Chapter 43: Structure, Function, and Disorders of the Integument

The integumentary system comprises the skin, hair, nails, and cutaneous glands, functioning as the body's primary barrier against environmental threats while maintaining homeostasis and enabling sensory perception. The skin consists of three distinct layers that work coordinately to provide comprehensive protection and physiological regulation. The epidermis, the outermost layer, contains multiple specialized cell types including keratinocytes that form the structural foundation, melanocytes that synthesize protective pigmentation, Langerhans cells that initiate immune responses, and Merkel cells that facilitate tactile sensation. The epidermis is organized into five stratified layers that undergo continuous cellular turnover through keratinization, a process that gradually transforms living cells into the protective proteins that form the barrier. Melanin production not only determines skin coloration but also shields deeper tissues from ultraviolet radiation damage. Beneath the epidermis lies the dermis, a connective tissue layer rich in collagen and elastin fibers that provide structural support and elasticity. The dermis houses blood vessels and lymphatic vessels regulating temperature and immune function, sensory nerve endings responsive to pressure and pain, and appendages including hair follicles and glands. The deepest layer, the hypodermis, consists primarily of adipose tissue that provides thermal insulation, energy reserves, and mechanical cushioning for underlying structures. The skin performs multiple critical functions beyond protection, including thermoregulation through coordinated vasoconstriction and vasodilation, immune defense via Langerhans cell activation and antimicrobial peptide secretion, vitamin D synthesis upon ultraviolet exposure, and psychosocial functions related to identity and appearance. Hair grows cyclically through anagen, catagen, and telogen phases from follicles associated with sebaceous glands and muscle. Nails develop from the nail matrix and provide protection and tactile enhancement. Sebaceous glands secrete lipid-rich sebum for skin lubrication and antimicrobial activity, eccrine sweat glands enable thermoregulation through watery secretions, and apocrine sweat glands produce thicker secretions influenced by hormonal and emotional stimuli. Wound healing progresses through inflammation, proliferation involving granulation tissue formation and angiogenesis, and remodeling with collagen maturation. Age-related changes significantly impact integumentary function, with infants exhibiting thinner epidermis and dermis, compromised barrier integrity, and immature temperature regulation, while older adults experience dermal thinning, reduced vascularity and collagen content, impaired wound healing capacity, diminished glandular function, and increased vulnerability to desiccation and infection.