Skin Structure

Epidermis

•    Stratified squamous epithelium

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)

•    Cell types:

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

Dermis

•    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

Appendages (Skin adnexal structures)

•    Hair follicles

•    Sebaceous glands

o    Holocrine secretion, produce sebum → lubrication

•    Sweat glands

o    Eccrine: thermoregulation (widely distributed)

o    Apocrine: axillae, groin; body odour

Skin Functions

•    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

Lesion Terminology

Primary lesions

•    Macule: flat, non-palpable, <1 cm

o    Example: vitiligo, freckles

•    Patch: flat, non-palpable, >1 cm

o    Example: large café-au-lait spot

•    Papule: raised, solid, <1 cm

o    Example: wart, lichen planus

•    Plaque: raised, flat-topped, >1 cm

o    Example: psoriasis

•    Nodule: solid, deeper and larger than papule (>1 cm)

o    Example: rheumatoid nodule

•    Vesicle: fluid-filled, <1 cm

o    Example: herpes simplex

•    Bulla: fluid-filled, >1 cm

o    Example: bullous pemphigoid

•    Pustule: pus-filled, <1 cm

o    Example: acne

Secondary lesions

•    Ulcer: loss of epidermis and dermis

•    Crust: dried serum, blood, or pus

•    Scale: flaking stratum corneum

•    Lichenification: thickening with exaggerated skin lines (chronic rubbing)

Extra Revision Pearls

•    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)