Memory tricks for skin, hair, nails, and glands

Epidermis: outermost layer — stratified squamous epithelium, avascular (no blood vessels). Produced by keratinocytes. Deepest layer (stratum basale) divides continuously → cells pushed up → die and fill with keratin → shed (desquamation). Dermis: middle layer — dense irregular connective tissue with collagen and elastin. Contains blood vessels, hair follicles, sweat glands, sebaceous glands, nerve endings. Papillary layer (loose CT, fingerprint ridges) and reticular layer (dense CT, structural strength). Hypodermis (subcutaneous layer): deepest — adipose and loose CT. Anchors skin to underlying muscle. Not technically part of the skin — insulation, energy storage, shock absorption.

Stratum Corneum (outermost): 20-30 layers of dead, flattened, fully keratinized cells — the waterproof barrier. Shed continuously. Stratum Lucidum: only in thick skin (palms, soles) — clear layer of dead cells. Stratum Granulosum: cells begin dying, keratohyalin granules form — transition zone. Stratum Spinosum: several layers of living cells with desmosomes — gives "spiny" appearance. Contains Langerhans cells (immune). Stratum Basale (deepest): single layer of stem cells that continuously divide — new keratinocytes produced here. Contains melanocytes (melanin pigment) and Merkel cells (touch receptors).

Keratinocytes (90% of epidermis): produce keratin — waterproof structural protein. Born in stratum basale → pushed upward → die → shed. Langerhans cells (in stratum spinosum): dendritic immune cells — capture antigens and present to T cells. First immune line in skin. Melanocytes (in stratum basale): produce melanin via melanosomes → transferred to keratinocytes via long dendrites → absorbs UV radiation, protects DNA. Everyone has same number of melanocytes — skin color difference is melanin AMOUNT. Merkel cells (stratum basale): touch receptors — detect light touch and pressure. Associate with sensory nerve endings.

Protection: physical barrier against microbes, UV, chemicals, mechanical injury. Waterproofing: keratin prevents water loss and entry — prevents dehydration. Thermoregulation: sweat glands cool the body, blood vessel dilation/constriction, arrector pili muscles (goosebumps — vestigial hair raising for insulation). Sensation: Meissner's corpuscles (light touch), Pacinian corpuscles (pressure and vibration), free nerve endings (pain, temperature). Vitamin D synthesis: UV light converts cholesterol derivative → Vitamin D3 → liver and kidney activate → essential for calcium absorption. Excretion: small amounts of waste (urea, NaCl) in sweat.

Eccrine sweat glands: most numerous — all over body, most on palms and soles. Secrete watery sweat (99% water + NaCl + urea) directly to skin surface via duct. Thermoregulation — activated by heat and emotional stress. Innervated by sympathetic cholinergic fibers. Apocrine sweat glands: in armpits, groin, areola, pubic region. Larger than eccrine — open into hair follicles. Produce thicker, protein-rich secretion → bacteria metabolize → body odor. Activated at puberty and by stress/sexual arousal. Sebaceous (oil) glands: everywhere except palms and soles. Produce sebum (oil) — keeps skin and hair soft, antibacterial (lowers pH), prevents drying. Holocrine secretion (whole cell ruptures). Plugged sebaceous gland = acne.

Hair shaft: visible part above skin. Hair root: below skin surface, within follicle. Hair follicle: tube of epithelium extending into dermis — wall cells produce hair. Inner structure (deep to superficial): medulla (central core, pigment), cortex (bulk of hair, keratin), cuticle (outer scale-like layer — overlapping). Hair matrix: rapidly dividing cells at base of follicle bulb. Hair papilla: supplies nutrients via blood vessels. Hair growth cycle: anagen (active growth, 2-6 years), catagen (transition, 2-3 weeks), telogen (resting, 3 months → hair shed). Arrector pili: smooth muscle attached to follicle — contracts → goosebumps + hair stands up (piloerection).

Nails are scale-like modifications of the epidermis — heavily keratinized. Nail plate: visible hard portion — dead, keratinized cells. Nail bed: epidermis beneath nail plate — highly vascular (pink color from blood). Nail matrix: proximal region beneath nail (under the cuticle) — actively divides to produce nail plate. Lunula: whitish crescent at base — visible part of matrix (less vascular). Eponychium (cuticle): narrow band of epidermis covering nail root. Nail growth: ~3 mm/month fingernails, 1 mm/month toenails. Clinical: clubbing (O2 deficiency), koilonychia (spoon nails — iron deficiency), splinter hemorrhages (endocarditis), yellow nails (fungal infection).

Basal cell carcinoma (BCC): most common skin cancer (80%) — from stratum basale. Pearly, raised nodule with rolled border. Grows slowly, rarely metastasizes. Best prognosis. Squamous cell carcinoma (SCC): from stratum spinosum — scaly, ulcerated lesion. Can metastasize — second most common. Melanoma: from melanocytes — least common but most dangerous. Highly metastatic. ABCDE warning signs: Asymmetry (one half unlike other), Border (irregular), Color (multiple colors), Diameter (>6 mm = pencil eraser), Evolving (changing over time). Risk factors: UV exposure, fair skin, family history, many moles. SPF sunscreen blocks UV.

First-degree (superficial): only epidermis damaged. Red, dry, painful. Sunburn. Heals in 3-5 days with no scarring. Second-degree (partial thickness): epidermis + part of dermis. Blisters, very painful (nerve endings intact and exposed). Heals in 2-3 weeks — may scar. Third-degree (full thickness): epidermis, dermis, and possibly hypodermis destroyed. Leathery, waxy, or charred — NOT painful (nerves destroyed). Requires skin grafting. Rule of Nines: used to estimate burned body surface area — head 9%, each arm 9%, each leg 18%, anterior trunk 18%, posterior trunk 18%, perineum 1%. Critical burns: >25% BSA second-degree, any third-degree. Major risk = infection and fluid loss.