Skin Anatomy and Function – Key Points

Cutaneous Membrane and Basic Organization

• Skin = cutaneous membrane; largest organ.
• Two layers: $\text{epidermis}$ (outer) and $\text{dermis}$ (inner);.$-$ Hypodermis (subcutaneous) under skin; not technically part of skin but contains blood vessels and nerves; common injection site.

Epidermis and Dermis: Composition

• Epidermis: stratified squamous keratinized epithelium; keratin = waterproof protein; avascular.
• Dermis: dense irregular + areolar tissue; two sublayers: $\text{papillary layer}$ (superficial) and $\text{reticular layer}$ (deeper).

Epidermal Layers (superficial to deep)

• Generally $4$ epidermal layers; thick skin has $5$ layers.
• Layers (superficial to deep): $\text{stratum corneum},\ \text{stratum lucidum (thick skin only)},\ \text{stratum granulosum},\ \text{stratum spinosum},\ \text{stratum basale}$.

Dermal Anatomy and Fingerprints

• Dermal papillae project into epidermis; form undulating border and create fingerprints (unique, formed in utero).
• Papillary layer: areolar tissue; highly vascularized; contains Meissner's corpuscles (touch).
• Reticular layer: dense irregular tissue; houses sweat glands, sebaceous glands; contains Pacinian corpuscles (pressure).
• Friction can separate epidermal ridges from dermis to form a blister.

Skin Function and Blood Flow

• Functions: insulation, cushioning, protection from mechanical/chemical/thermal damage; prevents water loss.
• Regulates heat via capillary dilation/constriction (flushed with heat; paleness when cold).
• Epidermis is avascular; dermis is vascularized.

Melanocytes and Pigmentation

• Melanocytes produce melanin; melanin provides UV protection; freckles = concentrated melanin.
• Melanoma risk exists; suspicious lumps should be checked.

Hair, Sebaceous Glands, and Sebum

• Hair structure: follicle, root (within follicle), shaft (outside).
• Arrector pili muscles cause goosebumps; contraction also helps expel sebum from sebaceous glands.
• Sebaceous glands produce sebum: oil that lubricates, moisturizes, protects; associated with hair follicle;
  present everywhere except palms and soles.
• Sebaceous glands are holocrine (whole cell disintegrates to release sebum).
• Vitamin E may aid in reducing oxidative damage and helping skin infections.

Sweat Glands

• Outlets: pores.
• Eccrine glands: small, tightly coiled; secrete clear perspiration (water, salt, urea); mainly for heat regulation; highest density in palms.
• Apocrine glands: larger; secrete milky, protein-lipid rich fluid; odor via bacterial breakdown; located in axillae and pubic regions; hormonally regulated.

Skin Model, Thick vs Thin Skin, and Microscopy

• Thick skin (palms/soles) contains $5$ epidermal layers; thin skin has $4$.
• Presence of $\text{stratum lucidum}$ helps identify thick skin.
• Dermal papillae and hair follicles visible in slides; Meissner's corpuscles in papillae; Pacinian corpuscles in reticular layer.
• Ringworm: hairs break easily; hair tips appear irregular under microscopy.
• Observational tips: typically view with power $4\times$ to $10\times$; inspect epidermis, dermis, and hair follicles.

Clinical Correlations and Quick Facts

• Suturing: dermis provides most holding strength; intradermal sutures anchor within the dermis.
• Psoriasis: abnormal epidermal turnover leading to plaques due to improper migration of keratinocytes.
• Ringworm (dermatophyte infection): hair shaft breakage and brittle hairs visible under microscope.