Skin Anatomy and Function

Skin

• The skin is an organ and part of the integumentary system.
• Functions of the skin:
  • Protection
  • Insulation
  • Storage
  • Regulation of body temperature
  • Sensation
  • Synthesis of Vitamin D
  • Excretion

Layers of the Skin

• Epidermis: The outer portion, composed of keratinized stratified squamous epithelium.
• Dermis: Connective tissue layer.
• Hypodermis: Also known as superficial fascia.

Integument Layers

The integument consists of the following layers:

• Epidermis
• Dermis
• Subcutaneous layer
  • Papillary layer
  • Reticular layer

Epidermis

• Outer portion of the skin.
• Major cell types include:
  • Keratinocytes: Produce keratin.
  • Melanocytes: Produce melanin.
  • Langerhans cells: Role in the immune system.
  • Merkel cells: Sense of touch.

Layers of Epidermis

• Stratum basale (germinativum):
  • Deepest layer.
  • High mitotic activity.
  • One layer thick.
  • Contains melanocytes.
• Stratum spinosum:
  • Several layers.
  • Contains keratinocytes.
• Stratum granulosum:
  • 3-5 layers.
  • Contains keratohyaline granules.
• Stratum lucidum:
  • Only in thick skin.
  • 2-3 rows.
• Stratum corneum:
  • Outermost layer.
  • 20-30 layers of dead cells.
  • Contains keratin.

Melanocytes

• Melanocytes produce melanin, which is then transferred to keratinocytes.
• Melanin pigment protects the skin from UV radiation.

Dermis

• Two parts:
  • Papillary region:
    • Upper 20% of the dermis.
    • Composed of areolar connective tissue.
    • Contains dermal papillae, which form fingerprints.
    • Contains nerve endings.
  • Reticular region:
    • Lower 80% of the dermis.
    • Contains blood vessels.

Integument Layers and Subcutaneous Layer

• Epidermis:
  • Stratum corneum: Most superficial layer; 20-30 layers of dead, flattened, anucleate, keratin-filled keratinocytes.
  • Stratum lucidum: 2-3 layers of anucleate, dead keratinocytes; only in thick skin (palms, soles).
  • Stratum granulosum: 3-5 layers of keratinocytes with distinct granules; keratinization begins here.
  • Stratum spinosum: Several layers of keratinocytes attached by desmosomes; epidermal dendritic cells present.
  • Stratum basale: Deepest, single layer of cuboidal to low columnar keratinocytes; cell division occurs; contains melanocytes and tactile cells.
• Dermis:
  • Papillary layer: Superficial layer of areolar connective tissue; forms dermal papillae; houses capillaries and tactile receptors.
  • Reticular layer: Deeper layer of dense irregular connective tissue; supports hair follicles, sebaceous and sweat glands, nerves, and blood vessels.
• Subcutaneous layer:
  • Not part of the integument; deep to the dermis; composed of areolar and adipose connective tissue.

Epidermis Functions

• Protection:
  • Provides layers of protection against harmful chemicals, toxins, microbes, and excessive heat or cold.
  • Protects deeper tissues from UV radiation as melanocytes produce more melanin.
• Prevention of water loss and water gain:
  • The epidermis is water-resistant.
• Metabolic regulation:
  • Keratinocytes produce vitamin D upon exposure to UV rays, and melanocytes produce more melanin.
• Secretion and absorption:
  • Materials (sebum, sodium, water, urea) secreted by dermal structures are released onto the epidermal surface.
  • The skin is selectively permeable.
• Immune function:
  • Epidermal dendritic cells engulf and destroy pathogens and alert the immune system.

Dermis Functions

• Temperature regulation:
  • Dilating blood vessels release heat; constricting vessels conserve heat.
  • Sweat glands release fluid onto the skin surface; the body cools off by evaporation.
• Sensory reception:
  • Sensory receptor structures detect and relay pain, heat, cold, touch, pressure, and vibration.
• Secretion and absorption:
  • Sweat glands secrete sodium, water, and urea to maintain electrolyte homeostasis.
  • Sebaceous glands secrete sebum, which lubricates the skin and hair and makes the integument water-resistant.

Glands

• Sweat (sudoriferous) glands:
  • Eccrine (merocrine) glands: Simple coiled tubular gland.
  • Apocrine glands: In axillary and genital areas.
• Sebaceous (oil) glands:
  • Holocrine glands; secrete sebum.
  • Most associated with hair follicles.

Hair

• Shaft: Complete keratinization; projects from the skin.
• Root.
• Hair follicle: Hair bulb; vascular and nerve supply.
• Arrector pili: Muscles attached to the follicle that pull on follicles to produce goosebumps.

Hair Structure

• Hair consists of:
  • Cuticle
  • Medulla
  • Cortex
• Hair follicle root sheath
  • Epithelial tissue
  • Connective tissue
• Matrix
• Hair papilla