ABSTRACT
INTRODUCTION When epidermal stem cells divide and detach from the basal layer, they follow the route of programmed differentiation. The generated transitional keratinocytes are preprogrammed into a stratification process throughout the epidermal layers (Fig. 1), leading to physiologic cell death and stratum corneum (SC) formation (Fig. 2). At the leading edge of the epidermis, the SC generates an efficient barrier that protects the internal milieu of the organism from the external desiccating environment (1). However, to accomplish this function, the epidermis constantly replenishes itself by a process of homeostasis. During this process, the dividing innermost stem cell (i.e., basal layer) will again execute the programmed differentiation, move outwards, and desquamate (2) (Fig. 2). As keratinocytes differentiate, they express specific proteins. For example, the expression of different types of keratins (K) is used as a hallmark to stage/identify keratinocyte differentiation/proliferation state. However, keratins are not the only differentiation-specific proteins. Hence, we will describe in this chapter the changing faces of certain proteins that accompany a keratinocyte from its embryonic origin to its desquamatory detachment. This chapter also addresses other epidermal layer-specific molecules corresponding to the different stages of keratinocyte maturation and a brief overview of their function.
