The skin (cutis, integument) has two layers of different embryological origin firmly bound together. The superficial epidermis is a keratinised stratified squamous epithelium derived from surface ectoderm; the deeper dermis is a vascular connective tissue derived from mesoderm (and neural crest in the face). Beneath them lies the hypodermis (subcutaneous tissue, superficial fascia), a layer of loose connective and adipose tissue that is not strictly part of the skin but anchors it to deep fascia and is studied with it. The skin covers ≈1.5–2 m² in an adult, accounts for roughly 16% of body weight (the heaviest single organ) and varies in thickness from ≈0.5 mm on the eyelids to more than 4 mm on the upper back, mainly because of differences in dermal thickness.
Skin appendages — hair follicles, sebaceous glands, eccrine and apocrine sweat glands and nails — all arise from epidermis that grows down into the dermis during fetal life. Histologically, skin is classified as thick (glabrous) skin on the palms and soles, which has a thick epidermis with a stratum lucidum, friction ridges and many sweat glands but no hair or sebaceous glands, and thin (hairy) skin everywhere else, which has a thinner epidermis without a lucidum and contains hair follicles with their sebaceous glands and arrector pili muscles.
The epidermis is avascular: its cells are nourished by diffusion from capillary loops in the dermal papillae. It is continuously renewed — stem cells in the stratum basale divide, and their daughter keratinocytes move outward, fill with keratin, die and are shed as flattened corneocytes about four weeks later. Scattered among the keratinocytes are pigment-producing melanocytes (neural crest), antigen-presenting Langerhans cells (immune) and mechanosensory Merkel cells. The undulating dermoepidermal junction, with interlocking epidermal ridges and dermal papillae, anchors the epidermis via hemidesmosomes and anchoring fibrils and resists shearing forces.
The dermis provides strength (type I collagen), recoil (elastic fibres) and hydration (glycosaminoglycans). It is divided into a thin superficial papillary layer of loose connective tissue and a thick deep reticular layer of dense irregular connective tissue. The dermis carries two horizontal vascular plexuses, lymphatics, sensory receptors and autonomic nerve fibres. Because skin blood flow can rise from ≈0.3 L/min at rest to 6–8 L/min in severe heat stress, the cutaneous circulation is a major effector of thermoregulation, under hypothalamic control via sympathetic nerves; eccrine sweating, uniquely, is driven by sympathetic cholinergic fibres.
The skin is also an immune organ (keratinocyte antimicrobial peptides, Langerhans and dermal dendritic cells, mast cells, resident memory T cells), an endocrine organ (synthesis of cholecalciferol under UVB light) and a sensory organ (encapsulated mechanoreceptors and free nerve endings mapped in dermatomes). Clinically it is a diagnostic window: cyanosis, jaundice, pallor, rashes, nail changes and finger clubbing often reveal systemic disease, and skin cancer is the commonest malignancy in fair-skinned populations.
With ageing, the epidermis thins, the dermoepidermal junction flattens (so the skin tears and blisters easily), collagen and elastin decline, and sweat and sebum production fall, impairing thermoregulation and barrier function. Chronic ultraviolet exposure superimposes photoageing: solar elastosis, wrinkling, irregular pigmentation and actinic (solar) keratoses, the precursors of squamous cell carcinoma.
Development
The epidermis develops from surface ectoderm. By week 4–5 it is a single layer covered by a transient periderm; a basal (germinative) layer proliferates to produce an intermediate layer, and by the end of the fourth month all definitive strata are present. Shed periderm cells mix with sebum to form the protective vernix caseosa that coats the fetus. Melanocytes are neural crest cells that migrate into the basal epidermis from about week 8 and begin producing pigment before birth; Langerhans cells arise from haematopoietic (yolk-sac and fetal-liver) precursors; Merkel cells derive from the epidermal lineage.
The dermis arises from mesoderm: the dermomyotome of the somites forms the dermis of the back, lateral plate (somatic) mesoderm that of the limbs and body wall, and neural crest that of the face and much of the neck. Epidermal (friction) ridges form between ≈10 and 17 weeks and fix the individual fingerprint pattern. Hair follicles appear from ≈week 9–12 as epidermal buds pushing into the dermis over a mesodermal dermal papilla; the first fine hairs (lanugo) are mostly shed before or soon after birth and replaced by vellus hair. Sebaceous glands bud from the follicle wall, apocrine glands also bud from follicles, whereas eccrine glands grow directly from the epidermis. Nails begin at ≈week 10 as thickenings (nail fields) on the dorsal digit tips and reach the fingertips by ≈32 weeks and the toe tips by ≈36 weeks — a sign of fetal maturity. Defects of ectodermal derivatives (hair, teeth, nails, sweat glands) occur together in the ectodermal dysplasias.