Skin, and How the Body Forms

Part 1 · General anatomy

Skin layers and repair, the germ layers, and why one nerve root has both a muscle and a skin patch

Two subjects in one chapter, and why they belong together

Skin is the organ you touch every time you treat a patient, the organ that fails first in the immobile, and the organ whose repair you either help or hinder. Embryology is what explains why the body is organised the way it is — why a nerve root supplies both a strip of skin and a group of muscles, why cardiac pain is felt in the left arm, why the ureter and the testis share a pain pathway, and why the thumb is lateral and the great toe medial.

They belong in one chapter because the answer to the chapter’s subtitle question — why one nerve root has both a muscle and a skin patch — comes from development, and is the single most clinically useful idea in either subject.

Learning outcomes

• Describe the layers of the epidermis and dermis and their cell populations.
• Describe skin appendages and skin blood supply, and explain thermoregulation.
• Describe the phases and timeline of wound healing, and distinguish healing by primary and secondary intention.
• Explain scar maturation and the difference between hypertrophic and keloid scar.
• Classify burns and pressure injuries and state the physiotherapy implications of each.
• Describe the formation of the three germ layers and name the derivatives of each.
• Describe somite formation and differentiation into sclerotome, myotome and dermatome.
• Explain segmentation, and define dermatome, myotome, sclerotome and viscerotome.
• Explain referred pain and the shoulder-tip / cardiac / diaphragmatic referral patterns from developmental principles.
• Describe limb development, including the AER, ZPA and limb rotation.
• Recognise the common congenital conditions a physiotherapist will meet.

Part A · Skin

The integument in numbers

Figure 1 · What skin does

Skin is the largest organ: 1.5–2 m² in an adult, ~15% of body weight, thickness from 0.5 mm on the eyelid to 4–5 mm on the sole and palm.

Its functions: protection (mechanical, chemical, microbial, ultraviolet), thermoregulation, sensation, vitamin D synthesis, fluid and electrolyte conservation, immune surveillance, excretion, absorption (the basis of transdermal drug delivery and of many topical physiotherapy agents), and social and communicative function.

Skin is classified as thick (palms and soles: thick epidermis with a stratum lucidum, no hair follicles or sebaceous glands, abundant sweat glands, epidermal ridges producing fingerprints) or thin (everywhere else: thinner epidermis, hair follicles and sebaceous glands present).

Structure

Figure 2 · The structure of the skin

8.3.1 Epidermis — stratified squamous keratinised epithelium, ectodermal, avascular

From deep to superficial:

Epidermal turnover is approximately 28–40 days from basal cell to desquamation — accelerated dramatically in psoriasis (to ~4–7 days), which is why psoriatic plaques are scaly.

Non-keratinocyte cells:

8.3.2 Dermis — mesodermal, vascular

The predominant collagen bundle orientation in the reticular dermis creates lines of cleavage (Langer’s lines). Incisions parallel to them gape less and scar better; incisions across them gape and produce wider scars. Wound orientation relative to these lines partly predicts scar quality — a point of direct relevance in scar management.

Ageing skin shows epidermal thinning, flattening of the dermo-epidermal junction (reducing adhesion and increasing shear vulnerability — the reason elderly skin tears with adhesive tape removal), collagen and elastin loss, reduced vascularity, reduced sweat and sebaceous output, and slower healing.

8.3.3 Appendages

8.3.4 Blood supply and thermoregulation

Two horizontal plexuses — deep (at the dermo-hypodermal junction) and superficial (subpapillary) — connected by perforating vessels, with arteriovenous anastomoses (glomus bodies) in the digits, palms, soles, ears and nose.

Skin blood flow can vary from about 0.5% to 30% of cardiac output. Sympathetic vasoconstrictor tone controls it; heat causes vasodilatation and sweating, cold causes vasoconstriction and arteriovenous shunting to preserve core temperature.

