Porth's Essentials of Pathophysiology, 4e

1142

Integumentary Function

U N I T 1 3

Stratum corneum Stratum lucidum Stratum granulosum Epidermis

Stratum germinativum Stratum spinosum

Basement membrane zone

Epidermis lifted to reveal papillae of the dermis

Dermis

Papillae

Dermis

Arrector pili muscle

Blood vessel

Sebaceous gland

Subcutaneous tissue

Nerve endings

Nerve to hair follicle

Sweat gland

FIGURE 45-1. Three-dimensional view of the skin.

layers: a superficial and thinner epithelial tissue layer, called the epidermis , and a deeper and thicker connec- tive tissue layer, called the dermis (Fig. 45-1). The base- ment membrane zone is an interface between the dermis and epidermis. A layer of subcutaneous tissue, some- times called the hypodermis, serves as a storage site for fat and contains large blood vessels that supply the skin. Epidermis The functions of the skin depend largely on the properties of its epidermis, which is composed of stratified keratin- ized epithelium containing four types of cells: keratino- cytes, melanocytes, Langerhans cells, and Merkel cells. Keratinocytes The keratinocytes are the predominant cell type of the epidermis. They produce a fibrous protein called keratin, which is essential to the protective function of skin and maybe involved inthe immune systemandwoundhealing. As they divide and mature, the keratinocytes form five

distinct layers, or strata : the stratum germinativum, the stratum spinosum, the stratum granulosum, the stratum lucidum, and the stratum corneum. The deepest layer, the stratum germinativum or stra- tum basale, consists of a single layer of basal cells that are attached to the basal lamina in the basement mem- brane zone (to be discussed). The basal cells are the only epidermal cells that are mitotically active. All cells of the epidermis arise from this layer. As new cells form in the basal layer, the older cells change shape and are pushed upward toward the skin surface (Fig. 45-2). As these cells approach the skin surface, their cytoplasm becomes converted to keratin and they form flattened plates of dead cells on the skin surface. It normally takes 3 to 4 weeks for the epidermis to replicate itself. The rate of cell division in the stratum germinativum is greatly accelerated when the outer layers of the epider- mis are stripped away as occurs in abrasions and burns. The remaining layers of epidermis are formed as cells from the basal cell layer move upward toward the skin surface. The second layer, the stratum spinosum, is two to four layers thick. The cells of this layer are commonly