Chapter 28: Basic Sciences

INTRODUCTION

• Skin is the largest organ in the body. It weighs over 5 kg in a 70-kg individual and contains a variety of complex adnexal structures, including hair, nails, glands, and specialized sensory structures

• The anatomic structures of the skin are similar in most regions of the body

• Specialized regions of skin have modified forms of these structures, such as the palms, soles, genitalia, and scalp

• Human skin consists of a stratified, cellular epidermis, and an underlying dermis of connective tissue. Beneath the dermis is a layer of subcutaneous fat

EPIDERMIS AND DERMIS

Epidermis

• The epidermis contains 3 major resident populations of cells: keratinocytes, melanocytes, and Langerhans cells

• Approximately 0.4 to 1.5 mm thick and consisting mostly of keratinocytes (Fig. 28-1)

• Epidermis is formed by division of cells in the basal layer which give rise to the spinous layer

• Under basal conditions, differentiated keratinocytes require approximately 2 weeks to reach the stratum corneum (SC) and an additional 2 weeks to be shed from the SC

• Keratinocytes have the capacity to increase rates of proliferation and maturation to levels far greater than this, when stimulated to do so by injury, inflammation, or disease

• Divided into 4 main layers with characteristic cell shape, specialized intracellular structures, types of keratin, accessory cells, and proteins (Table 28-1): SC, stratum granulosum, stratum spinosum, stratum basale

• Stratum disjunction: outer SC cells are more prone to desquamation

• Stratum compactum: cells of the lower SC; thicker cells and more densely packed, organized parallel arrays of keratin filaments, more fragile cornified envelope

• Stratum lucidum is an additional layer present between the strata granulosum and corneum in palmoplantar skin. It appears as an electron-lucent zone and contains nucleated cells

• Differentiation from basal cell to corneocyte involves the loss of the nucleus and extrusion of cellular contents except for keratin filaments and filaggrin matrix

• The most obvious function of epidermis lies in the SC, a semipermeable surface which serve as a physiologic barrier