skin lecture

Definition: The integumentary system consists of the skin, certain glands, toenails, and fingernails, making it a complex structure.

Epidermis: The outer layer of the skin, primarily composed of epithelial tissue.
- Thinnest Areas: Eyelids have the thinnest skin.
Dermis: Beneath the epidermis, rich in connective tissue and vascular with blood vessels, making it substantially thicker than the epidermis.
Hypodermis: Not officially part of the skin, containing areolar and adipose tissue, functions as connective tissue.

Largest Organ: The skin is the largest organ in the body by area and weight, approximately 22 square feet and weighs about 10-11 pounds, accounting for around 16% of total body weight.
Variability: The thickness of skin varies across different body parts, being thinnest on eyelids and thickest on palms and soles.

Thermoregulation: Maintains body temperature through sweating and adjusting blood flow to the dermis.
Blood Reservoir: Serves as a reservoir for blood and can redirect blood flow during physical activity.
Protection: Provides multiple types of protection:
- Barrier against pathogens: Keratin in epidermis prevents the entry of harmful substances.
- Chemical protection: Acidic sweat kills certain microbes.
- UV protection: Melanin absorbs UV radiation to protect the underlying tissues.
Sensory Reception: Contains different sensory receptors for touch, pressure, temperature, and pain.
Excretion and Absorption: Eliminates certain waste through perspiration; allows some substances to be absorbed (e.g., lidocaine patches).
Vitamin D Synthesis: UV light activates the production of vitamin D, crucial for calcium absorption in the intestines.

Epidermis: Avascular (no blood vessels) made up of keratinized stratified squamous epithelium.
- Comprised of 5 layers in thick skin:
  - Stratum Corneum: Outermost layer of dead keratinocytes, protects underlying layers.
  - Stratum Lucidum: Present only in thick skin, provides an additional protective layer.
  - Stratum Granulosum: Cells begin to die and flatten; responsible for keratin hydration and waterproofing.
  - Stratum Spinosum: Cells provide strength and flexibility to the skin.
  - Stratum Basale: Deepest layer containing actively dividing cells (keratinocytes).
Dermis: Vascular, consisting of:
- Papillary Region: Contains dermal papillae that increase surface area for nutrient transfer and sensation.
- Reticular Region: Dense irregular connective tissue provides strength and elasticity to the skin.

Keratinocytes: Most abundant cell type, responsible for producing keratin; provides waterproofing properties.
Melanocytes: Produce melanin, a pigment that protects against UV light; about 8% of epidermal cells.
Dendritic Cells (Langerhans Cells): Immune response functions, originating from bone marrow.
Tactile Cells (Merkel Cells): Function in touch sensation, located at the epidermal-dermal junction.

Hair: Composed of dead keratinized cells, functions include protection and sensation.
Structure of Hair: Comprised of three layers—medulla (inner), cortex (middle), and cuticle (outer). Hair originates from hair follicles that include dermal and epithelial root sheaths.
Types of Hair:
- *Lanugo: Fine hair on fetuses.
- Vellus: Fine, short body hair.
- Terminal Hair: Coarser, longer hair, typically found on the scalp and during puberty.
Nails: Protect fingertips, composed of keratin as well.

Primary Pigments: Melanin, carotene, and hemoglobin contribute to skin color.
- Melanin Types: Eumelanin (brown/black), pheomelanin (yellow/red).
Skin Color Variance: Determined by the type and amount of melanin produced; everyone has similar counts of melanocytes.
Freckles: Concentration of melanin in specific areas.

Epidermal Ridges: Increase grip on surfaces, contribute to fingerprints.
Stretch Marks (Striae): Result from stretching the skin beyond its limit; commonly occur during pregnancy.
Tension Lines: Surgeons must consider these when making incisions to promote better healing.

Keratinization: The process of cell maturation and keratin production, crucial for skin's protective capabilities.
Hair Growth Cycle: Comprising growth (anagen), regression (catagen), and resting (telogen) phases.