Unit
- Epidermal layers: The epidermis consists of five layers: stratum basale, spinosum, granulosum, lucidum (in palms and soles), and corneum. Each layer has distinct cell types and functions that contribute to barrier integrity.
- Keratinocytes: These cells produce keratin filaments and undergo terminal differentiation from basal layer to desquamation. This process takes about 28 days in healthy skin and is accelerated in conditions like psoriasis.
- Melanocytes: Located in the stratum basale, melanocytes produce melanin and transfer it to keratinocytes via dendrites. Melanin provides photoprotection and determines skin pigmentation, with variations in number and activity across ethnic groups.
- Langerhans cells: These antigen-presenting cells reside in the suprabasal epidermis and play a key role in cutaneous immune surveillance. They capture pathogens and migrate to lymph nodes to initiate adaptive immune responses.
- Barrier function: The stratum corneum provides a physical and chemical barrier. Its lipid matrix, composed of ceramides, cholesterol, and free fatty acids, regulates transepidermal water loss and prevents entry of microbes and irritants.
- Desmosomes: These intercellular junctions connect keratinocytes, providing mechanical strength. Cadherin proteins form the adhesive core, and their loss contributes to blistering disorders like pemphigus.
- Epidermal turnover: Normal renewal involves coordinated proliferation, differentiation, and desquamation. Disruption leads to scaling (e.g., ichthyosis) or hyperkeratosis (e.g., calluses).
- Acid mantle: The skin's surface pH is slightly acidic (4.5-5.5), which inhibits pathogen growth and maintains barrier function. Alkaline cleansers can disrupt this balance, increasing susceptibility to infection.
- Stratum corneum structure: The stratum corneum consists of 15 to 30 layers of corneocytes embedded in a lipid matrix. Corneocytes are flattened anucleate cells filled with keratin filaments and surrounded by a cornified envelope. The intercellular lipids are organized into lamellar bilayers that are critical for barrier function.
- Tight junctions: Located in the stratum granulosum, tight junctions form a second barrier within the epidermis. Proteins like claudin-1 and occludin seal intercellular spaces, restricting paracellular diffusion of water and solutes. Defects in tight junctions contribute to atopic dermatitis.
- Natural moisturizing factor (NMF): NMF consists of amino acids, lactic acid, urea, and other hygroscopic compounds derived from filaggrin breakdown. It maintains stratum corneum hydration. Mutations in filaggrin lead to dry skin and increased risk of eczema and allergies.
- Skin microbiome: The skin hosts diverse bacteria, fungi, and viruses that influence barrier health. Commensal organisms like Staphylococcus epidermidis produce antimicrobial peptides and inhibit pathogen colonization. Disruption of the microbiome is linked to inflammatory skin diseases.
Scope of Practice
Role: Dermatology Clinical Assistant
Within scope:
- Measuring transepidermal water loss using handheld devices
- Documenting skin lesion descriptions using standardized terminology
- Educating patients on gentle cleansing and moisturizing routines
- Preparing skin for allergy patch testing procedures
Outside scope:
- Interpreting biopsy results or diagnosing barrier disorders
- Prescribing topical barrier repair products
- Performing skin stripping or tape stripping for research
- Modifying treatment plans based on barrier function tests
Quick Revision
- The epidermis is a stratified epithelium with five layers, each contributing to barrier function and immune defense.
- Keratinocytes, melanocytes, and Langerhans cells work together to protect the body and regulate pigmentation.
- The stratum corneum lipid matrix is essential for preventing water loss and microbial entry.
- Clinical assistants can support barrier assessment and patient education but not independent diagnosis or prescribing.