls the overall rate at which chemicals can permeate
across the skin, other factors can contribute to the extent of such permeation.
Figure 1 shows that a number of major secondary factors governing the extent of
percutaneous absorption include permeant clearance from the skin (11) and
desquamation (12), as well as response and toxicity. It is not the purpose of this
chapter to reiterate the information already adequately reviewed elsewhere by
colleagues and by us. In this introductory chapter, therefore, we have elected to
cover some recent information relevant to the subject matter of the book, dermal
absorption and toxicity assessment. Thus we discuss new insights in skin biology
and barrier formation including the initiation of keratinocyte migration and
differentiation in the basal layer of the epidermis, the constituents and morphology
of the intercellular lipids and the process of desquamation (Fig. 2).
