Skin
Cutis
Also known as: integument, cutaneous membrane, cutis
The skin is the body's largest organ: a two-layered sheet of epidermis (epithelium) and dermis (connective tissue) that covers the whole external surface and protects, senses, thermoregulates and synthesises vitamin D.
Overview
The skin consists of a superficial epidermis of keratinised stratified squamous epithelium firmly attached at an undulating dermoepidermal junction to the underlying dermis, a vascular, innervated connective tissue. It rests on the hypodermis (subcutaneous tissue), which binds it loosely to the deep fascia and allows it to move. Skin appendages — hair follicles, sebaceous glands, sweat glands and nails — are epidermal down-growths that extend into the dermis and hypodermis.
Two histological types exist. Thick (glabrous) skin of the palms and soles has a five-layered epidermis up to ≈1.5 mm thick with a stratum lucidum, prominent friction ridges and abundant eccrine glands, but lacks hair and sebaceous glands. Thin skin covers the rest of the body; its epidermis is only ≈75–150 µm thick and has four layers, but it carries hair follicles, sebaceous glands and arrector pili. Overall skin thickness (≈0.5–4 mm) depends mostly on the dermis, which is thickest on the back.
At body openings the skin becomes continuous with mucous membranes at mucocutaneous junctions (lips, nostrils, eyelid margins, anus and urogenital orifices). Its lines of tension (Langer lines), creases and friction ridges are clinically and forensically important.
- Location
Covers the entire external body surface; continuous with mucous membranes at the lips, nares, eyelid margins, anal canal and urogenital openings. Thickest on the upper back, palms and soles; thinnest on the eyelids, postauricular skin and external genitalia.
Parts & subdivisions
- Avascular keratinised stratified squamous epithelium (surface ectoderm) — the barrier layer.
- Basement membrane zone with interlocking rete ridges and dermal papillae.
- Papillary (loose) and reticular (dense irregular) connective tissue carrying vessels, nerves and appendages.
- Subcutaneous fat and loose connective tissue; not strictly part of the skin.
- Hair follicles, sebaceous glands, eccrine and apocrine sweat glands, nails.
Anatomy
- Innervation
Sensory: cutaneous branches of spinal nerves arranged in dermatomes, and the trigeminal nerve for the face. Autonomic: sympathetic postganglionic fibres only (no parasympathetic) — adrenergic to arterioles and arrector pili, and cholinergic to eccrine sweat glands.
- Arterial supply
Perforating arteries reach the skin as direct cutaneous, musculocutaneous and fasciocutaneous perforators (the basis of surgical flaps and "angiosomes"). They form a deep (cutaneous) plexus at the dermis–hypodermis junction and a superficial (subpapillary) plexus, from which capillary loops ascend into each dermal papilla.
- Venous drainage
Venous plexuses parallel the arterial plexuses and drain to superficial subcutaneous veins (e.g. great saphenous, cephalic, basilic veins) and to venae comitantes of the perforating arteries.
- Lymphatic drainage
Blind-ended lymphatic capillaries in the papillary dermis drain to a dermal plexus and then to the regional nodes: head and neck → cervical nodes; upper limb and trunk above the umbilicus → axillary nodes; lower limb, perineum and trunk below the umbilicus (including the gluteal region and lower anal canal) → superficial inguinal nodes. This "watershed" at the umbilicus predicts nodal spread of skin infections and cancers.
- Histology
Thin skin: epidermis 4 layers (basale, spinosum, granulosum, corneum), with hair follicles and sebaceous glands. Thick skin: 5 layers (adds lucidum), thick corneum, no hair follicles. Dermis: papillary layer of loose connective tissue with capillary loops and Meissner corpuscles; reticular layer of dense irregular connective tissue with coarse type I collagen.
- Development
Epidermis from surface ectoderm; dermis from mesoderm (dermomyotome, lateral plate) and neural crest (face); melanocytes from neural crest; Langerhans cells from haematopoietic precursors.
Function
- Barrier to microbes, chemicals, UV radiation and mechanical trauma
- Limits water loss (transepidermal water loss ≈300–400 mL/day)
- Thermoregulation via blood flow and sweating
- Somatic sensation (touch, pressure, vibration, temperature, pain, itch)
- Vitamin D3 synthesis
- Immune surveillance and antimicrobial peptide production
- Physiology
Integrates barrier, thermoregulatory, sensory, endocrine and immune roles — see the dedicated entries for barrier function, thermoregulation, vitamin D synthesis, pigmentation and wound healing.
Clinical significance
Dermatomes and shingles
Reactivation of varicella-zoster virus latent in a dorsal root or trigeminal ganglion causes a painful vesicular rash confined to one dermatome that does not cross the midline. Ophthalmic (V1) zoster with vesicles on the tip of the nose (Hutchinson sign, nasociliary nerve) warns of eye involvement.
Skin as a diagnostic window
Systemic disease is often first visible in the skin: cyanosis, jaundice, pallor, spider naevi (liver disease), acanthosis nigricans (insulin resistance, gastric cancer), butterfly rash (SLE), erythema nodosum (sarcoidosis, IBD, TB), purpura (thrombocytopenia, vasculitis) and finger clubbing.
Skin ageing and photoageing
Intrinsic ageing thins the epidermis and flattens the dermoepidermal junction so that elderly skin tears and bruises easily (senile purpura). UV-driven photoageing adds solar elastosis (degenerate elastic material in the dermis), coarse wrinkles, lentigines and actinic keratoses.
Test yourself
Which epidermal layer is found only in thick (glabrous) skin?
- Stratum spinosum
- Stratum granulosum
- Stratum lucidum
- Stratum basale
Answer: Stratum lucidum
The stratum lucidum — a thin, clear band of flattened dead keratinocytes rich in eleidin — is present only in the thick skin of the palms and soles. Thin skin has four strata.
The main reason for differences in skin colour between ethnic groups is variation in:
- The number of melanocytes per mm²
- The amount of carotene in the dermis
- Melanosome number, size, distribution and rate of degradation
- Density of dermal capillaries
Answer: Melanosome number, size, distribution and rate of degradation
All humans have similar melanocyte densities at a given site. Darker skin has more, larger, individually dispersed and more slowly degraded melanosomes containing more eumelanin.
A patient has flaccid oral and skin blisters that extend when lateral pressure is applied to normal-looking skin. Autoantibodies are most likely directed against:
- BP180 (collagen XVII) in hemidesmosomes
- Desmoglein 3 in desmosomes
- Collagen VII in anchoring fibrils
- Tissue transglutaminase
Answer: Desmoglein 3 in desmosomes
Pemphigus vulgaris: anti-desmoglein 3 IgG causes suprabasal acantholysis, fragile intraepidermal blisters, mucosal involvement and a positive Nikolsky sign. Anti-BP180 causes tense subepidermal blisters of bullous pemphigoid.
Compiled from standard anatomy and physiology references — see the Integumentary System sources. For education only; not medical advice.