Clinical implications for physiotherapy: all thermal and electrophysical agents act on and through this system. In any patient with impaired sensation, impaired circulation or impaired cognition, the protective withdrawal from a damaging thermal stimulus is absent — which is why thermal agents are contraindicated over areas of sensory loss, arterial insufficiency, or in a patient who cannot report sensation reliably. Burns from hot packs and paraffin in neuropathic feet remain a persistent and entirely avoidable source of harm.

8.3.5 Cutaneous sensation

Two-point discrimination — approximately 2–4 mm on the fingertip, 40–50 mm on the back — reflects receptor density and cortical representation, and is a standard clinical measure of peripheral nerve regeneration.

Wound healing

The phases

Tensile strength timeline (a frequently examined sequence): ~5% at 1 week, ~20% at 3 weeks, ~50–60% at 6 weeks, and a plateau at ~70–80% of the original at 6–12 months. Scar never regains normal strength, and it lacks appendages, elastic recovery and normal sensation.

Primary versus secondary intention: a clean approximated wound heals by primary intention — minimal granulation, minimal contraction, a fine scar. A wound with tissue loss or contamination heals by secondary intention — abundant granulation, marked wound contraction, prolonged epithelialisation and a broad scar. Secondary healing is where contracture, and therefore physiotherapy, matters most.

Factors impairing healing: age; malnutrition (protein, vitamin C, zinc); diabetes; smoking; corticosteroids and immunosuppression; ischaemia and venous insufficiency; infection; repeated mechanical shear and pressure; radiotherapy; and foreign material.

Abnormal scar

Scar management principles for physiotherapy: sustained low-load stretch and positioning (exploiting creep — Chapter 7), pressure therapy (24 mmHg or more, worn 23 hours a day for months in burns), silicone gel or sheeting, massage once epithelialisation is complete, splinting in the anti-deformity position, and desensitisation. All of this works during the remodelling phase, which is the window that closes.

Burns

Depth classification

Extent is estimated by the rule of nines in adults (head 9%, each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, perineum 1%) — modified in children, whose head is proportionally larger; the Lund and Browder chart is more accurate.

Physiotherapy in burns is dominated by two aims: preventing contracture (positioning in the anti-deformity position — neck extension, shoulder abduction to ~90° with slight horizontal flexion, elbow extension and forearm supination, wrist slight extension with MCPs flexed to 70–90° and IPs extended, hips extended and abducted, knees extended, ankles neutral — which is the opposite of the comfortable flexed posture the patient adopts), and maintaining respiratory and functional capacity. Splinting, sustained stretch, early mobilisation, and later pressure therapy and scar management are the core interventions, delivered over months.

Pressure injury

Mechanism. Sustained pressure exceeding capillary closing pressure (~32 mmHg) occludes perfusion; shear compounds it by distorting and occluding vessels obliquely; moisture macerates and reduces tissue tolerance; friction removes the stratum corneum. Muscle is more susceptible than skin, so damage is frequently deeper than it appears — the “iceberg” pattern.

Sites: sacrum, ischial tuberosities, greater trochanters, heels, lateral malleoli, occiput, scapulae, elbows; and, increasingly, device-related sites (oxygen tubing, catheters, casts).

Staging (NPIAP/EPUAP):

Prevention is the physiotherapy contribution, and it is substantial: repositioning schedules, pressure-redistributing surfaces, teaching independent pressure relief (lift-offs or leans every 15–30 minutes in wheelchair users), wheelchair seating and cushion prescription, transfer technique that eliminates dragging, maintaining mobility and nutrition, and skin inspection education. A stage 4 pressure injury in a spinal cord injured patient can cost a year of rehabilitation progress.

Part B · How the Body Forms

The first three weeks

Figure 3 · Neurulation and the neural crest

Gastrulation is the pivotal event: it converts a two-layered disc into a three-layered embryo with an established body plan and axes. Every tissue in the body traces to one of the three layers formed in that week.

