Skin Biology Quiz: Melanin and Epidermis

Integumentary System Overview

• The integumentary system includes the skin and its accessory organs (hair, nails, and glands).
• It's the largest and heaviest organ of the body, covering 1.5-2.0 m² and 15% of body weight.
• Skin thickness ranges from 0.5-6 mm, thinner skin (0.1 mm) covers most of the body, while thick skin (0.5 mm) is on the palms and soles.

Learning Outcomes

• Students will be able to discuss the major functions of the skin.
• Students will be able to describe the structures of the integumentary system.
• Students will be able to outline the structure and cells of the skin layers.
• Students will be able to describe appendages of skin (hair and nails).
• Students will be able to name and describe two types of sweat glands.
• Students will be able to describe cancers involving skin cells.
• Students will be able to discuss the skin healing process after injury.

Skin Functions

• Resistance to trauma and infection: keratin, acid mantle
• Water barrier: prevents water loss, with tight junctions between skin cells. This helps prevent dehydration.
• UV radiation protection
• Defense against harmful chemicals:
• Vitamin D synthesis: a first step, liver & kidneys complete the process.
• Sensation: extensive sense organ, receptors for temperature, touch, pain, regulating temperature (vasoconstriction/vasodilation), perspiration
• Nonverbal communication: facial expression and its role in social acceptance and self-image

Skin Structure and Layers

• Three main layers: epidermis (outer, stratified squamous epithelium), dermis (middle, deeper connective tissue layer), hypodermis (deep, connective tissue layer below the dermis)
• Epidermis: keratinized stratified squamous epithelium, keratin protein for toughness, layers (strata).
• Main cell type: keratinocytes
• Lacks blood vessels, nutrients diffuse from underlying connective tissue, ending in nerve endings & receptors for touch & pain

Layers of the Epidermis

• Stratum basale: deepest layer, single layer of stem cells and keratinocytes resting on the basement membrane

• Stem cells: divide and form keratinocytes (migrate to surface to replace lost cells), melanocytes and tactile cells

• Stratum spinosum: Several layers of keratinocytes connected by desmosomes and tight junctions.

• Stratum granulosum: Three to five layers of flat keratinocytes, contain dark-staining keratohyalin granules

• Stratum lucidum: only in thick skin, thin, pale layer of keratinocytes packed with clear protein eleidin (converts to keratin at surface).

• Stratum corneum: surface layer, several layers of dead scaly keratinized cells that resist abrasion, penetration, and water loss.

Cells of the Epidermis

• Keratinocytes: majority, synthesize keratin, produced by mitosis in stratum basale.
• Melanocytes: synthesize pigment melanin, shields DNA from UV radiation, occur in stratum basale.
• Dendritic cells: macrophages, originate in bone marrow, guard against pathogens, in stratum spinosum and stratum granulosum
• Tactile cells: touch receptor cells, associated with dermal nerve fibers, basala layer

Skin Disorders

• Ichthyosis: Genetically inherited (or acquired), shedding process inhibited, build-up of dead cells, can result in overheating
• Eczema: Chronic, inherited inflammatory skin condition, symptoms include dry, itchy, and reddened skin
• Exfoliative dermatitis: Excessive shedding of skin

Dermis

• Connective tissue layer below epidermis, 0.2 mm (eyelids) to 4 mm (palms, soles), mainly collagen, rich in blood vessels, sweat glands, sebaceous glands, nerve endings, hair follicles & nail roots, skeletal muscles of facial expression.
• Two layers: -Papillary layer: superficial, thin areolar tissue near the dermal papillae; allows mobility of leukocytes. -Reticular layer: deeper, thicker, dense, irregular connective tissue, stretch marks (striae) due to tears in collagen fibers from stretching.

Hypodermis

• Subcutaneous tissue, more areolar and adipose tissue than in the dermis, pads body & binds skin to underlying tissues, abundant blood vessels (site of drug injection).
• subcutaneous fat: energy reservoir, thermal insulation, thicker in women, thinner in infants and elderly

Skin Appendages: Hair and Nails

• Hair: slender filament of keratinized cells growth from follicle.
• covers most of body, 100,000 hairs on scalp, 55-70 hairs per cm² on limbs & trunk, 10x on face.
• Three zones:
• Bulb: swelling at base, living cells.
• Root: remainder in follicle.
• Shaft: portion above skin surface
• Nail: finger/toe nails - clear, hard derivatives of stratum corneum, composed of thin, dead cells packed with hard keratin. -Functions: Grooming, food manipulation, sensitivity enhancement. (nail plate, free edge, nail body, nail root)

Skin Cancer

• Most cases caused by UV radiation.
• Common sites: head, neck, hands.
• Common in fair skin people & elderly.
• Three main types: basal cell carcinoma, squamous cell carcinoma, malignant melanoma

Skin Healing

• Stages of a wound healing:
  • Bleeding into the wound.
  • Scab formation (macrophages, fibroblasts, clotting).
  • Granulation tissue formation (new capillaries, fibroblasts).
  • Epithelial regeneration/remodeling (scarring, weeks to years).

Glands of the Skin

• Sweat glands: Two main kinds: apocrine and merocrine
  • Apocrine: found in groin, anal area, armpits, secretions milky, contain fatty acids, become active during puberty.
  • Merocrine: most numerous, especially on palms, soles, and forehead, simple tubular glands, watery sweat helps cool the body, have myoepithelial cells which squeeze secretions.
• Sebaceous glands: flask-shaped, short ducts to hair follicles, secrete sebum, oily secretion, keeps skin/hair from becoming dry/brittle/cracked (lanolin).

Other Information

• Skin color variations determined by melanin production and distribution.
• Environmental factors (sun exposure) affect melanin production, pigmentation.
• Other pigments (hemoglobin, carotene) also contribute to skin color (e.g., jaundice=bilirubin in blood).
• Conditions affecting nails: different potential issues (not elaborated).

Burns

• Leading causes of accidental deaths, Fires, spills, sunlight, ionization, strong acids/bases, electrical shocks.
• Can result in primary fluid loss, infection & toxic effects of eschar.
• Debridement is removing the eschar.
• Different degrees of burns (first, second, third-degree) are categorized by tissue involvement, symptom differences, and treatment requirements.

Stages in the Healing Process

• Defined stages leading to full wound healing.