o Layers (deep to superficial):
1. Stratum basale (mitotically active, stem cells, melanocytes)
2. Stratum spinosum (desmosomes, Langerhans cells)
3. Stratum granulosum (keratohyalin granules)
4. Stratum lucidum (only in palms and soles)
5. Stratum corneum (dead keratinocytes — barrier layer)
o Keratinocytes: produce keratin → mechanical protection
o Melanocytes: derived from neural crest, produce melanin → UV protection
o Langerhans cells: antigen-presenting cells → cutaneous immunity
o Merkel cells: touch sensation
• Composed of collagen (type I and III), elastin, extracellular matrix
• Contains:
o Blood vessels
o Lymphatics
o Nerve endings
o Immune cells (mast cells, macrophages)
• Provides mechanical strength and elasticity
o Holocrine secretion, produce sebum → lubrication
o Eccrine: thermoregulation (widely distributed)
o Apocrine: axillae, groin; body odour
• Barrier
o Against pathogens, UV radiation, water loss
• Temperature regulation
o Via sweating, vasodilation/constriction
• Sensation
o Pain, touch, temperature, vibration
• Immune defence
o Langerhans cells, local cytokine production
• Vitamin D synthesis
o UVB converts 7-dehydrocholesterol → vitamin D₃
o Example: vitiligo, freckles
o Example: large café-au-lait spot
o Example: wart, lichen planus
o Example: psoriasis
o Example: rheumatoid nodule
o Example: herpes simplex
o Example: bullous pemphigoid
o Example: acne
• Ulcer: loss of epidermis and dermis
• Crust: dried serum, blood, or pus
• Scale: flaking stratum corneum
• Lichenification: thickening with exaggerated skin lines (chronic rubbing)
• Koebner phenomenon: new lesions at sites of trauma (psoriasis, lichen planus, vitiligo)
• Auspitz sign: pinpoint bleeding on scale removal (psoriasis)
• Blanching vs non-blanching: helps differentiate purpura/petechiae (non-blanching) from erythema (blanching)
• Darier sign: urtication after stroking lesion (mastocytosis)
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Author & Educational Disclaimer
Author:
Dr Phillip Cockrell BM FRCP DipClinEd
Dr Phillip Cockrell is a UK Consultant Physician in Internal Medicine, currently working at Queen Alexandra Hospital, Portsmouth University Hospitals NHS Trust. He has previously worked as a registrar across Intensive Care Medicine, Gastroenterology, Cardiology, Stroke Medicine, Acute Medicine, and Respiratory Medicine.
He has held senior leadership roles including Associate Clinical Director of the Acute Medical Unit, Clinical Director of Internal Medicine, and Chief of Medicine. Dr Cockrell has over 15 years’ experience in postgraduate medical education, having lectured extensively across the MRCP syllabus and contributed to MRCP revision teaching and course development.
Dr Cockrell holds a Bachelor of Medicine (BM), Fellowship of the Royal College of Physicians (FRCP), and a Diploma in Clinical Education (DipClinEd). His teaching approach is based on structured consolidation of complex medical topics to support efficient and effective revision for postgraduate examinations.
Purpose of this content:
The material on this page is intended solely for educational purposes to support revision for the MRCP (UK) Part 1 examination. It reflects examination-relevant principles of internal medicine and is designed to aid learning and pattern recognition.
Medical disclaimer:
This content is designed for postgraduate medical examination revision and does not constitute medical advice, diagnosis, or treatment guidance and must not be used as a substitute for professional clinical judgement, local guidelines, or specialist consultation. Clinical decisions should always be made in the context of individual patient circumstances and current national guidance.