The three germ layers and their derivatives

Figure 4 · The three germ layers and what each becomes

Neural crest — the “fourth germ layer”

Neural crest cells delaminate from the crests of the neural folds and migrate widely. Because they produce structures as unrelated as the facial skeleton, the adrenal medulla and skin melanocytes, a neurocristopathy produces baffling multi-system combinations: Hirschsprung disease (absent enteric ganglia), neurofibromatosis, Waardenburg syndrome (pigmentary changes with deafness), DiGeorge syndrome (thymic, parathyroid and conotruncal cardiac defects), and CHARGE syndrome. If you ever wonder why a child with a cardiac outflow defect also has a facial anomaly and pigmentary change, the answer is neural crest.

Somites and segmentation — the key to clinical anatomy

Figure 5 · What a somite becomes

From day 20, paraxial mesoderm on each side of the notochord segments into somites, forming craniocaudally at roughly three pairs per day, reaching 42–44 pairs (4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 8–10 coccygeal, several of which regress). Somite count is the standard measure of embryonic age in weeks 4–5.

Each somite differentiates into three parts:

Each somite is supplied by its own spinal nerve, and that relationship is retained for life however far the tissue migrates. This is the whole basis of segmental clinical anatomy.

The four “-tomes”

Dermatome maps do not agree with each other (Keegan and Garrett vs Foerster vs the modern evidence-based maps), because there is substantial overlap between adjacent dermatomes — usually enough that section of a single dorsal root produces little detectable sensory loss, and at least two adjacent roots must be lost for a definite area of anaesthesia. Use dermatomes as a probabilistic guide, not a map with fixed borders. The most reliable single points (C5 lateral elbow, C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L1 groin, L3 medial knee, L4 medial malleolus, L5 dorsum of foot / first web space, S1 lateral heel) are the ones worth memorising, and are the ASIA key sensory points.

Referred pain, explained developmentally

Visceral afferents accompany sympathetic fibres back to the spinal cord and converge on the same second-order neurons as somatic afferents from the corresponding dermatome. The brain, which receives far more somatic than visceral input over a lifetime, misattributes the signal to the skin — the convergence–projection theory.

Why this matters in a physiotherapy clinic

You will see patients presenting with musculoskeletal-sounding pain of visceral origin. Left shoulder or interscapular pain that is exertional, unrelated to position or movement, and accompanied by breathlessness or nausea is a cardiac presentation until proved otherwise. Right shoulder-tip pain with abdominal tenderness may be hepatobiliary or a ruptured viscus. Thoracic pain with a band-like distribution and no mechanical behaviour may be visceral or neoplastic.

The screening rule: musculoskeletal pain has a mechanical behaviour — it is reproduced and eased by specific positions and movements. Pain that is entirely unrelated to position, load or movement, or that is accompanied by systemic features, requires medical screening before it receives a mechanical diagnosis.

Limb development

Figure 6 · Limb development and rotation

Critical period: the limbs are most vulnerable to teratogens in weeks 4–8, which is exactly when thalidomide produced phocomelia and amelia.

Congenital conditions a physiotherapist will meet

Where students consistently go wrong

• Forgetting the stratum lucidum is only in thick skin.
• Thinking skin colour reflects melanocyte number. It reflects melanin type, amount and distribution.
• Assuming scar regains normal strength. It plateaus at 70–80% and lacks appendages and elasticity.
• Missing the deep-partial burn. Reduced sensation and blotchy non-blanching appearance means likely grafting and near-certain hypertrophic scarring.
• Positioning the burned patient comfortably. The comfortable position is the contracture position; use the anti-deformity position.
• Underestimating a pressure injury. Muscle dies before skin; the visible lesion is the tip.
• Using thermal agents over insensate skin. A persistent and avoidable source of harm.
• Confusing dermatome, myotome and sclerotome. Skin, muscle and bone from the same somite and root.
• Treating dermatome maps as exact. Adjacent dermatomes overlap substantially.
• Forgetting the diaphragm’s cervical origin. C3–C5 explains shoulder-tip pain and, more importantly, why a lesion above C3 abolishes spontaneous ventilation.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 8 of 24 · Human Anatomy · Physiotherapist India End of Part 1 — General anatomy. Next: Chapter 9 — Bones of the Upper Limb, opening Part 2.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents