Preface

Keratinocytes are the main components of the epidermis, the outermost tissue of the human body. Epidermal keratinocytes are essential for maintaining the physiological function of the epidermis, which protects the inner body from environmental insults, including microorganisms, ultraviolet irradiation, mechanical injury, and thermal and chemical stresses. The biology of keratinocytes has been studied for a long time, and structural, chemical, and molecular studies have revealed their characteristic features. Keratinocytes respond to environmental stimuli by producing various cytokines, chemokines, antimicrobial peptides, and other molecules, inducing weak inflammation to fight against environmental stimuli; however, this inflammatory reaction ceases to maintain a normal physiological condition. Studies imply that keratinocytes have dual properties, as proinflammatory at the moment of insults, and anti-inflammatory at the later stage of inflammation. In some pathological conditions such as psoriasis and atopic dermatitis, subtle insults induce pathological pathways to develop new skin lesions because of the enhanced proinflammatory properties of keratinocytes. The anti-inflammatory property in these inflammatory conditions may be suppressed or defective and thus may result in enhanced inflammation.

It is unique in keratinocytes, and potentially other tissue cells such as endothelial cells and fibroblasts, to show both pro-inflammatory and anti-inflammatory properties because many immune cells are divided into subtypes that exhibit only certain aspects of immune functions, such as effector T cells that show proinflammatory function and regulatory T cells that show anti-inflammatory function.

It is important to study the anti-inflammatory aspects of keratinocytes and the mechanisms of ceasing the inflammatory reaction provoked by environmental stimuli to elucidate the mechanisms of maintaining the homeostasis of physiological conditions.

Section 1, composed of two chapters, discusses the structural components of keratinocytes, such as keratins and attachment molecules, and their importance in maintaining epidermal homeostasis. This section also reviews the importance of neighboring cells, such as melanocytes. Section 2 is composed of three chapters. The first chapter discusses the inflammatory aspects of keratinocytes as the main components of cutaneous phenotype formation in inflammatory skin diseases such as psoriasis and atopic dermatitis. The second chapter discusses stem cells in the epidermis and skin regeneration and aging as aspects of epidermal keratinocytes. The final chapter discusses keratinocytes and skin disorders, focusing on genetic abnormalities and barrier function of the epidermis and skin disorders.

It was an honor to edit this volume and I hope it will be a useful resource for those interested in keratinocyte biology and cutaneous disorders.

> **Mayumi Komine** Department of Dermatology, Jichi Medical University, Shimotsuke, Tochigi, Japan

Section 1
