GF

Skin Anatomy and Melanin (Chapters 1–7)

Skin anatomy overview

  • Skin is the largest organ; weight about 10\text{-}11\ \text{lb} and makes up about 16\% of body weight.
  • Two main skin regions: epidermis (superficial) and dermis (deeper). Subcutaneous layer (hypodermis) sits beneath the dermis and connects to underlying tissue.
  • Thin skin vs thick skin: thin skin has 4 epidermal layers; thick skin (palmar/plantar) has 5 layers.

Epidermis

  • Tissue type: \text{keratinized stratified squamous epithelium}; avascular.
  • Cells in the epidermis:
    • \text{Keratinocytes} — produce keratin; form water-repellent barrier; most abundant.
    • \text{Melanocytes} — produce melanin; located in the deepest layer; pigment protects from UV.
    • \text{Langerhans cells} — dendritic immune cells in the stratum spinosum.
    • \text{Merkel cells} — tactile receptors in the basal layer.
  • Epidermal layers (bottom to top) with key features:
    • Stratum basale (basal layer): single cell layer; where stem cells reside; melanocytes and Merkel cells present.
    • Stratum spinosum: several cell layers; contains Langerhans cells.
    • Stratum granulosum: keratinization begins; cells start dying.
    • Stratum lucidum: clear layer present only in thick skin.
    • Stratum corneum: outermost, several layers of dead, keratin-filled cells; provides barrier.
  • Important notes:
    • Keratinocytes produce keratin; keratin provides waterproof, protective barrier.
    • Melanin protects DNA from UV-induced mutations; amount determines skin tone; melanocytes reside in the deepest epidermal layer.
    • The epidermis is avascular; it relies on the underlying dermis for oxygen/nutrients via blood vessels.
    • The epidermis is constantly renewing from the basal layer; turnover from basal to corneum takes about 7\text{-}10\ \text{days}; cells in corneum can remain for a couple of weeks before shedding.

Dermis

  • Two sublayers:
    • Papillary dermis: areolar connective tissue; forms finger-like dermal papillae that interface with epidermal ridges.
    • Reticular dermis: dense irregular connective tissue; rich in collagen and elastic fibers.
  • Functions: structural strength, elasticity, houses blood vessels, nerves, and most accessory structures.
  • Accessory structures in the dermis include hair follicles, sebaceous glands, sweat glands, and the erector pili muscle.
  • The dermis is the main site for vascular supply to the skin; unlike the epidermis, it contains blood vessels.

Subcutaneous layer (hypodermis)

  • Not a true skin layer; primarily connective tissue and adipose tissue.
  • Connects dermis to underlying tissue (usually muscle).
  • Major fat depot; about 80\% of body fat stored here.

Accessory structures

  • Hair follicle: produces hair; rapidly dividing cells in the bulb are targeted by chemotherapy (hair loss).
  • Sebaceous gland: secretes sebum to lubricate hair and skin; glands are often associate with hair follicles; activity increases at puberty.
  • Sweat glands: regulate temperature and excretion.
  • Erector pili muscle (smooth muscle): causes goosebumps when stimulated by cold or emotion.

Skin markings and patterns

  • Dermal papillae and epidermal ridges create fingerprints/footprints; pattern is unique to each person.

Skin color determinants

  • Primary factors:
    • Melanin type and amount (produced by melanocytes in the epidermis; absorbs UV light).
    • Hemoglobin content and blood flow (pink/reddish tones in lighter skin).
    • Carotene (yellow-orange pigment from diet can deposit in the stratum corneum).
  • Melanin and UV protection:
    • More melanin provides greater UV protection but requires longer sun exposure for vitamin D synthesis.
  • Tan vs. tattoo:
    • Tan: epidermal pigmentation increase due to increased melanin production; fades as epidermal turnover sloughs off keratinocytes.
    • Tattoo: pigment deposited in the dermis; generally permanent because the dermis does not slough off.

Practical exam notes: thick vs thin skin and layer identification

  • Mnemonic for epidermal layers (bottom to top):
    • Stratum Basale (basal), Stratum Spinosum, Stratum Granulosum, Stratum Lucidum (present only in thick skin), Stratum Corneum.
  • Basal layer: single cell layer; contains stem cells, melanocytes, Merkel cells.
  • Spinosum: several cell layers; contains Langerhans cells.
  • Granulosum: cells begin dying; keratinization starts.
  • Lucidum: present only in thick skin; dead, flattened cells.
  • Corneum: outermost; dead, keratin-filled; protective barrier.

Clinical and lifestyle notes

  • Ulcers (decubitus ulcers): result of prolonged pressure cutting off dermal blood supply.
  • UV exposure accelerates aging by damaging collagen and elastin; can lead to wrinkles.
  • Smoking adversely affects skin health and aging.