WCC – Wound Care Certified

The skin is the largest organ of the body, accounting for roughly 15% of body weight in adults. Every wound assessment, staging decision, and dressing choice depends on knowing which layers are involved and how those layers repair. Partial-thickness wounds regenerate from epidermal cells; full-thickness wounds must fill with granulation tissue and close by contraction and scar. The WCC exam tests skin anatomy directly and indirectly through staging, skin tear classification, and lifespan questions.

The integument consists of the epidermis (outer, avascular epithelium), the dermis (vascular connective tissue), and the underlying hypodermis (subcutaneous tissue). The epidermis and dermis are joined at the dermal–epidermal junction (DEJ), a basement membrane zone in which downward projections of the epidermis (rete ridges) interlock with upward projections of the dermis (dermal papillae). This interlocking increases surface area for nutrient diffusion and resists shear. Flattening of the DEJ is the single most important structural reason older adults develop skin tears and blisters.

The epidermis is a stratified squamous keratinized epithelium with no blood vessels; it receives oxygen and nutrients by diffusion from dermal capillaries. Thickness ranges from about 0.05 mm on the eyelids to about 1.5 mm on the palms and soles. New keratinocytes form in the basal layer and migrate upward, flattening, losing their nuclei, and filling with keratin until they are shed from the surface. In healthy adults this turnover takes roughly four weeks; it slows with aging.

The dermis is 2–4 mm thick in most areas and gives skin its strength, elasticity, and blood supply. Its main cell is the fibroblast, which synthesizes collagen (predominantly type I in mature dermis), elastin, and ground substance (glycosaminoglycans and proteoglycans that bind water). Mast cells, macrophages, and lymphocytes provide local defense. The dermis houses blood vessels, lymphatics, sensory nerve endings, hair follicles, and glands.

The hypodermis is not technically part of the skin but anchors it to underlying fascia and muscle. It is composed of adipose lobules separated by fibrous septa and carries larger blood vessels and nerves. Functions include insulation, energy storage, and cushioning over bony prominences. Subcutaneous fat has relatively poor blood supply, which is why obesity increases the risk of surgical site infection and dehiscence, and why fat is less tolerant of ischemia than skin. Visible adipose tissue in a wound bed signals a full-thickness wound (Stage 3 pressure injury or deeper).

The skin surface is mildly acidic, with a pH of approximately 4 to 6 (often cited as 4.5–5.5). This acid mantle is formed by sebum, sweat, free fatty acids, and breakdown products of keratinization. An acidic surface supports resident flora, inhibits pathogenic bacteria and yeast, and optimizes the enzymes that maintain barrier lipids. Alkaline soaps, urine (which converts to ammonia), and fecal enzymes raise skin pH, weaken the stratum corneum, and increase susceptibility to incontinence-associated dermatitis and infection.

Full-term neonates have a functional but immature barrier. Preterm infants, especially those born very early, have few layers of stratum corneum and an underdeveloped DEJ, so the barrier matures rapidly over the first weeks after birth. At birth, skin pH is near neutral and becomes acidic during the first weeks. Vernix caseosa, the protective coating on the newborn, has moisturizing and antimicrobial properties and should be left to wear off naturally.

Children's skin reaches adult barrier function during early childhood, but sebaceous activity remains low until puberty. Pressure injuries in children are strongly associated with medical devices and critical illness. Pediatric-specific risk tools, such as the Braden Q, are used instead of the adult Braden Scale.

Intrinsic aging and cumulative sun exposure produce predictable changes in every skin layer. These changes reduce tissue tolerance to pressure, shear, friction, and moisture and slow each phase of healing. Wound care clinicians must anticipate fragility rather than wait for injury.