Chapter 4: The Integumentary System

The Integumentary System, or integument, encompasses the skin (cutaneous membrane) and its derivatives: hair, nails, and various exocrine glands, serving vital roles in physical protection from environmental hazards, sensory detection, temperature regulation (thermoregulation), excretion, and the synthesis and storage of lipid reserves and Vitamin D3. The skin comprises the superficial epidermis, a keratinized stratified squamous epithelium, and the underlying connective tissues of the dermis. The epidermis relies on four cell types: the most numerous keratinocytes, which undergo keratinization to form the tough, durable outer layers; melanocytes which produce melanin pigment to shield underlying tissues from damaging ultraviolet (UV) radiation; touch-sensitive Merkel cells; and wandering phagocytic Langerhans cells (dendritic cells) important for the immune response. The epidermis typically exhibits five layers in thick skin (palms and soles): the regenerative stratum basale, the stratum spinosum, the stratum granulosum, the translucent stratum lucidum, and the outermost, protective stratum corneum. Skin color results from a combination of the dermal blood supply, thickness of the stratum corneum, and variable amounts of pigments like yellow-orange carotene and protective melanin; a bluish discoloration known as cyanosis can indicate reduced oxygen in the dermal blood supply. Deep to the dermis is the subcutaneous layer (hypodermis or superficial fascia), composed of loose connective tissue and numerous adipocytes, functioning to stabilize the skin and provide insulation and energy reserves. The dermis itself is divided into the superficial papillary layer and the deeper reticular layer, an extensive meshwork of collagen and elastic fibers whose parallel alignment establishes tension lines (cleavage lines) significant for surgical incision and healing. Accessory structures include hair follicles, which produce hairs that provide sensation via the root hair plexus and are elevated by the arrector pili muscle; sebaceous glands which secrete oily sebum onto hair shafts and the epidermis for lubrication and antibacterial action; and sweat glands, which include limited apocrine glands producing a potentially odorous secretion and widespread merocrine (eccrine) sweat glands that produce watery sensible perspiration vital for cooling the body. Skin repair following injury involves clot formation (scab), the production of granulation tissue, and ultimately, the formation of inflexible, noncellular scar tissue, which often replaces lost hair follicles and glands; excessive scar formation can lead to a keloid. With aging, the integument experiences reduced activity in melanocytes and glands, epidermal thinning, a substantial decline in Vitamin D production (around 75 percent), and a decrease in the strength and resiliency of the dermis, collectively impairing thermoregulation and making older individuals more prone to injury and infection.