Skin (integumentary system)

Introduction

This most extensive organ system has the skin and accessory structures, including hair, nails, glands (sweat and sebaceous), and specialized nerve receptors for stimuli (changes in internal or external environment) such as touch, cold, heat, pain, and pressure. Its functions include protection of internal structures, prevention of entry of disease-causing microorganisms, temperature regulation, excretion through perspiration, pigmentary protection against ultraviolet sunrays, and production of vitamin D. The body stores about half its fat in the underlying hypodermis.

Skin: epidermal layers

The skin is the largest organ of the body, with a surface area of 18 square feet. Its two main layers are the epidermis (outer layer) and dermis (inner layer). The epidermis has several strata (layers) that contain four cell types. Keratinocytes produce keratin, a protein that gives skin its strength and flexibility and waterproofs the skin surface. Melanocytes produce melanin, the dark pigment that gives skin its color. Merkel's cells are probably involved with touch reception. Langerhans' cells help the immune system by processing antigens (foreign bodies).

The deepest layer of the epidermis, the stratum basale, is a single layer of cells resting on a basement membrane (layer between the dermis and epidermis). The stratum basale cells divide continuously. As new cells form, older ones are pushed toward the skin surface.

The epidermis does not have a direct blood supply; all nutrients that feed these cells come from the dermis. Only the deepest cells of the stratum basale receive nourishment. The cells that are pushed away from this layer die. When the cells reach the skin surface, they are sloughed off in a process called desquamation.

The next layer, the stratum spinosum, consists of spiny prickle cells that interlock to support the skin. The stratum granulosum, the thin middle layer, initiates keratinization (production of keratin). This process starts the death of epithelial cells (the cell type that makes up skin).

During desquamation, keratinocytes are pushed toward the surface. These cells begin to produce the keratin that eventually will dominate their contents. When these cells reach the epidermis outer layer, they are little more than keratin-filled sacs. Millions of these dead cells are worn off daily, creating a new epidermis every 35 to 45 days.

The stratum lucidum protects against sun ultraviolet-ray damage. This thick layer appears only in frequently used areas such as palms of the hands and soles of the feet. Thick skin epidermis has all five strata. Thin skin covers thinner epidermal areas such as eyelids. Thin skin has three or four of the five strata; it never has stratum lucidum.

The stratum corneum, the fifth, outermost layer is thick with rows of dead cells. These cells contain soft keratin, which keeps the skin elastic and protects underlying cells from drying out.